Current Induced Spin Orbit Torque Switching of Perpendicularly Magnetized Ta/MgO/CoFeB and MgO/CoFeB/Ta Structures

• Main Authors: Liou-Chuan Yu, 游劉傳
• Other Authors: Gung Chern
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/95499854569280234147

碩士 === 國立中正大學 === 物理學系暨研究所 === 103 === When a current passes through thin films of heavy metals such as Ta or Pt a spin current is generated along the transverse direction due to spin Hall effect. This spin current may exert a torque on the adjacent ferromagnetic layer and causes a magnetic switching of the ferromagnetic layer. This mechanism introduces an alternative way to design a new process of magnetic devices, such as magnetic random access memory (MRAM), with a current controlling mode. In this thesis, we fabricated a series of MgO/CoFeB/Ta, which is perpendicularly magnetized and is used as a core structure in the MRAM devices. We did Hall resistance measurements on these films. Samples are divided in two groups: Top structure: Sub/Ta(5)/MgO(1)/CoFeB(1.4)/Ta(x), x= 2 ,3 ,5 nm. This series changes the thickness of the cap layer (Ta) and the effect on spin Hall resistance is studied. Bottom structure: Sub/Ta(5)/CoFeB(y)/MgO(1)/Ta(5), y = 0.9 , 1.1 , 1.3 nm. This series changes the thickness the ferromagnetic layer (CoFeB) and the on spin Hall resistance is studied. All samples were prepared by nanoetching in which these structures were etched into a size 10 μm × 60μm and 15 μm × 150μm cell, respectively. Current is then applied along the longitudinal (x) direction when applying a magnetic field either along the and the z-direction (perpendicular) or x direction (in plane). The experiments are carried under two different conditions: (1). Current sweep with fixed external magnetic field: The current range varies between between ±15 mA and the change of the applied magnetic field (± 2000 Oe). (2). Field sweep with fixed current density: The magnetic field varies in the range of ± 400 Oe) and the change of current density (± 20 mA/cm2). The main results include: Top structures: First, the opposite effect with respect to the polarity pair of (J, Hx) is confirmed. Since the direction of the effective field of spin Hall effect (Hshe) is defined by (m ⃑ x( j) ⃑ x z ⃑) the interface of top structure (+z) and bottom (-z) introduces an opposite effect on Hshe. The confirmation of this polarity dependence provides extra evidence of the interface effect. Second, the magnetic switching of the thicker Ta sample shows distorted loop in that the magnetic states tend to switch between Mz (-Mz) and in-plane direction at field Hx> 200 Oe (< - 200 Oe) due to probably weaker perpendicular magnetic anisotropy. However, this distortion is recovered by reducing the Ta thickness. Third, these distorted loops also shift with respect to the J=0 and the switching to in-plane direction occurs at lower current density relative to the current switching to the perpendicular direction. Bottom structures: (1) the magnetic states deviate from the perpendicular direction toward to the in-plane direction and (2) the critical current also reduces as the magnetic state approaches to in-plane direction and results in asymmetrical loop with respect to the J= 0. Keyword: Current sweep with fixed external magnetic field、Field sweep with fixed current density、Spin Hall effect