Nanomaterials by design: a review of nanoscale metallic multilayers

Nanoscale metallic multilayers have been shown to have a wide range of outstanding properties, which differ to a great extent from those observed in monolithic films. Their exceptional properties are mainly associated with the large number of interfaces and the nanoscale layer thicknesses. Many studies have investigated these materials focusing on magnetic, mechanical, optical, or radiation tolerance properties. Thus, this review provides a summary of the findings in each area, including a description of the general attributes, the adopted synthesis methods and most common characterization techniques used. This information is followed by a compendium of the material properties and a brief discussion of related experimental data, as well as existing and promising applications. Other phenomena of interest, including thermal stability studies, self-propagating reactions and the progression from nano multilayers to amorphous and/or crystalline alloys, are also covered. In general, this review highlights the use of nano multilayer architectures as viable routes to overcome the challenges of designing and implementing new engineering materials at the nanoscale.


Introduction
Over the past four decades, the synthesis, characterization and applications of materials at the nanoscale has been the focus of vast amounts of research. This is in part due to the continuous progress in science and technology, which enabled the development of engineered materials with atomic-level precision [1][2][3][4][5][6]. The interest in materials synthesized at the nanoscale has remained active, since nanoscale features generally lead Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
to unique physical and chemical properties [1,[7][8][9][10][11]. In the present day, the synthesis and application of thin films is one of the broadest research areas in materials research.
The term 'thin film' refers to a layer of material overlaying a surface [12,13], whose purpose is property optimization and/or to provide a specific functionality to a host substrate [13]. Depending on the field, the thickness of a 'thin film' ranges between a few atomic layers to a micron (1 × 10 −6 m) [13][14][15], at which point it is usually agreed that this thickness represents a coating or foil. These foils or coatings may themselves be composed of thin films, as in the case of nano multilayers, which will be discussed in further detail throughout this review. Overall, thin films have been incorporated in multiple technological and commercial areas, including, but not Schematic highlighting the promising mechanical, optical, magnetic and radiation tolerance properties observed in NMMs, which meet the requirements for the development of novel nanostructures (as represented by the outer circle). Center of the figure displays a system comprising Hf-Ta/Hf [49] layers, which exhibits the general configuration and morphology of the stacking of individual layers with different chemical compositions. limited to, diffusion barriers in integrated circuits, data storing devices, food packaging and smart textiles [13,[16][17][18][19][20].
To date, numerous investigations examining the properties of NMMs and relating them to microstructure and composition have been presented. However, most of the available reviews summarizing the current and previous work in the field focus on the description of their magnetic properties [1,3,4,25,[39][40][41][42][43][44][45][46][47][48]. This focus can be attributed to the rapid commercialization and evolution of magnetic devices that are based on NMMs [3]. Therefore, the aim of this review is to present an overview of a wider range of applications and properties for which NMMs play a promising role, while also expanding on some new research areas. The schematic in figure 1 can be used as a guide to summarize the areas that will be covered in this review, where each quadrant represents a focus on their mechanical, optical and magnetic properties, as well as their radiation tolerance, while the outer circle outlines the use of NMMs as precursors for the development of new nanostructures. The first two sections of this review will comprise general information about NMMs and their attributes, followed in the third section by a brief description of the most common synthesis methods and characterization techniques. The fourth section will present a diverse number of NMM systems, which will be classified based on the aforementioned properties, and will depict some of the current and potential applications that have been developed based on their outstanding features. Other emerging topics of interest, such as thermal evolution in NMMs and the formation of crystalline and amorphous alloys, will also be explored. Although this manuscript will not necessarily include all multilayer systems available, it does provide a comprehensive review, which is designed to highlight a wide range of available NMM systems across multiple research fields. The aim is to present the reader with new connections and ideas that will inspire future studies and designs for the synthesis and application of novel nanomaterials.
Most of the NMM systems that have been investigated contain two different alternated layers of metal. Such bimetallic structures are classified according to the interfaces formed between both components as coherent, semi-coherent or incoherent (non-coherent) systems [23,115,116]. Figure 2 presents a schematic illustration of the different interfaces that could be developed in NMMs. In coherent systems (also called superlattices), the two metal components have the same type of crystal structure and a small lattice mistmatch (in general, on the order of a few percent) [10,32,98,117,118]. In semicoherent systems, the crystal structure of the components is the same, but the lattice mismatch is larger [108,118]. Thus, misfit dislocations are formed in order to accommodate the mismatch. Incoherent systems, on the other hand, are formed of materials that have different crystalline structures, resulting in a larger lattice mismatch [23,98,118].
Interfaces within the multilayer can also be compositionally abrupt or compositionally graded over some distance in the growth direction [5,10]. Compositionally graded means that the change in composition between the two distinct layers (material A into material B) does not occur at a sharp point, implying that atoms from material A migrated into material B through diffusion. If diffusion is not observed within the multilayers, then it is understood that an abrupt (sharp) interface exists [5]. In addition, as each of the layers deposits to form a resulting multilayer, the individual surface morphology contributes to the progression of differing levels of roughness, which affects the quality of the interfaces [5]. Further aspects of the influence of diffusion and roughness and the resulting effects on properties of interest will be discussed in section 4.

NMMs versus monolithic films
As mentioned earlier in the text and according to extensive experimentation, simulations and theoretical studies, the enhanced properties observed in NMMs are mainly associated with the large number of interfaces, but they are also a consecquence of the combined effects of the specific characteristics of the individual layers [5,10,25,34,35,39,95,107,112,[119][120][121][122][123][124][125][126][127][128][129][130]. In contrast to monolithic films, the properties of NMMs are a function of the thicknesses of the layers rather than a function of the grain size [8,10,11,25,53,54,56,60,62,106,115,122,[131][132][133][134][135][136]. For instance, Ni/Cu systems with high periodicity and layer thicknesses in the range of 1.5-3 nm exhibit a different mechanical response to monolithic Ni or Cu thin films [137]. In addition, the properties further depend on the selected multilayer composition (combination of metals) and other intrinsic microstructural parameters including but not limited to roughness, texture, grain boundaries and grain morphology [4,11,34,61,81,108,138,139]. For example, surface roughness contributes to the magnetic transport properties due to the scattering process that occurs at the interfaces [3,25,39,82,131,140], while the use of a columnar grain morphology improves the thermal stability of the system and helps to better retain a multilayer configuration [93,96].

Methods for the synthesis of NMMs
A broad range of microstructures can be achieved in NMMs by means of various synthesis methods and parameters, which results in the ability to vary grain size, grain boundaries and grain orientations as well surface roughness within the layers. The vast number of multilayer systems reported to date were prepared via bottom-up or top-down techniques. Bottom-up methods include electrodeposition (or electroplating) [39,48,109,141] as well as physical vapor deposition methods (PVD) such as electron beam evaporation (thermal evaporation), sputtering, cathodic arc deposition, pulsed laser deposition and molecular beam epitaxy (MBE) [5,6,17,23,39,127,[142][143][144]. Electrodeposition is a technique in which metallic ions that are available in an electrolytic solution are reduced and then deposited onto a plating surface. This is achieved by applying an electric potential difference between a metallic substrate and a reference electrode [39]. In PVD processes, the coating is deposited in vacuum via condensation from a flux of neutral or ionized atoms of metals [12,17]. Regardless of the chemical or physical nature of these commonly used synthesis methods, the formation of NMMs (or single-component thin films and foils) is achieved through the same phase transformations: nucleation, coalescence and growth. During nucleation, atoms or molecules condense as solid entities and grow after further accumulation. Afterwards, these species aggregate (or coalesce) until the desired material forms a continuous film, leading to growth in the thickness of the film [12,39,145,146].
In earlier studies of NMMs, it was believed that controlling the individual layer thickness at the nanometer scale could only be achieved by PVD [48,53]. Recently, top-down techniques have proved to be effective for the production of NMMs, especially via methods classified under severe plastic deformation (SPD) [84,86,105,110,134,[147][148][149][150] such as accumulative roll bonding (ARB). For comparison, figure 3 highlights three different Cu-based NMM systems fabricated by distinct methods: (a) electrodeposition, (b) magnetron sputtering and (c) ARB. Although all three systems in the figure are polycrystalline, the shape, size and orientation of the grains differ in each case, and the contrast between atomically sharp and ordered interfaces that is found in materials synthesized by PVD is evident [86,110,116,150]. Irregular thicknesses and waviness are observed for the material prepared by electrodeposition (figure 3(a)), while homogeneous layers are visible in the sample prepared by sputtering ( figure 3(b)). In addition to these observations, larger grains are noted in the multilayer prepared via ARB (figure 3(c)). These microstructural differences are accompanied by variations in the crystallographic texture, as well as differences in the distribution of residual stresses [82,134,151]. All of these distinctive characteristics lead to fluctuations in the performance of NMMs and depend on the deposition technique, as is the case of the magnetic behavior of NMMs grown by electrodeposition compared to those grown by PVD [82,[152][153][154]. In addition to bottomup and top-down approaches, the flux melting technique commonly used for the synthesis of metallic alloys has recently been adopted as an alternative method for processing bulk nanolayered materials [155][156][157]. This was achieved by exposing the melt to an extremely high undercooling process and could be a feasible option for the synthesis of nanostructured multilayers despite the potential for solid solution formation during cooling [155][156][157].
A partial list of metal/metal nano multilayers classified by composition and accompanied by their respective synthesis method are shown in table 1. This information highlights the fabrication feasibility of various multilayer systems using different techniques. The selection of a particular synthesis methodology depends on several factors including: growth rates, growth morphology, residual stresses, impurities, reproducibility and microstructural imperfections, among others. As can be observed from table 1, and in agreement with other reviews [39,142,144], sputtering prevails as the most common synthesis technique for the growth of a wide range of NMM systems. This is partly due to the fact that almost all elements in the periodic table can be sputtered at high deposition rates, and target materials are generally commercially available or can be readly fabricated [39,142,144]. However, synthesis selection is also highly correlated to the end function of the NMM, which determines the requirements with regard to layer control and definition, i.e. high layer definition via PVD is required for most coatings or components for small-scale devices, while NMMs prepared by ARB are more appropriate for structural applications where bulk quantities of material are required [161,162].
For further guidance on synthesis technique selection, reviews have been grouped for a wide variety of systems according to their respective synthesis method and mechanical [116,130,225,226], magnetic [10,39,43,48,226], optical [73] and electron transport properties [226] or because of their ability to form either reactive multilayers [144] or amorphous metallic glasses (MGs) [227]. In addition, the advantages and disadvantages of several synthesis methods have been summarized in [39]. Tables and other information presented herein can be used to select the appropriate method for the fabrication of a system of interest.

Characterization of NMMs
3.2.1. Conventional characterization methods. As stated in section 2, extensive sample characterization is needed in order to correlate properties, synthesis and performance [61,145]. This is due to the fact that the physical properties of NMMs, which determine their functionality, are sensitive to microstructure [23,82,134] and therefore, to their processing method. Many characterization techniques exist to study the microstructural features of multilayered materials, such as the crystal structure, preferred orientation (texture), grain size and shape, lattice defects and interfaces, among others. Additional methods are also available to measure the resulting mechanical, optical and magnetic properties, etc. These techniques offer diverse capabilities and have different requirements and levels of complexity, which range from ex situ characterization (post-deposition techniques), to the evaluation of the final performance.
For NMMs, the microstructure is usually explored by conventional x-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM) [48,62,109,142,228]. XRD can quickly provide information about the crystal structure, texture and grain size without the need for special sample preparation. Various XRD techniques are also widely used to investigate additional aspects of the multilayers. Wideangle x-ray scattering yields information on the bilayer thickness and interplanar spacings, while small-angle x-ray scattering and x-ray reflectivity experiments offer the opportunity to characterize the continuity of the interfaces between constituent layers (e.g. interface roughness, thickness and chemical intermixing) [48,229]. SEM characterization is acquired in plane and cross-sectional view modes and is usually employed to reveal the morphology of the grains and the surface characteristics, as well as the thickness of the NMM systems. TEM characterization, which is typically performed in the crosssectional mode, is used for the direct observation of the thickness, crystallographic orientation and formation of defects within each of the constituent layers [228]. Further crystallographic information can be obtained in TEM by collecting selected area electron diffraction patterns. In SEM, the quantitative evaluation of the crystallographic structure and the orientation of polycrystalline materials can be obtained via electron backscatter diffraction (EBSD) [28]. Since the  [9,18,28,36,53,86,97,102,105,110,130,136,147,[185][186][187][188][189][190] Cu/Ni Electrodeposition/DC magnetron sputtering/evaporation [2,65,129,[191][192][193][194][195] Cu/V DC magnetron sputtering [166,190] Cu/Mo DC magnetron sputtering [190,196] Cu/Ta ARB/DC magnetron sputtering/RF magnetron sputtering [60,197,198] Cu/Ru RF and DC magnetron sputtering [199] Cu/Pd DC magnetron sputtering [191] [224] thicknesses of the individual layers in NMM systems are often too thin to be characterized by EBSD, the heterophase interface character distribution method was developed to quantify the distribution of the 3D heterophase interface normal vectors using 2D EBSD images [27,230]. By using both SEM and TEM techniques it is also possible to analyze the chemical composition using energy-dispersive x-ray spectroscopy (EDXS) [222,228]. Microanalysis of NMMs can be achieved via electron energy-loss spectroscopy in TEM. However, the accuracy of the measurement is sensitive to the thickness of the specimen [228]. Auger electron spectroscopy (AES or AESDP) is employed to obtain depth profiles of composition and to study the possible occurrence of diffusion in the multilayers [75,170,[231][232][233]. Similar to AES, x-ray photoelectron spectroscopy is another tool that can be used to quantify the elements and determine their chemical composition [234]. In addition, atom probe tomography offers high spatial resolution with respect to chemical composition in 3D, which helps to provide quantitative information on interfacial mixing, segregation and local composition [168]. As for the analysis of surface topography, scanning probe microscopy (SPM) [235] measures the 3D surface structure, providing information such as the area and volume of the particles and therefore, the roughness of the samples. SPM techniques include atomic force microscopy, scanning tunneling microscopy and more recently, magnetic force microscopy [235][236][237], which is used to reconstruct the magnetic structure of the surface. Profilometry, on the other hand, is able to provide information such as surface topography, thickness and the calculation of residual stresses after the deposition [36,238].
In addition to conventional post-deposition characterization methods, the investigation of the microstructure and properties can also be achieved with the implementation of in situ techniques. The importance of such methods relies on the fact that they provide valuable information about the sample in real time, which can be collected during deposition or when the sample is subjected to a specific set of conditions [31]. One of the most common methods is reflection high-energy electron diffraction, which is used to monitor the growth progress during material synthesis by PVD [6]. Growth stresses are another commonly studied property that may be monitored during deposition, and provide insight into the microstructure of the final NMMs [239]. In addition, in situ XRD tests have been used to study the evolution of the microstructure of NMMs upon exposure to ion irradiation [200,240] and to determine their deformation behavior upon heating/cooling [36,194]. Synchrotron XRD experiments have also been carried out during tensile loading to examine the gradual progression in yield behavior, and are interpreted in terms of residual stresses, as well as elastic and plastic anisotropy [136,139,241]. Time-resolved x-ray microdiffraction is used for the observation of self propagating reactions in reactive multilayers, which allows for the study of phase transformation and diffusion [31,127,128,[242][243][244]. In spite of their many advantages, in situ methods usually require access to sophisticated equipment and installations or demand a special sample size or shape preparation. Therefore, conventional ex situ characterization techniques prevail as the most common tools for correlating microstructural parameters to properties and performance.
Once extensive microstructural analysis has been carried out, and depending on the desired application, different characterization methods are available to evaluate the properties of interest. Nanoindentation tests have become one of the most widespread techniques for the mechanical testing of very small volumes of materials, a task that would be difficult or impossible with conventional mechanical testing methods due to the sample size restriction [15,238,245,246]. Hardness and elastic modulus, yield strength, strain hardening, adhesive strength and fracture toughness are some of the many mechanical properties that can be evaluated using nanoindentation [122,245,247,248]. In addition, tensile testing via electromechanical systems (micro tensile tests) can be used to provide a direct measure of a stress-strain curve, which serves to elucidate the onset of plastic deformation, work hardening rate, elongation to failure and fracture mode [188,189]. If freestanding films are obtained, bulge tests can be performed, allowing for measurements of the Young's modulus, residual stress and yield stress [18,38,191,249]. Micropillar compression tests can also be an appropriate approach to characterize the deformation of NMMs and to obtain stress-strain curves in a nominally homogeneous stress state [103,110,200,246,[250][251][252]. Synchrotron x-ray microdiffraction combined with micropillar compression experiments can provide quantitative measurements of dislocation densities in NMMs, which allows for the study of microstructural changes associated with plastic deformation [251,252].
In the field of magnetic materials, the magnetic anisotropy can be deducted from the dynamic or static response using ferromagnetic resonance, torque magnetometry, torsion oscillating magnetometry or the magneto-optical Kerr effect [39,131]. Correspondingly, a vast number of techniques are available for the indirect measurement of the magnetic moment, including vibrating sample magnetometry (VSM), superconducting quantum interference device (SQUID) magnetometry, fluxgate magnetometry, alternating gradient magnetometry and pendulum magnetometry, with VSM and SQUID being the most popular methods [4,39,253]. Spin-polarized electron energy loss spectroscopyand Brillouin scattering are useful for the detection of the Dzyaloshinskii-Moriya interaction (DMI) energy. Both techniques measure the spin-wave propagation, where the presence of the DMI breaks the degeneracy of the spin wave [254]. Photoemission electron microscopy is able to resolve the specific spin configuration of the constituent elements [236], while scanning transmission x-ray microscopy provides the opportunity to map the plane of magnetization in materials exhibiting the presence of skyrmions. The two techniques are carried out in synchrotron facilities [236,255]. Spin-polarized lowenergy electron microscopy (SPLEEM) and x-ray magnetic circular dichroism photoemission electron microscopy are high-resolution imaging techniques that are based on spatial magnetic imaging of electrons, which are either deflected or emitted from the sample [236,256]. In particular, SPLEEM is capable of quantifying the arbitrary orientation of spin states [236]. Further details about these and other magnetic imaging characterization techniques such as SEM with polarization analysis, magnetic transmission x-ray microscopy and Lorentz transmission electron microscopy, can be surveyed in [254].
For the particular case of NMMs for extreme ultraviolet (EUV) and soft x-ray (SXR) devices, the spectral reflectivity is measured to determine the functionality of the synthesized structure. Reflectivity measurements near normal incidence usually require the use of an intense and continuous light source and therefore, the characterization of such NMMs is mostly carried out using synchrotron beamlines [70,72,[257][258][259]. Customized reflectometers with x-ray tubes have also been developed in order to achieve reflectivity measurements [79]. Other dedicated techniques, such as hydrogenography [207] and elastic recoil detection analysis [209,210] have been implemented to monitor the optical changes during hydrogen absorption and desorption in metals, a phenomenon that is useful to evaluate the hydrogen storage capacity via metal hydride formation. Neutron reflectometry [260] has proved to be sensitive to local density changes induced by He implantation, which is useful to investigate the damage of materials under He irradiation. Differential scanning calorimetry (DSC) is used for the study of phase transformation and thermal stability of NMMs [168]. Nanocalorimetry, on the other hand, offers higher heating rates than the common DSC method and therefore, is used to provide deep insight into phase transformations of reactive multilayers [127,261]. Along with nanocalorimetry, dynamic TEM with microsecond-level time resolution has been developed to analyze the reaction propagation and rapid transformations after the ignition of reactive multilayers [31,243]. Figure 4 illustrates an overview of different techniques used to determine the microstructure, composition and the specific properties of a generic NMM system.

Properties and applications of NMMs
A vast amount of experimental work has shown that most of the metallic elements in the periodic table have been combined in a nanonscale multilayer configuration with the aim of exploring their microstructural variations and properties. In an attempt to present a more specific classification of different NMMs, figure 5 shows a compilation of metal/metal systems grouped according to their composition, highlighting their field of application. It can be observed that many NMM systems have been incorporated into a wide range of technological and scientific areas, with an emphasis on magnetic applications. Therefore, the following sections describe some of the most significant results obtained in a broad range of fields, including a summary of early and most recent works.  [262]. The basic principle stated that this could be achieved by preparing a composite material comprising alternated thin layers of material A and material B. The successful application of this idea was soon confirmed experimentally, with results showing that the outstanding mechanical behavior of NMMs is dominated by interfaces [8, 53-67, 111, 130], which act as sources, barriers, and preferred sites for storage and dynamic recovery of dislocations, as presented in [58,110,111,114,263].
The increased yield strength (σ), usually associated with the increase in hardness (H), is one of the most significant improvements obtained through implementing the NMM configuration. The general trend shows that higher hardness is achieved through the reduction of layer thickness (or interface spacing-h) and the strengthening effect is explained using the theories of dislocation motion. This is because dislocations are line defects that are mainly responsible for the plastic deformation of crystalline solids [114,264,265], and their movement is obstructed by interfaces. Various dislocation models have been developed in order to interpret the strengthening mechanisms as a function of layer thickness [30,53,63,108,111,116,134,[266][267][268][269]. As recently summarized in [270], the most accepted model indicates that there are three primary mechanisms to explain deformation in NMMs, and the mechanisms at play depend on the layer thicknesses. The first one is based on the Hall-Petch scaling law (σ = h 1/2 ), which is valid when dislocation pile-ups can be treated as a continuum, typically when h is in the micron to submicron scale. As h is reduced, the number of dislocations allowed in a pile-up will decrease until only one dislocation can be accommodated. The glide of this single dislocation bounded by the interfaces is dictated by the second mechanism, known as the confined layer slip model, and is valid for h in the range of tens of nanometers with the following scaling law: σ = (ln(h))/h. Once h is further reduced to a few nanometers, the hardness is governed by the third mechanism and is explained in terms of the interfacial barrier strength for dislocations cutting across the interface. Further details about deformation mechanisms can be found in [30,53,63,108,111,116,134,[266][267][268][269][270][271].
Multiple compositions of multilayer systems have been studied, showing an increase in hardness with the reduction of h. As such, table 2 presents a compilation of hardness values  of systems grown upon Si substrates with repeated bilayer thicknesses. For most of these NMMs, a significant change in the elastic modulus E was not observed as a function of the layer thickness (known as the supermodulus effect), and therefore, the elastic modulus values are not included. All systems in table 2 exhibit hardness values higher than those expected from their respective rule of mixtures. In addition, during the compilation of these results it was noted that the highest hardness values were reported for systems with the largest total thickness. For example, the Cu/Nb system with individual layer thicknesses of 20 nm and a total thickness of 1.6 µm, has a hardness of 5.4 GPa; a value that is larger than that measured for the same system with a total thickness of 1 µm (4.5 GPa). Such discrepancies could originate from the microstructural variations arising from the selected synthesis method and/or substrate contributions during the measurements. For example, as shown in table 2, Cu/W multilayers prepared via sputtering [37] yielded a hardness of 5.3 GPa, in contrast to a higher hardness of 7.25 GPa for a sample prepared by electron beam evaporation, for samples with a similar measured individual layer thickness [272]. It is important to note that for non-repeated bilayer thickness samples, a trend of higher hardness with smaller layer thickness has not been established [65]. Instead, the strengthening effect is attributed to the volume fraction of the coupled elements, which leads to an increase in the interfacial dislocation density. In addition, a strengthening effect in NMMs has been observed as a result of structural transitions during the growth of very thin layers (usually 2.5-5 nm), which can form metastable or pseudomorphic phases. For instance, higher indentation hardnesses have been reported for face-centered cubic (fcc) Cu and body-centered cubic (bcc) Nb [130] than in fcc/fcc Cu/Nb [195] samples of the same layer thickness (see table 2). The mechanical properties of other pseudormorphic phases, as well as a brief summary of available theories regarding the phase transformations at very small thicknesses, have been reviewed in the literature [271,276]. Special attention has been given to the hexagonal close-packed (hcp) to bcc transition of Mg and Zr in NMM systems because of the possible incorporation of these compositions in aerospace and automotive applications [112,206,276]. Furthermore, studies exploring the effects of thermal annealing on the mechanical properties of multilayers have also been carried out, with the majority of the results showing that the increase in temperature leads to a decrease in hardness [37,38,211,277].
For reference, compilations of hardness values obtained from multiple systems have been presented in [111,116,130,271]. These systems, along with the ones presented in table 2, can be used as a guideline to explore the mechanical properties of other NMMs.

Improved ductility and wear resistance in NMMs.
To date, the mechanical behavior of NMMs has mostly been determined from nanoindentation, which is a versatile tool for the estimation of mechanical properties in small volumes [116]. Nevertheless, in order to provide a comprehensive understanding of the mechanical behavior, a wide range of testing techniques are needed and should include stress-strain responses [188,189,278]. Therefore, micropillar compression and tensile experiments have been adopted in order to provide such information [271,278]. Micropillar compression tests can be performed in different configurations: with the compression axis parallel or normal to the surface with either a circular or squared cross section. From both tensile and compression tests, the plastic flow behavior as well as softening and hardening effects can be elucidated from the stressstrain response [188,278,279]. However, ductility values from tensile experiments have been shown to exhibit smaller values than those obtained via micropillar compression. This can be attributed to variations in sample preparation, the geometry of the system or the loading conditions of a given testing method. Overall, the number of studies presenting the deformation of NMMs remains limited, with most of the experiments performed using Cu/Nb systems [116,188,189,250,271,278,279], although other works have been carried out for Ag/Cu [280] multilayers and more recently for Mg/Nb [274] and Zr/Cu [159] systems. The experimental results obtained from those studies have shown that in general, the strength and ductility of materials are mutually exclusive, meaning that the ductility of multilayer systems drops with decreasing the layer thickness [199,271,281]. In many of these reports, the correlation factor for the flow strength (σ f = H/ 2.7 or σ f = H v /3) has been used to compare and validate the measured data using the hardness values obtained from nanoindentation [188,250,278,279] or Vickers indentation [280] .
Wear studies provide further mechanical behavior insight into deformation and material loss under sliding contact [67], as well as its effects on the material's servicability and durability [282]. Wear resistance is known to depend on hardness and therefore, a suitable approach to improved wear resistance has been achieved using systems with layers a few nanometers in thickness, for which less volume loss due to wear and deformation has been observed [67]. However, it has been seen that the applied load, velocity, elastic properties of the constituent elements [67,264,282,283] and the presence of internal stresses [264] affect the wear response. The available studies employ the ratio of hardness over modulus H/E (named elastic energy) as the key parameter to describe the tribological properties of multilayers. The basic criterion indicates that the higher the magnitude of H/E ratio, the better the wear resistance of the coating [67,264,282,283]. Examples of multilayer structure response explored under cyclic sliding include Cu/Au [284], Cu/Ag [283] and Cu/Nb systems [67].

Mirrors for the EUV and SXR regions.
Multilayer structures exhibiting enhanced reflectivity near normal incidence in the range of EUV and SXR (1-60 nm), are in high demand for the design and fabrication of optical elements in lithography systems, spectroscopes, microscopes and xray free-electron lasers [68][69][70][71][72][73][74][75][76][77]. In contrast to monolithic films, multilayered optical materials offer improved performance at near-normal incidence. This is because the small reflections that occur at each of the interfaces add coherently in phase, producing a high reflectance over a narrow range of wavelengths [288,289]. Such wavelengths are then used to match the needs for specific practical applications, for instance, EUV lithography systems for chip fabrication typically operate in the range from 6.7 nm to 14 nm [290], while telescopes for solar astronomy work in the spectral range from 1030 nm [291][292][293]. Biological microscopes, on the other hand, operate in the water window region between 2.3-4.4 nm, where the contrast between carbon and water does not limit the imaging of the samples [72,291]. As an illustrative example for the use of optical mirrors, figure 6 shows a collection of NMMs that can be used for the observation of different spectral lines emitted by the sun. As noted at the bottom part of figure 6, the imaging of single active regions is achieved in real time, providing specific information about the processes occurring in the solar atmosphere. The importance of monitoring solar activity relies on the fact that variations in solar radiation drive changes in the density and ionization in Earth's thermosphere and ionosphere, which affects the performance of ground-based communications systems and spacecraft in low-Earth orbit [293].
The performance of multilayered mirrors has been shown to strongly depend on the composition of the multilayers, since in order to maximize the reflectance, elements with the highest possible optical contrast (absorbers) and the lowest possible absorption (spacers) have to be coupled [294]. Optical contrast indicates a material with sufficiently different values of the index of refraction and extinction coefficient, while for the spacer element, minimal absorption at the target spectral range is needed [69,295,296]. Depending on the application, peak reflectivity (maximum reflectance or reflection efficiency) as high as 70% and a narrow spectral band are required. The reflection efficiency influences the temporal resolution and defines the sensitivity of the instrument (meaning that acceptable clearer images/signals are achieved with shorter exposure times), while the spectral bandwidth diminishes the contribution from spectral lines that are contiguous [79]. In addition, the overall performance of these mirrors is a function of the reduced thickness of the layers, quality of the interfaces, surface characteristics of both the NMMs and the substrate, as well as by the intrinsic stresses that develop during deposition [77,182]. The variation of any of these parameters has been shown to have either a positive or negative effect on the optical response of the multilayers. For example, by adjusting the bilayer thickness, one can tune the peak reflectivity of the multilayer and match a desired wavelength [71,74,75,289]. In contrast, the existence of roughness and diffusion at the interfaces has been shown to dramatically degrade the reflectance and reduce the optical contrast [69,71,182,291,297], while residual stresses can cause deformation in the projected beam [71,79]. These negative effects can be minimized by using super-polished substrates [69,76,77,80,214] and suitable buffer layers [79], respectively.
A compilation of successfully synthesized NMM mirrors appears in table 3, showing the experimentally measured peak reflectivity. The systems are grouped by chemical composition and are accompanied by the angle at which the maximum reflectivity was achieved, as well as the corresponding target wavelength. As shown in table 3, a maximum reflectance of 70.2% was observed in a Mo/Be system, but the health risks associated with the use of Be can restrict its usage [71,298]. It should be noted that the peak reflectivity and target wavelengths vary from system to system, but this information is useful when comparing the performance of different mirrors and selecting possible candidates that can be suitable for several applications.
Other systems not presented in table 3, including Pd/Y [291] and La/B [307,308], have also been studied as possible reflective mirrors, but their multilayer structure disappears due to intermixing of the components. This is caused by the poor thermal and chemical stability of the combined element system. Thermal evolution tests are required in order to ensure the functionality at operation temperature of optical mirrors used in astronomical observation and synchrotron radiation, since they are exposed to a high flux of incident photons, which can lead to degradation of the structure upon heating [71,305,309,310]. As an example of sample degradation, Nechay et al studied the effect of annealing and surface oxidation on the reflectivity of Mo/Be multilayers [310]. They observed a reduction of reflectivity as a consequence of diffusion upon annealing, but also noted that oxidation had a major impact on lowering the optical response. Similar findings were reported for a Mo/Y system when exposed to a photon flux of lesser intensity than that expected under normal operational conditions [212]. Several approaches have been adopted in order to minimize the chemical interaction and degradation of the layers. Notable examples include the reduction of diffusion and inhomogeneous crystallization during the synthesis of an Al/Zr system by doping the Al layers [311], while the stabilization of Y/La mirrors was achieved by introducing N 2 Figure 6. Primary ion emission chart associated to specific spectral lines to trace active regions of the solar atmosphere. Chart includes different multilayer compositions that could be suitable for the reflection of various wavelengths in the SRX and EUV regions. Customized figure is based on the selection chart for primary ions presented in [69], and the table for emission lines for active solar emission wavelengths shown in [293]. Images courtesy of NASA/SDO and the AIA, EVE and HMI science teams.
into the sputtering chamber (passivation via nitridation) [291]. Other extrinsic parameters during operation have been shown to cause the loss of reflectivity, as in the case of surface oxidation and contamination due to exposure to ambient conditions. This is the case for Mo/Sr multilayers that oxidized when exposed to ambient atmosphere, for which long-term functionality was ensured by using C as a capping layer [302]. The surface oxidation of a Mo/Y system showed a drop of only a 4% in reflectivity after a period of 1 year by using Mo as a capping layer [214], highlighting an attractive strategy to avoid degradation. The synthesis of a three-component multilayer, for example, in the case of Mo or Zr layers inserted as diffusion barriers in between the layers of a Mg/Co system, has also served as a mirror stabilization strategy. For this particular case, results showed no degradation of the microstructure or the optical properties when Zr was used as an interlayer, even when the sample was annealed up to 400 • C [309]. The three-layered configuration (using Mo as an interlayer material) was also adopted for the synthesis of a Cr/Sc-based mirror in order to reduce the compressive residual stresses developed during deposition [306]. In addition to these observations, the purity of the targets (oxygen content) has been shown to influence the composition, microstructure and optical response of multilayer systems grown under identical conditions [214].
A number of other NMMs have been proposed theoretically as possible optical systems, promising high reflectivity in the SXR and EUV regions [69,295,303,304,[312][313][314]. For most of these cases (including the systems in table 3), the theoretical reflectivity differs to a great extent from that measured experimentally, but such information could serve as guidelines for further exploration. Other considerations for the development of efficient, high-quality multilayer optics have been covered extensively in the literature and can be surveyed elsewhere [69,73,74,79,80,289,295,296,315].

Optical switches for hydrogen storage.
The increasing interest in hydrogen as a cleaner and more efficient energy source has led to a fast-growing research area, which includes research of materials for its storage. The hydrogen uptake (hydrogenation) process, observed first in Y-and La-based thin films, has been considered as a feasible method to prepare devices that are able to both store and quantify the hydrogen uptake [316]. Hydrogenation depends on the metal to hydride formation, which usually leads to phase transitions, meaning that the material changes from a metal to a semiconductor [317] and therefore, the metallic film becomes transparent [318]. Under these circumstances, the optical and electronic properties could be switched between those of the highly reactive metallic state and the semiconducting hydride states by subsequent loading and unloading of hydrogen [209,319], allowing for real-time visualization of transmittance, reflectance and resistivity changes during hydrogen incorporation into the lattice structure [319]. The early studies of switching materials were carried out using monolithic thin films of alloys. Later on, van der Sluis et al reported that the switching effect in multilayers (kinetics of hydrogenation) was much faster than that observed in alloys of identical composition [320] and thus, NMM systems became attractive as possible optical switches. Outcomes have shown that tuning of the optical properties is possible by varying the layer thickness, while tuning the switching kinetics is attainable by varying the number of multilayers [320]. In addition, NMMs do not exhibit the disadvantageous hysteretic effects to the transmittance that monolithic thin films do, and this demonstrated reversible kinetics upon hydrogen loading and unloading ensures the reproducibility of the results during continuous operation [318].
Some of the requirements for the development of efficient multilayer optical switches include high gravimetric capacity and the ability to rapidly take up and release hydrogen (determined by the kinetic response) [321]. Since oxidation is much more favorable than hydrogenation (even at room temperature (RT)), hydrogen absorption experiments have to be carried out in ultra-high vacuum conditions and the final multilayer structure must incorporate a capping layer to prevent surface oxidation and/or to enhance the exchange of hydrogen.
To date, very few NMM systems have been explored as possible switchable mirrors, including Y/Mg [318], Mg/Ti [207], Pd/Mg [209], Gd/Mg and La/Ce [320], thus leaving an opportunity for the study of other compositions. The discovery of the GMR effect, first observed in a trilayered Fe/Cr/Fe system [51,52] and later in Fe/Cr multilayers [50], is perhaps the most remarkable example of how the properties of NMMs prompted a fast transition from scientific investigation to the commercialization of devices [3,50,[322][323][324]. Since then, a global widespread interest in the study and application of these nanostructures has emerged, accompanied by a great number of reported studies. Therefore, we limit our discussion to present qualitative aspects about GMR with further fundamental concepts, properties, preferred compositions, synthesis methods, applications, limitations and future trends available in the literature [3,25,41,48,82,140,322,[324][325][326][327][328][329].
The GMR effect can be described as the change in electrical resistance in response to an applied magnetic field [3] . It is observed in NMMs composed of alternating layers of ferromagnetic (FM) and non-magnetic (NM) elements. The basic principle of this phenomenon relies on the fact that the magnetic moments of the FM layers can be aligned in parallel or antiparallel to each other by applying a magnetic field. When the magnetic moment is aligned in parallel, the scattering of the carriers is minimized, and the system achieves its lowest resistance. In contrast, if the magnetic moments of the FM layers are anti-aligned, the scattering is maximized and the resistance reaches the maximum value [330]. The accumulated experimental data indicates that the GMR depends mainly on the thickness of the NM layers (because the strength of the MR oscillates with its thickness) [3,25,48,82,140,322,[324][325][326][327][328]330], but that it is also influenced by the composition of the multilayer, the roughness, presence of defects (such as grain boundaries) and diffusion [1,3,140]. Depending on the application, one or multiple of these parameters may serve as determining factors for the performance of the multilayer system.
According to the literature, the systems that exhibit the greatest values of GMR include: Fe/Cr, Co/Ru, Co/Cr, Co/Cu, Co/Ag and Ni/Fe alloys, while very low GMR has been measured in Ni/Ag, Ni/Cu, Fe/Mo, Fe/Au, Co/Al and Co/Ir alloy systems [3,48,140,324,331]. Large GMR systems are attractive for the fabrication of hard disk read heads, memory chips and magnetic recording disks [1,82,140,322,324,328,332], but thanks to their small size, high sensitivity and low power consumption, giant magnetoresistive multilayers are also suitable for the fabrication of sensors for use in other applications including transportation systems, flexible electronics, biology and healthcare [16,140,323,333]. In such cases, the composition of the stacked films is not limited to metals and/or to the assembly of periodic layers. It is now worth mentioning that the GMR effect has also been observed in systems with a reduced number of layers comprised of semiconductor materials or nanoparticles embedded in a matrix [323,329]. These GMR systems are known as spin valves, granular multilayers and magnetic tunnel junctions (MTJs). For reference, a comparative table showing the measured properties within different GMR systems can be surveyed in [329]. In addition, the work presented by Zheng et al [323] summarizes the transition from NMMs to spin valves, granular multilayers and MTJs, and includes the quantification of patents and publications that have become available since 1988. Much effort, directed towards the exploration of the GMR, has also enabled the control of an electron's motion by acting on the orientation of its spin, which led to the emergence of the field of spintronics [41,140,330].

Magnetic anisotropy in
NMMs. FM materials exhibit easy or hard axis magnetization, which refers to the application of a small or large magnetic field to reach the saturation of magnetization [39]. The energy required to achieve saturation depends on the direction of the applied magnetic field relative to the crystal axes and is called magnetic anisotropy. This direction can lie in the plane of the substrate or along the surface normal to the substrate (perpendicular magnetic anisotropy (PMA)) [39,43,141]. If easy magnetization is achieved in the direction perpendicular to the plane of a thin film then PMA is observed. PMA results from a combination of factors: the contributions from the shape of the nanostructure (shape anisotropy), strains associated with the deposition method (magnetoelastic anisotropy) and crystalline structure of the material (magnetocrystalline anisotropy). In NMMs, the magnetic anisotropy reaches larger values due to the presence of symmetry-breaking elements, such as planar interfaces and surfaces [39]. Moreover, it has been possible to tailor the strength and occurrence of PMA by changing the thicknesses of the individual layers and by choosing appropriate materials [39,43,141].
A large number of systems employed in the study of magnetic anisotropy, as well as additional details about this phenomena, can be found in [131,334]. Some of the most recent works in this area include the analysis of Co/Ni systems, which is of special interest for spintronic applications, since this composition exhibits several magnetic properties that are not attainable with other combinations of metals (e.g. high spin polzarization and low intrinsic magnetic damping) [131,179,334]. Co/Ni multilayers are in fact suitable candidates for the fabrication of spin-transfer torque magnetic random access memories (STT-MRAM), STT oscillators and bit-patterned media [39,43,131,334]. The work of Arora et al [131] showed experimentally and theoretically how variations in the type of substrate, buffer layer and number of stacked layers, help to adjust the magnetic anisotropy of Co/Ni systems. Similar experimental findings have been obtained by other authors for the same composition [334]. Further improvements to PMA have been achieved with the synthesis of Cobased multilayers. For example, an increase in the amplitude of PMA has been possible for a Co/Au system by promoting the formation of sharper interfaces and stress relaxation upon thermal annealing [335], while the measured PMA of a Co/Pt multilayer was doubled by accommodating thin layers of Cu and forming a three-layered structure (Co/Cu/Pt) [336]. In this case, the incorporation of the Cu layer obstructed the diffusion of Co and Pt.

Magnetic skyrmions in multilayered systems.
The study of nanoscale magnetic materials stacked in a multilayer configuration led to the discovery of magnetic skyrmions. As depicted by Jang et al [236], naturally occurring nanoscale noncollinear spin textures (such as magnetic domain walls and magnetic vortices) are present in bulk or magnetic nanostructured materials. They originate from the competition between different energy contributions: magnetic anisotropies, dipole interactions and exchange interactions [236]. In particular, the Néel-type and Bloch-type textures are of special interest. In the Néel-type textures, the spins inside the wall rotate as cycloidal spirals within the domain wall region, while in Bloch-type textures, the spins rotate as helical spirals [236,337]. Both topological configurations of the magnetic moments are called magnetic skyrmions [338] and have been observed in NMMs as well as in ultrathin monolithic magnetic films [236,255,256]. They are stabilized in most cases by the Dzyaloshinskii-Moriya interaction energy, which results from spinorbit effects in the absence of inversion symmetry. At the atomic scale, the DMI energy is defined as E DMI = D(S 1 × S 2 ), where D is the DMI vector and S 1 and S 2 are two coupled spins. This interaction favors a perpendicular orientation of the spins, which matches the spatial distribution of the magnetic moments [256].
The reduced size of skyrmions and the possibility of controlling their motion by applying an electrical current of small density, makes them promising candidates for several types of non-volatile magnetic memory devices [230,236,254,256,[337][338][339][340][341][342][343]. In such applications, the information could be coded by skyrmions in a magnetic nanoribbon (similar to the one employed in racetrack memories), and the spacing between bits could be of the order of magnitude of a few nanometers (close to the diameter of the skyrmion) [254,256,342]. Nevertheless, before moving forward to the fabrication of devices, several issues with regards to stabilization should be addressed, e.g. RT stability without the application of an external magnetic field [237,343]. Until now, there are some successful reports showing the stabilization of magnetic skyrmions in NMMs [237,337,341], but only isolated skyrmions have been observed in such systems. Examples of promising multilayer systems include: Ir/Co/Pt [255,344,345], Pd/Co [237], Pt/Co/Ta [346], Ir/Fe/Co/Pt [341] and Cr/Fe/Cr/Ga [340]. Similar to the properties of GMR multilayers and PMA, the thickness of the individual layers and the symmetry at the interfaces play a crucial role for the detection of magnetic skyrmions [236]. Readers interested in further exploring specific details about this topic can survey [236,254,256,337,338,341,342].

NMMs for radiation tolerance applications
The continuous exposure of materials to highly energetic particles (neutrons, ions, electrons and gamma rays) is a topic of interest since these forms of radiation are all capable of displacing atoms from their crystalline lattice sites [113,240,347]. Such interactions modify the microstructure, composition and properties, and lead to detrimental long-term performance of the materials. These deleterious effects are of special importance for structural materials employed in the design of nuclear reactors [113,162,347].
Several studies, including a recent and extensive review by Zhang et al [90] have shown that the interfaces, grain boundaries and free surfaces in nanostructured materials act as sinks for radiation-induced effects [86,91,113,114,162,260,348,349]. However, unlike point defect impurities, these features cannot be annihilated [84,87,88]. Therefore, if He is introduced into the structure of a metal (even at trace quantities), it precipitates in the form of bubbles [85,87,88,348,350,351]. Above a critical diameter (about 10 nm), He bubbles grow to capture vacancies and develop into voids that directly affect the integrity of the material, typically causing an increase in yield strength or embrittlement [114,271,348]. However, below the critical diameter, the bubbles remain stable and may be relatively benign [85,114,347] and can lead to an increase in hardness, since bubbles act as obstacles that impede dislocation motion [89,114,162,348,352], and increase in electrical resistivity [353]. Because of the multiple effects of He, ions of this element are usually employed to evaluate the radiation damage of materials [87,354]. Another advantage offered by He bombardment is that at sufficiently high concentrations, He bubbles are resolved under focused TEM [85,260,351,355], making it possible to trace the resulting impact by direct observation.
According to multiple studies, the existance of a significant number of interfaces in NMMs has been shown to enhance the tolerance of such structures to the damage caused by radiation [85,88,90,260,350]. However, the different interfaces exhibit different sink efficiencies due to the intrinsic defects that are inherent to their internal structure, which in turn are a direct consequence of their crystallographic character (as can be seen in figure 2 with the formation of different interfaces) [86,350]. The available literature has concluded that semicoherent interfaces are more efficient for alleviating He bubble formation [85,90,114,157,240,260,350] because they consist of alternating regions of coherency separated by networks of intrinsic defects known as misfit dislocations [155]. These dislocations are high-energy regions where He bubbles preferentially migrate and aggregate [91,260,350,351,356]. As a matter of fact, it has been observed that the density of misfit dislocations controls how much He can be stabilized and trapped at the interfaces [85,260]. Contrasting studies have proposed that coherent multilayers comprising immiscible metals could be considered as efficient materials to mitigate radiation damage [90,178,204,240]. This has been correlated to several factors, including: the possible creation of sinks due to the interaction of induced defects with coherent interfaces, the promotion of defect migration due to coherency stresses, and the alignment of He bubbles along the interfaces [90,178]. Interestingly, the work of Chen et al [204] presented evidence for the possible effectiveness of coherent interfaces by comparing the radiation damage of coherent and incoherent Cu/Fe multilayers, which were formed with layer thicknesses below 2.5 nm and above 5 nm, respectively. In their work, the density of He bubbles in the coherent Cu/Fe system was similar to that of the monolithic Cu layers, but the bubble diameter was smaller, indicating that radiation damage could be mitigated by reducing the size of the resulting bubbles. In addition, it has been determined that the radiation damage tolerance could be related to a decrease in the thickness of the multilayers, since the diffusion distance to the nearest sink is shortened [90]. The compilation of several NMMs that have been used to explore radiation damage is shown in table 4. For comparative purposes, the maximum computed values for dpa and He concentration (by SRIM) that could be accommodated within each system are presented. The classification of radiation-tolerant NMMs summarized in [90] could be used to complement the information presented herein.
The information presented in table 4 shows that the differences in the experimental conditions directly influence the response of the materials, which is reflected in the different ranges of damage (in dpa) and He concentrations that were measured in each case. The table also shows that similar irradiation damage and He concentrations were achieved by using Ag/Ni, Cu/Co, Cu/Fe and Cu/Mo multilayers, which were tested under identical conditions of fluence, implantation energy and repeated layer thickness (5 nm). Such outcomes prove the importance of defining standard experimental protocols to perform reliable comparative studies, which lead to the proper selection of promising systems for practical applications (including the use of NMMs for the fabrication of fuel cladding materials in nuclear reactors, as shown in figure 7(a)). Radiation damage studies have also been carried out using Al/Nb multilayers. However, this is a miscible system, which has been shown to result in the formation of intermetallic precipitates along the interfaces and therefore, its usage in this type of application is not recommended [167].
In general, most radiation studies have been carried out at RT, with the exception of some works exploring the behavior of Cu/Nb systems (performed at 450 • C and 480 • C). At RT, studies performed by Li et al [87] indicate that the implanted area (obtained by SRIM) can be partitioned into three regions. In Region I (RI), defects are a consequence of the formation of vacancies. In Region II (RII), the formation of defects stems from the synergetic contributions of He atoms and vacancies (also matching the peak concentration of He), and in Region III (RIII) defects are dominated by the implantation of He atoms. All three phenomena lead to the development of clearly distinctive zones, which in figure 7(b) appear marked as areas where bubbles were observed or areas where bubbles were not observed. However, it has been shown that the radiation damage in NMMs is a temperature-dependent phenomenon that is associated with the mobilities of vacancies and implanted atoms. When the temperature during irradiation is increased, the evolution of these distinguishable regions takes place, which implies that the microstructure of the multilayer system changes and that the resulting radiation damage becomes more evident. This effect can be observed in figure  7(c) for a Cu/Nb system. In this case, as the temperature goes from RT up to 480 • C, the shape and size of the He bubbles  increased until the regular spherical bubbles transformed into irregular faceted cavities [162]. In the same way, with the rise in temperature, the coverage of the bubbles expanded continuously, thereby affecting a greater number of layers within the multilayer system, thus leading to the degradation of the structure. In contrast to high-temperature experiments, it has been observed that for the majority of the NMM systems studied at RT, the He bubbles preferentially develop at the interfaces or in one of the metals within the multilayer. Overall, studies suggest that it is necessary to perform further experimental tests at higher temperatures (in the range of those reached or expected during normal operating conditions), in order to understand the effects of radiation damage under these working conditions. Other irradiation studies involving heavy ions of Kr ++ [354], Ar + [361] and Cu 3+ [240], as well as protons [357], have also been performed for the study of ion implantation in Ag/Ni, Ta/Ti and Cu/Fe systems. All of the aforementioned studies presented similar findings to when He ions are used [90], where the multilayer configuration provides sites to accommodate the implanted particles, thus stabilizing the structure upon irradiation. Notwithstanding, the nature of the particles used for irradiation led to the existence of different defects compared to those found in the Ag/Ni multilayer system. Dislocation loops were observed after the Ag/Ni multilayer was irradiated with Kr ++ ions [354], while interstitials, vacancy loops and He bubbles were formed when the system was irradiated with protons [357] and He ions [357], respectively. The development of these contrasting defects could have different effects on the properties, e.g. deformation. Thus, exploring the response of equivalent systems upon irradiation using various energetic particles could be useful to achieve a deep understanding of microstructural and property alterations. Extensive information covering further details about damage in nuclear reactors [347], the design of radiation tolerant materials [84,86,90], as well as defect interface interactions [114], is available to complement the results and observations summarized in this section.

Emerging properties and applications for NMMs
Apart from their outstanding mechanical, optical and magnetic properties, NMMs have been shown to be suitable structures to explore other phenomena and applications of interest. Figure 8 presents a schematic illustration showing additional capabilities such as reactive multilayers, model systems to achieve and explore thermal stability in nanocrystalline materials, and as precursors for the development of new nanocrystalline and amorphous systems via thermal annealing and ion beam mixing (IBM). Further details about these areas are discussed in the following sections.

Reactive multilayers
Reactive NMMs are energetic materials that store an excess of chemical energy, which originates from the energy of elastic stresses and free energy of the interfaces [127,128,144]. The accumulated energy can be released in an abrupt emission of light and heat when stimulated by an external source [31]. They usually comprise pure metals such as Al and Ni, and can be prepared by multiple synthesis methods, including sputtering, evaporation, ARB and electrodeposition [31,121,127,128,144,362,[365][366][367][368]. The release of energy in reactive multilayers can be initiated by several techniques, including mechanical loading, thermal heating, laser heating or electrostatic discharge [31,127,128,144,[365][366][367][368]. The ignition starts a self-propagating reaction that is characterized by a high-temperature wavefront that propagates through the multilayer, which then transforms reactants into products [121,127,128,144]. As a result of the rapid release of energy, reactive NMMs have the potential to be used in brazing, localized soldering, fabrication of igniters, flares, neutralization of biological hazards, long-term thermal batteries and possibly as car airbag initiators [31,127,128,144,[365][366][367][368]. Another promising application is for facilitating the synthesis of intermetallic materials that cannot be fabricated by other methods, e.g. the metastable PtAl 5 [128] and Al 9 Ni 12 [127].
The velocity of propagation (wavefront velocity or rate) and the maximum combustion temperature of reactive multilayers depend on the thickness of the constituent layers and varies with different modes of initiation [31,128,144,[365][366][367][368]. Systems with bilayer thicknesses above 1000 nm exhibit slower propagation rates than systems with thinner layers, but it has been noted that systems with bilayer thicknesses below 40 nm, where diffusion at the interface usually takes place during growth, can display a reduction of the stored chemical energy. The intermixed interfaces can also act as diffusion barriers or modify the thermal properties of the systems [127,144,369]. It has also been observed that the energy required to ignite the NMMs depends on the thickness of the layers and on the composition [31,128,144,362,[365][366][367][368][369], while the total heat output can be tailored by the choice of the number of repeated bilayers [127,144].
Comprehensive reviews that include and compare multiple reactive multilayers, and cover details about the synthesis conditions, common characterization techniques, and the influence of intermixing, individual layer thicknesses and residual stresses on the reactivity of the systems, can be surveyed elsewhere in [31,128,144,244,261]. Of these results, the Al/Ni system is by far the most studied composition and was in fact the first material in which reactive ignition was observed [127,128,244,261,366]. Both Al/Ni as well as Ni 0.91 V 0.09 /Al multilayers are commercially available [31,127,362] and are both employed in soldering applications (see figure 8(a)). Other studied reactive systems include Ag/Ni [128], Ti/Al [370], B/Ti [371] and Ru/Al [368,372], and their fabrication has been directed towards tailoring of the ignition temperature. For example, the recent work by Pauly et al [368] showed the possible reduction of the temperature required for ignition (with respect of a bimetal Ru/Al system) by up to 150 • C, and a further reduction by up to 230 • C by the use of three-layered systems comprising Ru/Al/Ni and Ru/Al/Pt. These results suggest methodologies for controlling and/or customizing the ignition temperature of reactive systems by coupling more than two metals in a multilayer structure.

Thermal stability and high-temperature properties
Thermal stability plays an important role in retaining and prolonging the functionality of materials [35, 49, 93-96, 363, 373-377]. NMMs display the same thermal stability issues observed in nanocrystalline materials because of the presence of a high density of interfaces, which result in an excess of free energy [34,377], and makes them prone to thermal transformations when subjected to an increase in temperature [35, 93-96, 375, 376]. Different interfaces are expected to have different levels of stability depending on the initial microstructure, the thermal properties of the individual layers [94,95], as well as on their corresponding interface energies [378,379]. Immiscible multilayer systems (with a positive enthalpy of formation), have been shown to degrade mainly by grain boundary grooving, which often results in pinch-off of the layers, followed by the spheroidization of the discontinuous layers [96,160,380]. Meanwhile, miscible systems degrade by interdiffusion across the interfaces, a process that often results in the formation of solid solution or intermetallic phases [96] due to the intermixing driving force associated with their negative enthalpy of formation [96,[381][382][383]. Multiple studies have investigated the thermal stability of NMMs and the majority of the reports have focused on monitoring the instability mechanisms involving interdiffusion, chemical reactions, phase transformations and deterioration of the properties upon exposure to temperature. Examples of such studies include the reported deterioration of the layered structure observed in Cu/Ni [384], Cu/V [380], Cu/Ta [160,385], Cu/W [34,377], Co/Cu [173] and Mo/V [386] systems, the partial oxidation of Nb/Ti [387] and Zr/Nb [222,223] multilayers (see figure 8(b)), the formation of intermetallic phases in Nb/Ni [224,277,388], the loss of hardness reported for Pt/Mo [38], Cu/W, Cu/Cr and Cu/Mo systems [37,222], as well as the decay of the magnetic moment in Co/Cu [104].
More specific studies have been carried out with the aim to explore and achieve the thermal stability of NMMs by employing kinetic and thermodynamic approaches that are used for the stabilization of nanocrystalline materials. The kinetic approach attempts to retard or stop the mobility of grain boundaries by adding second-phase particles or solutes, while the thermodynamic approach promotes the reduction of the driving force for grain growth by inducing solute segregation to the grain boundaries [49,374,375]. Misra et al [94] observed that the realignment of triple junctions in a zig-zag pattern effectively prevented pinch-off of Cu/Nb multilayers. Such stabilization processes take place only when the initial microstructure displays a specific stacked morphology, which affects the movement of triple junctions that is driven by the imbalance in tensions between the interphase and grain boundaries [95,96]. In addition, Ma et al [93] observed that the stabilization of the highly coherent Cu/Ag system occurred by two different effects that depended on the heating temperature: at about 200 • C the development of grooves reached an equilibrium angle, then acting as drags to inhibit the boundary migration; above 300 • C, the interface energy was lowered because twins developed at the interface. In a more recent study, Riano et al [49] showed that the stabilization of the Hf/Hf-Ta system was possible by developing a bimodal multilayer structure comprising columnar and brick-like grains. The stability of the columnar grains was increased by the presence of quadrupole points that locked the grain boundaries, as well as by the presence of semi-coherent Hf interfaces, which prevented recrystallization. These research studies highlight the existence of feasible routes to achieve NMMs that are stable even at elevated temperatures, thus, presenting strategies that could stimulate new studies and applications.

Nano multilayers as precursors for the synthesis of amorphous and nanocrystalline alloys
NMMs have been shown to be useful precursors for the synthesis of MGs with different compositions, as well as for the development of nanocrystalline alloys. Amorphous metallic alloys, characterized by a dense and disordered atomic structure and the absence of grain boundaries, have resulted in several outstanding properties, including good softmagnetic properties, high specific strength, large elastic limits and improved wear and corrosion resistance [227,[389][390][391][392][393][394]. It is important to note that the term metallic glass is often used interchangeably to refer to all synthesized amorphous alloys. According to the literature, the term metallic glass is used to refer to non-crystalline structures that result from the rapid cooling of a liquid melt (liquid-solid transformation), while the term amorphous alloy is generally employed when dealing with non-crystalline structures resulting from solid-solid or vapor-solid transitions e.g. sputtering, SPD, etc. Readers are encouraged to survey [394][395][396] for a complete description of MGs and amorphous alloys prepared via different processing methods and their respective characteristics and behavior. Additional information about the properties and applications of bulk MGs and thin-film MGs can be found in [389,391,397,398] .
The first amorphous alloy obtained from a multilayer system was produced in 1983 by Schwarz and Johnson via medium-temperature annealing using an initial La/Au structure [391,399,400], but soon after, IBM proved to be a more effective route for the synthesis of MGs [227,364,401]. The IBM method utilizes an ion beam (typically of inert gases) to induce atomic collisions that result in the intermixing of the metallic layers. Upon the removal of the source, the resulting mixture undergoes rapid relaxation, from which only amorphous or simple structure alloys can be obtained via solid-state phase transformations [227,401]. The use of NMMs was rapidly considered a feasible strategy for the formation of MGs, since the desired alloys could be achieved at low temperatures (less than 100 • C) [394] and because of the vast range of compositions that could be realized through tuning the total thicknesss of the films, relative thickness of the constituent metals and thickness of each layer [227,391,402]. In fact, the glass formability of different multilayers, which is based on their enthalpy of mixing and maximum possible amorphization range, has been reviewed in detail by Liu et al [227,401]. This work includes a list of experimental reports in which MGs have been achieved via IBM from initial NMM systems such as Al/Ni, Au/Ti, Co/Mo, Fe/W and Nb/Zr.
Other quasicrystalline and metastable binary alloys have been obtained via IBM from Cu/Zr [364], Cu/W [403], Fe/Ag [404], Fe/Pt [253], Ag/Co and Ag/Ni multilayers [405]. For all of these systems, the composition of the formed phases was tuned by changing the individual layer thicknesses, number of layers and irradiation doses [364,403,404]. Thermal annealing has also been used for the same purpose. For instance, magnetic L1 1 CoPt and L1 0 FePt nanocrystalline and metastable alloys have been prepared from Co/Pt [406] and Fe/Pt [402,[407][408][409] systems, respectively, while TiNi alloys have been obtained from Ti/Ni multilayers [133,410]. The fast formation of ordered phases was made possible due to the existance of shorter diffusion paths between the coupled metals, which were made available by using individual layer thicknesses in the order of only a few nanometers (0.2-3 nm) [405,408,409,411].
More recently, equiaxed grain nanostructured alloys have resulted after annealing Hf-Ti/Ti [374,376], Mo-Au/Au [363], Ta-Hf/Hf [49] and NiW/NiAlW [412] systems. For these compositions, the progression from nano multilayers to nanocrystalline films is driven by different thermally activated transformations (grain boundary relaxation, grain growth, recrystallization, solute segregation), which depend on the initial microstructure and composition of the multilayers (see figures 8(c) and (d)). According to those studies, the stability of the final microstructures was achieved either due to the addition of solutes at an equilibrium composition, which effectively decreased the grain boundary energy and prevented grain growth [374,376], or due to the occurrence of grain relaxation and phase separation [412]. It is worth mentioning that the selection of the Hf-Ti/Ti [374,376], Mo-Au/Au [363] and Ta-Hf/Hf [49] systems was based on the thermodynamic stability maps presented by Murdoch and Schuh [413,414], which predict the feasiblility of a binary alloy having a stable nanograin configuration; while the synthesis of NiW/NiAlW [412] was chosen due to the high solubility of W in Ni, which is known to facilitate solid solution strengthening. Collectively, these studies show that NMMs could be useful for the synthesis of nanostructured and amorphous alloys, as well as model systems for studying fundamental science phenomena such as thermal processes and mechanisms that influence nanograin stability.

Summary and outlook
Thorughout this review, a wide range of studies have demonstrated the advantages of leveraging the stacked configuration of nanoscale metallic layers in different technological areas. This is in great part due to advances in the synthesis methods, which have allowed for the preparation of stacked layers with high purity and atomic-level precision. Along with the evolution of the synthesis routes, advances in the characterization methods have led to the development of dedicated techniques, enabling extensive characterization of the microstructure, properties and performance. The resulting NMMs can lead to strong materials, which exhibit high values of hardness and resist dislocation motion. Furthermore, magnetoresistance up to an order of magnitude larger than that measured in monolithic films has been obtained with the proper combination of magnetic and non-magnetic metals. The usage of crystalline metals in NMMs has enabled the development of highly reflective mirrors (with reflectivities in the range of 70%), which have improved the resolution of optical devices such as telescopes and microscopes. Radiation-tolerant materials have been developed to take advantage of misfit dislocations in semi-coherent interfaces, which act as sinks for impurities and defects. In addition, novel phenomena and properties in NMMs, such as the formation of magnetic skyrmions or the feasible release of stored chemical energy in multilayers, are promising for magnetic data storage and micro soldering applications, respectively. Amorphous and nanocrystalline alloys have been effectively formed by inducing solid-state reactions upon the exposure of NMMs to thermal annealing or IBM. Multilayer systems have been shown to be useful as thermodynamic stability models, since thermally stable nanostructures have been obtained by selecting the appropriate compositions. These outcomes suggest the possibility to synthesize materials that can maintain and prolong their properties even when subjected to elevated temperatures.
Overall, the current review highlights the properties and behavior observed in NMMs and how specific design factors play different roles in determining a particular behavior or property. However, independent of application or function, parameters such as composition, interface type, total number of layers, surface roughness, grain morphology and residual stresses simultaneously contribute to the performance of NMMs. Therefore, in order to fully take advantage of, as well as expand and improve the properties and applications offered by multilayer systems, a comprehensive understanding of the role of each of the afformentioned parameters is imperative, as discussed throughout this manuscript.