Band match enhanced current-in-plane giant magnetoresistance in epitaxial Co50Fe50/Cu multilayers with metastable bcc-Cu spacer

Although current in-plane giant magnetoresistance (CIP-GMR) is widely used as various magnetic field sensors, a higher magnetoresistance (MR) ratio is still required to improve their sensitivity and detectivity for certain applications. Here, we report dramatic enhancement of the MR ratio up to 26.5% in a spin valve device and 40.5% in an antiferromagnetically coupled trilayer device using fully epitaxial Co 50 Fe 50 /Cu/Co 50 Fe 50 structures with metastable bcc-Cu spacer layers. Transmission electron microscopy analysis indicated that the metastable bcc-Cu had a perfect lattice match at the bcc-Co 50 Fe 50 /bcc-Cu interfaces. First-principles calculations showed good electronic band matching that induces a large spin asymmetry of the electron transmittance in the in-plane direction. The combination of this substan-tial lattice match and electronic band match is attributed to the large MR ratio, suggesting that exploring the use of metastable structure in ferromagnetic/nonferromagnetic multilayers will lead to further enhancement of CIP-GMR. on both Co 50 Fe 50 and Fe any dislocations; we found a large difference in the MR ratio between the spin-valve CIP-GMR device with Co 50 Fe 50 layers (26.5%) and ones with Fe layers (5%). First-principles calculations for the spin-dependent transmittance at Co x Fe 1 − x /Cu interfaces indicated a strong enhancement in the transmittance of majority-spin electrons for bcc-Co 50 Fe 50 /bcc-Cu/Co 50 Fe 50 but a rapid drop in transmittance to zero in bcc-Fe/bcc-Cu/Fe near the boundary of the two-dimensional Brillouin zone. This result clearly explains the difference between the observed MR ratios between the Co 50 Fe 50 and Fe-based devices. It further suggests that a calculation of ballistic transmittance in a GMR stack can predict the MR ratio of CIP-GMR devices, which would be a guiding principle for further enhancement of the MR ratio in these devices. Accordingly, we achieved the MR ratio of 40.5% in an antiferromagnetically coupled trilayer bcc-Co 50 Fe 50 /bcc-Cu/Co 50 Fe 50 including a specular reflec- tion layer, which is the highest MR ratio ever reported for the trilayer CIP-GMR. However, although the MR ratio is large, current bcc-Co 50 Fe 50 /bcc-Cu/Co 50 Fe 50 cannot be used as a magnetic field sensor as is. Further study is required to increase the sensitivity of the device, for example, by using a softer ferromagnetic layer having a similar bcc structure as Co 50 Fe 50 to achieve more linear response instead of a jump. More exploration in bcc-Co 50 Fe 50 and bcc-Cu devices using an industrially more viable polycrystalline structure with Si wafer substrates also have to be considered for future sensor application.


I. INTRODUCTION
Since the discovery of current in-plane giant magnetoresistance (CIP-GMR) in 1988, an enormous number of the studies have been performed on this phenomena and various applications have been developed. 1,2 The GMR effect was first found in ferromagnetic (FM)/nonmagnetic (NM) multilayers, in which the ferromagnetic layers magnetically interact with each other by interlayer exchange coupling through the nonmagnetic layer. 3 The discovery of a large exchange bias effect from antiferromagnetic (AFM) materials to pin the magnetization of the ferromagnetic (FM) layers led to the development of spin valve (SV) type CIP-GMR, 4 which allows the FM layer to switch at a small external magnetic field to be suitable for magnetic field sensor applications with high magnetic field sensitivity, such as read heads for hard disk drives (HDDs). Multilayer and SV-type CIP-GMR devices were developed for a variety of sensing applications, including earth magnetic field sensors, speed-rotation-position sensors in automobiles, 5 and detectors of magnetic beads used in biomedical applications. 6 Here, magnetic encephalography and cardiography, which require much lower noise, would be promising applications because CIP-GMR has intrinsically small 1/f noise and the device resistance is tunable. 7,8 For all these magnetic sensing applications, a large magnetoresistance (MR) ratio is always beneficial for improving sensitivity and detectivity.
Based on various experimental studies and theoretical treatments using ab initio calculations for electronic structures and the classical Boltzmann equation for spin-dependent transport, [9][10][11][12][13] it has been recognized that the most important factor for enhancing the MR ratio is the lattice and electronic band matchings between the ferromagnetic (FM) layer and nonmagnetic layer. For example, in the case of Fe/Cr multilayers, Fe and Cr both have a body centered cubic (bcc) structure with a very small lattice mismatch of 0.63%, 14 which minimizes the additional spin-independent scattering at the interface caused by structural strain and dislocations. In addition, the band dispersions of the minority-spin electrons in Fe are Cr similar, while those of the majority-spin electrons are not, which give rise to spin asymmetry of electron scattering at the Fe/Cr interface. In particular, MR ratios of 220% at 1.5 K and 42% at room temperature have been reported in Fe/Cr multilayers. 15 Similarly, the good lattice matching and band matching of majority-spin electrons explains the large MR ratio in CIP-GMR devices with Co/Cu and Co 90 Fe 10 /Cu multilayers. Co 90 Fe 10 /Cu multilayers have been used in read heads for HDDs because of the soft magnetic properties of this Co 90 Fe 10 . [16][17][18][19] However, after the CIP-GMR read head was replaced by the tunneling MR (TMR) read head, the research activity for CIP-GMR has been diminished. Since then, little progress has been made in increasing the MR ratio of CIP-GMR because no other combination of conventional FM and NM materials showing superior lattice and band matchings has been found.
As a motivation of the present work, we noticed an interesting study reporting a highly lattice matched all-bcc-Fe/Cu/Fe layered film with a metastable bcc Cu spacer mimicking the structure of bcc Fe. 20 Although such a metastable bcc Cu is expected to grow on any Co 1−x Fex having a bcc structure (x ≳ 0.25), 21 the CIP-GMR properties of all-bcc Co 1−x Fex/Cu/Co 1−x Fex have not been investigated systematically. Thus, we fabricated fully epitaxial CIP-GMR spin valve devices with Co 1−x Fex layers of varying x to investigate the relationship between metastable bcc growth of Cu on Co 1−x Fex and the MR properties. Interestingly, we observed the largest MR ratio ever reported for CIP-GMR at room temperature in the devices with bcc Co 1−x Fex layers (0.25 ≤ x ≤ 0.67). Our first-principles calculations of the band dispersions and the interfacial transmittance of conduction electrons explain the mechanism behind these excellent MR properties. Moreover, we found that the addition of a specular reflection oxide layer 22 and fabrication of a CIP-GMR multilayer increased the MR ratio even further.

A. Experimental details
CIP-GMR spin-valve films consisting of Co 1−x Fex (6 nm)/Cu (t nm)/Co 1−x Fex (6 nm)/IrMn (8 nm) were deposited on top of the MgO (001) substrate. The deposition was done using magnetron sputtering in an ultrahigh vacuum system with a base pressure lower than 5 × 10 −7 Pa. The surface of the MgO substrate was etched by Ar ion milling in the sputtering chamber before depositing the Co 1−x Fex film to obtain (001)-oriented epitaxial growth. Different Co 1−x Fex alloy targets (x = 0.10, 0.25, 0.5, 0.67, and 1) were used to study the CoFe composition dependence. All films had the same CoFe compositions in the top and bottom layers. The Cu thickness t was varied from 0 to 5 nm by making a wedge shape structure. To make samples showing a larger MR ratio, a trilayer structure with Co 50 Fe 50 (3 nm)/Cu (1.6 nm)/Co 50 Fe 50 (3 nm)/MgO (2 nm) and a multilayer with the structure of [Co 50 Fe 50 (3 nm)/Cu (1.6 nm)] 33 Co 50 Fe 50 (3 nm)/MgO (2 nm) were fabricated under the same conditions as the previous samples. The Co 50 Fe 50 thicknesses were reduced from 6 nm to 3 nm to minimize current shunting through Co 50 Fe 50 layers which will increase the MR ratio. All samples were annealed in a 239 kA/m constant magnetic field at 250 ○ C, holding this temperature for 1 h. Photolithography and physical etching were used to pattern the film into wires. The magnetoresistance properties of each wire were measured using the standard dc four probe method with the applied external magnetic field parallel to the current direction.
The microstructure of the spin-valves with Fe, Co 50 Fe 50 , and Co 90 Fe 10 compositions was compared by transmission electron microscopy. An aberration-corrected (scanning) transmission electron microscope [(S)TEM; FEI Titan G2 80-200] was used to analyze the microstructures of the samples, especially the Cu spacer and interfaces between the Co 1−x Fex and the Cu spacer. The specimens for the (S)TEM observation were prepared with the lift-out technique using a dual-beam focused ion-beam/scanning electron microscope (FIB/SEM; FEI Helios G4).

B. First-principles calculation details
The transport calculations were performed with the QUAN-TUM espresso code 23 using the generalized gradient approximation 24 for the exchange-correlation energy. The number of k points was 15 × 15 × 1, and a broadening parameter of 0.01 (Ryd) was used. A B2-CoFe(or Fe)/Cu/B2-CoFe(or Fe)(001) trilayer was constructed in a tetragonal supercell, where the in-plane lattice parameter of the supercell was fixed at 2.86 Å. We prepared a supercell, consisting of a multilayer containing 9 atomic layers of Cu spacer and 15 atomic layers of CoFe (or Fe) for the Co-and Fe-terminated interfaces, in which the atomic positions were fully optimized. Furthermore, we prepared the fcc-Co 90 Fe 10 /fcc-Cu/fcc-Co 90 Fe 10 (011) trilayer with 9 atomic layers of fcc-Cu and 15 atomic layers of fcc-Co 90 Fe 10 , where the in-plane lattice parameters are fixed to that of fcc-Cu lattice constant, 3.61 Å and 3.61 × √ 2 Å. For the transport calculations, we considered an open quantum system consisting of a scattering region corresponding to Cu spacers and a junction with CoFe (or Fe) attached to left and right semi-infinite electrodes corresponding to bulk. The transmittance was obtained by solving the scattering equation with infinite boundary conditions in which the wave function of the scattering region and its derivative were connected to the Bloch states of each electrode. 25,26 The potential in the scattering equation was obtained from self-consistent electronic structure calculations for the supercell containing a left and a scattering region. Since our system was repeated periodically in the xy plane, propagating states were assigned an in-plane wave vector k || = (kx, ky) index.

III. RESULTS AND DISCUSSION
A diagram illustrating the stacking of the fabricated films and x-ray diffraction (XRD) patterns of the respective samples are shown in Figs. 1(a) and 1(b). As expected, the films with Co75Fe 25 , Co 50 Fe 50 , Co 33 Fe 67 , and Fe having a bcc structure all had a peak around 66 ○ , i.e., the (002) peak from bcc Co 1−x Fex, indicating (001)-oriented growth of CoFe and Fe on the MgO substrate. The film with Co 90 Fe 10 did not have this particular peak; it is known to have a stable fcc phase. 21 Figures 2(a) and 2(b) show the Cu spacer thickness dependence of the MR ratio and Co:Fe composition ratio dependence of the MR ratio at each MR maximum and at t Cu = 2.5 nm. The MR ratio was much larger in the devices with bcc Co 33 Fe 67 , Co 50 Fe 50 , and Co75Fe 25 than in fcc Co 90 Fe 10 . Co 50 Fe 50 had the largest MR ratio, 26.5%, at room temperature, which is about 2.6 times larger than the 10.3% of Co 90 Fe 10 , a material often used in magnetic sensors. There was a dramatic enhancement in the MR ratio from 5.4% for   without any visible misfit dislocations throughout a wide region, suggesting that there was almost no lattice mismatch at the interfaces. Additional TEM images taken from a different zone axis direction of Co 50 Fe 50 (Fig. S1) confirmed the formation of bcc Cu. Therefore, we conclude that the nearly complete in-plane lattice matching in both Co 50 Fe 50 /Cu/Co 50 Fe 50 and Fe/Cu/Fe arises from the formation of a metastable bcc Cu spacer. We also took TEM images of a Co 50 Fe 50 /Cu/Co 50 Fe 50 sample with a thicker Cu region (∼4.3 nm) in which the MR ratio decayed to 15% (see Fig. S2). The metastable bcc Cu structure remained even at this thickness, but we found the misfit dislocations at the Co 50 Fe 50 interface and distortion of the lattice of the Cu spacer. Such deterioration of the interfacial lattice matching with growing Cu thickness seems to be the cause of the rapid reduction in the MR ratio against Cu thickness in Co 33 Fe 67 , Co 50 Fe 50 , and Co75Fe 25 . We investigated the interlayer exchange coupling between two Co 50 Fe 50 layers through a bcc Cu spacer to see if we could get a further enhancement of the MR ratio by making a multilayer CIP-GMR structure. The investigation of the Cu thickness dependence of the magnetization curve for a device with a MgO-subs./Co 50 Fe 50 (3 nm)/Cu/Co 50 Fe 50 (3 nm)/MgO capping layer structure revealed a clear antiferromagnetic coupling with a coupling field of 12 mT for a 1.6-nm-thick Cu spacer, as shown in Fig. 4(a). The MR ratio of CIP-GMR for the corresponding sample reached 40.5%, which is nearly twice that of SV-type CIP-GMR and the largest value at room temperature ever reported for trilayer CIP-GMR devices (reported MR ratios in CIP-GMR is summarized in Fig. S5). This improvement relative to the SV type can be explained by the specular reflection of the conductive electrons at the Co 50 Fe 50 /MgO capping layer interfaces. 17 Both MgO substrate and MgO capping layer interfaces reflect electrons back into the FM/NM layers without losing kinetic energy, leading to additional spin-dependent scattering; this increases the MR ratio. Note that we observed smaller MR ratio of about 30% in the another device with 6 nm-thick bottom and top Co 50 Fe 50 layers, which suggests the contribution of the specular reflection from MgO on the enhancement of the MR ratio. A [Co 50 Fe 50 (3 nm)/Cu(1.6 nm)] 33 /Co 50 Fe 50 multilayer structure was also fabricated. The M-H curve showed almost zero magnetization at zero field, which clearly indicates an antiparallel magnetization configuration [ Fig. 4(b)]. The MR ratio was up to 73.3% at room temperature. The HAADF-STEM image in Fig. 4(c) shows that the Cu had a well-grown epitaxial bcc structure throughout the sample. This huge multilayer structure maintains the interfacial coherency from the bottom to the top layer through the metastable growth of bcc Cu, which contribute to the large MR ratio.
Finally, to understand the mechanism behind the behavior of the MR ratio, we performed first-principles calculations of the electronic band dispersion of Co 50 Fe 50 , Fe, and bcc Cu (see the supplementary material, Fig. S3). The dispersive 4s bands that would hold the conduction electrons at the Fermi level in bcc Cu are unoccupied in both the majority-and minority-spin bands for Fe. In contrast, only the majority-spin band in Co 50 Fe 50 has a similar occupied 4s band at E F because the position of the Fermi level in Co 50 Fe 50 is higher than in Fe. To elucidate the effect of this difference in band matching between Fe/Cu and Co 50 Fe 50 /Cu on In the majority-spin transmittance, fcc-Co 90 Fe 10 /Cu/Co 90 Fe 10 and B2-Co 50 Fe 50 /Cu/Co 50 Fe 50 show large transmittances around the zone boundaries, the four corners of (kx, ky) ≈ (±0.5, ±0.5), and over a wide range of kx ≈ ±0.5 and ky ≈ ±0.5. In the ballistic transport calculation based on the Landauer formula, the z components of the wave-vector of incident electrons perpendicular to the plane are determined by the kz-band crossing points at the Fermi level of the electrode material. Furthermore, the total incident energy of conductive electrons is given by Thus, the majority-spin transmittance with a large in-plane wave vector (kx, ky) corresponds to that of the incident electrons with a glancing angle to the plane. In the CIP-GMR effect, the momentum vectors of conductive electrons are almost parallel to the plane, indicating that the transport properties of electrons with a glancing angle are very important. Therefore, the large majority-spin transmittance G ↑ > 2G 0 and the small minority-spin transmittance G ↓ < 0.5G 0 around the boundary edge of the 2D Brillouin zone of fcc-Co 90 Fe 10 /fcc-Cu/fcc-Co 90 Fe 10 and B2-Co 50 Fe 50 /bcc-Cu/B2-Co 50 Fe 50 provide high spin asymmetry to conductive electrons parallel to the plane, resulting in a large CIP-GMR ratio. On the other hand, the majorityspin transmittance of Fe/bcc-Cu/Fe abruptly decreases to zero around the edge of the 2D Brillouin zone, leading to low spin asymmetry in the conductive electrons parallel to the plane. These calculated results are consistent with the experimental results. There are two possible reasons for higher MR ratio in B2-Co 50 Fe 50 /bcc-Cu/Co 50 Fe 50 than that in fcc-Co 90 Fe 10 /fcc-Cu/Co 90 Fe 10 . The first is the formation of dislocation-free perfectly coherent interfaces that suppress spin-independent electron scattering at the interfaces. Another is the much smaller transmittance of the minority-spin electron in the wide region of (kx, ky) plane that can be seen by com- the bcc-Co 50 Fe 50 /Cu/Co 50 Fe 50 film to confirm the validity of this analysis (shown in the supplementary material, Fig. S4). The CPP-GMR device had a much smaller MR ratio (about 4% with the value of ΔRA is 1.8 mΩ μm 2 at RT) than that of the CIP-GMR device. This suggests that a large spin-asymmetry of the transmittance cannot be obtained for electrons propagating in the perpendicular direction, as predicted by the calculations illustrated in Figs. 5(b) and 5(e). These results also suggest that first-principles calculations of ballistic transmittance in a 2D Brillouin zone can be used to predict whether various stacking structures intended for exploiting CIP-GMR will have a high MR ratio.

IV. CONCLUSIONS
In conclusion, we thoroughly investigated the CIP spindependent transport properties through microstructure analyses and first-principles calculations of Co 1−x Fex layers with a metastable bcc Cu spacer layer. A perfectly lattice-matched metastable bcc Cu ARTICLE scitation.org/journal/apm layer was grown on both Co 50 Fe 50 and Fe layers without any notable misfit dislocations; however, we found a large difference in the MR ratio between the spin-valve CIP-GMR device with Co 50 Fe 50 layers (26.5%) and ones with Fe layers (5%). First-principles calculations for the spin-dependent transmittance at CoxFe 1−x /Cu interfaces indicated a strong enhancement in the transmittance of majorityspin electrons for bcc-Co 50 Fe 50 /bcc-Cu/Co 50 Fe 50 but a rapid drop in transmittance to zero in bcc-Fe/bcc-Cu/Fe near the boundary of the two-dimensional Brillouin zone. This result clearly explains the difference between the observed MR ratios between the Co 50 Fe 50 and Fe-based devices. It further suggests that a calculation of ballistic transmittance in a GMR stack can predict the MR ratio of CIP-GMR devices, which would be a guiding principle for further enhancement of the MR ratio in these devices. Accordingly, we achieved the MR ratio of 40.5% in an antiferromagnetically coupled trilayer bcc-Co 50 Fe 50 /bcc-Cu/Co 50 Fe 50 including a specular reflection layer, which is the highest MR ratio ever reported for the trilayer CIP-GMR. However, although the MR ratio is large, current bcc-Co 50 Fe 50 /bcc-Cu/Co 50 Fe 50 cannot be used as a magnetic field sensor as is. Further study is required to increase the sensitivity of the device, for example, by using a softer ferromagnetic layer having a similar bcc structure as Co 50 Fe 50 to achieve more linear response instead of a jump. More exploration in bcc-Co 50 Fe 50 and bcc-Cu devices using an industrially more viable polycrystalline structure with Si wafer substrates also have to be considered for future magnetic sensor application. Finally, we conclude that although CIP-GMR devices are the original spintronics form that was thoroughly studied from its discovery, there is still a room for enhancement of their MR properties by exploring new materials including metastable structures such as bcc-Cu to achieve a large spin asymmetry at interfaces and that such development may lead to a new class of sensitive magnetic sensors.

SUPPLEMENTARY MATERIAL
The supplementary material for the additional STEM result, the first-principles calculation of band structures, and the comparison with CPP-GMR and MR ratio roadmap is available online.