利用臨場加電壓量測Fe/ZnO自旋元件之介面電子結構與自旋傳輸性質

Abstract

本研究目標在利用同步加速器X光吸收光譜(X-ray Absorption Spectrum，XAS)、X光磁圓偏振二向性光譜(X-ray Magnetic Circular Dichroism，XMCD)，搭配電性量測系統，以臨場電壓控制Fe/ZnO異質結構之介面電子結構組態，以達到研究自旋電子傳輸特性。在超高真空環境中(<10-10 torr)，改變施加電壓於Fe/ZnO結構異質介面，控制局部介面鐵原子的電子組態。在施加臨場電壓之XAS光譜，觀察到鐵氧鍵結經歷兩階段變化，第一階段隨著施加電場由0 V至70 V鐵氧鍵結隨電場增加而上升；而第二階段，當施加臨場電場大於70 V，發生相反趨勢的變化，即氧化鐵態至金屬鐵的變化。由XMCD在鐵元素的L3 峰值變化，得知在Fe/ZnO介面處，鐵的自旋電子組態會隨著外加場增加而降低。磁特性的表現上，同時量測由鐵的XMCD磁滯曲線以及異向磁阻(Anisotropic magnetoresistance，AMR)，皆觀察到矯頑磁場(coercive field ，Hc)在平行膜面方向隨著外加場增加而上升。而在實驗中同時利用光譜以及電性磁阻監測試片在同一個點的變化，並結合了光譜的元素選擇性以及對於表面微觀訊號敏感特性，配合宏觀的電性量測，以實現元件作動下的光譜特徵及磁特性之量測。

This work demonstrates the interface oxidation control in relation to spin-transport properties of a Fe/ZnO heterostructure device, with the use of a unique x-ray setup that is capable of performing in-situ x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) with electrical control. The heterostructure with oxidized Fe was initially prepared, yet the Fe chemical state was modified by applying voltage on a Hall-bar based Fe/ZnO device in an ultra-high vacuum condition (<10-10 torr). In-situ XAS shows that the Fe layer underwent an oxidized (0 V~70 V) to metallic transition (70 V~120 V) with increasing applied voltage. A voltage-induced enhancement of coercivity (Hc) was also observed by in-situ XMCD. This suggests the modifications of Fe’s spin-electronic state as a result of voltage-driven reduction of the Fe state specifically occurring at Fe/ZnO interface. Element-specific anisotropy magnetoresistance (AMR) measurement was operated on the Fe layer by fixing photon energy at Fe L3 absorption edge upon magnetic field reversal. The increase of switching field is consistent with Hc enhancement, which indicates a different reversal mechanism of the Fe layer enabled by applied voltage. This work enables a straightforward detection of interface-state in conjunction with spin-transport and magnetic-reversal properties of the versatile ferromagnet-semiconductor system, by taking advantage of x-ray’s element-specificity in combination with electrical control characterizations.