(A) study on spin-orbit torques in structures involving ferrimagnets with perpendicular magnetic anisotropy

Abstract

Recently, a manipulation of magnetization by in-plane current utilizing spin-orbit coupling in heavy metal / ferromagnet bilayer structures has been receiving a great deal of attention due to its possible applications in spintronic devices. However, ferromagnetic materials are not only sensitive to the external magnetic field but also they are known to produce stray fields, which are likely to cause crosstalk issues. In addition, since the magnetization dynamics of the ferromagnetic material is known to be slower than that of the antiferromagnetic material, it is important to overcome these problems in order to realize high density, high performance device. In this thesis, I propose a new scheme for solving these problems by using a structure containing a rare earth-transition metal ferrimagnetic alloy in which the magnetic moments are aligned in opposite directions. First, the spin-orbit torque property in structure containing ferrimagnetic alloy was evaluated and it was confirmed that the spin-orbit torque was greatly enhanced in the vicinity of the magnetization compensation point, which is the point at which the magnetic moments of the rare earth and the transition metal are canceled out resulting in zero magnetization, regardless of the bottom heavy metal layer. Moreover, it was also found that the spin-orbit torque itself occurs in the ferrimagnet single layer without bottom heavy metal layer. Finally, in order to control the magnetic properties of the ferrimagnet, electric field was applied on the ferrimagnet and it was confirmed that the magnetic properties were successfully controlled through electric field application by measuring the change in the transport properties with respect to the gating voltages.