Shunsuke Fukami

Magnetization switching by spin-orbit torque in an antiferromagnet-ferromagnet bilayer system

Spin-orbit torque (SOT)-induced magnetization switching shows promise for realizing ultrafast and reliable spintronics devices. Bipolar switching of the perpendicular magnetization by the SOT is achieved under an in-plane magnetic field collinear with an applied current. Typical structures studied so far comprise a nonmagnet/ferromagnet (NM/FM) bilayer, where the spin Hall effect in the NM is responsible for the switching. Here we show that an antiferromagnet/ferromagnet (AFM/FM) bilayer system also exhibits a SOT large enough to switch the magnetization of the FM. In this material system, thanks to the exchange bias of the AFM, we observe the switching in the absence of an applied field by using an antiferromagnetic PtMn and ferromagnetic Co/Ni multilayer with a perpendicular easy axis. Furthermore, tailoring the stack achieves a memristor-like behaviour where a portion of the reversed magnetization can be controlled in an analogue manner. The AFM/FM system is thus a promising building block for SOT devices as well as providing an attractive pathway towards neuromorphic computing.

Properties of magnetic tunnel junctions with a MgO/CoFeB/Ta/CoFeB/MgO recording structure down to junction diameter of 11 nm

We investigate properties of perpendicular anisotropy magnetic tunnel junctions (MTJs) with a recording structure of MgO/CoFeB/Ta/CoFeB/MgO down to junction diameter (D) of 11 nm from 56 nm. Thermal stability factor (Δ) of MTJ with the structure starts to decrease at D = 30 nm. D dependence of Δ agrees well with that expected from magnetic properties of blanket film taking into account the change in demagnetizing factors of MTJs. Intrinsic critical current (IC0) reduces with decrease of D in the entire investigated D range. A ratio of Δ to IC0 shows continuous increase with decrease of D down to 11 nm.

A spin-orbit torque switching scheme with collinear magnetic easy axis and current configuration.

Spin-orbit torque, a torque brought about by in-plane current via the spin-orbit interactions in heavy-metal/ferromagnet nanostructures, provides a new pathway to switch the magnetization direction. Although there are many recent studies, they all build on one of two structures that have the easy axis of a nanomagnet lying orthogonal to the current, that is, along the z or y axes. Here, we present a new structure with the third geometry, that is, with the easy axis collinear with the current (along the x axis). We fabricate a three-terminal device with a Ta/CoFeB/MgO-based stack and demonstrate the switching operation driven by the spin-orbit torque due to Ta with a negative spin Hall angle. Comparisons with different geometries highlight the previously unknown mechanisms of spin-orbit torque switching. Our work offers a new avenue for exploring the physics of spin-orbit torque switching and its application to spintronics devices.

MgO/CoFeB/Ta/CoFeB/MgO Recording Structure in Magnetic Tunnel Junctions With Perpendicular Easy Axis

Junction size (D) dependence of thermal stability (Δ) factor and intrinsic critical current (IC0) were investigated for MgO/CoFeB/Ta/CoFeB/MgO recording structure in magnetic tunnel junctions (MTJs) having a CoFeB reference layer and a synthetic ferrimagnetic (SyF) reference layer. Δ of the recording structure shows almost constant value down to 40 nm, whereas IC0 shows a linear dependence on the recording layer area, as similarly observed in recording structure with single-interface. Average absolute intrinsic critical current density is 3.5 MA/cm2, which is comparable to previously reported value for recording structure with single-interface. A MgO/CoFeB(1.4)/Ta(0.4)/CoFeB(1.0)/MgO double-interface recording structure in MTJ with SyF reference layer shows Δ of 59 at D = 29 nm.

10.5 A 90nm 20MHz fully nonvolatile microcontroller for standby-power-critical applications

Recently there has been increased demand for not only ultra-low power, but also high performance, even in standby-power-critical applications. Sensor nodes, for example, need a microcontroller unit (MCU) that has the ability to process signals and compress data immediately. A previously reported 130nm CMOS and FeRAM-based MCU features zero-standby power and fast wakeup operation by incorporating FeRAM devices into logic circuits [1]. The 8MHz speed, however, was not sufficiently high to meet application requirements, and the FeRAM process also has drawbacks: low compatibility with standard CMOS, and write endurance limitations. A spintronics-based nonvolatile integrated circuit is a promising option to achieve zero standby power and high-speed operation, along with compatibility with CMOS processes. In this work, we demonstrate a fully nonvolatile 16b MCU using 90nm standard CMOS and three-terminal SpinRAM technology. It achieves 20MHz, 145μW/MHz operation with a 1V supply in the active state, and 4.5μW intermittent operation with 120ns wakeup time and 0.1% active ratio, without forwarding of re-boot code from memory. The features provide sufficiently long battery life to achieve maintenance-free sensor nodes.

Spin-orbit torque induced magnetization switching in nano-scale Ta/CoFeB/MgO

We study the device size dependence of spin-orbit torque induced magnetization switching in a Ta/CoFeB/MgO structure with perpendicular easy axis. The miniaturization of the device from micrometer-sized wire to 80-nm dot results in the increase of the threshold current density Jth by one order, whereas Jth increases only slightly with further reducing the device size down to 30 nm. No significant increase in Jth is seen, as the current pulse width decreases from 100 ms down to 3 ns. We reveal that the switching in devices at reduced size is reasonably well explained by the macrospin model, in which the effects of both the Slonczewski-like torque and field-like torque are included.

High-speed simulator including accurate MTJ models for spintronics integrated circuit design

An extremely practical simulation program with integrated circuits emphasis (SPICE) incorporating model parameters of magnetic tunnel junction (MTJ) was developed. The simulator provides reliable simulation results in spintronics circuit design because it can accurately calculate various MTJ characteristics that actual devices have, that considerably influence the operation margin and power dissipation. It can also accelerate the simulation speed, which makes it possible to simulate three times or more large-scale circuits than when a conventional macro-model is used.

Comprehensive study of CoFeB-MgO magnetic tunnel junction characteristics with single- and double-interface scaling down to 1X nm

We study characteristics of CoFeB-MgO magnetic tunnel junction with perpendicular easy-axis (p-MTJ) at a reduced dimension down to 1X nm fabricated by hard-mask process. CoFeB-MgO p-MTJ with double-interface shows higher thermal stability down to 1X nm than that with single-interface. Thermal stability factor of 58 and intrinsic critical current of 24 μA are obtained in the CoFeB-MgO magnetic tunnel junction with perpendicular easy-axis using double-interface structure at a diameter of 20 nmφ.

Depinning probability of a magnetic domain wall in nanowires by spin-polarized currents

Current-induced magnetic domain wall motion is attractive for manipulating magnetization direction in spintronics devices, which open a new era of electronics. Up to now, in spite of a crucial significance to applications, investigation on a current-induced domain wall depinning probability, especially in sub-nano to a-few-nanosecond range has been lacking. Here we report on the probability of the depinning in perpendicularly magnetized Co/Ni nanowires in this timescale. A high depinning probability was obtained even for 2-ns pulses with a current density of less than 10¹² A m⁻². A one-dimensional Landau-Lifshitz-Gilbert calculation taking into account thermal fluctuations reproduces well the experimental results. We also calculate the depinning probability as functions of various parameters and found that parameters other than the coercive field do not affect the transition width of the probability. These findings will allow one to design high-speed and reliable magnetic devices based on the domain wall motion.

Analogue spin–orbit torque device for artificial-neural-network-based associative memory operation

We demonstrate associative memory operations reminiscent of the brain using nonvolatile spintronics devices. Antiferromagnet–ferromagnet bilayer-based Hall devices, which show analogue-like spin–orbit torque switching under zero magnetic fields and behave as artificial synapses, are used. An artificial neural network is used to associate memorized patterns from their noisy versions. We develop a network consisting of a field-programmable gate array and 36 spin–orbit torque devices. An effect of learning on associative memory operations is successfully confirmed for several 3 × 3-block patterns. A discussion on the present approach for realizing spintronics-based artificial intelligence is given.