Manipulation of Unconventional Spin Polarization in Non-Collinear Exchange-Spring Magnetic Structures.

Manipulating the polarization of spin current is essential for understanding the mechanism of charge-to-spin conversion and achieving efficient electrically driven magnetization switching. Here, a novel exchange-spring magnetic structure is introduced formed by the coupling of perpendicular magnetic anisotropy (PMA) CoTb and in-plane magnetic anisotropy (IMA) Co films. When a spin current with the polarization along the y-direction flows through this exchange-spring (x-z plane) structure, the interaction between the y-spin and the local exchange field with a non-collinear spatial distribution gives rise to substantial unconventional spin polarizations in the x- and z-directions, enabling field-free spin-orbit torque driven perpendicular magnetization switching at room temperature. More importantly, the polarization directions of this unconventional spin current can be reversed depending on whether the interfacial exchange coupling is ferromagnetic or antiferromagnetic. This work establishes a platform to explore emergent mechanisms for manipulating unconventional spin polarization with rich non-collinear magnetic exchange spin structures.