40 research outputs found
Tuning a binary ferromagnet into a multi-state synapse with spin-orbit torque induced plasticity
Inspired by ion-dominated synaptic plasticity in human brain, artificial
synapses for neuromorphic computing adopt charge-related quantities as their
weights. Despite the existing charge derived synaptic emulations, schemes of
controlling electron spins in ferromagnetic devices have also attracted
considerable interest due to their advantages of low energy consumption,
unlimited endurance, and favorable CMOS compatibility. However, a generally
applicable method of tuning a binary ferromagnet into a multi-state memory with
pure spin-dominated synaptic plasticity in the absence of an external magnetic
field is still missing. Here, we show how synaptic plasticity of a
perpendicular ferromagnetic FM1 layer can be obtained when it is
interlayer-exchange-coupled by another in-plane ferromagnetic FM2 layer, where
a magnetic-field-free current-driven multi-state magnetization switching of FM1
in the Pt/FM1/Ta/FM2 structure is induced by spin-orbit torque. We use current
pulses to set the perpendicular magnetization state which acts as the synapse
weight, and demonstrate spintronic implementation of the excitatory/inhibitory
postsynaptic potentials and spike timing-dependent plasticity. This
functionality is made possible by the action of the in-plane interlayer
exchange coupling field which leads to broadened, multi-state magnetic reversal
characteristics. Numerical simulations, combined with investigations of a
reference sample with a single perpendicular magnetized Pt/FM1/Ta structure,
reveal that the broadening is due to the in-plane field component tuning the
efficiency of the spin-orbit-torque to drive domain walls across a landscape of
varying pinning potentials. The conventionally binary FM1 inside our
Pt/FM1/Ta/FM2 structure with inherent in-plane coupling field is therefore
tuned into a multi-state perpendicular ferromagnet and represents a synaptic
emulator for neuromorphic computing.Comment: 37 pages with 11 figures, including 20 pages for manuscript and 17
pages for supplementary informatio
Tuning a binary ferromagnet into a multi-state synapse with spin-orbit-torque-induced plasticity
Ferromagnets with binary states are limited for applications as artificial synapses for neuromorphic computing. Here, it is shown how synaptic plasticity of a perpendicular ferromagnetic layer (FM1) can be obtained when it is interlayer exchange‐coupled by another in‐plane ferromagnetic layer (FM2), where a magnetic field‐free current‐driven multistate magnetization switching of FM1 in the Pt/FM1/Ta/FM2 structure is induced by spin–orbit torque. Current pulses are used to set the perpendicular magnetization state, which acts as the synapse weight, and spintronic implementation of the excitatory/inhibitory postsynaptic potentials and spike timing‐dependent plasticity are demonstrated. This functionality is made possible by the action of the in‐plane interlayer exchange coupling field which leads to broadened, multistate magnetic reversal characteristics. Numerical simulations, combined with investigations of a reference sample with a single perpendicular magnetized Pt/FM1/Ta structure, reveal that the broadening is due to the in‐plane field component tuning the efficiency of the spin–orbit torque to drive domain walls across a landscape of varying pinning potentials. The conventionally binary FM1 inside the Pt/FM1/Ta/FM2 structure with an inherent in‐plane coupling field is therefore tuned into a multistate perpendicular ferromagnet and represents a synaptic emulator for neuromorphic computing, demonstrating a significant pathway toward a combination of spintronics and synaptic electronics
Adjustable current-induced magnetization switching utilizing interlayer exchange coupling
Electrical current-induced deterministic magnetization switching in a magnetic multilayer structure without external magnetic field is realized by utilizing interlayer exchange coupling. Two ferromagnetic Co layers, with in-plane and out-of-plane anisotropy respectively, are separated by a spacer Ta layer, which plays a dual role of inducing antiferromagnetic interlayer coupling, and contributing to the current-induced effective magnetic field through the spin Hall effect. The current-induced magnetization switching behavior can be tuned by pre-magnetizing the in-plane Co layer. The antiferromagnetic exchange coupling field increases with decreasing thickness of the Ta layer, reaching 630 ±5 Oe for a Ta thickness of 1.5nm. The magnitude of the current-induced perpendicular effective magnetic field from spin-orbit torque is 9.2 Oe/(107Acm-2). The large spin Hall angle of Ta, opposite in sign to that of Pt, results in a low critical current density of 9×106A/cm2. This approach is promising for the electrical switching of magnetic memory elements without external magnetic field
Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors
A critical challenge for the integration of the optoelectronics is that
photodetectors have relatively poor sensitivities at the nanometer scale. It is
generally believed that a large electrodes spacing in photodetectors is
required to absorb sufficient light to maintain high photoresponsivity and
reduce the dark current. However, this will limit the optoelectronic
integration density. Through spatially resolved photocurrent investigation, we
find that the photocurrent in metal-semiconductor-metal (MSM) photodetectors
based on layered GaSe is mainly generated from the photoexcited carriers close
to the metal-GaSe interface and the photocurrent active region is always close
to the Schottky barrier with higher electrical potential. The photoresponsivity
monotonically increases with shrinking the spacing distance before the direct
tunneling happen, which was significantly enhanced up to 5,000 AW-1 for the
bottom contacted device at bias voltage 8 V and wavelength of 410 nm. It is
more than 1,700-fold improvement over the previously reported results. Besides
the systematically experimental investigation of the dependence of the
photoresponsivity on the spacing distance for both the bottom and top contacted
MSM photodetectors, a theoretical model has also been developed to well explain
the photoresponsivity for these two types of device configurations. Our
findings realize shrinking the spacing distance and improving the performance
of 2D semiconductor based MSM photodetectors simultaneously, which could pave
the way for future high density integration of 2D semiconductor optoelectronics
with high performances.Comment: 25 pages, 4 figure
Deterministic Magnetization Switching Using Lateral Spin–Orbit Torque
Current-induced magnetization switching by spin-orbit torque (SOT) holds
considerable promise for next generation ultralow-power memory and logic
applications. In most cases, generation of spin-orbit torques has relied on an
external injection of out-of-plane spin currents into the magnetic layer, while
an external magnetic field along the electric current direction is generally
required for realizing deterministic switching by SOT. Here, we report
deterministic current-induced SOT full magnetization switching by lateral
spin-orbit torque in zero external magnetic field. The Pt/Co/Pt magnetic
structure was locally annealed by a laser track along the in-plane current
direction, resulting in a lateral Pt gradient within the ferromagnetic layer,
as confirmed by microstructure and chemical composition analysis. In zero
magnetic field, the direction of the deterministic current-induced
magnetization switching depends on the location of the laser track, but shows
no dependence on the net polarization of external out-of-plane spin currents.
From the behavior under external magnetic fields, we identify two independent
mechanisms giving rise to SOT, i.e. the lateral Pt-Co asymmetry as well as
out-of-plane injected spin currents, where the polarization and the magnitude
of the SOT in the former case depends on the relative location and the laser
power of the annealing track. Our results demonstrate an efficient field-free
deterministic full magnetization switching scheme, without requiring
out-of-plane spin current injection or complex external stack structures.Comment: 39 Pages, 9 Figure
Complementary Lateral‐Spin–Orbit Building Blocks for Programmable Logic and In‐Memory Computing
Current-driven switching of nonvolatile spintronic materials and devices based on spin-orbit torques offer fast data processing speed, low power consumption, and unlimited endurance for future information processing applications. Analogous to conventional CMOS technology, it is important to develop a pair of complementary spin-orbit devices with differentiated magnetization switching senses as elementary building blocks for realizing sophisticated logic functionalities. Various attempts using external magnetic field or complicated stack/circuit designs have been proposed, however, plainer and more feasible approaches are still strongly desired. Here we show that a pair of two locally laser annealed perpendicular Pt/Co/Pt devices with opposite laser track configurations and thereby inverse field-free lateral spin-orbit torques (LSOTs) induced switching senses can be adopted as such complementary spin-orbit building blocks. By electrically programming the initial magnetization states (spin down/up) of each sample, four Boolean logic gates of AND, OR, NAND and NOR, as well as a spin-orbit half adder containing an XOR gate, were obtained. Moreover, various initialization-free, working current intensity-programmable stateful logic operations, including material implication (IMP) gate, were also demonstrated by regarding the magnetization state as a logic input. Our complementary LSOT building blocks provide a potentially applicable way towards future efficient spin logics and in-memory computing architectures.
Electric field control of deterministic current-induced magnetization switching in a hybrid ferromagnetic/ferroelectric structure
All-electrical and programmable manipulations of ferromagnetic bits are highly pursued for the aim of high integration and low energy consumption in modern information technology1, 2, 3. Methods based on the spin–orbit torque switching4, 5, 6 in heavy metal/ferromagnet structures have been proposed with magnetic field7, 8, 9, 10, 11, 12, 13, 14, 15, and are heading toward deterministic switching without external magnetic field16, 17. Here we demonstrate that an in-plane effective magnetic field can be induced by an electric field without breaking the symmetry of the structure of the thin film, and realize the deterministic magnetization switching in a hybrid ferromagnetic/ferroelectric structure with Pt/Co/Ni/Co/Pt layers on PMN-PT substrate. The effective magnetic field can be reversed by changing the direction of the applied electric field on the PMN-PT substrate, which fully replaces the controllability function of the external magnetic field. The electric field is found to generate an additional spin–orbit torque on the CoNiCo magnets, which is confirmed by macrospin calculations and micromagnetic simulations