Py/Au/Py和Co/Au/Co非局域自旋閥自旋電子在金中擴散長度和異向性磁阻之研究

Abstract

巨磁阻效應的發現讓自旋電子學展開蓬勃的發展，結構是以磁性/非磁性/磁性堆疊的多層膜結構，依自旋相依散射使得元件電阻大幅的變化。 我研究的主題以這種磁性/非磁性(金)/磁性結構，讓電流通過磁性材料到金中，產生自旋極化電流注入，造成金的上下自旋電子能態匹裂，因上下自旋電子濃度不平衡，而在通道中擴散，可從另一端磁性材料偵測到，利用兩鐵磁磁矩平行和反平行時偵測到電壓差，這個電壓差反映出金上下自旋電子能態匹裂程度。自旋電子在通道中擴散距離L大於自旋電子擴散長度λ_s時，會因自旋鬆弛而翻轉，偵測到的自旋擴散電流會隨擴散距離越長呈e^(-L/λ_s )衰減，分析金通道長度和偵測電壓差的關係，以此可以估算出金的自旋電子擴散長度約50-130nm與文獻上所量測的大致相同。 我們也量測到在鐵磁材料中的異向性磁阻效應，與鐵磁磁矩和電流間的相對方向有關，由電流方向與鐵磁材料極化方向的夾角不同會造成材料電阻的改變，實驗中外加磁場與電流平行至反平行或反平行至平行時，會有一電阻變小的現象來自異向性磁阻效應，實驗量測到的磁性材料異向性磁阻效應的磁阻變化MR%小於1.5%。

Spintronics have been developed rapidly ever since the discovery of the giant magnetoresistance (GMR) effect, which is observed in structures consisting of alternating ferromagnetic and non-magnetic conductive layers. By applying an external magnetic field, the overall resistance changes dramatically. The effect is based on spin-dependent scattering of electrons. In this work, we probe the ferromagnetic/non-magnetic/ferromagnetic sandwich structure. A current is driven through a ferromagnetic film to a non-magnetic film and returned to the current source. The current passing through the ferromagnetic film into the gold, which is non-magnetic, will be spin polarized. This leads to a non-equilibrium population of spins that split the spin energy level of the gold. The spins diffuse a distance in gold, and are then detected by another ferromagnetic film used as a spin detector. In an external magnetic field, the voltage detected, which is sensitive to the parallel or antiparallel alignments of the magnetization of the two ferromagnetic films reflects the extent of the splitting. The detected voltage decays exponentially with the distance between the two ferromagnetic films, since the spins are lost by random relaxation in the gold. According to this property, we calculate the spin diffusion length in gold to be 50-130nm, in agreement with published values. We also find the anisotropic magnetoresistance (AMR) effect in ferromagnetic films. The electrical resistance depends on the angle between the direction of electric current and direction of the magnetization. On switching from parallel to antiparallel applied magnetic field, the change in the magnetoresistance is less than 1.5%.