Study on field-free magnetization switching in spin-orbit torque based magnetic memory

Abstract

Efficient magnetization switching in magnetic memory devices is essential for the high speed and reliable non-volatile memory. Recently, it has been demonstrated that the manipulation of magnetization in a magnetic layer sandwiched between heavy metal and oxide layer can be achieved by applying in-plane current. Since the spin orbit coupling play a crucial role for manipulation, these torque related with magnetization switching is referred to as the spin-orbit torque (SOT). SOT has attracted great interest as a potential write method for switching perpendicular magnetic layer in memory device, which allows for a lower write energy and higher writing speed compared with the conventional STT. However, the switching with SOT is not purely electrical in laterally homogeneous structures, thus additional in-plane magnetic field is indeed required to achieve deterministic switching which is an obstacle for device application. In this thesis, I tried to remove the external magnetic field via the approaches of the materials and structural engineering. Firstly, antiferromagnets was introduced instead of heavy metal (HM) that is typically used in study of SOT. Exchange bias field generated between antiferromagnet (AFM) and ferromagnet (FM) was used as an alternative for external magnetic field, resulting in perpendicular magnetization switching without magnetic field. Secondly, interface-generated spin current which could have the z-component spin polarization was proved, and field-free switching was achieved in the sample structure of FM/Ti/FM/oxide where bottom (top) FM layer has the in-plane (perpendicular) magnetic anisotropy. Furthermore, SOT in the FM/HM/FM structure was investigated for the purpose of enhancement of SOT, because, besides SOT originated from spin Hall effect in the HM, there are additional source of SOT including bottom FM.