Ag Spacer Layer Research Articles

Growth of [001]-oriented polycrystalline Heusler alloy thin films using [001]-textured Ag buffer layer on thermally oxidized Si substrate for spintronics applications

To utilize highly spin-polarized Heusler alloys in practical spintronic devices, the realization of highly textured and structurally ordered polycrystalline thin films under limited annealing temperatures (TA) is critical. Compared to the natural [110]-texture of Heusler alloys, the [001]-texture is considered to be favorable for current-perpendicular-to-plane giant magnetoresistance devices due to the reduced lattice misfit with the face-centered-cubic Ag spacer layers. In this study, we fabricated [001]-oriented polycrystalline Co2FeGa0.5Ge0.5 (CFGG) Heusler alloy films epitaxially grown on a [001]-oriented polycrystalline Ag buffer layer on a thermally oxidized Si substrate, and the microstructure of the [001]-oriented Ag/CFGG bilayer film was investigated in detail. The [001]-oriented Ag films were obtained by introducing N2 into Ar during the sputtering process. The [001]-oriented CFGG films exhibited smooth interfaces, B2 ordering, and a high saturation magnetization close to the theoretical value under relatively low annealing at TA = 300 °C, which are critical for industrial applications such as read heads of hard disk drives.

Independence of the spin current from the Néel vector orientation in antiferromagnet CoO

Spin pumping from ferromagnetic Fe into antiferromagnetic CoO across a Ag spacer layer was studied using ferromagnetic resonance (FMR) in Py/CoO/Ag/Fe/Ag(001). The thin Py film on top of CoO permits an alignment of the CoO N\'eel vector through field cooling in two otherwise equivalent [110] and $[1\overline{1}0]$ crystalline axes which are parallel and perpendicular to the Fe magnetization direction, respectively. Fe FMR linewidth is measured as a function of Ag thickness in 10--20-GHz frequency range and in 180--330 K temperature range. We find that there exists an anisotropy in the Fe FMR damping for parallel and perpendicular alignment of the Fe and CoO spins. However, such anisotropic damping exists only at thin Ag thickness where there exists a magnetic interlayer coupling between Fe and CoO, and vanishes at thick Ag thickness where the interlayer coupling becomes negligible but permitting spin-current transmission into CoO. Our result indicates the absence of anisotropic spin current for parallel and perpendicular alignment of the Fe and CoO spin axes.

Fully epitaxial C1b-type NiMnSb half-Heusler alloy films for current-perpendicular-to-plane giant magnetoresistance devices with a Ag spacer.

Remarkable magnetic and spin-dependent transport properties arise from well-designed spintronic materials and heterostructures. Half-metallic Heusler alloys with high spin polarization exhibit properties that are particularly advantageous for the development of high-performance spintronic devices. Here, we report fully (001)-epitaxial growth of a high-quality half-metallic NiMnSb half-Heusler alloy films, and their application to current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with Ag spacer layers. Fully (001)-oriented NiMnSb epitaxial films with very flat surface and high magnetization were prepared on Cr/Ag-buffered MgO(001) single crystalline substrates by changing the substrate temperature. Epitaxial CPP-GMR devices using the NiMnSb films and a Ag spacer were fabricated, and room-temperature (RT) CPP-GMR ratios for the C1b-type half-Heusler alloy were determined for the first time. A CPP-GMR ratio of 8% (21%) at RT (4.2 K) was achieved in the fully epitaxial NiMnSb/Ag/NiMnSb structures. Furthermore, negative anisotropic magnetoresistance (AMR) ratio and small discrepancy of the AMR amplitudes between RT and 10 K were observed in a single epitaxial NiMnSb film, indicating robust bulk half metallicity against thermal fluctuation in the half-Heusler compound. The modest CPP-GMR ratios could be attributed to interface effects between NiMnSb and Ag. This work provides a pathway for engineering a new class of ordered alloy materials with particular emphasis on spintronics.

Current perpendicular to film plane type giant magnetoresistance effect using a Ag–Mg spacer and Co2Fe0.4Mn0.6Si Heusler alloy electrodes

The current perpendicular to film plane type giant magnetoresistance (CPP-GMR) effect in Co2Fe0.4Mn0.6Si/Ag83Mg17/Co2Fe0.4Mn0.6Si junctions was investigated. An epitaxially grown 5-nm-thick Ag83Mg17 film having partially ordered L12 structure was fabricated on the Co2Fe0.4Mn0.6Si layer by magnetron sputtering. CPP-GMR effects were observed in the submicrometer-sized junctions, and the maximum value of the observed (intrinsic) MR ratio was 40% (48%) at room temperature. The average change in the resistance–area product was 23 mΩ·µm2 for the Ag–Mg junctions, which was higher than those of conventional CPP-GMR junctions using a Ag spacer layer.

Current-perpendicular-to-plane giant magnetoresistance using Co2Fe(Ga1−xGex) Heusler alloy

We investigated the current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) of pseudo spin valves (PSVs) using ferromagnetic (FM) Heusler alloy Co2Fe(Ga1−xGex) (x = 0, 0.5, and 1) layers and an Ag spacer layer. The FM layer with x = 0.5 gave rise to the highest MR output, ΔRA = 8.7 mΩ·μm2, at room temperature with 10 nm thick Heusler alloy layers. The MR output increased with increasing annealing temperature, Ta, with a maximum at Ta = 500 °C. Transmission electron microscopy showed no visible changes in the layered structure; however, energy dispersive x-ray analysis indicated a considerable diffusion of Ga and Ge into the other layers by annealing at Ta = 550 °C. The ΔRA dependence on the thickness of the FM layers indicated that both high bulk and interface spin asymmetries contribute to the high MR output.

Enhancement of interfacial specific resistance by the insertion of Fe(001) layers between alternate monatomic [Fe/Co]n superlattices and Ag spacer

The bulk and interface spin scattering asymmetric coefficients, βF and γF/N, respectively, as well as the interfacial specific resistance, AR*F/N, contribute to the total resistance change-area product, ΔRA, in current-perpendicular-to-plane (CPP) geometry. In this paper, we report the effect of the insertion of a 1 nm Fe layer into ferromagnetic (F)/non-ferromagnetic (N) interfaces on the spin dependent transport properties. Using the Valet-Fert theory based on the two-current model in which ΔRA is obtained as a function of thicknesses of the ferromagnetic layer (3, 4, and 5 nm), we experimentally deduced the values of βF, γF/N, and AR*F/N of exchange-biased spin valves (EBSVs) for an artificially ordered B2 state Fe50Co50 alloy as a magnetic layer and an Ag spacer layer.

Oscillatory antiferromagnetic interlayer exchange coupling in Co2Fe(Al0.5Si0.5)/Ag/Co2Fe(Al0.5Si0.5) films and its application to trilayer magnetoresistive sensor

We report oscillatory antiferromagnetic (AFM) interlayer exchange coupling (IEC) in a trilayer film with Co2Fe(Al0.5Si0.5) (CFAS) Heusler alloy ferromagnetic layers and a Ag spacer layer. The AFM-IEC was observed only in the films annealed at >400 °C, suggesting the enhancement of the interfacial spin-dependent scattering by improved B2 order in the CFAS film is its origin. Using the AFM coupled trilayer, we demonstrated a scissors-type trilayer CPP-GMR device with relatively large ΔRA of 4.5 mΩ μm2 and MR ratio of 24%.

Atomistic Modeling of the Interlayer Coupling Behavior in Perpendicularly Magnetized $L1_{0}$-FePt/Ag/$L1_{0}$-FePt Pseudo Spin Valves

An atomistic model based on a classical spin Hamiltonian and a Landau-Lifshitz-Gilbert (LLG) equation was utilized to simulate and gain understanding of the magnetic, interfacial, and reversal properties of perpendicular anisotropy L10-FePt/Ag/L10-FePt pseudo spin valves, with different interfacial roughness, representing the experimentally observed behavior of the interface where the Ag spacer layer was postannealed at different temperatures. Simulation results showed that the influence of the Ag spacer on the independent switching of the FePt layers became stronger with a greater degree of interlayer mixing under higher temperature treatment. This was the result of an increased magnetic polarization of Ag with a decrease in Ag spacer thickness. Furthermore, with greater intermixing the magnetization reversal of the harder fixed FePt layer also changed from a coherent reversal process to one which took place via nucleation and propagation of reversed domains.

Spin scattering asymmetric coefficients and enhanced specific interfacial resistance of fully epitaxial current-perpendicular-to-plane giant magnetoresistance spin valves using alternate monatomic layered [Fe/Co]n and a Ag spacer layer

Using current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) measurement, we have evaluated the bulk and interface spin scattering asymmetric coefficients, βF and γF/N and the specific interfacial resistance, AR*F/N, for exchange-biased spin-valves consisting of artificially ordered B2 structure Fe50Co50 and Ag spacer layer. Artificially epitaxial ordered Fe50Co50 superlattices have been successfully fabricated on MgO (001) substrate by alternate monatomic layer (AML) deposition at a substrate temperature of 75 °C. The structural properties of the full epitaxial trilayer, AML[Fe/Co]n/Ag/AML[Fe/Co]n, on the Ag electrode have been confirmed by in situ reflection high-energy electron diffraction and transmission electron diffraction microscopy. A considerably large resistance-area product change and MR ratio (ΔRA > 3 mΩμm2 and MR ratio ∼5%) were confirmed even at thin AML[Fe/Co]n layer at room temperature (RT) in our spin-valve elements. The estimated values of βF and γF/N were 0.80 and 0.84 ± 0.02, respectively, from the Valet–Fert theory analysis of ΔRA as a function of thickness of the ferromagnetic layer (3, 4, and 5 nm) on the basis of the two-current model.

Bulk and interfacial scatterings in current-perpendicular-to-plane giant magnetoresistance with Co2Fe(Al0.5Si0.5) Heusler alloy layers and Ag spacer

We report the transport properties of a current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) device with Co2Fe(Al0.5Si0.5) (CFAS) Heusler alloy ferromagnetic layers and a Ag spacer layer. The CPP-GMR devices showed relatively high ΔRA values and MR ratios up to 17 m Ω μm2 and 80% at 14 K, and 8 m Ω μm2 and 34% at 290 K. The spin diffusion length ∼3 nm and the bulk spin asymmetry ∼0.77 for the CFAS alloy at 14 K were estimated by the Valet–Fert model, indicating a large contribution of the interfacial scattering.

Influence of metal spacer on the properties and microstructure of multilayer films and analyses

Ta/NiFe/nonmagnetic metal spacer/FeMn/Ta films were prepared by magnetron sputtering. The dependence of the exchange coupling field ( H ex) between the FeMn and NiFe layers on the thickness of a nonmagnetic metal spacer layers was investigated systematically. The results show that H ex decreases rapidly with increasing thicknesses of the Bi and Ag spacer layers. It decreases gradually, however, with an increase in the thickness of the Cu spacer layer. We found empirically that H ex corresponds to the lattice match between spacer layer atoms and NiFe layer atoms. However, the results of X-ray photoelectron spectroscopy show that when a small amount of Bi atoms are deposited on the NiFe/FeMn interface, they migrate to the FeMn layer surface and hardly influence H ex.

Microstructures of NiFe/nonmagnetic metal spacer/FeMn films and their influences on exchange coupling

Ta/NiFe/nonmagnetic metal spacer/FeMn films were prepared by magnetron sputtering. The dependences of the exchange coupling field (H ex) between an antiferromagnetic FeMn layer and a ferromagnetic NiFe layer on the thickness of nonmagnetic metal spacer layers were systematically studied. The results show that the H ex dramatically decreases with the increase in the thicknesses of Bi and Ag spacer layers. However, it gradually decreases with the increase in the thickness of a Cu spacer layer. For a Cu space layer, its crystalline structure is the same as that of NiFe and the lattice parameters of them are close to each other. The Cu layer and FeMn layer will epitaxially grow on the NiFe layer in succession, so the (111) texture of the FeMn layer will not be damaged. As a result, the H ex gradually decreases with the deposition thickness of a Cu layer. For an Ag space layer, its crystalline structure is the same as that of NiFe, but its lattice parameter is very different from that of NiFe. Thus, neither an Ag nor an FeMn layer will epitaxially grow on the NiFe layer and the (111) texture of the FeMn layer will be damaged. The H ex rapidly decreases with the increase in the deposition thickness of an Ag layer. For a Bi spacer layer, not only its crystalline structure but also its lattice parameter is greatly different from that of NiFe. For the same reason, the Bi and FeMn layer cannot epitaxially grow on the NiFe layer. The texture of the FeMn layer will also be damaged. Therefore, the H ex rapidly decreases with the increase in the deposition thickness of a Bi layer as well. However, the research result of X-ray photoelectron spectroscopy indicates that a very small amount of surfactant Bi atoms will migrate to the FeMn layer surface when they are deposited on the NiFe/FeMn interface. Thus, the H ex will hardly decrease.

Size effects and giant magnetoresistance in unannealed NiFe/Ag multilayer stripes

We have observed giant magnetoresistance (GMR) in unannealed NiFe/Ag multilayer thin-film stripes. Rectangular stripes having constant thickness and a constant 11:1 length-to-width aspect ratio, but varying widths down to 0.5 μm, were measured. Two types of multilayer configurations were tested, a system of five NiFe/Ag bilayers with 5.5-nm-thick Ag spacer layers, and a system of nine bilayers with 4.4-nm-thick Ag layers. In contrast to the characteristic of annealed NiFe/Ag multilayer stripes, the unnannealed stripes produced increasing GMR ratios for decreasing stripe sizes, with the 0.5-μm-wide stripe of the five-bilayer system exhibiting a ΔR/R of 2.5%. Barkhausen noise and response broadening also increased with decreasing stripe size, however. The results are discussed in terms of magnetostatic coupling of the NiFe layers within the stripes.

Biquadratic exchange coupling in Fe/Cr/Fe(100) and Fe/Ag/Fe(100) thin film sandwiches (abstract)

We have investigated the magnetic exchange coupling between an Fe film and an Fe(100) single crystal whisker substrate separated by varying thickness Cr or Ag spacer layers. The magnetization of the Fe was measured using scanning electron microscopy with polarization analysis. Reflection high-energy electron diffraction and scanning tunneling microscopy (STM) were used to analyze the film structure. In addition to the oscillatory thickness-dependent bilinear exchange coupling, biquadratic exchange coupling was observed in both films. At spacer thicknesses that correspond to the transitions between ferromagnetic and antiferromagnetic coupling the Fe film magnetization was in-plane and orthogonal to the ferromagnetic alignment. Our observations generally agree with the model proposed by Slonczewski1 in which the biquadratic coupling originates from the bilinear coupling through spatial fluctuations of the spacer thickness. If these fluctuations occur over small enough length scales and the bilinear coupling varies between ferromagnetic and antiferromagnetic with monolayer thickness changes, then the Fe film intralayer exchange stiffness forces biquadratic coupling of the film to the substrate. We have observed the necessary short-period coupling in both Fe/Cr/Fe and Fe/Ag/Fe and we have measured the length scale of the thickness variations on a similar sample using a STM. One unexpected result in Fe/Ag/Fe was that the amount of biquadratic coupling increased with increasing top Fe layer thickness. No similar dependence was observed for Fe/Cr/Fe.