以磁電阻量測探討次微米磁性圓盤之磁區結構

Abstract

鐵磁性物質的幾何性狀大小會決定其內部的磁區結構，對於次微米磁性圓盤而言，直徑和厚度會決定其磁區結構為單一磁區(single domain)，漩渦磁區(vortex domain)或是混合磁區(multi-domain)，我們實驗主要的目的是利用磁電阻量測的方式來探討漩渦磁區的物理特性。 透過電子束微影技術製作了直徑範圍從0.5um到1.9um，厚度48nm以及38nm的permalloy磁性圓盤，在這個範圍內的磁性圓盤在沒有外加磁場時，內部磁矩排列呈現漩渦磁區結構。我們將圓盤製作在30nm厚的下電極金上，金的寬度等於圓盤直徑，並利用四點量測法量測電阻；磁電阻量測包含兩種安排：電流平行外加磁場(LMR)和電流垂直外加磁場(TMR)。 漩渦磁區隨著外加磁場而產生的變化透過異向性磁阻效應(AMR effect)會充分的反映在我們的磁電阻量測上，並以此我們可以得知漩渦磁區的annihilation field和nucleation field；在我們的量測結果中，annihilation field以及nucleation field會隨著圓盤直徑的增加而減少。 當漩渦磁區的核(vortex core)在外加磁場下移動時，我們將其視為剛體，表示漩渦磁區的移動並不會破壞其磁矩和核(vortex core)的相對位置和方向；利用剛體模型算出的annihilation field，可以在定量上解釋由磁電阻遲滯曲線量測所得到的annihilation field對圓盤直徑的關係，雖然用剛體模型所算出的nucleation field只有磁電阻遲滯曲線得到的四分之一，但是在定性上理論所預期的nucleation field對圓盤直徑的關係和磁電阻量測所得到的是一致的。

The magnetic domain structure of a magnetic disk depends on its geometrical factors such as thickness and diameter and can be single domain, vortex domain or multi-domain. The vortex domain structure is mostly observed in a sub-micron permalloy disk. In this thesis, we investigate the vortex domain by the magneto-resistance measurement. We prepare two series of permalloy disks with diameter ranging from 0.5um to 1.9um by e-beam lithography. The thickness of one series is 38nm and the other is 48nm at remanence. The domain structure of permalloy disk with geometrical like this is vortex domain. In order to make an electric measurement, several identical disks are distributed atop a 30nm thick Au strip with a width same as the disk diameter. Contact configuration is arranged for a four-terminal electrical measurement. The magneto-resistance measurement are made in two configurations. In LMR, current is parallel to external magnetic field. In TMR current is perpendicular to external field. Magnetization can be well described by anisotropic magneto-resistance effect (AMR) resulting in the exploration of magnetization reversal of domain configuration of the permalloy disk. We can obtain annihilation field and nucleation field from the measurement. Our data show both nucleation field and annihilation field increases with decreasing disk diameter. The domain of disk in applied magnetic field is treated as off-center rigid vortex structure; i.e., the vortex keep its spin distribution while being displaced. The rigid vortex model yields the analytical expressions for the size dependent vortex nucleation and annihilation fields. The simulation gives a quantitative agreement with the vortex annihilation field that obtained form MR curves. The vortex nucleation field from the experiment data is about one quarter of the theoretical prediction, however, it has a qualitative dependence on its geometry as expected.