Memristors, demonstrated by solid-state devices with continuously tunable resistance, [1][2][3][4][5][6][7] have emerged as a new paradigm for self-adaptive networks that require synapse-like functions (artificial synapse, for example). Spin-based memristors offer advantages over other types of memristors because of their significant endurance and high energy efficiency. [8,9] Yet, it remains a challenge to build dense and functional spintronic memristors with structures and materials that are compatible with existing ferromagnetic devices. [10] Here, a memristive device based upon Ta/CoFeB/MgO heterostructures is demonstrated, which are commonly used in out-of-plane magnetized magnetic tunnel junctions. [11] To achieve the memristive function, a domain wall (DW) is driven back and forth in a continuous manner in the CoFeB layer by applying in-plane positive or negative current pulses along the Ta layer, utilizing the spin-orbit torque (SOT) that the current exerts on the CoFeB magnetization. [12][13][14][15][16][17] Hence, Memristors, demonstrated by solid-state devices with continuously tunable resistance, have emerged as a new paradigm for self-adaptive networks that require synapse-like functions (artificial synapse, for example). Spin-based memristors offer advantages over other types of memristors because of their significant endurance and high energy efficiency. Yet it remains a challenge to build dense and functional spintronic memristors with structures and materials that are compatible with existing ferromagnetic devices. Here, a memristive device based upon Ta/CoFeB/MgO heterostructures is demonstrated, which are commonly used in out-of-plane magnetized magnetic tunnel junctions (MTJ). To achieve the memristive function, a domain wall (DW) is driven back and forth in a continuous manner in the CoFeB layer by applying in-plane positive or negative current pulses along the Ta layer, utilizing the spin-orbit torque (SOT) that the current exerts on the CoFeB magnetization. Hence, the magnetization and consequently the anomalous Hall effect (AHE) resistance are modulated in an analog manner, being controlled by the pulsed current characteristics including amplitude, duration, and repetition number. The quasi-continuous AHE resistance variation is explained by the SOT-induced DW creep motion. These results pave the way for developing SOT-based energy-efficient neuromorphic systems.

Multilevel remanence states have potential applications in ultra-high-density storage and neuromorphic computing. Continuous tailoring of the multilevel remanence states by spin-orbit torque (SOT) is reported in perpendicularly magnetized Pt/Co/IrMn heterostructures. Double-biased hysteresis loops with only one remanence state can be tuned from the positively or negatively single-biased loops by SOT controlled sign of the exchange-bias field. The remanence states associated with the heights of the sub-loops are continually changed by tuning the ratio of the positively and negatively oriented ferromagnetic domains. The multilevel storage cells are demonstrated by reading the remanent Hall resistance through changing the sign and/or the magnitude of current pulse. The synaptic plasticity behaviors for neuromorphic computing are also simulated by varying the remanent Hall resistance under the consecutive current pulses. This work demonstrates that SOT is an effective method to tailor the remanence states in the double-biased heavy metal/ ferromagnetic/antiferromagnetic system. The multilevel-stable remanence states driven by SOT show potential applications in future multilevel memories and neuromorphic computing devices.

Single-atoms (SAs) strategies have been proved to be effective in modulating electromagnetic wave (EMW) absorption, however, the establishment of a definitive relationship between metal SAs electronic configurations and physical loss mechanisms has been still absent, especially on the atomic scale. Herein, stable Ni-SAsx/N-doped carbon (NC) absorbers are fabricated with the strategy of ligand polymerization. The morphology, composition, electrical conductivity, defects, and electronic interactions of the material can be well tailored by Ni species modulation engineering. Theoretical and experimental results show that the atomically dispersed individual Ni atoms contribute to enhanced EMW absorption performance through excess Ni 3d orbital unpaired electron induced polarization loss. Benefiting from it, Ni-SAs3/NC with the highest Ni SAy-Nx (y > 1, x > 1) polar/defect centers exhibit excellent EMW absorption with an effective absorption bandwidth of 7.08 GHz at a matched thickness of 2.50 mm. Radar cross-section simulations further demonstrate its potential for practical application as EMW absorber. This study reveals the continuous evolution of microscopic electromagnetic loss mechanism (i.e., conduction loss→ unique polarization loss→ conduction loss) for the first time, which provides insight into the deep design of absorbers from atom-scale view.

Magnetization switching via charge current induced spin-orbit torques (SOTs) in heavy metal/ferromagnetic metal/heavy metal heterostructures has become an important issue due to its potential applications in high stability and low energy dissipation spintronic devices. In this work, based on a Pt/Co/Ta structure with perpendicular magnetic anisotropy (PMA), we report the effect of inserting a non-metal C interlayer between Co and Ta on the current-induced magnetization switching. A series of measurements based on the extraordinary Hall effect were carried out to investigate the difference of the anisotropy field, switching field, and damping-like and field-like SOT-induced effective fields as well as the current-induced spin Hall effect (SHE) torque after C decoration. The results show that PMA can be reduced by C decoration and the ratio of the effective SHE torque per unit current density and anisotropy field plays an essential role in the switching efficiency. In addition, the obtained switching current density has a quite low value around the order of 106 A/cm2. Our study could provide a way for achieving the low switching current density by manipulating PMA in SOT-based spintronic devices through interface decoration.

Current-induced magnetization reversal via spin-orbit torques (SOTs) has been intensively studied in heavy-metal/ferromagnetic-metal/oxide heterostructures due to its promising application in low-energy consumption logic and memory devices. Here, we systematically study the function of Joule heating and SOTs in the current-induced magnetization reversal using Pt/Co/SmOx and Pt/Co/AlOx structures with different perpendicular magnetic anisotropies (PMAs). The SOT-induced effective fields, anisotropy field, switching field and switching current density (Jc) are characterized using electric transport measurements based on the anomalous Hall effect and polar magneto-optical Kerr effect (MOKE). The results show that the current-generated Joule heating plays an assisted role in the reversal process by reducing switching field and enhancing SOT efficiency. The out-of-plane component of the damping-like-SOT effective field is responsible for the magnetization reversal. The obtained Jc for Pt/Co/SmOx and Pt/Co/AlOx structures with similar spin Hall angles and different PMAs remains roughly constant, revealing that the coherent switching model cannot fully explain the current-induced magnetization reversal. In contrast, by observing the domain wall nucleation and expansion using MOKE and comparing the damping-like-SOT effective field and switching field, we conclude that the current-induced magnetization reversal is dominated by the depinning model and Jc also immensely relies on the depinning field.