以磁電阻量測探討鎳鐵平板線之磁區結構翻轉

Abstract

在本論文中，我們利用量測磁電阻的方式研究鎳鐵平板線其磁區結構翻轉之空間分佈。 我們製作並量測了一系列厚度為30nm，長度20μm，寬度從0.2μm至8.5μm的鎳鐵平板線，利用四點量測技術測量位在外掛電磁鐵的He4低溫系統中的樣品磁電阻，此外加磁場與線表面平行，且與電流方向(沿著長軸方向)夾角為θ。 樣品磁電阻主要是由於s-d軌域間自旋軌道耦合與異向性散射機制造成的，此種磁電阻稱為異向性磁阻，而異向性磁阻決定於磁化向量與電流方向的夾角，因此我們可以用來探討樣品的磁區結構。 線寬小於1.9μm的樣品具有磁矩會快速翻轉的磁區結構，我們稱之為單磁區。對於線寬0.6μm至1.5μm的樣品，同線寬不同區域的磁電阻特性曲線會有明顯差異，雖然樣品的瞬間翻轉場幾乎未改變，但是在線邊緣區域卻出現與其他區域不一致的磁區結構，且隨著樣品寬度增加，不一致的磁區其範圍會持續擴大；而在線寬小於0.5μm的樣品，其內有90﹪以上的磁區則維持一致性。 我們發現磁區變化之空間分佈與樣品的形狀異向性能有關。對於較小線寬的樣品，其形狀異向性能較大，能克服鎳鐵平板線邊緣所產生的靜磁能，使得所有磁矩仍然平行長軸；反之，較大線寬的樣品其形狀異向性能較小，轉而由邊緣的靜磁能主導，使得邊緣磁區產生磁壁造成不一致的磁區分佈，我們推論邊緣處不一致的磁區分佈與樣品本身的形狀異向性能有關。

In this work, we study the spatial distribution of magnetization reversal in patterned permalloy planar wires using magneto-transport properties. A series of sample of 30nm thick, 20μm long, and various widths (0.2μm<w<8.5μm) were fabricated and systematically investigated. The magnetoresistance was carried out by a four-probe technique in a pumped 4He cryostat equipped with an electromagnet. Here, the magnetic field is applied in the plane of the wire and makes an angle θ with the current direction which is along the long axis of the wire. Magnetoresistances of there samples result mainly from the spin-orbital coupling and the anisotropic scattering mechanism of s- and d- electrons, and are referred as anisotropic magnetoresistance (AMR). The effect depends on the orientation of magnetization with respect to the direction of electrical current and hence, can be used to explore the magnetization configuration of the sample. Wire of width less than 2μm is the so called single domain wire with typical abrupt switching in magnetization reversal process. For wires with widths between 1.5μm and 0.6μm, the in-plane MRs of different segments of the wire behave very differently. Although the switching fields change slightly, the magnetization configuration is inhomogeneous from both ends up to somewhere of the wire. The region expands with increasing wire width. However, wire with width less than 0.5μm demonstrates a nearly homogeneous magnetization configuration, over 90﹪of the wire. We find that the evolution of the spatial distribution of magnetization is closely related to the anisotropy energy of the individual wire. For narrower wires, the anisotropy energy is bigger and overcomes the magnetostatic energy due to the edges in the end regions forcing all moments still in parallel with the long axis of the wire. While, for wider wires of less anisotropy energy, the effect of magnetostatic energy due to the edges in the end regions dominates resulting in the formation of domain wall with an inhomogeneous distribution of magnetization. We suggest that the end domain region of inhomogeneous magnetization is determined by the shape induced anisotropic constant.