Current-in-plane Giant Magnetoresistance Research Articles

Analysis of current-in-plane giant magnetoresistance using Co2FeAl0.5Si0.5 half-metallic Heusler alloy

There has been renewed interest in current-in-plane giant magnetoresistance (CIP-GMR) devices for high-sensitivity magnetic sensors. However, further improvement in magnetoresistance (MR) ratio is necessary to achieve sufficient magnetic field sensitivity. Use of a half-metallic Co-based Heusler alloy ferromagnetic (FM) layer has been demonstrated to be effective in enhancing GMR in the configuration with current perpendicular to the plane; however, only small MR ratios are obtained in the CIP configuration. To understand the origin of the disappointingly low MR in the CIP configuration when using Heusler alloy FM layers, we investigated the magnetotransport properties of CIP-GMR devices using half-metallic Co2FeAl0.5Si0.5 (CFAS) Heusler alloy and conventional CoFe alloy as the FM layers in combination with Ag or Cu as a nonmagnetic (NM) spacer layer. Regardless of the high lattice and electronic band matching at the CFAS/Ag interface, CFAS/Ag CIP spin valves (SVs) show a MR ratio of only 1.2% at room temperature, which is much smaller than those of reference CoFe/Cu and CoFe/Ag SVs (21.6% and 8.4%, respectively). Current density distribution simulations suggest that large current shunting occurs in the Ag layer due to the significant resistivity gap between CFAS and Ag, which limits the generation of highly spin-polarized current from the CFAS layer, resulting in the very small MR ratios. To enhance the MR ratio in CIP-GMR using half-metallic materials, resistivity matching between FM layers and the NM layer is required, in addition to the high electronic band match that has been considered, as a key factor to obtain a high MR ratio in CIP-GMR devices.

Effects of (Ni0.8Fe0.2)100−xCrx seed layer on microstructure, magnetic properties, and giant magnetoresistance of [FeCoNi/Cu] multilayer films

We studied the effect of the (Ni0.8Fe0.2)100−xCrx seed layer on the current-in-plane giant magnetoresistance (CIP-GMR) in Fe16Co66Ni18/Cu multilayer films. Both the GMR ratio (ΔR/R) and interlayer exchange coupling (IEC) between the FeCoNi layers showed strong dependences on the Cr concentration x in the seed layer. The highest ΔR/R of ∼30% was obtained for x = 38 (at. %) due to a comparatively strong antiparallel IEC resulting from a strong [111]-out-of-plane-texture of the [FeCoNi/Cu] multilayer. This confirms that a strong [111] texture is essential for the GMR in antiferromagnetically coupled multilayers as reported in previous works. However, we found that the strong [111] texture is not sufficient to obtain large ΔR/R. For x = 48, the [FeCoNi/Cu] multilayer showed a strong [111] texture as that for x = 38; however, the observed ΔR/R and antiparallel IEC for x = 48 were much smaller than those for x = 38. Detailed microstructural and chemical analyses by scanning transmission electron microscopy revealed that the fluctuation of the thickness of the Cu spacer layer can explain the reduction of antiparallel IEC that gave rise to the decrease in ΔR/R in the case of x = 48. We also discuss the effect of IEC energies on the sensitivity of GMR sensors.

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 Co50Fe50/Cu/Co50Fe50 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-Co50Fe50/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 substantial 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.

Effect of Short Annealing Times on the Magnetoelectronic Properties of Co/Pd-Based Pseudo-Spin-Valves

We investigated the effects of short annealing times on the magnetoelectronic properties of pseudo-spin-valves (PSV) with perpendicular magnetic anisotropy based on Co/Pd multilayers using a contact hot plate. In order to study the time scale at which the degradation of film properties occurs for possible application in perpendicular MgO-based magnetic tunnel junctions (MTJ), the results were compared against our previous study of Co/Pd PSV based on vacuum annealing. With contact annealing for up to 90 s, no significant changes to the current-in-plane giant magnetoresistance (CIP-GMR), interlayer coupling, sheet resistance and layer coercivities were observed for up to 200 degrees C. At 350 degrees C, a 39 to 46% decrease in CIP-GMR was observed for annealing times of 30 to 90 s, respectively, slightly lower than that observed for vacuum annealing at 230 degrees C for 1 h. Similar results were also obtained for interlayer coupling, sheet resistance and layer coercivities, indicating that short annealing times allow for reduced interlayer diffusion at higher temperatures. However, it is clear that significant degradation of GMR performance occurs at 350 degrees C and above even for annealing times as short as 30 s, indicating the potential difficulty of realizing Co/Pd-based perpendicular MgO-MTJ.

Magnetoresistance and Switching Properties of Co–Fe/Pd-Based Perpendicular Anisotropy Single- and Dual-Spin Valves

Switching characteristics and magnetoresistance of spin valves with perpendicular anisotropy based on Co-Fe/Pd multilayers deposited by ultrahigh vacuum (UHV) sputtering have been studied. In unpatterned thin films without exchange bias, high current-in-plane giant magnetoresistance (CIP-GMR) of 9.7% and 15.2% in single-spin valves (SSVs) and dual-spin valves (DSVs) was measured, a significant improvement over previous work. We describe the effects of a Ta seed layer and postdeposition annealing on the perpendicular anisotropy and magnetoresistance of Co-Fe/Pd spin valves, which can be attributed to improvements in the fcc (111) orientation of Pd and the formation of Co-Pd alloys at the Co-Fe/Pd interfaces, respectively. We also show that the coercivity of the layers can be tuned by varying the Co-Fe alloy composition in the multilayers, and describe the minor loops of perpendicular DSVs that exhibit four distinct resistance states, which is a potential structure for multistate storage devices.

The effect of microstructure on the temperature dependence of the interlayer coupling in Co/Cu multilayers

Three classes of giant magnetoresistance Co(1 nm)/Cu(2.1 nm) multilayers were sputter grown with different microstructures in respect to grain size and interface roughness, depending on deposition conditions. Magnetization and current in-plane giant-magnetoresistance (GMR) isothermal loops reveal an unusually high increase of coercivity from 280 down to 5 K. In addition, a systematic variation was observed in the temperature dependence of the indirect exchange coupling as the Co–Cu layering is modified in the three classes of Co/Cu multilayers. Specifically, the temperature dependence of the saturation (switching) field in the GMR-loops, and the indirect coupling strength, vary as (T/T0)/sinh(T/T0) whereas the spin-blocking temperature T0 is found equal to 84(4), 96(11), and 105(10) K for class A, B, and C multilayers, respectively. These results indicate that the desirable low hysteresis appears in the GMR loops at room temperature because the spin structure becomes unstable above the obtained T0 due to domain wall fluctuations. Such magnetic fluctuations define a short–range order state above T0 that depends on Co–Cu intermixing and geometric factors of the grains.

Improvement of giant magnetoresistance performance of a metal layered structure

A metal layered structure consisting of Cr/Cu buffer layer and two Co layers with different thickness, separated by a Cu spacer, was deposited on Si wafers using magnetron sputtering. The influence of the individual layer’s thickness on the magneto transport and magnetic hysteresis was studied. Optimization of the magnetoresistance performance was supported by the current in-plane giant magnetoresistance (GMR) modeling based on the solution to the Boltzmann transport equation. The best structures demonstrated very sharp magnetization reversal of the lower Co layer at about 100 Oe accompanied by resistivity change with a rate of 0.9%/Oe. Maximum GMR effect measured at 5 K was 9.6%. Samples with capping layers exhibited substantially smaller switching fields of the upper Co layer than the uncapped samples with a minute amount of CoO.

Current-in-Plane Magnetoresistance in a Magnetic Sandwich Structure with the Interface Inter-Diffusional Roughness

By introducing a thin transition sublayer to describe the interface structure with large inter-diffusion, the impurities and other defects at it, current-in-plane (CIP) giant magnetoresistance (GMR) is calculated based on the Kubo formalism in real spaces. The oscillations of CIP GMR versus the thicknesses of various layers are predicted due to the quantum size effect. The contribution of the interface inter-diffusional roughness on the GMR effect is discussed with the variations of the thicknesses of ferromagnetic and nonmagnetic layers.

A unified theory of CIP and CPP giant magnetoresistance in magnetic sandwiches

A theory of giant magnetoresistance (GMR) in magnetic sandwiches F/P/F for current in plane (CIP) and current perpendicular to plane (CPP) geometries is developed. We adopted the free electron model described by four parameters: mean free paths and scattering amplitudes (coherent potentials) at the interfaces for spin-up and spin-down electrons. For both CIP and CPP geometries, we calculated the conductivities and GMR using Kubo formalism and the Green function technique in mixed real space-momentum representation. The final expressions for GMR in both geometries were obtained using the same microscopic parameters. Main attention was paid to the relative role of spin-dependent bulk and interfacial scattering. It was shown that increasing of surface scattering for fixed spin asymmetry leads to non-monotonic behaviour of CIP GMR due to renormalization of the scattering amplitude. In the case of CPP geometry the dependence of GMR on interfacial scattering amplitude is monotonic.

Effects of bulk and interface scattering on giant magnetoresistance in magnetic multilayers

The effects on the giant magnetoresistance (GMR) of various types of randomness in magnetic multilayers are studied using a simple two-band (s + d) model for FM/Cu/FM-type trilayers, where FM is the ferromagnetic metal or alloy. Three types of randomness: (i) spin-independent randomness, (ii) spin-dependent bulk and (iii) spin-dependent interface randomness, are taken into account to calculate CIP (current in plane to layers) GMR. The conductivity is calculated in the Kubo formalism. It is shown that the dependence of GMR on the spin-independent randomness is rather strong and that both bulk and interface randomness contribute to GMR in permalloy/Cu-type multilayers. Based on the calculated results, we analyze the concentration dependence of the GMR of CoNi/Cu multilayers and find that the interface randomness governs the basic feature of the concentration dependence of GMR in these multilayers. The other types of randomness, however, are necessary to obtain quantitative agreement between the experimental and theoretical values of the MR ratio. The implications of GMR effects observed in amorphous FM/Cu multilayers are also presented.

Quantum statistical theory of giant magnetoresistance in magnetic heterogeneous alloys

We present a quantum-statistical theory of giant magnetoresistance in magnetic heterogeneous alloys, consisting of small particles of ferromagnetic metal embedded in a nonmagnetic conducting matrix. The paper focuses on the spin-dependent size-effect on the conductivity of the system, i.e. the influence of the relative orientation of the magnetization in neighboring particles on their conductivity as a function of the size and distance between the particles. It is shown that the conductivity of heterogeneous alloys is not self-averaging when the particle sizes and/or distances are comparable to the mean-free paths. As for current in-plane giant magnetoresistance, the electron mean-free paths are the relevant length scale parameters. When the size-effects due to the bulk and interfacial spin-dependent electron scattering are taken into account, the giant magnetoresistance amplitude exhibits a maximum as a function of the average radius of the particles. The optimum radius value is determined by a balance between the interfacial and bulk scattering.

Iterative semiclassical solutions of CIP giant magnetoresistance in magnetic multilayers with mixed layers taken into account

The problem of solving semiclassically the current in-plane (CIP) giant magnetoresistance in magnetic multilayers with mixed layers taken into account is comprehensively studied. The solution of this problem is attributed to the solution of G-coefficients. A new choice of local spin quantization-axes is adopted so that the number of G-coefficients is reduced from Johnson-Camley's 4(5n + 1) to our 4(4n + 1). Furthermore, we show that actually only a half number of G-coefficients need be sloved for the symmetric structure and a superlattice can be simplified to a symmetric penta-layered structure. The main result of this paper is the establishment of the iteration method for solving the G-coefficients. Associating this method with that of numerical integration, for which a formulism is developed, the GMR can be conveniently calculated.

Recent results on the giant magnetoresistance in magnetic multilayers (anisotropy, thermal variation and CCP-GMR)

We present some recent results obtained on the electrical transport properties in magnetic multilayers. Three points are addressed. The first one is an experimental demonstration of the existence of an intrinsic anisotropy of the giant magnetoresistance (GMR). The experiments have been carried out on spin-valve samples for which there is no contribution of the usual anisotropic magnetoresistance to the observed magnetoresistance. The GMR amplitude is found to be larger (lower) in the direction perpendicular (parallel) to the sensing current. The second point concerns a quantitative analysis of the thermal variation of the CIP (current-in-plane) GMR in magnetic multilayers. This analysis is based on a semi-classical theory including the spin-intermixing due to spin-flip scattering by magnons. This approach allows quantitatively evaluation of the respective weights of the various contributions to the thermal decrease in GMR: (i) scattering by magnons in the bulk of the ferromagnetic layers; (ii) phonon scattering in the non-magnetic spacer layer; and (iii) interfacial scattering by paramagnetic interfacial layers which may form as the temperature is increased. The third point is a theoretical investigation of the CPP (current perpendicular to the plane) electrical transport through an interface between two semi-infinite metallic materials. It is shown that when a potential step U exists at such an interface, this step gives rise to an interfacial resistance proportional to U 2. It also leads to the existence of large oscillations in the electric fields on both sides of the interface.