Currently, the commercialization of SOT-MRAM is hampered by high switching current density due to relatively low switching efficiency. One of the key parameters determines the switching efficiency is the spin hall angle (SHA, θSH). The maximum θSH of commonly used materials in foundries are: Ta (~-0.12), Pt (~0.1), and β-W (~-0.3) 1-3. Even though there are promising SOT channel materials, like topological insulators (TI), showing high θSH > 1, the resistivity is several orders higher than above mentioned normal heavy metals (HM). Meanwhile, TI is hard for manufacturing as it consists of materials not friendly to be used in silicon-based fabs. Then, it is important to pursue high θSH for HM for the SOT-MRAM. The purpose of our study is to re-visit at the θSH of modified tungsten-based SOT channels. We demonstrated relatively high θSH up to -0.44 and low resistivity ~130 μΩcm in the nitrogen-doped W buffer (WN) /W interlayer/ferromagnetic hybrid system measured by spin-torque ferromagnetic resonance (ST-FMR). The θSH of the sample with WN/W SOT channel was nearly twice as large as 5nm β-W and 5nm β-WN (Fig. 1). The magnetic tunnel junctions (MTJ) with three kinds of SOT channels were also fabricated in order to check the device performance. Fig. 2 shows the typical R-J curves of MTJ devices by DC measurements. The switching current density of MTJs with the 3WN/2W SOT channel is 16.6 MA/cm2, which is 36.4% and 26.2% smaller than 26.1 MA/cm2 and 22.5 MA/cm2 respectively in the devices with 3.5W and 3.5WN SOT channels. The Damping like torques ξDL = TintθSH estimated from the switching current density is 0.28 for the system with 3WN/2W SOT channel, which is close to 2 times of that for the sample with 3.5W. The results from MTJ devices are consistent with values obtained from ST-FMR. Our promising results give potential solutions to the low switching efficiency issue for the W-based SOT-MTJ devices.
1 Luqiao Liu, R. A. Buhrman, et al. Spin-torque switching with the giant spin hall effect of tantalum, Science 336 (6081) (2012) 555–558 2 Minh-Hai Nguyen, R. A. Buhrman, et al. Spin torque study of the spin hall conductivity and spin diffusion length in platinum thin films with varying resistivity, Phys. Rev. Lett. 116 (2016) 126601 3 Chi-Feng Pai, R. A. Buhrman, et al. Spin transfer torque devices utilizing the giant spin Hall effect of tungsten, Appl. Phys. Lett. 101 (2012) 122404