Current-induced Magnetization Switching Research Articles

Slater-Pauling behavior of interfacial magnetic properties of 3d transition metal alloy/Pt structures

Ferromagnet (FM)/heavy metal (HM) bilayers are core structures for current-induced magnetization switching and chiral magnetic structure generation. Static and dynamic properties of the FM moment depend substantially on interfacial perpendicular magnetic anisotropy (iPMA) and interfacial Dzyaloshinskii-Moriya interaction (iDMI). Therefore, it is of crucial importance to control iPMA and iDMI, and to understand their underlying physics. Here we experimentally show that both iPMA and iDMI exhibit similar Slater-Pauling-like dependence on the FM variation as the saturation magnetization (${M}_{S}$) does. We measure ${M}_{S}$, iPMA, and iDMI of the FM/HM bilayers with HM fixed to Pt and FM varied from Mn (electron number $Z=25$) to Ni $(Z=28)$, including their alloys for fractional $Z$. Our result indicates that the density of states structure important for the Slater-Pauling dependence is crucial also for iPMA and iDMI. This provides a useful method to engineer chiral magnetic textures.

Large unidirectional spin Hall and Rashba−Edelstein magnetoresistance in topological insulator/magnetic insulator heterostructures

The unidirectional spin Hall and Rashba−Edelstein magnetoresistance is of great fundamental and practical interest, particularly in the context of reading magnetization states in two-terminal spin–orbit torque memory and logic devices due to its unique symmetry. Here, we report large unidirectional spin Hall and Rashba−Edelstein magnetoresistance in a new material family—magnetic insulator/topological insulator Y3Fe5O12/Bi2Se3 bilayers. Such heterostructures exhibit a unidirectional spin Hall and Rashba−Edelstein magnetoresistance that is about an order of magnitude larger than the highest values reported so far in all-metal Ta/Co bilayers. The polarized neutron reflectometry reveals a unique temperature-dependent magnetic intermediary layer at the magnetic insulator–substrate interface and a proximity layer at the magnetic insulator–topological insulator interface. These polarized neutron reflectometry findings echo the magnetoresistance results in a comprehensive physics picture. Finally, we demonstrate a prototype memory device based on a magnetic insulator/topological insulator bilayer, using unidirectional spin Hall and Rashba−Edelstein magnetoresistance for electrical readout of current-induced magnetization switching aided by a small Oersted field.

Current-induced magnetization switching in perpendicularly magnetized V/CoFeB/MgO multilayers

The perpendicular magnetic anisotropy properties of CoFeB/MgO films on $3d$ light-metal vanadium have been experimentally investigated. The interfacial magnetic anisotropy is $1.4\phantom{\rule{0.16em}{0ex}}\mathrm{erg}/\mathrm{c}{\mathrm{m}}^{2}$ for the V/CoFeB/MgO multilayers annealed at 240 \ifmmode^\circ\else\textdegree\fi{}C. A sign change of the effective torque fields with the increase of V thickness has been observed, which is confirmed by current-induced magnetization switching measurements, suggesting a competition of multiple effects on current-induced torques in the weakly spin-orbit coupled V/CoFeB/MgO multilayers. These findings have implications for the fundamental understanding of current-induced torque and further device applications using $3d$ light transition-metal based heterostructures.

Enhanced Spin-Orbit Torque and Low Critical Current Density in Pt100-xRux/[CoNi]/Ru Multilayer for Spintronic Devices.

Using a heavy-metal (HM) alloy layer in spin-orbit torque (SOT)-based devices is an effective method for obtaining a high current-spin conversion efficiency θSH. In this work, SOT-based spintronic devices with a Pt100-xRux-alloyed HM layer are studied by applying harmonic Hall measurements and magneto-optical Kerr effect microscopy to detect the θSH and to observe the process of current-induced magnetization switching. Both the highest θSH of 0.132 and the lowest critical current density (Jc) of 8 × 105 A/cm2 are realized in a device with x = 20, which satisfies the high SOT efficiency and low energy consumption simultaneously. The interfacial Dzyaloshinskii-Moriya interaction can be overcome by increasing the in-plane assist field. Meanwhile, the minimum in-plane field required for current-induced complete switching can be reduced to ±60 Oe. Our study reveals that using the Pt-Ru alloyed HM layer is an effective route for SOT application with enhanced performance.

Temperature dependence of intrinsic critical current in perpendicular easy axis CoFeB/MgO magnetic tunnel junctions

Current induced magnetization switching in CoFeB/MgO-based magnetic tunnel junctions (MTJs) with a perpendicular easy axis is studied above room temperature. The intrinsic critical current IC0 of the MTJs decreases with increasing temperature. From a vector-network-analyzer ferromagnetic resonance measurement with a heating system, temperature dependence of magnetic anisotropy and damping constant is evaluated. We find that the reduction of IC0 at elevated temperature is mainly due to a decrease in magnetic anisotropy. A slight increase in the damping constant with temperature rise is also observed, consistent with the mechanism considering electron scattering through the inter-band transition.

Highly efficient spin-current generation from Pt/Ru multilayers

Increasing spin Hall angle of heavy metals has attracted considerable attention due to their potential applications in spintronic technology. Here, we demonstrate that the spin Hall angle could be enhanced dramatically in the Ta/[Pt/Ru]n/Pt/Co/Ta multilayers. The spin Hall angle reaches a maximum value of 0.28 when n = 5 from the loop-shift method with a relative low resistivity of approximately 48 μΩ cm. Meanwhile a minimum critical switching current density approximately 3.2 × 106 A/cm2 under an in-plane bias magnetic field around 5% of the perpendicular anisotropy field was obtained for the sample with n = 5 in current-induced magnetization switching experiment. We find that the extrinsic spin Hall mechanism changes from the spin skew scattering when n < 2 to spin side-jump mechanism when n > 2. This work provides an effective way in increasing spin Hall angle for low-power consumption spintronics devices.

Current-induced perpendicular magnetization switching without external magnetic field in gate-induced asymmetric structure

We demonstrate current-induced switching of perpendicular magnetization without any external magnetic field by introducing lateral structural asymmetry through gate-induced redox reactions. A gate electrode was fabricated to cover only half of a heavy metal/ferromagnet/oxide wire, allowing a gate voltage to form an in-plane oxidation gradient. The essential role of structural asymmetry is evidenced by the fact that external-field-free deterministic switching is absent when the gate electrode covers an entire wire. We confirm that the current-induced perpendicular effective field deterministically drives the magnetization direction at zero field. Magnetic domain imaging and transport measurements clarify that the origin of the perpendicular effective field is not the Rashba effect, but the Oersted field at the wire edges.

Magnetoresistance effect realized in current-in-plane Van der Waals spin valve structure by electrically switchable magnetization

We present a device concept for magnetoresistance (MR) effect in which two Van der Waals (vdW) spin valves are used as the left and right ferromagnetic (FM) leads connected by a bilayer graphene as the channel material of the central scattering region. Unlike conventional current-perpendicular-to-plane magnetic tunnel junction consisting of two FM thin films with a thin insulating barrier sandwiched between them, the FM leads in our proposed current-in-plane MR device are vdW spin valves. This is important because the application of an out-of-plane electric field allows control of the direction and magnitude of the magnetization in vdW spin valves. Moreover, we show that the oscillatory behavior is found in the MR and conductance as the height (depth) of the barrier (well) of the scattering region with the p(n)-doping increase, or the width of the scattering region increase. Remarkably, when the potential barrier is present, the oscillation magnitude of MR is considerable and can be changed from positive to negative value, whereas for the potential well, the oscillation amplitude is relatively small and is always positive. Therefore, we hope that this device configuration with electrically tunable large MR can open up new possibilities for future lower power magneto-electric devices since no current-induced magnetization switching.

Efficient current-driven magnetization switching owing to isotropic magnetism in a highly symmetric 111-oriented Mn4N epitaxial single layer

We investigated in-plane current-induced magnetization switching in a Mn4N epitaxial single layer. Efficient magnetization switching was detected via the measurement of anomalous Hall resistivity after the application of current pulses, with a duration of 1 s, to the 111-oriented Mn4N film compared with a reference 001-oriented Mn4N film. The threshold current density of magnetization switching with 0.5 s pulse durations, Jc ≈ 1 MA/cm2, was relatively low compared with that reported for magnetic tunnel junctions and/or ferromagnet/heavy metal bilayer systems. The relatively low Jc in the 111-oriented film was attributed to the low magnetic anisotropy on the (111) plane of Mn4N owing to the isotropic crystal symmetry as revealed by x-ray diffraction and transmission electron microscopy as a reduced switching barrier boosts the probability of magnetization switching. It was concluded that manipulation of the magnetic anisotropy based on the crystal orientation is one of the promising approaches to develop materials suitable for application in highly efficient switching devices with Mn4N layers.

Probing the magnetization switching with in-plane magnetic anisotropy through field-modified magnetoresistance measurement

Effective probing current-induced magnetization switching is highly required in the study of emerging spin–orbit torque (SOT) effect. However, the measurement of in-plane magnetization switching typically relies on the giant/tunneling magnetoresistance measurement in a spin valve structure calling for complicated fabrication process, or the non-electric approach of Kerr imaging technique. Here, we present a reliable and convenient method to electrically probe the SOT-induced in-plane magnetization switching in a simple Hall bar device through analyzing the MR signal modified by a magnetic field. In this case, the symmetry of MR is broken, resulting in a resistance difference for opposite magnetization orientations. Moreover, the feasibility of our method is widely evidenced in heavy metal/ferromagnet (Pt/Ni20Fe80 and W/Co20Fe60B20) and the topological insulator/ferromagnet (Bi2Se3/Ni20Fe80). Our work simplifies the characterization process of the in-plane magnetization switching, which can promote the development of SOT-based devices.

Current-Induced Magnetization Switching in Seed Optimized Perpendicular Magnetic Tunnel Junctions with Enhanced Spin-Transfer-Torque Efficiency and Uniformity

A NiCr film is introduced in magnetic tunnel junction (MTJ) seed layer to improve the smoothness of multilayer and to enhance the perpendicular magnetic anisotropy (PMA) of the pinning layer. Espec...

The deterministic field-free magnetization switching of perpendicular ferrimagnetic Tb-Co alloy film induced by interfacial spin current

Current-induced magnetization switching in compensated ferrimagnetic materials by the spin–orbit torque (SOT) effect is promising for the next generation information storage devices. In this work, we report the current-induced deterministic field-free magnetization switching of the perpendicular Tb-Co ferrimagnet layer in a Co/Ti/Tb-Co trilayers. We found that the switching proportion and polarity of the Tb-Co ferrimagnet depend on the magnetization direction of the in-plane Co layer. The switching process revealed by magneto-optical Kerr microscope imaging further confirmed the current-induced field-free switching of the Tb-Co layer. We also demonstrated the large SOT effective field and the perpendicular effective field acting on the Tb-Co layer, by utilizing the second harmonic voltage measurement and the current-induced loop shift method. The large interfacial SOT efficiency and deterministic field-free magnetization switching in the trilayers structure may accelerate the application of ferrimagnet in SOT memory devices.

Current-induced magnetization switching at charge-transferred interface between topological insulator (Bi,Sb)2Te3 and van der Waals ferromagnet Fe3GeTe2

Ferromagnetic two-dimensional van der Waals materials attract enormous interest as a platform to explore spin-related quantum phenomena, especially in conjunction with other quantum materials. Topological insulator is one such candidate to form the junction, because the spin-polarized nature of the surface or interface Dirac states enables the highly efficient spin-charge conversion. Here, we report the current-driven magnetization switching in the bilayer film of a van der Waals ferromagnetic semimetal Fe3GeTe2 (FGT) and a topological insulator (Bi1−xSbx)2Te3 (BST). We observed the current-induced magnetization switching via the Edelstein effect in a wide temperature range, whose threshold current density is as small as that reported for the heterostructure of FGT with a Pt layer. By analyzing the transport properties in heterostructures with different Fermi level (EF) positions in the BST layer, we found that the EF position of the charge-transferred interface Dirac states causes the significant variation of the threshold current density with a Bi/Sb ratio. The present result may promise spintronic phenomena in heterostructures of 2D van der Waals ferromagnets with topological insulators/semimetals.

Magnetization switching and deterministic nucleation in Co/Ni multilayered disks induced by spin–orbit torques

We present experimental and numerical results on the magnetization reversal induced by spin–orbit torques of micronic disks of a ferromagnetic multilayer with perpendicular magnetic anisotropy on top of a Pt track: Pt (6 nm)/[Co(0.2 nm)/Ni(0.6 nm)] × 5/Al(5 nm). The current induced magnetization switching process is probed by anomalous Hall effect measurements and Kerr microscopy. The electrical characterization reveals the critical current for the complete reversal to be about 3×1011 A/m2, and Kerr microscopy uncovers a deterministic nucleation that depends on current and field polarity. Through the use of experimental switching phase diagrams coupled to micromagnetic simulations, we evaluated the field-like to damping-like torque ratio to be 0.73 ± 0.05, which is in good agreement with experimental values observed by second harmonic measurements. These measurements emphasize an unexpectedly large field-like contribution in this relatively thick Co/Ni multilayer (4 nm). In light of these experiments and simulations, we discuss the key parameters needed to understand the magnetization reversal, namely, the field and damping-like torques and the Dzyaloshinskii–Moriya interaction.

Observation of current-induced switching in non-collinear antiferromagnetic IrMn3 by differential voltage measurements

There is accelerating interest in developing memory devices using antiferromagnetic (AFM) materials, motivated by the possibility for electrically controlling AFM order via spin-orbit torques, and its read-out via magnetoresistive effects. Recent studies have shown, however, that high current densities create non-magnetic contributions to resistive switching signals in AFM/heavy metal (AFM/HM) bilayers, complicating their interpretation. Here we introduce an experimental protocol to unambiguously distinguish current-induced magnetic and nonmagnetic switching signals in AFM/HM structures, and demonstrate it in IrMn3/Pt devices. A six-terminal double-cross device is constructed, with an IrMn3 pillar placed on one cross. The differential voltage is measured between the two crosses with and without IrMn3 after each switching attempt. For a wide range of current densities, reversible switching is observed only when write currents pass through the cross with the IrMn3 pillar, eliminating any possibility of non-magnetic switching artifacts. Micromagnetic simulations support our findings, indicating a complex domain-mediated switching process.

Influence of thickness on current-induced magnetization switching in L10-FePt single layer* *Project supported by the National Key Research and Development Program of China (Grant No. 2019YFB2005801), the National Natural Science Foundation of China (Grant Nos. 51971027, 51731003, 51971023, 51927802, and 51971024), and the Fundamental Research Funds for the Central Universities, China (Grant No. FRF-TP-19-001A3).

The thickness dependent spin–orbit torque (SOT) in an L10-FePt single layer is investigated in this work. As the thickness increases from 8 nm to 16 nm, the magnetization switching ratio in the L10-FePt film with higher chemical ordering becomes smaller. It is noted that compared with 3-nm-thick L10-FePt film, 8-nm-thick L10-FePt film can switch much magnetization with the increase of chemical ordering. When the FePt film is thick enough, the SOT in FePt is closely related to the L10-ordered structure, which indicates a bulk nature. Therefore, the disordering plays an important role in the magnetization switching only for the ultra-thin FePt films, while the structural gradient may play an important role for thicker films. However, both of the two mechanisms cannot fully explain the process of magnetization switching and the spin current generation. Although many factors influence SOT, here in this work we emphasize only the bulk nature of strong SOC in L10-FePt through density functional theory calculations, which should generate large spin current due to spin Hall effect.

Thermal Effect in Current-Induced Magnetization Switching and Out-of-Plane Effective Field Measurements

We study the current-induced spin–orbit torque (SOT) switching and out-of-plane (OOP) effective field in a CuPt/CoPt heterostructure. By using three measurement methods (“continuous current” mode, “pulsed current+nonequilibrium” mode, and “pulsed current+equilibrium” mode), we found their switching loops show distinct behaviors and their OOP effective fields have different dependencies on the current. Our observation identified the important role of the thermal effect in the SOT process with large current.

The optimized memristor performance by utilizing the constrained domain wall motion in Pt/Co–Tb/Ta structure

Spintronic devices can realize multilevel state storage and mimic the properties of the synapse, which enables their potential application in the field of artificial neural networks. In this paper, we demonstrate the existence of a large intermediate transition zone in current-induced magnetization switching curves of Pt/Co–Tb/Ta structures, and the number of states in the transition zone that can be manipulated by changing the Co content. The magneto-optical Kerr microscope imaging indicates that this property is related to the constrained domain wall motion in the Co–Tb films with large Co content. We also demonstrate the multilevel state storage properties of the sample by applying a sequence of current pulses. The synaptic plasticity behaviors were mimicked in these samples through regulating the value of Hall resistance by current pulses. The constrained domain wall motion supplies a simple but effective way to achieve multilevel state storage and show potential applications in neuromorphic computing.

Charge–spin conversion in Pt1−xBix alloys for spin–orbit torque switching

We have studied the spin torque efficiency of Pt1−xBix/Co based heterostructures for spin–orbit torque (SOT) switching devices. While the field-like component of the SOT shows a relatively small change with the Bi concentration (x), the damping-like component (ξDL) abruptly increases with increasing x. The increase in ξDL with x is largely associated with the change in the Pt1−xBix layer resistivity (ρxx). The spin Hall conductivity (σSH) estimated from ξDL and ρxx remains nearly constant at ∼830 (ℏ/e) Ω−1 cm−1 for 0 < x ≲ 0.6. Further substitution of Bi results in the formation of PtBi2, which causes smaller σSH than that of alloys with smaller x. The magnetic easy axis of a 0.6 nm thick Co layer points along the film normal when x is smaller than ∼0.7. Exploiting these characteristics, we show current-induced magnetization switching in the heterostructures. The efficiency to switch a perpendicularly magnetized Co layer is the largest when x ∼ 0.5. These results demonstrate the potential of Pt1−xBix alloys for spin–orbit torque magnetization switching devices.

Topological insulators for efficient spin–orbit torques

Current-induced magnetic switching via spin–orbit torques has been extensively pursued for memory and logic applications with promising energy efficiency. Topological insulators are a group of materials with spin-momentum locked electronic states at the surface due to spin–orbit coupling, which can be harnessed to reach strong spin–orbit torques. In this paper, we summarize and compare the methods for calibrating the charge-spin conversion efficiency in topological insulators, with which topological insulators are identified as outstanding spin–orbit torque generators compared with the well-studied heavy metals. We then review the results of magnetic switching under reduced current density in topological insulator/ferromagnet heterostructures. Finally, we provide insights on current challenges as well as possible exploration directions in the emerging field of topological spintronics.