Investigation of spin-orbit torque using current-induced magnetization curve

Abstract

Manipulation of magnetization using current-induced torque is crucial for magnetic recording devices. Recently, the spin-orbit torque (SOT) that emerges in a ferromagnetic thin film on a heavy metal is focused as a new scheme for magnetization switching in perpendicularly magnetized systems. Since the SOT provides a perpendicular effective field to the system, the formation of a magnetic multiple domain state because of Joule heating is supressed in the magnetization reversal process. This means that high reliable switching is possible using the SOT. Here, by utilizing the SOT induced domain stability, we show that an electrical current directly injected to a perpendicularly magnetized Pt/Co/Pd system can magnetize itself, that is, current-induced magnetization process from multi to single domain state. A quantitative determination of the SOT is performed using the current-induced magnetization curve. The present results are of great importance as another approach to evaluate the SOT effect, as well as a demonstration of domain state switching caused by the SOT.

Figure 1

Current-induced magnetization process and schematic of device structure. (a) When electrical current is injected into thin ferromagnet/Pt layered structure, the system is gradually magnetized and finally, single magnetic domain structure appears. (b) Device image taken by an optical microscope. A Pt/Co/Pd Hall bar structure with 5 μm width is prepared. Grey regions indicate Cr/Cu electrodes. Current source and voltage meter are connected as shown in the figure. Directions of external magnetic fields (μ ₀ H _x, μ ₀ H _y and μ ₀ H _z) are indicated by white arrows. The sign of electrical current is defined as positive when the current flows from left to right.

Figure 2

Anomalous Hall measurement under various dc currents. (a) Results of anomalous Hall resistance R _Hall measurement by sweeping perpendicular field μ ₀ H _z. Densities of dc current J _dc used here are +2.8 × 10⁹ A/m². Sweep rate of μ ₀ H _z was ~0.04 mT/s. Measurements were performed at stage temperature of 304 K. Red arrows indicate the sweep direction. (b,c) R _Hall as a function of J _dc obtained under μ ₀ H _y (b) and μ ₀ H _x (c) of +38 mT. Shaded area indicates the J _dc region where multiple domain state appears under y-field. The error bar, which is the standard deviation of two data points, is smaller than the symbols.

Figure 3

Current sweep measurement under 343 K. (a) μ ₀ H _z – R _Hall curve obtained at stage temperature T _d of 343 K. J _dc of +2.8 × 10⁹ A/m² is used for the measurement. (b–d) J _dc dependences of R _Hall measured under (b) 0 T, (c) μ ₀ H _y, and (d) μ ₀ H _x of +38 mT, respectively. Yellow (purple) points indicate results for the positive (negative) current sweep. (e) Results of T _d cooling experiment. T _d is reduced from 343 K to 304 K with injecting J _dc of −0.25 × 10¹¹ (red) and −2.8 × 10⁹ A/m² (blue) under μ ₀ H _x = +38 mT. The error bar, which is the standard deviation of two data points, is smaller than the symbols.

Figure 4

Quantitative determination of SOT effective field. (a) Normalized R _Hall as a function of μ ₀ H _z in range from 0 to +4.0 mT at 343 K. (b) Negative J _dc dependence of normalized R _Hall under μ ₀ H _x of +38 mT at 343 K. Zeeman energy gains by μ ₀ H _z and perpendicular component of the SOT effective field μ ₀ H _eff correspond to areas of coloured region indicated in a and b, respectively. The error bar, which is the standard deviation of two data points, is smaller than the symbols. (c) Schematic illustration displaying directions of the longitudinal effective field due to the Slonczewski-like torque μ ₀ H _SL and μ ₀ H _eff with respect to the magnetization direction m. Electrical current flows in +x direction.

Figure 5

Alternative domain state switching. (a) Sequence of pulsed J _dc injection. J _dcs of +0.4 × 10¹¹, +2.8 × 10⁹, and −0.4 × 10¹¹ A/m² are repeatedly injected into the device. The pulse duration of each J _dc is 1.0 s. (b) Monitored R _Hall during alternative J _dc injection. Dashed lines indicate the saturation values of R _Hall in single domain state.