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The bit-to-bit distribution of a critical current density for magnetic domain wall (DW) motion is studied using Co/Ni wires with various wire widths (ws). The distribution inherently decreases with the w, and the ratio of standard deviation to average is 9.8% for wires with w=40 nm. It is found that a self-distribution within one device, which is evaluated through repeated measurement, is a dominant factor in the bit-to-bit distribution. Micromagnetic simulation reveals that the distribution originates from DW configuration, which varies with device size. [ABSTRACT FROM AUTHOR]

Antiferromagnet (AFM)/ferromagnet (FM) heterostructures with broken inversion symmetry are perceived to open new opportunities for nonvolatile spintronic devices. Previous studies of such systems have demonstrated an emergence of spin-orbit torques (SOTs) in the heterostructures which are strong enough to bring about magnetization reversal. The impact of broken inversion symmetry and spin-orbit coupling also leads to an emergence of the Dzyaloshinskii-Moriya interaction (DMI) which governs the magnetic configuration and magnetization reversal. In this work, we study the SOT-induced effective fields and DMI in a heterostructure with an antiferromagnetic PtMn layer and a ferromagnetic [Co/Ni] multilayer and compare the results with a reference Pt/ [Co/Ni] system. Magnetotransport measurements reveal the same sign and similar magnitude of SOT-induced effective fields for the two systems while current-induced domain wall motion measurements under in-plane fields reveal the opposite sign and smaller magnitude of DMI at the PtMn/ [Co/Ni] interface compared to the Pt/[Co/Ni]. The obtained results offer in-depth information concerning the manifestations of spin-orbit interactions in AFM/FM systems, which is key to understanding of static magnetic configuration and magnetization reversal for their possible applications in antiferromagnetic spintronics. [ABSTRACT FROM AUTHOR]

The spin-dependent scattering in ferromagnet/nonmagnet/ferromagnet (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As trilayer structures was studied. An increase of sheet resistance was observed when the magnetizations of the two ferromagnetic (Ga,Mn)As layers were aligned anti-parallel, which was realized by the different coercivity of the two (Ga,Mn)As layers with different compositions. This is the first demonstration of spin-dependent scattering in magnetic multilayer structures made of semiconductor-materials alone. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigate the magnetization-reversal mode in CoFeB/MgO magnetic tunnel junctions with perpendicular easy axis from the magnetic-field-angle dependence of coercivity. The reversal in a free layer with diameter of ~20 nm is found to be in a good agreement with coherent reversal mode for a device with a reference layer much larger than the free layer. However, the reversal mode is quite different in a junction in which the two layers are almost the same size. From micro-magnetic simulation, the difference is attributed to the reduction of the magnetic anisotropy in the vicinity of the device edge. [ABSTRACT FROM AUTHOR]

We study spin-orbit torque induced magnetization switching in devices consisting of an antiferromagnetic PtMn and ferromagnetic Co/Ni multilayer with sizes ranging from 5 lm to 50 μm. As the size decreases, switching behavior changes from analogue-like to stepwise with several intermediate levels. The number of intermediate levels decreases with the decreasing size and finally evolves into a binary mode below a certain threshold. The results are found to be explained by a unique reversal process of this system, where ferromagnetic domains comprising a number of polycrystalline grains reverse individually and among the domains both out-of-plane and in-plane components of exchange bias vary. [ABSTRACT FROM AUTHOR]

We observe magnetic domain structures of MgO/CoFeB with a perpendicular magnetic easy axis under an electric field. The domain structure shows a maze pattern with electric-field dependent isotropic period. The analysis of the period indicates a major role of the electric-field modulation of interfacial magnetic anisotropy for the observation and possible contribution from electric-field modulation of the exchange stiffness constant. [ABSTRACT FROM AUTHOR]

We investigate the electric-field effect on the exchange stiffness constant in a CoFeB/MgO junction through the observation of the spin-wave resonance in a nanoscale magnetic tunnel junction. We evaluate the electric-field dependence of the stiffness constant from the separation between resonance fields for the Kittel and spin-wave modes under electric fields. The obtained stiffness constant increases when the interfacial electron density is decreased. This dependence is consistent with that determined from the observation of electric-field dependent domain structures. [ABSTRACT FROM AUTHOR]

We investigated electrical endurance of perpendicularly magnetized Co/Ni wires, which are a promising candidate material system for current-induced domain wall motion device. Monitoring the wire resistance while applying dc stress is shown to be a promising way to evaluate the electrical breakdown. An electromigration model describes well the observed time-to-failure as a function of temperature and current density. The dc stress current density which leads to 10-yr lifetime with 50% failure at 150 °C was twice as large as the threshold current density for domain wall motion, suggesting that the device with Co/Ni wire is highly durable against electrical stresses. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys, L10-ordered (Co, Fe)–Pt alloys and Co/(Pd, Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB–MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 μA. [ABSTRACT FROM AUTHOR]

Magnetic information storage relies on external magnetic fields to encode logical bits through magnetization reversal. But because the magnetic fields needed to operate ultradense storage devices are too high to generate, magnetization reversal by electrical currents is attracting much interest as a promising alternative encoding method. Indeed, spin-polarized currents can reverse the magnetization direction of nanometre-sized metallic structures through torque; however, the high current densities of 107-108?A?cm-2 that are at present required exceed the threshold values tolerated by the metal interconnects of integrated circuits. Encoding magnetic information in metallic systems has also been achieved by manipulating the domain walls at the boundary between regions with different magnetization directions, but the approach again requires high current densities of about 107?A?cm-2. Here we demonstrate that, in a ferromagnetic semiconductor structure, magnetization reversal through domain-wall switching can be induced in the absence of a magnetic field using current pulses with densities below 105?A?cm-2. The slow switching speed and low ferromagnetic transition temperature of our current system are impractical. But provided these problems can be addressed, magnetic reversal through electric pulses with reduced current densities could provide a route to magnetic information storage applications. [ABSTRACT FROM AUTHOR]

We review recent progress made in the field of semiconductor spintronics, a branch of semiconductor electronics where both charge and spin degrees of freedom play an important role in realizing unique functionalities. We first describe the new spin-dependent phenomena found in semiconductors including carrier-induced ferromagnetism in III-V compounds, followed by an account of our current understanding of such spin-dependent phenomena. Then we summarize the challenges the semiconductor spintronics has to meet in order for it to be a success as "electronics". [ABSTRACT FROM PUBLISHER]

Discusses the origin of ferromagnetic interaction. What ferromagnetic interaction is; How they are used in semiconductors; How spin-dependent resonant tunneling is involved.

We have compared the scaling exponents in scaling formula for magnetic domain wall creep by measuring the magnetic-field induced domain wall velocity of (Ga,Mn)As layers grown on (In,Al)As semistep-graded buffer layer and (In,Ga)As buffer layer. The different critical exponents for the two (Ga,Mn)As layers indicate that the observed creep motions belong to different universality classes, which are found to be governed by the degree of surface roughness due to crosshatch dislocation introduced during epitaxial growth. Domain wall creep motion in (Ga,Mn)As layer grown on (In,Al)As with flatter surface belongs to random-field disorder, whereas that in (In,Ga)As to random-bond disorder. [ABSTRACT FROM AUTHOR]

Switching of tunnel magnetoresistance in magnetic tunnel junctions (MTJs) has been achieved by magnetic domain wall motion in one of the electrodes. The fabricated devices have two (Ga,Mn)As-based MTJs with a common bottom (Ga,Mn)As electrode, in which the domain wall motion takes place both by magnetic field and by current. This configuration allows not only the observation of switching but also time-resolved detection of the position of the domain wall by the tunnel magnetoresistance. [ABSTRACT FROM AUTHOR]

The authors report tunnel magnetoresistance (TMR) ratios as high as 472% at room temperature and 804% at 5 K in pseudo-spin-valve (PSV) CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) annealed at 450 °C, which is approaching the theoretically predicted value. By contrast, the TMR ratios for exchange-biased (EB) SV MTJs with a MnIr antiferromagnetic layer are found to drop when they are annealed at 450 °C. Energy dispersive x-ray analysis shows that annealing at 450 °C induces interdiffusion of Mn and Ru atoms into the MgO barrier and ferromagnetic layers in EB-SV MTJs. Mechanisms behind the different annealing behaviors are discussed. [ABSTRACT FROM AUTHOR]

The effect of low-temperature annealing on (Ga,Mn)As/GaAs/(Ga,Mn)As trilayer structures is studied. Low-temperature annealing significantly increases the ferromagnetic transition temperature T[sub C] of top (Ga,Mn)As layers, reaching as high as 160 K, whereas no apparent effect is observed on bottom (Ga,Mn)As layers. The annealing effect on Be-doped trilayers is also presented. [ABSTRACT FROM AUTHOR]

A fourteen-fold anisotropy in the spin transport efficiency parallel and perpendicular to the charge transport is observed in a vertically biased (Ga, Mn)As-based spin-polarized light emitting diode. The spin polarization is determined by measuring the polarization of electroluminescence from an (In, Ga)As quantum well placed a distance d (20-420 nm) below the p-type ferromagnetic (Ga, Mn)As contact. In addition, a monotonic increase (from 0.5% to 7%) in the polarization is measured as d decreases for collection parallel to the growth direction, while the in-plane polarization from the perpendicular direction (∼0.5%) remains unchanged. [ABSTRACT FROM AUTHOR]

We observe a transition of domain-wall (DW) dynamics in ferromagnetic wires made of Co/Ni multilayers by use of transport measurement. As the wire width reduces, DW dynamics exhibits a transition from dendrite growth to pure DW motion. The threshold width is found to be about 300 nm and strongly depends on the relative dragging direction of the magnetic field and the current on DW: parallel (antiparallel) direction results in much smaller (larger) threshold width. It should be considered as a building block for DW-motion-based device applications. [ABSTRACT FROM AUTHOR]

We have investigated the magnetic and magnetotransport properties of (Ga, Mn)As/(Al, Ga)As/(Ga, Mn)As semiconductor-based magnetic trilayer structures. We observe a weak ferromagnetic interlayer coupling between the two ferromagnetic (Ga, Mn)As layers as well as magnetoresistance effects due to spin-dependent scattering and to spin-dependent tunneling. Both the coupling strength and the magnetoresistance ratio decrease with the increase of temperature and/or the increase of Al composition of the nonmagnetic (Al, Ga)As layer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have performed low-temperature scanning Hall probe microscopy on a ferromagnetic semiconductor (Ga[sub 0.957]Mn[sub 0.043])As. The observed magnetic domain structure is a stripe-shaped pattern as has been observed in conventional nonsemiconductor ferromagnetic materials, and the measured magnetic field from the sample surface was small, reflecting the weak magnetization of (Ga, Mn)As. The domain width increased and the measured magnetic field decreased with raising temperature, which are consistent with calculated results, in which the exchange interaction between Mn spins deduced from the Curie temperature is assumed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the quasibinary ferromagnetic semiconductor (Ga[sub 0.98]Mn[sub 0.02])As by magnetic circular dichroism in x-ray absorption. We find a richly structured Mn absorption spectrum typical for localized 3d electrons. An analysis of the magnetization-averaged and dichroism line shapes shows a local Mn moment of 4.6μ[sub B], which is close to the Hund's rule moment for the half-filled 3d shell. The magnitude of the dichroism reveals that only about 1/7 of the Mn atoms participate in the ferromagnetic order. Our experiment does not show a distinction between the ferro- and paramagnetic Mn atoms. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: We have grown a 5-nm-thick (Ga,Mn)Sb with Mn composition of 0.032, and investigated its magnetic properties and their electric-field dependence. The sample shows ferromagnetism below about 25K, whose magnetic easy axis is perpendicular to plane. By utilizing a field-effect transistor structure, we show that the Curie temperature and magnetic anisotropy field can be controlled by the application of electric field. [ABSTRACT FROM AUTHOR]

Magnetic domain wall motion induced by magnetic fields and spin-polarized electrical currents is experimentally well established. A full understanding of the underlying mechanisms, however, remains elusive. For the ferromagnetic semiconductor (Ga,Mn) As, we have measured and compared such motions in the thermally activated subthreshold, or ‘creep,’ regime, where the velocity obeys an Arrhenius scaling law. Within this Law, the clearly different exponents of the current and field reflect different universality classes, showing that the drive mechanisms are fundamentally different. [ABSTRACT FROM AUTHOR]

We report molecular beam epitaxy of a ferromagnetic semiconductor (Ga,Mn)Sb, which is a single crystal without detectable second phases. We report also the details of magnetotransport properties of (Ga,Mn)Sb and the effects of electric fields on them. The difference between the properties observed here and properties of those reported earlier for a ferromagnetic semiconductor (Ga,Mn)As, provides information critical for further understanding of fundamental and device physics of ferromagnetic semiconductors. [ABSTRACT FROM AUTHOR]