自旋軌道矩於鎢/鐵磁與鎢/亞鐵磁異質結構中的效率比較

Abstract

在磁阻式隨機存取記憶體(magnetoresistive random access memory, MRAM)中，透過自旋軌道矩(spin-orbit torque, SOT)來翻轉磁矩方向是一個非常有效率的控制機制。在這種記憶體薄膜結構中，主要由兩大部分組成，包含作為自旋軌道矩來源的薄膜層以及儲存訊息的磁性層。在自旋軌道矩的材料中，原子序較大的過渡金屬因為有較強的自旋霍爾效應而備受青睞。至於磁性層，具有垂直異向性(perpendicular magnetic anisotropy, PMA)的薄膜材料則是我們實驗室的研究重點，主要是其在儲存密度與穩定性上可以提供更好的效果。因此，在我的研究中，主要著重於自旋軌道矩在不同垂直磁性多層膜異質結構中的作用效用之比較。 在我的研究中，Ｗ被選為產生自旋軌道矩之材料，因為它具有較強的自旋霍爾效應(spin-Hall effect, SHE)。這使其作為一個底層結構能夠提供相鄰磁性層高效率的damping-like spin-orbit torque (DL-SOT)。在這項研究中，有兩種磁性曾被拿來做比較，包含鐵磁材料和亞鐵磁材料，而這兩者都具備十分優秀的垂直異向性。實驗中的自旋軌道矩效率(DL-SOT efficiency )是透過磁滯曲線對不同外加電流的偏移量進行分析而得，從結果我們可以得到不管是在W/ 或W/ 的系統中，都可以顯示對Ｗ的微結構有明顯的相關性，而在鐵磁和亞鐵磁兩個系統中，自旋軌道矩的作用效率可達 和 。這些數據提供我們了解到非晶質Ｗ可以在鐵磁和亞鐵磁材料中提供有效的自旋軌道矩之作用。

Spin-orbit torque magnetoresistive random access memory (SOT-MRAM) is a promising memory device. Its basic structure is composed of two major parts, spin-Hall induced layer and magnetic layer. For spin-Hall materials, heavy transition metals with large atomic numbers are the most popular candidates due to their large spin-Hall ratios. As for magnetic layers, materials with perpendicular magnetic anisotropy (PMA) is our priority since this property can provide higher memory density and thermal stability for MRAM devices. Thus my investigation focuses on the comparative study on spin-orbit torque efficiencies from different magnetic heterostructures with PMA. In my investigation, W is chosen for its largest spin-Hall ratio among all heavy transition metals, which makes it a good candidate for generating efficient damping-like spin-orbit torque (DL-SOT) acting upon adjacent ferromagnetic or ferrimagnetic layer. Here I provide a systematic study on the spin transport properties of W/FM magnetic heterostructures with the FM layer being ferromagnetic or ferrimagnetic with PMA. The DL-SOT efficiency , which is characterized by a current-induced hysteresis loop shift method, is found to be correlated to the microstructure of W buffer layer in both W/ and W/ systems. Maximum values of and are achieved when the W layer is amorphous in the W/ and W/ heterostructures, respectively. Our results suggest that the spin-Hall effect from resistive phase of W can be utilized to effectively control both ferromagnetic and ferrimagnetic layers through a DL-SOT mechanism.