No Arabic abstract
We numerically study ultra fast resonant spin torque (ST) magnetization reversal in magnetic tunnelling junctions (MTJ) driven by current pulses having a direct current (DC) and a resonant alternating current (AC) component. The precessional ST dynamics of the single domain MTJ free layer cell are modelled in the macro spin approximation. The energy efficiency, reversal time, and reversal reliability are investigated under variation of pulse parameters like direct and AC current amplitude, AC frequency and AC phase. We find a range of AC and direct current amplitudes where robust resonant ST reversal is obtained with faster switching time and reduced energy consumption per pulse compared to purely direct current ST reversal. However for a certain range of AC and direct current amplitudes a strong dependence of the reversal properties on AC frequency and phase is found. Such regions of unreliable reversal must be avoided for ST memory applications.
We analyse the effect of the applied field (Happl) on the current-driven magnetization reversal in pillar-shaped Co/Cu/Co trilayers, where we observe two different types of transition between the parallel (P) and antiparallel (AP) magnetic configurations of the Co layers. If Happl is weaker than a rather small threshold value, the transitions between P and AP are irreversible and relatively sharp. For Happl exceding the threshold value, the same transitions are progressive and reversible. We show that the criteria for the stability of the P and AP states and the experimentally observed behavior can be precisely accounted for by introducing the current-induced torque of the spin transfer models in a Landau-Lifschitz-Gilbert equation. This approach also provides a good description for the field dependence of the critical currents.
Among the magnetostrictive alloys the one formed of iron and gallium (called Galfenol from its U.S. Office of Naval Research discoverers in the late 90s) is attractive for its low hysteresis, good tensile stress, good machinability and its rare-earth free composition. One of its applications is its association with a piezoelectric material to form a extrinsic multiferroic composite as an alternative to the rare room temperature intrinsic multiferroics such as BiFeO$_3$. This study focuses on thin Fe$_{0.81}$Ga$_{0.19}$ films of thickness 5, 10, 20 and 60 nm deposited by sputtering onto glass substrates. Magnetization reversal study reveals a well-defined symmetry with two principal directions independent of the thickness. The magnetic signature of this magnetic anisotropy decreases with increasing FeGa thickness due to an increase of the non-preferential polycrystalline arrangement, as revealed by transmission electron microscopy (TEM) observations. Thus when magnetic field is applied along these specific directions, magnetization reversal is mainly coherent for the thinnest sample as seen from the transverse magnetization cycles. Magnetostriction coefficient reaches 20 ppm for the 5 nm film and decreases for thicker samples, where polycrystalline part with non-preferential orientation prevails.
Microwave assisted magnetization reversal has been investigated in a bilayer system of Pt/ferromagnet by detecting a change in the polarity of the spin pumping signal. The reversal process is studied in two material systems, Pt/CoFeB and Pt/NiFe, for different aspect ratios. The onset of the switching behavior is indicated by a sharp transition in the spin pumping voltage. At a threshold value of the external field, the switching process changes from partial to full reversal with increasing microwave power. The proposed method provides a simple way to detect microwave assisted magnetization reversal.
The ability to switch magnetic elements by spin-orbit-induced torques has recently attracted much attention for a path towards high-performance, non-volatile memories with low power consumption. Realizing efficient spin-orbit-based switching requires harnessing both new materials and novel physics to obtain high charge-to-spin conversion efficiencies, thus making the choice of spin source crucial. Here we report the observation of spin-orbit torque switching in bilayers consisting of a semimetallic film of 1T-MoTe2 adjacent to permalloy. Deterministic switching is achieved without external magnetic fields at room temperature, and the switching occurs with currents one order of magnitude smaller than those typical in devices using the best-performing heavy metals. The thickness dependence can be understood if the interfacial spin-orbit contribution is considered in addition to the bulk spin Hall effect. Further threefold reduction in the switching current is demonstrated with resort to dumbbell-shaped magnetic elements. These findings foretell exciting prospects of using MoTe2 for low-power semimetal material based spin devices.
Current-induced magnetization switching through spin-orbit torques (SOTs) is the fundamental building block of spin-orbitronics. The SOTs generally arise from the spin-orbit coupling of heavy metals. However, even in a heterostructure where a metallic magnet is sandwiched by two different insulators, a nonzero current-induced SOT is expected because of the broken inversion symmetry; an electrical insulator can be a spin-torque generator. Here, we demonstrate current-induced magnetization switching using an insulator. We show that oxygen incorporation into the most widely used spintronic material, Pt, turns the heavy metal into an electrically-insulating generator of the SOTs, enabling the electrical switching of perpendicular magnetization in a ferrimagnet sandwiched by electrically-insulating oxides. We further found that the SOTs generated from the Pt oxide can be controlled electrically through voltage-driven oxygen migration. These findings open a route towards energy-efficient, voltage-programmable spin-orbit devices based on solid-state switching of heavy metal oxidation.