Effect of δ-doping on GMR effect in a semiconductor 2DEG heterostructure modulated by two overlapping magnetic barriers

We theoretically investigate the spin transport behavior of multilayer [ Co 2 FeSi / Ag ] N structure for the application of next-generation read sensors in hard disk drive. To demonstrate the potential of the Heusler alloy-based current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) device, we employ an atomistic model coupled with a spin accumulation model including the effect of a diffuse interface. The dynamics of magnetization is observed in the atomistic model and the calculation of magnetoresistance (MR) and MR ratio of the magnetic structure can be achieved by the spin accumulation model enabling us to investigate the spin transport behavior within the structure. The MR value can be directly calculated from the gradient of spin accumulation and spin current. The effect of injected current density is first investigated. It is found that increasing the current density results in a high MR ratio. Subsequently, to achieve a high performance reader, the number of coupled layers (N) is varied up to 16 to study its effect on the MR ratio. The calculated results indicate that increasing the number of layers N gives rise to the enhancement of the resistance change and MR ratio. At the critical point N = 5, further increasing N does not affect the MR ratio, which remains relatively unchanged. Interestingly, the MR ratio is doubled for N > 5 compared to N = 1. Our results demonstrate the possibility of enhancing the performance of multilayer CPP-GMR devices.

Giant magnetoresistance effect in nanostructures consisting of magnetic–electric barriers

The GMR effect in nanogranular Ag 72 Co 28 films made by rf sputtering has been investigated in order to apply to the sensors in brushless DC motors. In a Ag 72 Co 28 film having the MR ratio of 9.5%, the Co particle size decomposed from a Ag matrix was estimated to be about 20 nm. The temperature dependence of resistivity linearly increases up to 573 K, whereas the MR ratio linearly decreases up to 573 K, that is, 12% at room temperature and 4% at 573 K. These behaviors are to the advantage of compensation both the temperature dependenees of the film sensor. In addition, the thermal stability of the MR effect is excellent. Accotdingly the nanogranular Ag 72 Co 28 film sensors have a potential to replace with conventional Hall sensors restricted below about 400 K in brushless DC motors. The nanogranular Ag 72 Co 28 film sensors attached to the bias permanent magnets set in a brushless motor are able to detect the rotating angle of the rotor together with the distinction between the polarity from the quiescent state and the rotating state. Consequently, the nanogranular GMR Ag 72 Co 28 thin films have excellent properties for rotating angle sensors at high temperatures in brushless motors.

Large negative magnetoresistance (MR) is observed in Fe–xCr–10Co (x=10,20,30,40,50,- 60,70, and 80 wt %) ternary heterogeneous alloy films prepared by the IBS (ion beam sputter) deposition process. All the as-deposited films consisted of the α phase alone. After aging treatment, the α phase is decomposited into two phases: the α1 (Fe–Co rich phase) and α2 (Cr rich phase). In the granular alloy, the GMR effect is obtained by virtue of the interface scattering between α1 and α2. After isothermal aging at 400 °C for 1 h, the largest MR of −27% appears around the Cr content of 60 wt % at 77 K and 14 kOe. The MR ratio of Fe–50Cr–10Co and Fe–70Cr–10Co is −12% and −20%, respectively, at the same condition. The MR ratio is also obviously effected by the Cr (or Fe) content in the alloy system.

A CPP (Current perpendicular to Plane)-GMR and a specular reflection effect have investigated for an improvement of GMR effect. Structures of films for CPP-GMR were Si/Cu(30nm)/Ta(5nm)/NiFe/CoFe/Cu/CoFe/IrMn/Ta/SiO2(20nm)/AlO(150nm)/Al(200nm) spin valve. The samples were prepared by RF/DC magnetron sputtering system with 10 cathodes. The GMR size is a few mm square. A contact hole was fabricated in a SiO2 layer using Ion milling and Photo lithography technique. A size of holes which were observed by SEM was much smaller than that of mask. Although the resistance of samples decreased with increasing the thickness of an under electrode, the GMR effect was not obtained. Therefore, the samples have the under electrode of 30 nm thick and their resistance is a few Ω. When the Cu layer thickness between two magnetic layers was varied, the change of resistance had a maximum value at 5 nm. AΔR(A is the size of the contact hole, which is regarded as the element area, and ΔR is the change of resistance.) was calculated 0.7 mΩμm2 from the dependence of the relationship between the resistance change and the inverse of A. The resistance change increased with increasing a pinned layer thickness and had a peak value with increasing a free layer thickness. For the total magnetic layer thickness, the resistivity change was obtained the maximum value at 14 nm thick. The specular reflection effect was also investigated for the improvement of the MR ratio. The thin oxidation layer prepared in the pined CoFe layer using a natural oxidation technique. When the CoFe layer was oxidized at the pressure of 0.026 and 0.065 Pa for about 5 minutes, MR ratio increased from 3.5 % to 6 %. The exchange coupling field between the antiferromagnetic layer and the pined layer did not decreased in this conditions.

The spin-dependent conductance and magnetoresistance ratio (MRR) for a semiconductor heterostructures consisting of two magnetic barriers with different height and space have been investigated by the transfer-matrix method. It is shown that the splitting of the conductance for parallel and antiparallel magnetization configurations results in tremendous spin-dependent MRR, and the maximal MRRs reach 5300% and 3800% for the magnetic barrier spaces W = 81.3 and 243.9 nm, respectively. The obtained spin-filtering transport property of nanostructures with magnetic barriers may be useful to magnetic-barrier-based spintronics.

Half-metallic ferromagnetic Heusler alloys having high spin polarization are promising candidates to realize large magnetoresistance (MR) ratio and high spin-transfer torque (STT) efficiency in next-generation spintronic devices. Since the Heusler alloy properties are sensitive to composition, optimizing the composition is crucial for enhancing device performance. Here, we report the fabrication of high-performance current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices using Co2MnxFe1−xGe (0 ≤ x ≤ 1) Heusler alloy, employing a high-throughput and detailed composition optimization method. The method combined composition-gradient films and local measurements to enable the composition variation from Co2FeGe to Co2MnGe to be efficiently studied on a single library sample with a small composition interval. The CPP-GMR devices fabricated from stacks annealed at 250 °C showed a clear composition dependence of MR with the maximum of MR ratio ∼38% in the Mn-rich region of x = 0.85. By increasing the annealing temperature to 350 °C, the MR ratio increased to ∼45% along with high STT efficiency ∼0.6 in the broad composition range of 0.2 ≤ x ≤ 0.7. The optimal composition for the highest MR changed with annealing temperature because of the stability of the GMR stack being higher in the lower x range. The record high MR for the all-metal CPP-GMR devices, at low annealing temperature of 250 °C was achieved by the detailed composition optimization. These results present the high potential of Co2MnxFe1−xGe and provide a comprehensive guidance on the composition optimization for achieving large MR ratio and high STT efficiency in the CPP-GMR devices.

The giant magnetoresistance (GMR) effect in a device, composed of nanosized ferromagnetic (FM)-Schottky metals (SM) and semiconductor heterostructure, is investigated theoretically. Experimentally, this GMR device can be realized by the deposition of two parallel FM strips and a SM stripe on the top of a GaAs heterostructure. It is shown that the GMR effect ascribes a significant electron transmission difference between the parallel and antiparallel magnetization configurations of two FM stripes in the device. It is also shown that the magnetoresistance (MR) ratio depends strongly on the magnetic intensity of the magnetic barrier (MB) and the electric-barrier (EB) height induced by an applied voltage to the SM stripe. Thus, this device can be used as a tunable GMR one, whose MR ratio can be switched by adjusting the applied voltage under the SM stripe or by changing the magnetic strength of the MB.

Based on the transfer-matrix method, we theoretically investigate the spin-dependent transport properties in magnetic silicene superlattice in the presence of extrinsic Rashba spin–orbit interaction (RSOI). It is found that the spin transmission probability and spin conductivities can be efficiently controlled by the number of magnetic barriers. As the number of magnetic barriers increases, spin conductivities strongly decrease, and reduce to zero in the large on-site potential difference between A and B sublattices (Δz) region. The results indicate that a magnetic silicene superlattice exhibits a remarkable wavevector-dependent spin filtering effect. Also, the magnetoresistance (MR) ratio exhibits an oscillatory behavior with the Fermi energy. The MR ratio can be tuned by the Fermi energy, number of magnetic barriers and extrinsic RSOI. Specifically, in the presence of magnetic field the spin polarization can be observed, and increases by increasing the magnetic field.

Multilayers composed of 30 [Ni–Fe/Cu] bilayers deposited on a 50-Å-thick Fe buffer layer were prepared by ion beam sputtering. The magnetoresistance (MR) ratio Δρ/ρ0 of the multilayers took high value at the Cu layer thickness δCu of 10, 20, and 32 Å for the multilayers with Ni–Fe layer thickness δNi–Fe of 10 Å. It was observed that (100) orientation of Ni–Fe and Cu crystallites were enhanced at δCu of 20 and 32 Å. The multilayered film with δCu of 10 Å exhibited Δρ/ρ0 as high as 12%. The multilayered film possessed good soft magnetic properties, and exhibited coercivity Hc and relative permeability μr of about 4 Oe and 800, respectively. The saturation magnetic field Hs was about 300 Oe. Δρ/ρ0, Hc, and Hs at δCu of 20 Å, i.e., at the ‘‘second peak,’’ were 6%, 10 Oe, and 25 Oe, respectively. The δCu dependence of Δρ/ρ0 seemed to be correspondent to that of the preferential orientation of the (100) plane. Ni–Fe/Cu multilayers with the same construction but without an Fe buffer layer did not reveal apparent (100) preferential orientation and did not exhibit Δρ/ρ0 as high as detectable. Then, Δρ/ρ0 was strongly dependent not only on interlayer thickness, such as δCu, but also (100) crystallite orientation.

A random access protocol with multi-packet reception (MPR) capability for infrastructure-less wireless autonomic networks is introduced and analyzed. In these networks mobile nodes may communicate with each other directly without a central entity (base station), where each mobile node either will be in a transmitting mode or in a receiving mode or in an idle mode. The throughput per node and the packet retransmission probability depend exclusively on the MPR capability and the ratio of the transmission probability and the receiving probability of each mobile node. For a given ratio of the transmission probability and the receiving probability of each mobile node, throughput-delay performance increases with the increase of MPR capability. In the proposed infrastructure-less networks, mobile nodes can control the network traffic very precisely by controlling the three parameters. These three parameters are transmission probability, receiving probability and idle mode probability of each mobile node. Since each mobile node can control the network traffic very precisely to obtain the maximum throughput, the network is autonomic, i.e., self-optimizing. The optimum transmission probability of each mobile node to obtain the maximum throughput is evaluated. The throughput utility increases with the increase of MPR capability. On the other hand, the cost per mobile node also increases with the increase of MPR capability. Therefore the MPR capability should be optimized to provide reasonable trade-off between the throughput per node and the cost per mobile node. The results of this study may be used for a system design of an infrastructure-less contention-based multiple access schemes with MPR capability.

In various material systems, an antiferromagnetic phase was found to coexist with a weak ferromagneticlike signal, while symmetry-based theoretical predictions indicate a possibility of a nonzero anomalous Hall effect (AHE) even in the absence of sample magnetization. This is the case of nominally collinear antiferromagnets, in particular, hexagonal MnTe, where the AHE and no detectable magnetization have been recently reported. To clarify the role of magnetization, we present a study of bulk MnTe samples, combining experiment and theory. We demonstrate that the existence of the AHE in the hexagonal MnTe is accompanied by the presence of a weak but detectable ferromagneticlike signal, vanishing at the Néel temperature. In contrast to thin layer samples, we find that the AHE hysteresis loop shows an opposite sign and Barkhausen-like jumps. We introduce a macrospin model involving the Dzyaloshinskii–Moriya type interaction, which explains the existence of a nonzero magnetic moment in the absence of external field and reproduces well hysteretic behavior of the AHE. Using analysis of Néel-vector-dependent Berry curvature, we show that the intrinsic AHE in hexagonal MnTe can be nonzero even when the magnetization vanishes and, also, that it changes sign depending on the Fermi energy position. Published by the American Physical Society 2024

We investigated the spin-polarized resonant transport in a hybrid high-electron-mobility transistor (HEMT) structure, with source and drain electrodes made of ferromagnetic (FM) material, while the channel consists of a highly doped ${n}^{++}$ AlGaAs-GaAs two-dimensional electron gas (2DEG). The electron transport in the FM layer is modeled using the spin-drift diffusion model, while across the 2DEG layer, ballistic transport is assumed, given the long mean free path within the 2DEG. By solving the two transport models self-consistently, we found that the transport properties of the device, such as the transmission probability, the spin injection (SI) efficiency, and the magnetoresistance (MR) ratio, all exhibit oscillatory behavior when the 2DEG layer width or the 2DEG Fermi energy is varied. The basis of these oscillations is the resonant transport across the 2DEG, which is reminiscent of the spin-polarized resonant tunneling (SPRT), observed recently in magnetic tunnel junctions (MTJs). The hybrid device has distinct advantages over the metal-based MTJ structures in the practical utilization of the SPRT effect. This is because the ballistic charge conduction through the 2DEG enables easy tunability of the MR ratio and SI efficiency, by varying the doping density and gate bias, while avoiding the exponential suppression of MR with barrier thickness, which occurs in MTJ devices. Numerically, the hybrid HEMT device is predicted to be capable of achieving maximum MR and SI ratios approaching 20% and 40%, respectively, at the crest of their respective oscillations.

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Controllable magnetoresistance effect in a δ-doped and magnetically-confined semiconductor heterostructure