09:30 〜 09:45
▼ [16a-C41-1] Sputtering condition dependence of spin-orbit torque induced magnetization reversal in W/CoFeB/MgO heterostructure
キーワード：spintronics
Recently, much attention has been focused on Spin-orbit torque (SOT) induced magnetization reversal in heavy-metal/ferromagnetic-metal/oxide heterostructrues, which is expected to be applied for a new switching scheme of magnetic tunnel junction devices. beta-W is a promising material for the heavy-metal layer due to its large spin Hall angle (~0.3). The SOT-switching properties are expected to depend on the structure or resistivity of W, which are strongly related to the deposition conditions. Here we investigate the efficiency of the SOT induced magnetization reversal in nano-scale W/CoFeB/MgO dots with perpendicular magnetic easy axis, which is deposited by magnetron sputtering with various deposition conditions. We note that the measurement on nano-scale devices is crucial for the accurate evaluation of SOT-switching properties.
We firstly investigate the crystalline structure and resistivity r of W by X-ray diffraction technique and standard four-point probe method, respectively. We find that lower sputtering power (PW) and higher gas pressure (pAr) promote the formation of b phase of W with higher r. For example, the W layer deposited at PW = 100 W and pAr = 0.06 Pa possesses r of 111 uWcm, which increases to 202 uWcm for the layer deposited at PW = 30 W and pAr = 0.17 Pa. We then fabricate devices consisting of a 120-nm diameter dots on a W Hall bar from W(5 nm)/CoFeB(1.3 nm)/MgO(2 nm)/Ta(1 nm) deposited under various PW and pAr. Magnetization reversal is induced by pulsed currents of width 10 ns under an in-plane magnetic field (20 mT) applied collinear to the current. As we decrease PW from 100 W to 30 W and increase pAr from 0.06 to 0.17 Pa, threshold current density Jth for magnetization reversal decreases from 2.5 to 1.8´1012 A/m2 and the effective anisotropy field HKeff (evaluated from a transport measurement) increases from 0.24±0.01 to 0.46±0.05 T. The effective spin Hall angle of W increases from 0.15±0.01 to 0.41±0.02, which is much larger than that for the devices with Ta as a heavy-metal layer, indicating that the W is more efficient source of SOT with some degree of the tunability of the efficiency.
We firstly investigate the crystalline structure and resistivity r of W by X-ray diffraction technique and standard four-point probe method, respectively. We find that lower sputtering power (PW) and higher gas pressure (pAr) promote the formation of b phase of W with higher r. For example, the W layer deposited at PW = 100 W and pAr = 0.06 Pa possesses r of 111 uWcm, which increases to 202 uWcm for the layer deposited at PW = 30 W and pAr = 0.17 Pa. We then fabricate devices consisting of a 120-nm diameter dots on a W Hall bar from W(5 nm)/CoFeB(1.3 nm)/MgO(2 nm)/Ta(1 nm) deposited under various PW and pAr. Magnetization reversal is induced by pulsed currents of width 10 ns under an in-plane magnetic field (20 mT) applied collinear to the current. As we decrease PW from 100 W to 30 W and increase pAr from 0.06 to 0.17 Pa, threshold current density Jth for magnetization reversal decreases from 2.5 to 1.8´1012 A/m2 and the effective anisotropy field HKeff (evaluated from a transport measurement) increases from 0.24±0.01 to 0.46±0.05 T. The effective spin Hall angle of W increases from 0.15±0.01 to 0.41±0.02, which is much larger than that for the devices with Ta as a heavy-metal layer, indicating that the W is more efficient source of SOT with some degree of the tunability of the efficiency.