鐵磁／反鐵磁雙層膜之反鐵磁序於Dzyaloshinskii-Moriya交互作用之影響及其憶阻性質應用於類神經元計算之研究

A spin-orbit torque switch at ferromagnet/antiferromagnet interface for memristivity via tailoring Dzyaloshinskii-Moriya interaction

本研究旨於探討如何於重金屬(Pt)/鐵磁(Co)/反鐵磁(IrMn)三層結構中，藉由控制反鐵磁層IrMn之反鐵序化度達到調控Dzyaloshinskii-Moriya交互作用(Dzyaloshinskii-Moriya interaction，DMI)的目的，此次實驗將採用自旋軌道矩(spin orbit torque，SOT)驅動翻轉法及迴圈偏移法(loop-shift method)量測此反鐵磁序影響DMI於此三層結構中，實驗上可透過增加反鐵磁層IrMn的厚度，增強其反鐵磁序化度並能呈現在交換偏壓(exchange bias，Hex)的變化，此反鐵磁序的增長能減弱系統中DMI強度，並進一步減少進行自旋軌道矩翻轉時所需之外加磁場的依賴程度，我們將此DMI的弱化歸因於反鐵磁序所產生的交換偏壓會引發鐵磁層Co的磁單向性，而抑制DMI所造成的相鄰間磁傾耦合。此外，藉由提升反鐵磁序化度亦可以改善自旋軌道矩翻轉的磁矩穩定度，能透過隨機脈衝讀寫測試的模組中加以驗證，而測試結果發現較強之反鐵磁序於此自旋軌道矩元件中，將展現出憶阻器(memristor)之特性而能應用於類神經網絡計算的研究領域；反之，較弱的反鐵磁序則展現磁矩翻轉之隨機性，其混沌位元(stochastic cell)特色可應用於亂數產生器的開發。本研究將展示如何藉由精準地調控反鐵磁序化度於Pt/Co/IrMn三層膜中，以調控DMI之強弱達成兩極端的自旋軌道矩翻轉切換，使其可分別應用於亂數產生器及具類比式憶阻器記憶體的開發。

This work reports the effect of how an antiferromagnetic (AFM) order modifies the strength of Dzyaloshinskii-Moriya interaction (DMI) in a heavy metal (Pt)/ferromagnet (FM, Co)/ antiferromagnet (AFM, IrMn) trilayer, studied by spin orbit torque (SOT) switching and loop-shift method. Increasing the AFM order reflected on exchange bias (Hex) through increasing the thickness of IrMn appears to significantly reduce the DMI strength of the trilayer, which leads to the reduction of the external field for yielding a complete SOT switching. The reduced DMI may be attributed to the Hex-hosted uni-directional anisotropy suppressing the neighboring spin canting in the FM layer. This stabilization for the magnetic moments via the Hex-hosted uni-directional anisotropy also improved the stability of SOT switching on the random read/write characterization, in which the strong AFM order would promote the memristivity and the weak AFM order would give rise to the stochasticity of the trilayer. This work demonstrates the significance of precise control over the AFM order would simultaneously modify the DMI for the SOT switching toward the small field application and then provide a switch to turn the SOT device into a stochastic/memristive cell.