Current-induced Magnetization Switching Research Articles

Magnetization switching driven by current-induced torque from weakly spin-orbit coupled Zr

The authors presents results that show sizable current-induced torque and a robust current-induced magnetization switching in weakly spin-orbital coupled Zr based perpendicular magnetized multilayers. Current induced effective torque field changes its sign as the Zr layer thickness varies, indicating the competition of multiple effects. Possible roles of the orbital Hall effect, which generates sizable torque as predicted by theoretical calculation, are discussed.

Ultrafast and energy-efficient spin–orbit torque switching in compensated ferrimagnets

Spin–orbit torque can be used to manipulate magnetization in spintronic devices. However, conventional ferromagnetic spin–orbit torque systems have intrinsic limitations in terms of operation speed due to their inherent magnetization dynamics. Antiferromagnets and ferrimagnets with antiparallel exchange coupling exhibit faster spin dynamics and could potentially overcome these limitations. Here, we report ultrafast spin–orbit torque-induced magnetization switching in ferrimagnetic cobalt-gadolinium (CoGd) alloy devices. Using a stroboscopic pump–probe technique to perform time-resolved measurements, we show that the switching time in the ferrimagnets can be reduced to the subnanosecond regime and a domain wall velocity of 5.7 km s‒1 can be achieved, which is in agreement with analytical modelling and atomistic spin simulations. We also find that the switching energy efficiency in the ferrimagnets is one to two orders of magnitude higher than that of ferromagnets. Time-resolved measurements show that current-induced magnetization switching in ferrimagnetic devices is faster and more energy-efficient than in ferromagnet devices.

Ultrathin perpendicular magnetic anisotropy CoFeB free layers for highly efficient, high speed writing in spin-transfer-torque magnetic random access memory

Perpendicular magnetic anisotropy (PMA) ferromagnetic CoFeB with dual MgO interfaces is an attractive material system for realizing magnetic memory applications that require highly efficient, high speed current-induced magnetic switching. Using this structure, a sub-nanometer CoFeB layer has the potential to simultaneously exhibit efficient, high speed switching in accordance with the conservation of spin angular momentum, and high thermal stability owing to the enhanced interfacial PMA that arises from the two CoFeB-MgO interfaces. However, the difficulty in attaining PMA in ultrathin CoFeB layers has imposed the use of thicker CoFeB layers which are incompatible with high speed requirements. In this work, we succeeded in depositing a functional CoFeB layer as thin as five monolayers between two MgO interfaces using magnetron sputtering. Remarkably, the insertion of Mg within the CoFeB gave rise to an ultrathin CoFeB layer with large anisotropy, high saturation magnetization, and good annealing stability to temperatures upwards of 400 °C. When combined with a low resistance-area product MgO tunnel barrier, ultrathin CoFeB magnetic tunnel junctions (MTJs) demonstrate switching voltages below 500 mV at speeds as fast as 1 ns in 30 nm devices, thus opening a new realm of high speed and highly efficient nonvolatile memory applications.

Strong long-range perpendicular exchange bias across a spacer layer

The perpendicular magnetic anisotropy (PMA) and perpendicular exchange bias (PEB) have been investigated in a CoPt/spacer/FeMn heterostructure by inserting Ta and Pt as spacer layers, respectively. First, we show that strong PMA can be obtained in (111)-oriented A1-CoPt single layer and CoPt/FeMn bilayer films on MgO (111) substrates. Then we demonstrate that the Ta and Pt spacer layers have largely different effects on the PMA and PEB of the CoPt/spacer/FeMn films. By increasing the thickness of the Ta spacer layer to 1 nm, the PMA and PEB drastically decrease. While, in the case of Pt, the PMA and PEB increase first, then slightly decrease. When the Pt spacer layer is 1 nm, a value of 85 Oe of the long-range PEB still can be obtained. The possible mechanisms were discussed to explain the different trends of PMA and PEB. Since Ta and Pt are the generally used materials in the spintronics, our study provides a piece of information for the control of the magnetic anisotropy and exchange bias in the current-induced magnetization switching of ferromagnet without external magnetic field.

Mechanism of Néel Order Switching in Antiferromagnetic Thin Films Revealed by Magnetotransport and Direct Imaging.

We probe the current-induced magnetic switching of insulating antiferromagnet-heavy-metal systems, by electrical spin Hall magnetoresistance measurements and direct imaging, identifying a reversal occurring by domain wall (DW) motion. We observe switching of more than one-third of the antiferromagnetic domains by the application of current pulses. Our data reveal two different magnetic switching mechanisms leading together to an efficient switching, namely, the spin-current induced effective magnetic anisotropy variation and the action of the spin torque on the DWs.

Current-induced magnetization switching in all-oxide heterostructures

The electrical switching of magnetization through spin-orbit torque (SOT)1 holds promise for application in information technologies, such as low-power, non-volatile magnetic memory. Materials with strong spin-orbit coupling, such as heavy metals2-4 and topological insulators5,6, can convert a charge current into a spin current. The spin current can then execute a transfer torque on the magnetization of a neighbouring magnetic layer, usually a ferromagnetic metal like CoFeB, and reverse its magnetization. Here, we combine a ferromagnetic transition metal oxide7 with an oxide with strong spin-orbit coupling8 to demonstrate all-oxide SOT devices. We show current-induced magnetization switching in SrIrO3/SrRuO3 bilayer structures. By controlling the magnetocrystalline anisotropy of SrRuO3 on (001)- and (110)-oriented SrTiO3 (STO) substrates, we designed two types of SOT switching schemes. For the bilayer on the STO(001) substrate, a magnetic-field-free switching was achieved, which remained undisturbed even when the external magnetic field reached 100 mT. The charge-to-spin conversion efficiency for the bilayer on the STO(110) substrate ranged from 0.58 to 0.86, depending on the directionality of the current flow with respect to the crystalline symmetry. All-oxide SOT structures may help to realize field-free switching through a magnetocrystalline anisotropy design.

All-electric magnetization switching and Dzyaloshinskii-Moriya interaction in WTe2/ferromagnet heterostructures.

All-electric magnetization manipulation at low power is a prerequisite for a wide adoption of spintronic devices. Materials such as heavy metals1-3 or topological insulators4,5 provide good charge-to-spin conversion efficiencies. They enable magnetization switching in heterostructures with either metallic ferromagnets or with magnetic insulators. Recent work suggests a pronounced Edelstein effect in Weyl semimetals due to their non-trivial band structure6,7; the Edelstein effect can be one order of magnitude stronger than it is in topological insulators or Rashba systems. Furthermore, the strong intrinsic spin Hall effect from the bulk states in Weyl semimetals can contribute to the spin current generation8. The Td phase of the Weyl semimetal WTe2 (WTe2 hereafter) possesses strong spin-orbit coupling6,9 and non-trivial band structures10 with a large spin polarization protected by time-reversal symmetry in both the surface and bulk states9-11. Atomically flat surfaces, which can be produced with high quality12, facilitate spintronic device applications. Here, we use WTe2 as a spin current source in WTe2/Ni81Fe19 (Py) heterostructures. We report field-free current-induced magnetization switching at room temperature. A charge current density of ~2.96 × 105 A cm-2 suffices to switch the magnetization of the Py layer. With the charge current along the b axis of the WTe2 layer, the thickness-dependent charge-to-spin conversion efficiency reaches 0.51 at 6-7 GHz. At the WTe2/Py interface, a Dzyaloshinskii-Moriya interaction (DMI) with a DMI constant of -1.78 ± 0.06 mJ m-2 induces chiral domain wall tilting. Our study demonstrates the capability of WTe2 to efficiently manipulate magnetization and sheds light on the role of the interface in Weyl semimetal/magnet heterostructures.

Spin–Orbit Torque-Driven Magnetic Switching of Co/Pt-CoFeB Exchange Spring Ferromagnets

We experimentally investigate spin–orbit torque (SOT) switching of a hybrid exchange spring system (ESS) in which a Co/Pt with perpendicular magnetic anisotropy (PMA) is coupled to a CoFeB thin film with an in-plane (IP) magnetic anisotropy (IMA). The magnetization tilt angle from the out-of-plane can be easily tuned by varying the thickness of the IMA layer due to the competition between PMA and IMA layers. We find that the tilt angle is highly tuned from 0° to 90° over a small range of CoFeB thicknesses, from 0 to 1 nm. The tilted films with high thermal stability are promising materials for a zero/low field operating SOT magnetic random access memory (MRAM) or for a bit patterned magnetic recording media. IP current-induced magnetization switching of Co/Pt-CoFeB ESS is also studied, under an IP magnetic field ( $H_{\mathrm {IP}}$ ). The current amplitude for the 45° tilt magnetization switching is almost independent of the $H_{\mathrm {IP}}$ ranged from 50 to 1000 Oe. This phenomenon might be due to the presence of the Dzyaloshinskii–Moriya interaction (DMI) and DMI effective field is expected to be less than 50 Oe.

Deterministic magnetic switching of perpendicular magnets by gradient current density

We investigated the current-induced magnetization switching in a perpendicular magnetized Pt/CoNiCo/Pt sandwich structure by spin-orbit torques under in-plane gradient current density. The gradient spin current density resulted from the spin Hall effect along the current direction was achieved by designing a bottom wedged Pt electrode. An in-plane effective field along the current direction is induced by the wedged Pt layer, which determines the direction of the current induced magnetization switching. The effective field increased with the slope of the wedged Pt, and is sufficient to induce deterministic switching of nearly 90% domains with the slope value of 0.21.

Multi-bit MRAM storage cells utilizing serially connected perpendicular magnetic tunnel junctions

Serial connection of multiple memory cells using perpendicular magnetic tunnel junctions (pMTJs) is proposed as a way to increase magnetic random access memory (MRAM) storage density. A multibit storage element is designed using pMTJs fabricated on a single wafer stack, with serial connections realized using top-to-bottom vias. The tunneling magnetoresistance effect above 130%, current induced magnetization switching in zero external magnetic field, and stability diagram analysis of single, two-bit, and three-bit cells are presented together with thermal stability. The proposed design is easy to manufacture and can lead to an increased capacity of future MRAM devices.

Spin-orbit torque and spin Hall magnetoresistance in Pt/Co/Ru/AlOx multilayers with modifying spin transport at interfaces

We report the spin-orbit torque and spin Hall magnetoresistance (SMR) in Pt/Co/Ru/AlOx multilayers with modifying spin transport at interfaces. Through current induced magnetization switching and harmonic measurements, we have found that the switching current is decreased and the damping-like effective field is enhanced after inserting Ru layer in Pt/Co/AlOx multilayers. This variation is firstly ascribed to the enhancement of the spin accumulation at the Co/Ru interfaces as discussed in our previous work. Then, the temperature dependence of the SMR in Pt/Co/AlOx and Pt/Co/Ru/AlOx multilayers has been compared. The unexpected temperature dependence of SMR in Pt/Co/Ru/AlOx multilayers is suggested to be the interfacial-generated spin current that transverse into Pt.

Field-Free Deterministic Magnetization Switching with Ultralow Current Density in Epitaxial Au/Fe4N Bilayer Films.

Current-induced magnetization switching was investigated in Au/Fe4N bilayer films grown by a plasma-assisted molecular beam epitaxy (PA-MBE) system. Depending on lattice distortion and interfacial coupling induced by substrates, the Fe4N layer could be divided into two sublayers having different magnetic anisotropies. The bottom sublayer shows perpendicular magnetic anisotropy (PMA), while the top one has in-plane magnetic anisotropy (IMA). Coupling between the two sublayers provides an extra in-plane effective field and enables a field-free magnetization switching in the bilayer films. By summarizing a series of Hall measurements, a switching phase diagram was obtained. Temperature-dependent switching behaviors demonstrate that the threshold current density for the field-free magnetization switching, which is much smaller than that of pervious reports, increases with decreasing temperature and shows similar temperature dependences to those of coercivity.

Crystal orientation dependence of spin-orbit torques in Co/Pt bilayers

We study the role of Pt crystal orientation in spin-orbit torques in Co/Pt bilayers by means of the harmonic Hall effect and current-induced switching measurements. Perpendicularly magnetized Co/Pt bilayers were fabricated with the Pt layer exhibiting either a polycrystalline grain structure or an epitaxial (111)-oriented film on MgO substrates by magnetron sputtering. We find that the damping-like spin-orbit torque is 1.3 times smaller in the epitaxial Co/Pt(111) bilayers compared to the polycrystalline films, whereas the field-like spin-orbit torque values are of comparable magnitude. Current-induced magnetization switching measurements show good agreement with the results of harmonic measurements of damping-like torque, i.e., the critical switching current is about 30% higher in epitaxial Co/Pt(111). These results highlight the importance of crystal orientation effects on spin-orbit torques in nominally identical bilayer structures.

Current induced magnetization switching in Pt/Co/Cr structures with enhanced perpendicular magnetic anisotropy and spin Hall effect

We study the perpendicular magnetic anisotropy (PMA) and spin–orbit torques (SOTs) in Pt/Co/Cr stacks as a function of Cr thickness. An enhanced PMA is observed and ascribed to the strong interfacial anisotropy between Co and Cr. Moreover, SOTs induced magnetization switching is achieved under relatively small critical current density in the order of 106 A cm−2 due to the enhanced effective spin Hall angle up to 0.15–0.27. Significantly, the linear dependence between and the square of longitudinal resistivity suggests that the spin Hall effect may arise from the intrinsic mechanism in our systems.

Modification of spin-obit torques using the Ta oxidation buffer layer

Abstract We carefully investigated the influence of the thickness of TaOx buffer layer on the perpendicular magnetic anisotropy and spin-obit torques in Pt/Co/Pt trilayers. Using the first and second harmonic Hall voltage measurements, we found that both the perpendicular magnetic anisotropy field and the damping-like field are sensitive to the thickness of the TaOx layer. The damping-like torque with various TaOx thickness changes synchronously with the resistance of the device, which can be explained by the buffer TaOx layer induced current distribution changes in the bottom and top Pt layers. Through the current-induced magnetization switching measurement, we found that an optimal switching efficiency can be obtained by tuning the thickness of TaOx buffer layer.

Enhancement of perpendicular magnetic anisotropy and spin-orbit torque in Ta/Pt/Co/Ta multi-layered heterostructures through interfacial diffusion

Heavy metal/ferromagnetic metal bi-layered structures that exhibit both strong perpendicular magnetic anisotropy (PMA) and large spin-orbit torque (SOT) efficiency have high potential in high-density, low-power memory, and logic device applications. Here, we report the enhancement of PMA and SOT in Ta/Pt/Co/Ta multi-layered heterostructures through interfacial diffusion. The structures can exhibit PMA fields of 9100 Oe at 300 K and 14100 Oe at 10 K and an effective spin Hall angle (SHA) of 0.61 ± 0.03 at 300 K. These values are larger than the corresponding values reported previously for similar heterostructures. The current-induced magnetization switching was demonstrated. The critical switching current density is on the order of 106 A/cm2, and the corresponding switching efficiency is higher than that reported for similar structures. X-ray absorption spectroscopy and high-angle annular dark-field scanning transmission electron microscopy analyses suggest a strong correlation between the observed PMA and SOT enhancement and the interfacial diffusion during the sputtering growth of the samples. It is very likely that the interfacial diffusion gives rise to enhanced spin-orbit coupling at the interface, while the latter results in enhancement in the PMA, SHA, and switching efficiency in the structure.

Observation of spin–orbit torque-induced magnetization switching in Gd-Fe perpendicular magnetized wire with in-plane exchange bias field

We fabricated a Ta/Gd-Fe/Ta and a Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire and investigated spin–orbit torque current-induced magnetization switching in these wires. Magnetization switching in the Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire can be observed by an electric current even if the external in-plane magnetic field is not applied at all. Moreover, we successfully observed the periodical magnetization switching in the Ta/Gd-Fe/Ir22Mn78/Co90Fe10/Ta multilayered magnetic wire in zero magnetic field. This indicates that the present wire is a promising material to realize magnetic random access memory with low power consumption.

A Strain‐Mediated Magnetoelectric‐Spin‐Torque Hybrid Structure

AbstractMagnetization dynamics induced by spin–orbit torques in a heavy‐metal/ferromagnet can potentially be used to design low‐power spintronics and logic devices. Recent computations have suggested that a strain‐mediated spin–orbit torque (SOT) switching in magnetoelectric (ME) heterostructures is fast, energy‐efficient, and permits a deterministic 180° magnetization switching. However, its experimental realization has remained elusive. Here, the coexistence of the strain‐mediated ME coupling and the SOT in a CoFeB/Pt/ferroelectric hybrid structure is shown experimentally. The voltage‐induced strain only slightly modifies the efficiency of SOT generation, but it gives rise to an effective magnetic anisotropy and rotates the magnetic easy axis which eliminates the incubation delay in current‐induced magnetization switching. The phase field simulations show that the electric‐field‐induced effective magnetic anisotropy field can reduce the switching time approximately by a factor of three for SOT in‐plane magnetization switching. It is anticipated that such strain‐mediated ME‐SOT hybrid structures may enable field‐free, ultrafast magnetization switching.

Modulated spin orbit torque in ultrathin ferromagnetic layer with different capping layers

In heavy metal (HM)/ferromagnetic metal (FM)/oxide heterostructures with perpendicular magnetic anisotropy (PMA), the magnetization can be efficiently controlled through spin orbit torque (SOT), which has attracted great attention since its applications in spintronic devices. In this work, we have investigated the SOT with modifying capping layers X in Pt/Co/X trilayers. In these systems, both the bottom Pt/Co and top Co/X interfaces are utilized to generate SOT via bulk SHE effect and/or Rashba effect. By the selecting of different capping layer X, one can expect an enhancement or cancellation of SOT contributions via interface effect, which provides an interesting prospect for enhancing the efficiency of SOT through a proper combination of materials in the structural engineering. Our experimental study explored the choice of the capping materials which influenced the magnetic behavior and current-induced magnetization switching process.

Spin–Orbit Torque Switching in a Nearly Compensated Heusler Ferrimagnet

Ferrimagnetic materials combine the advantages of the low magnetic moment of an antiferromagnet and the ease of realizing magnetic reading of a ferromagnet. Recently, it was demonstrated that compensated ferrimagnetic half metals can be realized in Heusler alloys, where high spin polarization, zero magnetic moment, and low magnetic damping can be achieved at the same time. In this work, by studying the spin-orbit torque induced switching in the Heusler alloy Mn2 Ru1- x Ga, it is found that efficient current-induced magnetic switching can be realized in a nearly compensated sample with strong perpendicular anisotropy and large film thickness. This work demonstrates the possibility of employing compensated Heusler alloys for fast, energy-efficient spintronic devices.