Electrical control of spin-orbit coupling for novel spintronics applications

Abstract

Spin-orbit torque (SOT) arises from spin-orbit coupling in heavy metal (HM)/ferromagnet (FM)/oxide structures, in which a spin current generated by the spin Hall effect in the HM and the Rashba effect at the HM/FM and/or FM/oxide interfaces. Providing fast and energy-efficient magnetization switching and domain wall motion, SOT has been intensively studied as an alternative technique to manipulate the magnetization for various spintronic devices including magnetic random access memories and spintronic logics and oscillators. However, in conventional HM/FM/oxide structures, deterministic field-free switching of perpendicular magnetization via SOTs is impossible without applying an in-plane external (or effective) magnetic field. This is due to a symmetry constraint of the spin-orbit coupling effect

a charge current flowing in the x-direction is limited to generate a spin current polarized along y-direction, which is orthogonal to inversion symmetry broken z-direction. In this thesis, I report that the Rashba effect in Pt/Co/AlOx structures is laterally modulated by electric voltages, generating out-of-plane SOTs. This enables field-free switching of the perpendicular magnetization and electrical control of the switching polarity. Changing the gate oxide reverses the sign of out-of-plane SOT while maintaining the same sign of voltage-controlled magnetic anisotropy, which confirms the Rashba effect at the Co/oxide interface is a key ingredient of the electric-field modulation. The electrical control of SOT switching polarity in a reversible and non-volatile manner can be utilized for programmable logic operations in spintronic logic-in-memory devices.