Spin-Orbit-Torque Memory Operation of Synthetic Antiferromagnets

In this Letter, we show the demonstration of a sequential antiferromagnetic memory operation with a spin-orbit-torque write, by the spin Hall effect, and a resistive read in the CoGd synthetic antiferromagnetic bits, in which we reveal the distinct differences in the spin-orbit-torque and field-induced switching mechanisms of the antiferromagnetic moment, or the Néel vector. In addition to the comprehensive spin torque memory operation, our thorough investigations also highlight the high immunity to a field disturbance as well as a memristive behavior of the antiferromagnetic bits.

Figure 1

The spin torque (ST) writing scheme and the samples. (a) The basic scheme of the ST rotation of the antiferromagnet using the spin Hall effect. The writing current $I_w$ invokes the spin Hall effect, and injects the spin currents toward the antiferromagnetic bit. The spin torques rotate the magnetizations in the same rotation direction. (b) Layer stacks of sample #1 consisting of a thermally oxidized Si substrate/ Pt 4 nm/ ${\mathrm{Co}}_{86}{\mathrm{Gd}}_{14}6\mathrm{nm}/{\mathrm{Co}}_{62}{\mathrm{Gd}}_{38}6\mathrm{nm}$/ Pt 4 nm and sample #2 consisting of a thermally oxidized Si substrate/ ${\mathrm{Co}}_{86}{\mathrm{Gd}}_{14}6\mathrm{nm}/{\mathrm{Co}}_{62}{\mathrm{Gd}}_{38}6\mathrm{nm}$/ Pt 4 nm. The blue and red arrows represent the Co and Gd moments, respectively. $m_1$ and $m_2$ are the net magnetization in each layer. (c) Microscope image of the Hall cross structure. Electrodes labeled with 1–4 are made of Ti $5\mathrm{nm}/\mathrm{Au}$ 100 nm. Definition of the field angle ${\theta }_H$ is also indicated. (d) The ST writing scheme. The directions of current flow up on writing “0” and “1” are presented. (e) Vectors of the net writing current $I_w$, polarization of the spin current $P_{s,1}$ and $P_{s,2}$, magnetizations in the CoGd layers $m_1$ (top) and $m_2$ (bottom), and the ST acting on the magnetizations ${\tau }_1$ and ${\tau }_2$ when considering sample #1. (f) The field writing scheme. The directions of the external field up on writing “0” and “1” are presented.

Figure 2

Electrical resistance measurements in a rotating in-plane magnetic field. (a) The Hall resistance $R_{\text{Hall}}$ as a function of ${\theta }_H$ with $H_{\mathrm{ex}}=3\mathrm{kOe}$ at various temperatures. $R_{\text{Hall}}$ curves are offset for visibility. Rotation of magnetizations with respect to the rotating $H_{\mathrm{ex}}$ direction (b) at $T\ne T_c$ and (c) at $T=T_c$. The net magnetizations of $m_1$ and $m_2$ are always either parallel or antiparallel to the rotating $H_{\mathrm{ex}}$ at $T\ne T_c$, while magnetizations $m_1$ and $m_2$ are always perpendicular to the rotating $H_{\mathrm{ex}}$ at $T=T_c$.

Figure 3

Sequential write and read operation for sample #1. (a), (b), and (e) show the results of the ST write-read, the field write-read, and the continuous ST write-read, respectively, at $T=283\mathrm{K}$ ($=T_c$). (c), (d), and (f) show the results of the ST write-read, the field write-read, and the continuous ST write-read, respectively, at $T=312\mathrm{K}$. The arrows on the top represent the write “1” and “0” operations described in Figs. 1 and 1. As indicated in the graph, $\mathrm{\Delta }{R}_{\text{Hall}}$ is defined as $\mathrm{\Delta }{R}_{\text{Hall}}=R_{\text{Hall}}(“1”)-R_{\text{Hall}}(“0”)$ where $R_{\text{Hall}}$ (“1”) and $R_{\text{Hall}}$ (“0”) represent the $R_{\text{Hall}}$ after the write “1” and the write “0”, respectively. For the continuous write-read tests, the blue and the red plot represent the continuous ST write “0” and the continuous write “1” operation, respectively, with $I_w=16\mathrm{mA}$. Note that the first 2 cycles of the blue data plot show a normal $“0”\to “1”$ writing operation. The dotted curves are the fitting with an exponential decay function $\mathrm{y}={\mathrm{y}}_0+A{e}^{-[(x+x_0)/\tau ]}$. $\mathrm{\Delta }{R}_{\text{Hall},\mathrm{sat}}$, indicated in (f), is determined by extrapolating the decay functions.

Figure 4

$I_w$ and $H_{\mathrm{ex}}$ dependence of $\mathrm{\Delta }{R}_{\text{Hall}}$. $\mathrm{\Delta }{R}_{\text{Hall}}$ as a function of the writing current $I_w$ (a) and the writing field $H_{\mathrm{ex}}$ (b) for sample #1 at various temperatures and $\mathrm{\Delta }{R}_{\text{Hall}}$ as a function of the writing current $I_w$ (c) and the writing field $H_{\mathrm{ex}}$ (d) for sample #2 at various temperatures.

Figure 5

Summary of the ST and the field writing. $\mathrm{\Delta }{R}_{\text{Hall},\mathrm{sat}}$ by the ST writing (red) and the $\mathrm{\Delta }{R}_{\text{Hall}}$ by the field writing (blue) as a function of temperature for sample #1 (a) and sample #2 (b). The presented values of $\mathrm{\Delta }{R}_{\text{Hall}}$ by the field writing are taken from the data shown in Fig. 4 at $H_{\mathrm{ex}}=4\mathrm{kOe}$. The temperature range where the sample becomes antiferromagnetic (AF) is indicated in the red area. Outside the temperature range, the sample is ferrimagnetic (F).