In all STOs studied to date, the active region where the negative ST damping exceeds the positive Gilbert damping of the ferromagnet was restricted to nanoscale dimensions. Spin orbit torques 19, 20, 21 in this system can arise from the spin Hall effect in Pt 22, 23, 24, 25, 26, 27, 28 and the Rashba effect at the Pt/Py interface 29, 30, 31, 32. Recently, a new type of STO based on current-induced spin orbit (SO) torques in a Permalloy(Py)/platinum(Pt) bilayer was demonstrated 16, 17, 18. ST oscillators (STO) have been realized in nanoscale spin valves 3, 5, 6, 7, point contacts to magnetic multilayers 8, 9, 10 and nanoscale magnetic tunnel junctions 11, 12, 13, 14, 15. Owing to its non-conservative nature, this spin torque (ST) can act as effective negative magnetic damping and thereby excite magnetization self-oscillations 3, 4. A current of spin angular momentum incident on a ferromagnet exerts torque on its magnetization and drives it out of equilibrium 1, 2.
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