Driving a uniform magnetization to a metastable, mixed state by Spin Hall Effect Spin Torque

Spin Hall effect based magnetic switching and domain wall motion have recently attracted a lot of attention both from a fundamental and an application perspective [1,2,3]. In that context it is important to understand how spin Hall current acts on a uniform magnetization in the absence of external magnetic field. We observe that in Hall bars made from a thin film stack of Ta (10 nm)/CoFeB (1 nm)/MgO (1 nm)/Ta (10 nm) a current pulse of magnitude 5x10$^6$ A/sq. cm. drives a uniformly polarized magnet to a metastable mixed state of up and down polarized domains. We have experimentally confirmed the mixed state through anomalous Hall effect measurement and magneto-optic Kerr effect imaging. The magnet breaks into domains due to nucleation of domain walls followed by free domain wall motion as a result of depinning of the domain walls from defects by the spin Hall torque.\\[4pt] [1] Liu, L. \textit{et al. Science }\textbf{336,} 555-558 (2012).\\[0pt] [2] Bhowmik, D. \textit{et al.} Nature Nanotechnology (2013), DOI:10.1038/nnano.2013.241.\\[0pt] [3] Emori, S. \textit{et al. }Nature Materials \textbf{12}, 611-616 (2013).