微結構鐵磁系統的磁矩翻轉和磁電傳輸性質

Abstract

此篇論文探討在鐵磁微結構中的磁矩翻轉和磁電傳輸性質，樣品的尺度和形狀在這些微結構中扮演著重要的角色。 在研究微磁學時，我們以平行膜面磁阻的量測和施加磁場下觀察磁區的磁力顯微來從事一系列鎳鐵合金毫微米線的磁矩翻轉研究，這一系列毫微米線的線長和厚度分別固定為20微米和30奈米，線寬則由10微米變化至0.1微米，線寬的改變將造成磁電傳輸行為和磁區結構有截然不同的結果。當線寬大於2微米是屬於多磁區結構，磁區翻轉是藉由已確知的磁區擴張來完成，而對應的磁相關行為也是可預期的，此篇論文將針對線寬小於2微米屬於所謂單一磁區的毫微米線研究其微磁結構，根據磁翻轉行為，單一磁區的毫微米線又可被區分為三種類型，當線寬小於0.5微米，在未達到磁瞬間翻轉場前，磁矩幾乎都平躺於長軸，只有在超過磁翻轉場後，磁矩才瞬間翻轉至反向，其餘的兩種類型，在零磁場時，線的兩端都有漩渦封閉磁區結構，對於線寬介於1.2和2微米，位於線兩端的磁區結構會隨著反向外加磁場的增加而擴及至樣品整體，在瞬間翻轉前會觀察到類似於180 度cross-tie的磁區壁結構，當線寬介於0.5和1.2微米，磁區擴張則不會擴及至樣品整體，導致在樣品不同部位觀察的磁電阻行為有明顯的差異，此外，當線寬小於1.2微米，瞬間翻轉磁場與磁場相對毫微米線角度的相互關係可以被建構在curling 翻轉方式的Aharoni模型描述，當線寬小於0.3微米，由於幾何形狀已經不完全符合curling翻轉方式的考量，使得大角度的數據和模型有明顯的偏離，我們發現當線寬小於1.2微米時，磁矩翻轉都是透過局部的翻轉搭配磁區壁的移動來完成。 以我們對毫微米線的了解也進一步的研究Néel磁區壁電阻，我們設計了一系列類似蜈蚣的多腳樣品，包含了許多相互垂直的鎳鐵毫微米線，這些線都因幾何形狀的選擇而有單一磁區結構和高異向性。如此一來，位於兩線交錯區的零磁場下磁矩方向將決定於這兩線的異向性能比值，因此創造不同角度的Néel磁區壁，利用簡單的電阻串聯模型配合異向性磁阻來分析磁場平行電流和磁場垂直電流的磁阻結果，磁區壁電阻即可被估算出來。我們發現Néel磁區壁電阻大約是毫歐姆且隨著磁區壁兩邊磁區的相對角度變小而變小。

We present the magnetization reversal and magneto-transport in patterned ferromagnetic systems where the dimensions and lateral shape of samples play significant roles. In the investigation of micromagnetism, in-plane magnetoresistance (MR) and in-field magnetic force microscopy (MFM) on a series of permalloy (Py) planar wires were performed. Here, the wire length and thickness are kept constant, 20μm and 30nm, respectively. The width of the wire spans from 10 to 0.1μm. The magneto-transport behavior and magnetic configuration are quite different regarding their widths, corresponding to the aspect ratio. For wire of large width (w>2μm), remanent state is a multi-domain configuration, the magnetization reversal is via the well-known domain expansion and all behaviors are under expectation. We focus on the narrow wire with w<2μm to explore the micromagnetic configuration of the so called “single domain” wire in which the abrupt switch of magnetization reversal occurs. According to their behaviors, we catalog them into three regions. For wire with width less than 0.5μm, a typical single domain state with moment along the long axis is observed. The moment barely rotates and switches suddenly to the opposite direction when magnetic field reaches the switching field. The other two kinds of wire have clear domains in both ends at remanence. When wire width is between 1.2 and 2μm, both end domain regions expand to the whole volume of the wire with increasing magnetic field in the opposite direction. A 180o cross-tie like wall forms right before switching. When wire width is between 0.5 and 1.2μm, the expansion of end domains does not extend to the whole volume before switching resulting in a spatial dependent in-plane MR behavior. Moreover, angular dependence of the switching field of wires less than 1.2μm can be described using Aharoni model under the consideration of curling for the ellipsoid. When a wire width less then 0.3μm, the condition for curling mode is no longer fulfilled, there are deviation at large angles. We find that the magnetization reversal for all narrower wires (w<1.2μm) originates from the same mechanism of local nucleation and the propagation of the domain wall. In the study of the angular dependence of Néel wall resistance, we create the centipede-like Py structure which consists of a central wire with numerous orthogonally bisecting finger wires. All Py wires were designed to have a single domain structure at remanence and high anisotropy by the geometric control. The remanent domain at the junction between the central and finger wires is determined by the anisotropy constants of both wires and hence, variable angles of Néel wall can be achieved. Developing a simple resistance-in-series model in corporation with the anisotropic MR effect, the analyses of the longitudinal and transverse MRs of the centipede-like structure give the domain wall resistance. Our results show that the Néel wall resistance is about milliohm and decreases with decreasing the relative angle between two domains.