Current-driven Magnetization Reversal Research Articles

In-plane current-induced magnetization reversal of Pd/CoZr/MgO magnetic multilayers

High critical current density (> 106 A/cm2) is one of major obstacles to realize practical applications of the current-driven magnetization reversal devices. In this work, we successfully prepared Pd/CoZr(3.5 nm)/MgO thin films with large perpendicular magnetic anisotropy and demonstrated a way of reducing the critical current density with a low out-of-plane magnetic field in the Pd/CoZr/MgO stack. Under the assistance of an out-of-plane magnetic field, the magnetization can be fully reversed with a current density of about 104 A/cm2. The magnetization reversal is attributed to the combined effect of the out-of-plane magnetic field and the current-induced spin-orbital torque. It is found that the current-driven magnetization reversal is highly relevant to the temperature owing to the varied spin-orbital torque, and the current-driven magnetization reversal will be more efficient in low-temperature range, while the magnetic field is helpful for the magnetization reversal in high-temperature range.

Field- and Current-Driven Magnetization Reversal and Dynamic Properties of CoFeB-MgO-Based Perpendicular Magnetic Tunnel Junctions* *Supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300803), the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology, the National Natural Science Foundation of China (Grant Nos. 11774150 and 11874135), and the Natural Science Foundation of Jiangsu Province of

We report a perpendicular magnetic tunnel junction (pMTJ) cell with a tunnel magnetoresistance (TMR) ratio of nearly 200% at room temperature based on CoFeB/Ta/CoFeB as the free layer (FL) and a synthetic antiferromagnetic (SAF) multilayer [Pt/Co]/Ru/[Pt/Co]/Ta/CoFeB as the reference layer (RL). The field-driven magnetization switching measurements show that the pMTJs exhibit an anomalous TMR hysteresis loop. The spin-polarized layer CoFeB of SAF-RL has a lower critical switching field than that of FL. The reason is related to the interlayer exchange coupling (IEC) through a moderately thick Ta spacer layer among SAF-RLs, which generates a moderate and negative bias magnetic field on CoFeB of RL. However, the IEC among RLs has a negligible influence on the current-driven magnetization switching of FL and its magnetization dynamics.

Field and Current Controlled Domain Wall Propagation in Twisted Glass-Coated Magnetic Microwires

The torsion effect on the field and current driven magnetization reversal and the associated domain wall velocity in cylindrical amorphous and nanocrystalline glass-coated microwires is reported. Samples from three representative compositions have been investigated: (1) amorphous Fe77.5Si7.5B15 with positive magnetostriction, λ ≅ 25 × 10−6, (2) amorphous Co68.18Fe4.32Si12.5B15 with nearly zero negative magnetostriction, λ ≅ −1 × 10−7, and (3) nanocrystalline Fe73.5Si13.5B9Cu1Nb3 (FINEMET) with small positive magnetostriction, λ ≅ 2.1 × 10−6, all having the diameter of the metallic nucleus, d, of 20 µm and the glass coating thickness, tg, of 11 µm. The results are explained through a phenomenological interpretation of the effects of applied torque on the anisotropy axes within the microwires with different characteristics. Among all the complex mechanical deformations caused by the application of torque on magnetic microwire samples, the most important are the axial compression – for axial field-driven domain wall motion, and the circumferential tension – for electrical current/circumferential field-driven domain wall motion. The Co68.18Fe4.32Si12.5B15 microwire, annealed at 300 °C for 1 hour and twisted at 168 Rad/m exhibits the optimum characteristics, e.g. the lowest switching current (down to 9 mA~2.9 × 10−3 A/cm2) and the largest domain wall velocity (up to 2300 m/s).

Current Driven Magnetic Damping in Dipolar-Coupled Spin System

Magnetic damping of the spin, the decay rate from the initial spin state to the final state, can be controlled by the spin transfer torque. Such an active control of damping has given birth to novel phenomena like the current-driven magnetization reversal and the steady spin precession. The spintronic devices based on such phenomena generally consist of two separate spin layers, i.e., free and pinned layers. Here we report that the dipolar coupling between the two layers, which has been considered to give only marginal effects on the current driven spin dynamics, actually has a serious impact on it. The damping of the coupled spin system was greatly enhanced at a specific field, which could not be understood if the spin dynamics in each layer was considered separately. Our results give a way to control the magnetic damping of the dipolar coupled spin system through the external magnetic field.

Current driven magnetization reversal in microstructured spin valve with current-in-plane configuration

The current induced magnetization reversals on the free layer of patterned spin valve ellipse have been investigated using differential magnetoresistance (MR) measurements, in which various direct currents were superimposed to 14 μA alternating current for the MR measurements. Conventional field scanned MR minor loops and subminor loops were measured under different scan ranges of magnetic field to verify the existing metastable states. The magnetization transitions between metastable states were realized in the dc scanned MR loop, in which the resistance levels were identified in accordance with the field scanned MR loop. This phenomenon is believed to be associated with the domain wall motion pushed by the direct current.

Current-driven magnetization reversal at extremely low threshold current density in (Ga,Mn)As-based double-barrier magnetic tunnel junctions

Current-driven magnetic orientation reversal at an extremely low threshold current density, as low as 2.0×104A∕cm2, has been achieved in (Ga,Mn)As-based double-barrier magnetic tunneling junctions (MTJs) sandwiched between top and bottom MTJs. The middle magnetic free layer thickness dependence clearly demonstrates that the low threshold current density is owing not only to the small magnetization of the magnetic free layer but also the enhancement of the spin torque caused by a spin-polarized current through the top and bottom MTJs.

Magnetization reversal driven by spin-polarized current in exchange-biased nanoscale spin valves

We report micromagnetic simulations of current-driven magnetization reversal of the free-layer nanomagnet in ${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Cu}∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanoscale spin valves for large current pulses and compare the results to experiments. Our simulations demonstrate that the mechanism of current-driven magnetization reversal in these samples can depend strongly on the equilibrium angle between the magnetizations of the pinned layer and the free layer of the spin valve. In the case of collinear equilibrium magnetizations, the reversal proceeds via nucleation of a vortex. However, even small misalignments of the magnetizations of the free and the pinned layers, on the order of 5\ifmmode^\circ\else\textdegree\fi{}--10\ifmmode^\circ\else\textdegree\fi{}, can result in magnetization reversal by a macrospinlike coherent precessional rotation. This result is in good agreement with recent experimental observations.

Pulse-Width Dependence in Current-Driven Magnetization Reversal Using GaMnAs-Based Double-Barrier Magnetic Tunnel Junction

We have investigated the current pulse width dependence on current-driven magnetization reversal in double-barrier structures using GaMnAs-based magnetic tunneling junctions (MTJ) in order to clarify the origin of low threshold current density for current-driven magnetization reversal. Comparing with the case of single-barrier MTJ, the pulse-width dependence reveals that threshold current density is reduced by double-barrier MTJ. We confirmed that the threshold current density in the order of 104 A/cm2 is estimated considering the effect of current pulse width.

Layer-thickness dependence in tunneling magnetoresistance and current-driven magnetization reversal using GaMnAs-based double-barrier structures

We have investigated the layer-thickness dependence on tunneling magnetoresistance (TMR) in double-barrier structures using GaMnAs-based magnetic tunneling junctions in order to clarify the origin of low threshold current density for current-driven magnetization orientation reversal. The TMR characteristics are well explained by assuming the spin configurations due to the difference in coercive force in each magnetic layer. Using the thin middle magnetic layer, current-driven magnetization reversal in low current density and spin accumulation are realized.

Domain Wall Creation in Nanostructures Driven by a Spin-Polarized Current

We report on current-driven magnetization reversal in nanopillars with elements having perpendicular magnetic anisotropy. Whereas only the two uniform magnetization states are available under the action of a magnetic field, we observed current-induced Bloch domain walls in pillars as small as 50 x 100 nm(2). This domain wall state can be further controlled by current to restore the uniform states. The ability to nucleate and manipulate domain walls by a current gives insight into the reversal mechanisms of small nanoelements and provides new prospects for ultrahigh density spintronic devices.

A comparison of spin-polarized current driven magnetization reversal in Co/Cu/Co magnetic multilayers

Co/Cu/Co multilayers have been used in order to build magnetic devices for spintronic applications. The anisotropy of thin Co nanostructures is not well known, so it is difficult to take it into account in a precise way for micromagnetic simulations. Starting from experimental data in order to determine the anisotropy constant of thin Co, we study and compare the fast switching magnetization processes of three different structures.

Current-driven magnetization reversal in exchange-biased spin-valve nanopillars

We have investigated the current-driven magnetization reversal of exchange-biased spin-valve giant magnetoresistive nanopillars with a magnetically pinned ferromagnetic layer. Current-driven magnetization reversal of a ferromagnetic layer with a smaller MV (M: magnetization, V: volume) value is found to take place even when the layer is pinned by the exchange bias induced by an antiferromagnet. The critical current density Jc of the spin-valve nanopillar with a MnIr layer adjacent to the current-driven free layer is of the same order as that of a Co/Cu/Co nanopillar (∼107A∕cm2).

Thermal activation in spin‐polarized current driven magnetization reversal process

Magnetization reversal by spin polarized current flowing perpendicular to a 10 nm Co/6 nm Cu/2.5 nm Co circular nanopillar of 130 nm diameter is studied using a micromagnetic model. In addition to the spin transfer torque term, the magnetostatic coupling with the pinned layer and the classical Ampere field created by the current are taken into account. Furthermore, thermal activation is also considered by means of a stochastic white-noise thermal field. The experimental parameters are J0 = 4.5 × 108 A/cm2, Bext = 0.14 T, T = 300 K, whereas the damping parameter is α = 0.005. Firstly computations without thermal activation were performed. Transitions from the parallel state (PS) to the antiparallel one (APS) and backwards were computed. In both cases, magnetization reversal occurs via highly inhomogeneous states revealing that these processes are not accurately described by single-domain models. Considering the non-equilibrium character of the switching process, the thermal activation simulations have to be carried out as many times as possible in order to average for obtaining quantitative results. Thermal activation favors the transition from PS to APS, a decrease in the switching time is observed and the intermediate state which appears in the computations without thermal field is suppressed. On the contrary the transition from APS to PS is not so influenced and an intermediate magnetization state remains stable even after averaging over 100 realizations. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Current-Driven Magnetization Reversal in a Ferromagnetic Semiconductor(Ga,Mn)As/GaAs/(Ga,Mn)AsTunnel Junction

Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5 x 0.3 microm2 device revealed that magnetization switching occurs at low critical current densities of 1.1-2.2 x 10(5) A/cm2 despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.

Magnetization dynamics with a spin-transfer torque

The magnetization reversal and dynamics of a spin valve pillar, whose lateral size is $64\ifmmode\times\else\texttimes\fi{}64{\mathrm{nm}}^{2},$ are studied by using micromagnetic simulation in the presence of spin-transfer torque. Spin torques display both characteristics of magnetic damping (or antidamping) and of an effective magnetic field. For a steady-state current, both $M\ensuremath{-}I$ and $M\ensuremath{-}H$ hysteresis loops show unique features, including multiple jumps, unusual plateaus, and precessional states. These states originate from the competition between the energy dissipation due to Gilbert damping and the energy accumulation due to the spin torque supplied by the spin current. The magnetic energy oscillates as a function of time even for a steady-state current. For a pulsed current, the minimum width and amplitude of the spin torque for achieving current-driven magnetization reversal are quantitatively determined. The spin torque also shows very interesting thermal activation that is fundamentally different from an ordinary damping effect.

Field dependence of magnetization reversal by spin transfer

We describe and analyze the effect of an applied field ${(H}_{\mathrm{appl}})$ on the current-driven magnetization reversal in pillar-shaped Co/Cu/Co trilayers. Depending on the magnitude of ${H}_{\mathrm{appl}},$ we observe two different types of transitions between the parallel (P) and antiparallel (AP) magnetic configurations of the the Co layers. If ${H}_{\mathrm{appl}}$ is smaller than some threshold value, the transitions between P and AP are relatively sharp and irreversible. For ${H}_{\mathrm{appl}}$ exceeding this threshold value, the transitions are progressive and reversible. We show that this behavior can be precisely accounted for by introducing the current-induced torque of the spin transfer models into a Landau-Lifshitz-Gilbert equation to determine the stability or instability of the P and AP states. This analysis also provides a good description for the field dependence of the critical currents.