(A) study on spin transport mechanism at non-magnetic/ferromaganetic interfaces by spin-orbit torque and spin hall magnetoresistance

Abstract

Magnetic random access memory (MRAM) has been expected to replace conventional memory device, such as DRAM and flash memory, in terms of ultra high density and/or ultra high speed. MRAM devices generally utilize magnetic tunnel junctions (MTJs) as a cell, which consist of insulating layer sandwiched by two ferromagnetic layer. The information of the cell is read by measuring tunneling magnetoresistance (TMR). For recording, the direction of magnetization in a FM electrode can be deliberately manipulated

so current-induced Oersted field or spin-transfer torque has been used, but they have difficulty in realizing currently required device. Recently, it has been demonstrated that current flows within underlayer could reverse the magnetization direction of FM in Ta/ CoFeB/ MgO or Pt/ Co/ $AlO_x$ heterostructure with perpendicular magnetic anisotropy (PMA), which is referred to as spin-orbit torques (SOTs). However, the origin of SOTs remains controversial between spin-Hall effect (SHE) and Rashba effect. SHE in the nonmagnetic conductor with large spin-orbit coupling, where electrons with distinct spin orientation are transported in the opposite direction to accumulate spin-polarized current at each side. This spin current, however, can be generated via Rashba effect in the heterostructure

of which structural inversion asymmetry generates electric field, and current feels the electric field as an effective magnetic field. Meanwhile, a new phenomena named spin-Hall magnetoresistance (SMR) was discovered in NM/ FM insulator bilayer. As reciprocal phenomenon of SOTs, the resistance of NM conductor changes due to the interaction between spin current generated by SHE and magnetization. In this study, we investigated SOTs and SMR in W/ CoFeB/ MgO heterostructure with PMA. The reason why we employed W as underlayer is that it has been known to be of the highest spin Hall angle so far. The materials were deposited by sputtering method, and cross structure called Hall bar was fabricated by photo-lithography. First, for the calculation of SOTs, magnetization behavior was analyzed by measuring the first and second harmonic of Hall voltage under magnetic field with AC current. Furthermore, direct current-induced magnetization switching was also conducted for the same samples. Subsequently, both longitudinal and transverse resistances were simultaneously measured under magnetic field with DC current source. As a results, we could observe lower switching current density and ten times larger SMR compared to the structure with Ta or Pt underlayer. Therefore, this study can be regarded as significant from both industrial and scientific point of view.