以磁電阻量測探討鎳鐵矩形環之磁矩翻轉

Abstract

某些微米磁性物質有較強的形狀異向性，其幾何形狀會影響其內部磁區結構。本研究主要是利用電子束微影技術製作厚度為30 nm，寬為0.3 um，外徑邊長分別為5 um，3 um，2 um的鎳鐵正方環以及3 um×5 um和2 um×3 um的鎳鐵矩形環，以四點探針法得到矩形環各區段之磁電阻曲線，並綜合各區段磁電阻曲線而推測矩形環的磁矩翻轉過程。

對次微米尺度的鎳鐵矩形環施加一外加磁場於環平面方向，當磁場沿著環其中兩對邊軸向時，則此平行兩邊因形狀異向性大部份磁矩會一致地排列於其軸向上，僅在翻轉場時做瞬間翻轉。至於環另外垂直於磁場的兩邊，若原本是單一平板線則所有磁矩應同步且漸序的轉動，從自身短軸方向(飽和磁場)翻轉至長軸方向(零磁場)，再到反向短軸方向(反向飽和磁場)，但在矩形環中可能是受到平行磁場的兩邊牽制，導致有數種可能翻轉形式。我們利用矩形環轉角因domain wall的瞬間轉變(transverse ⇌ 90° domain wall)造成其電阻驟升或驟降的現象，揭露五種磁區翻轉模式與殘磁下亞穩態的磁區分佈，以及平行磁場的兩邊可能因其兩端轉角影響，而有較低靜磁能的穩定磁態，導致其轉換場增大。

相較之下，當磁場稍偏離環其中兩對邊軸向時，矩形環各區段磁矩有特定的翻轉模式。於轉換場前後瞬間，矩形環磁區結構從對角線洋蔥結構(diagonal onion configuration)轉換成馬蹄形結構(horseshoe configuration)。

Some patterned magnetic matetial have relatively strong shape anisotropy, their magnetic domain structures depend on their geometry. In this work, e-beam lithography technique was used to fabricate Permalloy square rings with lateral size 5 um, 3 um, 2um, and rectangular rings with short and long outer edges of 3 um, 5 um and 2 um, 3um. All rings have the same width of 0.3 um, and the same thickness of 30nm. We obtained the MR curve of each section of the ring by using the standard four-point probe technique. Based on the AMR effect, we can speculate the magnetization reversal process of rectangular rings.

When magnetic field is applied in the plane and along the major axis of their sides of the rectangular ring, due to shape induced anisotropy, most of magnetic moments of each side will align to the major axis of its own, and to do 180 ° rotation instantly when the switching field reached. As for the other two sides perpendicular to the applied field, magnetization reversal should be in a coherent rotation, from their minor axis (at saturation field) to the major axis (at zero field), and to the counter minor axis (at opposite saturation field) when they are single wires. However, the impact of the two other sides of the ring parallel to the applied field may leads to several types of magnetization rotation form for rectangular rings. We investigate the abrupt resistance increase or decrease due to the domain wall switching at the corners of the rectagular ring to explore five kinds of magnetization reversal and magnetic domain structures at remanence. Moreover, perhaps because the edge region of the sides parallel to the field have lower magnetostatic energy and more stable domain structure, which results in the need for greater switching field to make the side parallel to the applied field to do 180 ° rotation instantly.

On the other hand, when the applied field misaligns slightly with the major axis of the two sides of the rectangular ring, there is a specific rotation form of magnetization of each section of the ring. Upon crossing the switching field, the domain structure of the ring changes from diagonal onion to horseshoe configurations.