Anisotropic spin polarization induced by Fermi surface manipulation

Sugimoto, Satoshi*; Araki, Yasufumi   ; Ieda, Junichi   ; Kasai, Shinya*

Current-induced spin-orbit torque (SOT) at heavy metal/ferromagnet (HM/FM) interfaces plays a critical role in modern spintronics. In addition to enhancement of the conversion efficiency, manipulation of the direction of spin polarization has been developed as the major challenge in SOT research until now. This challenge is aimed to improvement of all electric operations of the solid-state non-volatile memories. The unconventional out-of-plane (z) and in-plane (x) components of SOT break the geometrically invariant points in the magnetization switching process in various device configurations, and enable zero-field SOT switching of type-z and type-x configurations, respectively. Here, we propose a brand-new protocol to meet such demands by engineering the Fermi surface symmetry. A topological insulator BiTe shows the hexagonal warping structure due to the discrete rotational symmetry of crystal under spin-momentum locking. Such C3V structure can be further lowered by fabricating an atomic-scale thickness modulation. Thus, a trivial C1 structure can be realized in a simple layered device structure. This asymmetric Fermi surface leads to the distinctive three-dimensional spin polarization without building any complex devices. We demonstrated this type of Fermi surface manipulation protocol for R-3m BiTe (001)/CoFeB sputtered heterostructures via ST-FMR based torque measurements. Our results showed considerable improvements in the unconventional SOT efficiencies characterized by the unique nonreciprocal spectra. Both the out-of-plane and in-plane SOT components reached up to 10 percent in their conversion efficiencies, comparable to the champion values among the various device engineering works so far. Such remarkable improvement of unconventional SOT components enables unique zero-field SOT manipulations subsequently.