Nanostructures synthesized by the reverse microemulsion method and their magnetic properties

The reverse microemulsion (ME) based synthesis is a versatile route for obtaining a variety of nanomaterials with controlled size and shape. Optimization of the various components involved in the formation of a ME is necessary to obtain the desired morphology and size. Magnetic materials have great significance in various fields like data storage, biomedical imaging, therapeutics, catalysis and sensor applications to name a few. It is seen that the magnetic properties of the nanostructures synthesized by ME methods are distinct from the structures obtained by other methods. Further, even for the nanostructures synthesized by the ME method, the magnetic properties depend upon their size and morphology. Thus, tuning the size and morphology of the nanostructures to tailor the magnetic properties is a desirable objective of several research projects. This mini-review comprehensively discusses those nanostructures synthesized by the ME method whose magnetic properties were investigated.


Introduction
The reverse microemulsion (ME) based route is found to be highly versatile for the synthesis of a variety of nanomaterials as well-documented in several excellent reviews [1][2][3][4][5][6][7]. The effect of various ME parameters and components on the final size and morphology of these structures has been extensively investigated. Optimization of these components like surfactant, co-surfactant, organic phase and water to surfactant ratio (W 0 ) enables us to synthesize nanomaterials with controlled size and shape [6]. It is also well established that the physical properties, including their magnetic properties of nanostructures of the same composition differ with their size and morphology. Thus the control of shape and size in nanostructures is essential for obtaining the desired properties. Our group has over the past decade extensively investigated these systems by techniques like small angle x-ray scattering (SAXS), Fluorescence correlation spectroscopy (FCS) and time-resolved fluorescence studies by which we have been able to understand in detail the growth of bare ME systems as well as of the nanostructures that grow inside them [8][9][10][11]. The effect of various ME parameters were correlated with the final size and morphology of these structures. Nanostructured materials obtained by the ME method include metals, alloys, oxalates, carbonates, borates, succinates and oxides including niobates, tantalates and manganites.
Magnetic nanostructures have significant applications in several fields including data storage [12,13], biomedical [14][15][16][17][18] sensor [19][20][21] and catalytic [22,23] applications and thus it is highly desirable to tune the magnetic response of these nanostructures. Apart from the reverse-microemulsion (water-in -oil) systems (as we shall see in numerous examples below), the oil in water microemulsion [24] system has been extensively used for synthesis of nanostructures whose magnetic properties have been utilized for applications like magnetic resonance imaging (MRI) and drug delivery for therapeutic purposes [25,26]. This current mini-review discusses the work done in our laboratory and other laboratories on the nanostructures obtained by the ME method whose magnetic properties were investigated. In several cases it is seen that the magnetic properties of magnetization (∼125 emu g −1 ) which is higher than that of pure Cu-Co nanocomposite or alloy. This material is potentially useful for applications as a soft magnet.
Spherical FeCoB alloy nanoparticles that were magnetic in nature were synthesized by borohydride reduction in water/n-hexane (W/He) microemulsion systems [30]. There was a substantial difference in the size of the alloy particles synthesized by the microemulsion method (where the average size was approximately 10.7 nm) and those synthesized by a conventional aqua-solution method where the average size was 304.2 nm. On dispersing these FeCoB particles in W/microemulsion, water and silicone oil, three kinds of ferrofluids (FFs) were prepared. The W/He based FeCoB FFs are superparamagnetic and a saturation magnetization (Ms) that reached to 12.4 emu g −1 was observed. Additionally, as compared to the water-based and silicone oil-based FFs, W/He-based FeCoB FFs exhibit high stability, with magnetic weights decreasing slightly even under the magnetic field intensity of H=210 mT. The synthesis of Fe x Ni (1−x) bimetallic alloy nanoparticles was reported by the microemulsion method [31]. Variation of the microemulsion parameters such as water/surfactant/oil ratio, presence of co-surfactant and the concentration of the starting materials resulted in Fe x Ni (1−x) nanoparticles with different sizes, morphologies and compositions. The presence of butanol as co-surfactant led to chain-like arrangement of nanoparticles. The magnetic properties of these alloy nanoparticles were found to be affected by their composition, size and morphology. Spherical nanoparticles were superparamagnetic while chain-like ones were ferromagnetic.
Magnetic CoNi nanostructures were grown by the electrodeposition method using ME as templates [32]. The advantage of this method was that reducing agents were not required. The magnetic properties were dependent on the structure of the nanoparticles. In another study by the same group [33], it was seen that the presence of surfactants influences the magnetic properties with the coercivity decreasing in presence of the surfactant Triton-X. The electrodeposition was attempted in a bicontinuous ME and it was observed that this ME evolves into a 'percolated' W/O ME which, on consumption of ions further evolves into a non-percolated W/O ME phase. Each of these three types of ME results in different types of nanoparticles with the biocontinious, percolated W/O and non-percolated W/O respectively leading to porous films, small nanoparticles and aggregates of nanoparticles each with varying magnetic properties.

Metal carboxylates and metal oxides obtained from them
Metal oxides are another important class of compounds that have structural and physical properties. Often the ME method is used to synthesize precursor nanomaterials (e.g. metal carboxylates) with tailored shape and size. Calcination of these precursor nanomaterials yields the corresponding oxides. Transition metal oxalates, anhydrous manganese oxalate (MnC 2 O 4 ), nickel oxalate (NiC 2 O 4 ·2H 2 O) and copper oxalate (CuC 2 O 4 · H 2 O) nanorods were synthesized by the ME route [34][35][36]. The manganese oxalate nanorod precursors, under specific reaction conditions yielded single phase nanoparticles of various manganese oxides such as MnO, Mn 2 O 3 and Mn 3 O 4 [34]. The size of MnO nanoparticles was 28 nm while that of α-Mn 2 O 3 was 50 nm and both were cubic in phase. α-Mn 2 O 3 shows a weak antiferromagnetic transition with a Néel temperature (T N ) of 80 K, while the spinel Mn 3 O 4 particles (100 nm in size) show a ferrimagnetic transition at 43 K. In another report nickel oxalate and copper oxalate nanorods were synthesized [35][36][37]. The nickel oxalate nanorods have a diameter of 250 nm and a length of the order of 2.5 μm while for the copper oxalate nanorods, the diameter and length are 130 nm and 480 nm respectively. The nature of the solvent was found to influence the aspect ratio of the nanorods. Nickel oxalate nanorods exhibited an antiferromagnetic transition at T N =34 K while the copper oxalate nanorods show temperature independent paramagnetism. The magnetic properties were also found to be dependent on the synthesis techniques. For example in copper oxide nanoparticles synthesized by water-in-oil MEs with two different nonpolar solvents (isooctane and n-octane) the T N was found to be different [38]. The particles synthesized using isooctane have a grain size of 25-30 nm and exhibit a T N of 80 K while for the ones synthesized in the n-octane system have a grain size of 80-90 nm and a T N of ∼220 K, similar to that of bulk sized particles. In another study, iron oxalate dihydrate nanorods ∼470 nm long and having ∼70 nm diameter were synthesized by the reverse micellar route [39]. These rods show an antiferromagnetic ordering at 27 K. Depending on the decomposition conditions, these oxalate rods act as suitable precursor for obtaining a variety of iron oxide nanoparticles with varying magnetic properties. When the iron oxalate nanorods were decomposed in air at 500°C, spherical Fe 2 O 3 nanoparticles (∼50 nm) were obtained that exhibit a transition from a weakly-ferromagnetic to weakly anti-ferromagnetic behavior at ∼225 K which is indicative of a Morinlike transition. However, on decomposition of the oxalate nanorods at 500°C in vacuum (∼10 −5 torr) cuboidal Fe 3 O 4 nanoparticles (∼60-70 nm) were obtained which show a Verwey transition at 122 K [39].
We have also investigated the morphologies and properties of cobalt oxalate dihydrate nanorods obtained by the ME route [40]. The length of these rods can be modified with change in calcination temperature (from ∼6.5 μm at 50°C to ∼2.5 μm at 150°C) while maintaining a nearly constant diameter of 200-250 nm. Nanoparticles of Co 3 O 4 and Co are obtained on thermal decomposition of these rods in air and H 2 , respectively, while a mixture of Co and CoO nanoparticles is obtained by carrying out the decomposition in a helium atmosphere. Antiferromagnetic ordering at 54 and 35 K is seen for the oxalate rods and Co 3 O 4 nanoparticles respectively (figure 2). In another report [41] cobalt oxalate was obtained by the ME method that on decomposition at high temperatures yielded Co 3 O 4 nanoparticles. These Co 3 O 4 nanoparticles align under controlled kinetic parameters and specific decomposition temperatures to form anisotropic nanostructures with well-defined aspect ratios ranging from 1:5 to 1:13. It was observed that the temperature of decomposition influences the shape and size of the individual nanostructures while the cationic surfactant plays an important role in the assembly of these structures. The nanorods showed antiferromagnetic behavior and with decrease in the average size of the individual oxide nanoparticles the T N decreases proportionately. Nickel oxide (NiO) nanoparticles with an average size of 25 nm size having a narrow size distribution were obtained from nickel oxalate synthesized by the reverse-micellar route [42]. These particles exhibit a high magnetic moment as compared to the bulk NiO and also a nearly temperature independent paramagnetism.
Rod-shaped cobalt succinate sesquihydrate and spherical iron succinate trihydrate/pentahydrate were synthesized by the ME route [43]. These are excellent precursors for the synthesis of the respective metal and metal oxide nanoparticles and pure phase Co and α-Fe nanoparticles were obtained on heating in nitrogen atmosphere. Fe 3 O 4 cubes of edge length ∼150 nm and elongated particles of diameter ∼200 nm were obtained. Use of a longer chain dicarboxylate ligand results in shorter rods. The Fe nanoparticles exhibit nearly 100% super para magnetism. The Fe 2 O 3 nanoparticles show a Morin-like transition at 223 K while the Fe 3 O 4 nanoparticles exhibit a Verwey transition at 115 K.
Grass like Fe 3 O 4 nanostructures were reported by using a microemulsion assisted solvothermal method [44]. The magnetic hysteresis loops display superparamagnetic signature at room temperature with the saturation magnetization of 65.5 emu g −1 . Superparamagnetic silica modified iron oxide nanoparticles (8.8-12 nm) were synthesized [45] at room temperature in CTAB/butanol/cyclohexane/water ME system. The as synthesized particles exhibited fast response to applied magnetic fields and zero remanence and coercivity. In γ-Fe 2 O 3 nanoparticles, nanorods and multi-pod nanostructure prepared by variation of reaction conditions the magnetic properties were found to be influenced by the morphology [46].
Magnetic studies on Co 3 O 4 nanoparticles indicated an antiferromagnetic ordering at 20 K. ZnMn 2 O 4 nanoparticles with the spinel structure were obtained from the decomposition of metal oxalate precursors synthesized by (a) the ME and (b) the coprecipitation methods [47]. It was seen that the shape, size and morphology of precursors and oxides vary significantly with the method of synthesis. The ME synthesis method resulted in the micron sized zinc oxalate rods whereas the coprecipitation method yielded spherical nanoparticles of size 40-50 nm. Low temperature (∼450°C) decomposition of oxalate precursors obtained from either of the methods yielded phase pure ZnMn 2 O 4 nanoparticles. However, the size of the ZnMn 2 O 4 nanoparticles obtained from the precursors made by ME method are relatively much smaller (20-30 nm) as compared to those obtained from precursors made by the co-precipitation method (40-50 nm). Antiferromagnetic ordering in the range of ∼150 K was seen in nanocrystalline ZnMn 2 O 4 . NiMn 2 O 4 nanospheres and hexagonal particles were synthesized by tuning the morphology of the nickel manganese oxalate precursor [48]. Variation of the surfactant, co-surfactant and non-polar phase in the ME route lead to the tuning of the aspect ratio from 2 to 24 in these nickel manganese oxalate precursors. With the decrease in the aspect ratio of the nanorods the ferrimagnetic transition temperature Tc (126-72 K) and magnetization decrease. The lowest Tc (72 K) and magnetization is shown by the hexagonal nanoparticles of NiMn 2 O 4 . Ni 0.5 Mn 0.5 (C 2 O 4 ).2H 2 O was synthesized by the ME route and crystallized as nanorods [49]. With the increase in the chain length of the cosurfactants (1-butanol, 1-hexanol and 1-octanol), the aspect ratio of the nickel manganese oxalate increased by up to four times. This was attributed to the reduction of film flexibility of the micelle on increasing the chain length of the co-surfactant in the presence of a non-ionic surfactant (Tergitol). The use of a cationic surfactant led to highly uniform nickel manganese oxalate nanorods. These oxalate precursors on thermal decomposition yielded anisotropic nanostructures of nickel manganese oxide (NiMnO 3 ). It was observed that the NiMnO 3 nanostructures were all ferromagnetic. The Curie temperature ranged from 437 to 467 K and the saturation magnetization increased with increase in the aspect ratio of the nanorods. Nanosized complex manganites LaMnO 3 , La 0·67 Sr 0·33 MnO 3 and La 0·67 Ca 0·33 MnO 3 were synthesized using the ME route [50]. An average grain size of 68, 80 and 50 nm and ferromagnetic ordering at around 250 K 350 K and 200 K was observed for LaMnO 3 , La 0·67 Sr 0·33 MnO 3 and La 0·67 Ca 0·33 MnO 3 respectively. These Curie temperatures correspond well with those reported for bulk materials with similar composition.
Anisotropic nickel borate nanostructures were obtained from a nickel boron precursor synthesized using MEs by a precursor-mediated route [51,52]. Various co-surfactants (1-butanol, 1-hexanol and 1-octanol) were used and it was found a higher chain length leads to more uniform nanorods rather than nanospindles. The nanorods exhibit antiferromagnetic behavior with the T N ranging from 44 to 47 K. Additionally, the magnetic moment increases drastically with the anisotropy of nanorods (thinner rods) though there is no marked variation in T N .
Tb-doped BiFeO 3 (Tb x Bi 1−x FeO 3 ) nanoparticles were synthesized using the micro-emulsion route [53]. SEM indicated the formation of 80-120 nm particles. On substitution of Bi 3+ by Tb 3+ , the magnetic properties were found to be substantially altered. Among the synthesized compositions, the maximum saturation magnetization (Ms) of 0.6691 emug −1 was exhibited by Tb 0.02 Bi 0.98 FeO 3 while the maximum coercivity of 549 Oe was seen in BiFeO 3 . In another study Tb 3+ -doped 16-24 nm sized nanocrystalline zinc ferrites of a nominal composition of Zn 1−x Tb x Fe 2 O 4 (x=0, 0.03, 0.06, 0.09, 0.12 and 0.15) were prepared [54]. The magnetic studies showed that these nanocrystalline terbium doped zinc ferrites are ferrimagnetic. Hydroxyapatite-encapsulated cobalt ferrite (CoFe 2 O 4 ) nanopowders were synthesized [55]. The obtained phases, morphology and magnetic properties were found to depend on calcining temperature. Nanoparticles calcined at 700°C exhibit core-shell morphology. A maximum saturation magnetization of 7.8 emug −1 with a characteristic hysteresis loop was observed. On increasing the calcination temperature to 900°C, cobalt ferrite reacts with hydroxyapatite leading to a the formation of a new phase, Fe 12 (PO 4 ) 8 (OH) 12 which results in decrease in saturation magnetization. The ME method was used to synthesize porous nickel ferrite (NiFe 2 O 4 ) nanorods ∼200 nm in diameter and ∼2.5 to 3.0 μm long [56]. The structural defects like vacancies and surface disorder were investigated using positron annihilation spectroscopy while the cation distribution and magnetic hyperfine properties were probed by Mossbauer spectroscopy. As compared to the bulk composition, a reduced saturation magnetization (M s =20.3 emu gm −1 ) and an enhanced coercive field (H c =37 Oe) field was observed and this was attributed to the mixed spinel structure and the charge accumulated on the surface. The ME method followed by calcination was used to prepare MnFe 2 O 4 :xEu 3+ nanostructures that were irregular in shape with an average diameter of 30-80 nm [57]. It was observed that the magnetic properties were greatly influenced by amount of Eu 3+ doping and increasing it from 0 to 1.0 mol l −1 the saturation magnetization (Ms) values decrease from 42.06 to 0.69 emug −1 . CoFe 2 O 4 nanoparticles (∼49 nm) were prepared by the ME method [58]. At an applied magnetic field of 50 kOe and a temperature of 5 K, the coercivity and the madsximal magnetization were 15.1 kOe and 15.2 emu g −1 , respectively. The particles were found to be superparamagnetic at room temperature.
Yttrium iron garnet (YIG) and yttrium aluminum iron garnet (YAIG) nanoferrites were synthesized [59]. When sintered at 1100°C, YIG nanoparticles show higher initial permeability and Q factor as compared to the YAIG nanoparticles. The saturation magnetization, remanence, and coercivity of YIG and YAIG samples varied when sintered at 900, 1000, and 1100°C as seen from the hysteresis loops. The saturation magnetization and coercivity for the YIG and YAIG nanoparticles were observed in the range 11.56-19.92 emu g −1 and 7.30-87.70 Oe respectively. Spherical maghemite (γ-Fe 2 O 3 ) nanoparticles of size 3.5±0.6 nm were synthesized by a water-in-oil microemulsion method [60] were reported to have high saturation magnetization values. Mg 1-x Co x Cr x Fe 2x O 4 (x=0.0-0.5) were prepared by ME method [61]. Hysteresis loops were recorded at temperatures of 300, 200, and 100 K up to 50 kOe. With decrease in temperature, both the saturation magnetization and the anisotropy coefficient increase with maximum values obtained at x=0.3 and x=0.2, respectively and this was explained on the basis of site occupancy of the substituting atoms.
Crystalline λ-MnO 2 nanodisks synthesized by a microemulsion route using polyvinylpyrrolidone (PVP) as a capping layer [62]. Temperature-dependent magnetization studies were carried out during zero-field-cooling and field-cooling (ZFC and FC) modes from 5 to 300 K. A discrepancy between these two curves indicates that an anisotropic potential barrier is present that blocks the magnetization reversal process. This potential barrier was attributed to the magnetocrystalline anisotropy of Mn 3 O 4 nanoparticles (secondary phase) that was strong enough to camouflage the relatively weak antiferromagnetism of λ-MnO 2 . Orthorhombic MnO(OH) nanoparticles (∼7 nm) were obtained by a microemulsion method and were found to be ferromagnetic with a T c of 35 K and magnetic moment of ∼0.7 μB mol −1 [63].
LiNiCo spinel ferrite nanoparticles of sizes 28 nm to 70 nm doped with rare earth metals Nd and Pr were prepared by the micro-emulsion method [64]. Magnetic properties were analyzed by VSM within applied magnetic field of −10,000 Oe and 10,000 Oe. M S and M R values decreased with the increase in concentration. The ferrites LiNi 0.35−y Co 0.15 Pr y Nd x Fe 2−x O 4 were found suitable for applications in transformer core materials. X-type hexagonal nano-sized ferrites (Sr 2 NiCoFe 28 O 46 ) doped with Yb were synthesized by the microemulsion method [65]. In these 29-41 nm crystallites the lattice strain increased from 3.62×10 −2 to 3.73×10 −2 with increased Yb content. The doping of Yb 3+ lead to decrease in remanence and saturation magnetization ( figure 3). These ferrite materials would be useful for longitudinal recording media due to their thermal stability and higher coercivity. On substitution of Ca and Ni ions at Mg and Fe sites respectively single phase cubic spinel nano-ferrites with chemical formula Mg 1-x Ca x Ni y Fe 2y O 4 (x.0-0.6, y¼0.0-1.2) and size 29-45 nm were synthesized [66]. An increase in saturation magnetization (Ms) from 9.84 to 24.99 emu g −1 is seen up to x=0.  an FCC structure were synthesized [68]. Though no clear trend is indicated, in general, the saturation magnetization (Ms) decreases from 47.9 to 13.09 emu g −1 as x varies from 0 to 0.8. The high values of Ms of some compositions predicted the potential applications in high density recording media and microwave devices.
Silica-Fe 2 O 3 composite nanostructures [69] with iron oxide nanoparticles having a wide size (4-50 nm) and a diverse morphology (spherical, ellipsoidal and rod-like) distribution were synthesized using a combination of microemulsion and sol-gel methods. ε-Fe 2 O 3 , a hard magnet phase with coercivity ∼2.13 T, was identified as the dominant crystalline phase via magnetic measurements. The samples were subject to post-annealing thermal treatment at various temperatures and a decrease in coercivity was seen for samples treated up to a temperature of 750°C (to 1245 Oe). This was attributed to a change in phase from ε-Fe 2 O 3 to α-Fe 2 O 3 . However, in samples treated at higher temperatures (1000°C), an increase in coercivity H C ∼1.5 T was observed and this was attributed to the re-formation of the ε-Fe 2 O 3 structure. On treatment at higher temperatures (1100°C) the phase transformation (ε-Fe 2 O 3 → α-Fe 2 O 3 ) and crystallization of amorphous silica were observed.

Composites
Composite nanomaterials, due to a symbiotic effect among its components, may exhibit physical and chemical properties that are more suitable for specific applications than the individual components. Additionally, composites are of interest as they can be engineered to provide advantages like mechanical stability, corrosion resistance, increased solubility and alternate pathways for electron transfer to facilitate chemical, electrochemical and photoelectrochemical reactions. In this section, we shall discuss the composites prepared by the ME method whose magnetic properties were studied. Triton X-100 reversed-phase water-in-oil microemulsion encapsulation method was employed in coating the pre-prepared Fe 3 O 4 nanoparticles with chitosan [70]. The resultant magnetic chitosan coated particles were 50 to 92 nm in size and the spinel structure of the Fe 3 O 4 remained unchanged on coating. The saturated magnetization of these coated nanoparticles reached 18.62 emug −1 , and showed these characteristics of superparamagnetism. A hydroxyapatitemagnesiumferrite (HA-MgFe 2 O 4 ) nanocomposite was synthesized [71] by the ME route which exhibits a maximum saturation magnetization of 9.47 emu g −1 . Carbon nanotubes (CNTs) were decorated with magnetic M(II) Fe 2 O 4 (M=Co, Ni, Cu, Zn) nanoparticles of sizes 15 to 20 nm [72]. Magnetic hysteresis loop measurements indicated that the nanocomposites displayed ferromagnetic behavior at 300 K which can be manipulated using an external magnetic field (figure 4). The CoFe 2 O 4 /CNTs nanocomposites exhibited the maximum value of saturation magnetization (37.47 emu g −1 ).
A SDS/water/cyclohexane/n-pentanol microemulsion system was employed to synthesize functionalized polyaniline/Mn 0.6 Zn 0.4 Fe 2 O 4 nanocomposites (PANI/MZFO NCs) that exhibited a superparamagnetic behavior [73]. Superparamagnetic Fe 3 O 4 /Ag nanocomposites with self-aggregated branch like nanostructures with sizes in the range of 10±2 nm were synthesized by the ME route. The observed saturation magnetization of these nanocomposites were 40 emu g −1 . Co 0.5 Zn 0.5 Fe 2 O 4 /PANI nanocomposites were synthesized via ME method [74]. VSM measurements indicated ferromagnetic nature of the composite. A saturation magnetization of 3.95 emu g −1 and low coercive force (39 Oe) was observed. The formation of a composite with PANI nanofibers results in transition of the magnetic properties of Co 0.5 Zn 0.5 Fe 2 O 4 crystals from superparamagnetic to ferromagnetic.
Inorganic nanocrystals combined with anticancer drugs to construct multifunctional hybrid nanostructures are a powerful tool for cancer treatment and tumor suppression [24]. The critical challenge is to reproducibility synthesize compact, multifunctional nanostructures with improved functionality. Magnetite hybrid nanostructures employing Fe 3 O 4 nanoparticles to form multifunctional magnetite nanoclusters (NCs) were synthesized by combining an oil-in-water microemulsion assembly and a layer-by layer (LBL) method. These nanostructures were shown to serve as improved magnetic resonance imaging (MRI) contrast agent. Fe 3 O 4 nanoparticles and CdTe quantum dots (QDs) were directly incorporated into 50 nm diameter silica shell by reverse microemulsion method to produce water-soluble composites that were both magnetic and fluorescent [75]. By modifying the surface of MNs-QDs/SiO 2 composite nanoparticles with amino and methylphosphonate groups, biologically functionalized and monodisperse MNs-QDs/SiO 2 composite nanoparticles can be obtained. In this work, bi-functional composite nanoparticles were conjugated with FITC labeled goat antirabbit IgG, to generate novel fluorescent-magnetic-biotargeting tri-functional composite nanoparticles, which can be used in a number of biomedical application. Ferromagnetic Fe/Fe 3 C-MWCNT composites were prepared by calcination of Fe 2 O 3 -MWCNT composite in a H 2 atmosphere. By electron microscopy it was seen that the Fe 2 O 3 or Fe/Fe 3 C nanoparticles both fill and surface decorate the MWCNTs [76]. The saturation magnetization and anisotropy field are particularly strong for Fe/Fe 3 C-MWCNT.

Core-shell particles
Multiphasic, core-shell type nanoparticles comprise of an inner core structure and outer shell(s) with of different compositions. As a result of the composition of the core and the shell material as well as due to their design and geometry, these particles exhibit unique properties. Multifunctional nanocomplexes with a superparamagnetic iron oxide nanoparticle (SPIONs) core and an upconversion luminescent shell comprising of lanthanide ions were synthesized by the ME method [77]. Three distinct shells on SPIONs cores namely LaF 3 :Yb 3+ /Er 3+ , NaYF 4 :Yb 3+ /Er 3+ and NaYF 4 :Yb 3+ /Ho 3+ were synthesized of which the first was obtained using the microemulsion and coprecipitation methods while the latter was obtained using the thermal decomposition method. VSM studies revealed a superparamagnetic character of the nanocomplexes. The saturation magnetization was 10.2 emu·g −1 and 8.4 emu·g −1 for SPIONs@LaF 3 :Yb 3+ /Er 3+ nanocomplexes with and without heat treatment, respectively. Multiphoton microscopy confirmed the upconversion process in these materials. For SPIONs@NaYF 4 :Yb 3+ /Er 3+ , three peaks of emission were obtained, at 520 nm, 540 nm and 657 nm, while for SPIONs@NaYF 4 :Yb 3+ /Ho 3+ and SPIONs@LaF 3 :Yb 3+ /Er 3+ samples two peaks at 540 nm and 657 nm were observed. Xylan-Fe 2 O 3 (QX-Fe 2 O 3 ) core/shell nanocomposites were prepared by synthesis of γ-Fe 2 O 3 nanoparticles by a solvothermal process and coating with QX via a reverse ME method. The particles Gadolinium carbonate (Gd 2 (CO 3 ) 3 ) hollow nanospheres were synthesized via a reverse ME method and their suitability for drug transport and magnetothermally-induced drug release was evaluated [78]. Tris (tetramethylcyclopentadienyl)gadolinium(III) and CO 2 were used as the starting materials and hollow spheres with an outer diameter of 26±4 nm, an inner cavity of 7±2 nm, with a wall thickness of 9±3 nm are obtained. As a proof of concept study, the nanocontainers of Gd 2 (CO 3 ) 3 hollow nanospheres were filled with anti-cancerogenic agent doxorubicin (DOX), via a ME strategy. The resulting DOX@Gd 2 (CO 3 ) 3 nanocontainers provided the option of multimodal imaging including magnetic resonance imaging (OI, MRI), optical imaging as well as magnetothermal heating and drug release.
A dual contrast agent for computed tomography (CT) and magnetic resonance imaging (MRI) consisting of ∼7 nm Fe 3 O 4 particles forming the core with an iodine-carrying nanopolymeric shell, having an overall particle size ranging from 50 to 250 nm was synthesized via the microemulsion polymerization method. This contrast agent consists of (d=7 nm) with 2-Methacryloyloxyethyl(2,3,5-triiodobenzoate) was used as the monomer. The amount of the surfactant, sodium oleate was varied to control the overall particle size. The particles provided a highly visible contrast in CT and MR images. A template for biomedical applications was created by adding a comonomer and the particles were further functionalized with the somatostatin analogue Tyr3octreotate. The particles were tested for specific uptake into somatostatin receptor-positive AR42J cells.
Fe@Au core-shell nanoparticles were synthesized by the microemulsion methods [79]. Most reports of Fe core based magnetic nanoparticles with an Au shell have iron oxides as cores which have a large lattice mismatch with Au. A Fe core be more appropriate, however, this is prone to oxidation, so this work is quite significant as it describes a method to circumvent this to produce stable core-shell Fe@Au nanoparticles. These 6 nm particles with 3 nm core have a saturation magnetization of 1.13 emug −1 . Cobalt-silver (Co-Ag) core-shell nanoparticles (3-5 nm) with different shell thicknesses were synthesized in a two-step reduction process by the ME technique [80]. The observed magnetic properties were resultant of both superparamagnetic and ferromagnetic contributions. At room temperature, giant magnetoresistance values of 0.1% were observed. FeCo@C core-shell nanoparticles [81] were synthesized using a combination of the ME and catalytic CVD techniques. A lower saturation magnetization and higher natural-resonance frequency was observed in these core shell particles as compared with FeCo nanoparticles.
Biocompatible Superparamagnetic MnO@SiO 2 core/shell nanoparticles with a luminescent silica shell were synthesized by a combination of microemulsion techniques and common sol-gel procedures [82]. The presence of hydrophilic poly(ethylene glycol) (PEG) chains on the SiO 2 surface, makes these nanocomposites soluble and stable in aqueous media, including physiological saline, buffer solutions and human blood serum. It was seen that the presence of a silica shell did not change the magnetic properties significantly, and cytotoxicity and biocompatibility studies indicated that these material are suitable for applications in biological imaging applications. The MnO@SiO 2 nanocomposite particles showed a T1 contrast with relaxivity values comparable to those of the commonly used PEGylated MnO nanoparticles. Janus shaped plasmonic-magnetic silvermagnetite nanostructures were developed via a single phase microemulsion technique [83]. The measurements of response for bare magnetite as well as silver-magnetite nanoparticles in an alternating magnetic field reveal their suitability in hyperthermia applications.
Design of magnetic nanoparticles in useful in medicine as they can be directed to specific body sites by external magnetic fields. These magnetic nanoparticles may be used for magnetic resonance imaging and targeted drug delivery. Polymer coated manganese iron oxide nanoparticles were produced by the microemulsion method and used for treatment of medical device related infections [84]. Core-shell nanoparticles coated with the polymer, polyacrylamide (pAcDED), that is intrinsically antimicrobial, exhibits very desirable magnetic as well as antimicrobial properties. These particles were injected into the patients either soon after device implant or even after onset of infection and were able to prevent or remove the infection causing biofilm from the device.

Miscellaneous
A series of pentaalkylguanidinium-based magnetic room-temperature ionic liquids 1,1,3,3-tetramethyl-4alkylguanidinium bromotrichloroferrate(III) [CnTMG][FeCl 3 Br] (n=2, 4, 6, 8) were used as the polar phase in a microemulsion system that had different aliphatic oils as a nonpolar phase, a mixture of nonionic surfactant Triton X-100 (TX-100) and different short-chain aliphatic alcohols as a cosurfactants [85]. Magnetic susceptibility and rheological measurements indicated that these microemulsion systems had low viscosity and strong magnetic susceptibility over the composition range where the system was optically clear. The ME method has also been used for polymerization reactions to synthesize nanomaterials whose magnetic properties have been utilized for medical applications [86]. For example, glycidyl methacrylate was utilized to derivitize dextran for the purpose of anchoring vinyl groups on the polysaccharides backbone. This enabled its utilization along with acrylic acid as a monomers for miniemulsion polymerization to form hydrogels. Separately magnetite particles (∼5.2 nm) were synthesized by the co-precipitation method and encapsulated in the as prepared hydrogels. The magnetic nano-hydrogels were superparamagnetic and were stable under physiological conditions for a month.

Conclusions
The synthesis and characterization of a variety of nanomaterials by the microemulsion method and evaluation of their magnetic properties was reviewed. It was seen that the solvent and the decomposition temperature play a significant role in the size and morphology of the precursor oxalates and hence the magnetic properties of the resulting oxides. For example for copper oxide particles the solvent was seen to influence the size and corresponding magnetic properties. It is also observed that the decomposition temperature, (in iron oxide and cobalt oxide) and the surfactant, co-surfactant and the non-polar phase (for ZnMn 2 O 4 particles) was found to influence the magnetic properties. Core-shell nanoparticles comprising of two immiscible metals could be formed by this method and the magnetic properties of the resulting particles exhibit magnetic properties that are distinct from composites of such materials. For particles such as nickel borate the co-surfactant influences the anisotropy and hence their magnetic properties. Applications in directed drug delivery, digital storage media, biomedical imaging applications were highlighted in this review. A class of materials that has been demonstrated to be useful in all these applications are nanoferrites. Though various empirical relationships between the above factors and the size and morphology of the obtained nanomaterials have been deduced, it is seen that these relationships are usually composition dependent. Thus, more intensive research needs to be carried out, to deduce generalized conditions and protocols that may be applied to obtain nanostructures different compositions with well-controlled size and morphology.