氧化鎳與鈷鎳合金薄膜在銅001之磁性質

Magnetic Property of NiO on CoNi/Cu(001) Alloy Films

施英達(Ying-Ta Shih)

指導教授 ： 潘瑋

國立中正大學/理學院/物理學系/博士(2014年)

摘要

本篇論文致力於探討反鐵磁性氧化鎳薄膜之形成以及與鐵磁性鈷鎳合金之耦合現象。我們將鈷鎳/銅(001) 之磁性超薄膜於壓力 10-6 mbar 下鋪氧 1800 秒，在X光吸收光譜 (X-ray absorption spectroscopy, XAS) 得知此壓力大量曝氧的方式可以於表面形成氧化物。而且磁性並沒有破真空而造成磁性減少，這也指出表面氧化物具有保護作用。
　　在13層鎳於銅(001)上鋪氧之實驗，隨著外加磁場降低溫 (field cooling) 探討氧化層的物理性質，觀察道在低溫下有coercivity enhancement之現象。另外，在10層鎳上鍍上不同層數的鎳並隨之曝氧，研究這種氧化方式所造成的氧化層之厚度。得知此氧化鎳之層數為3層，並且觀察到在低溫下有coercivity enhancement 以及 exchange bias 兩種現象。從各溫度的磁性了解到氧化鎳的涅爾溫度 (Néel temperature, TN) 為200K 以及Block temperature, Tb 為190K。
　　在同步輻射05B2顯像式光電子顯微實驗站 (Photoemission electron microscopy, PEEM)拍攝氧化鎳/鈷鎳/銅(001) 之表面磁區 (magnetic domain)，使用Image J 對反鐵磁性與鐵磁性磁區統計灰階與磁性方向之關係，從此方法我們可以了解到氧化鎳這種破碎之反鐵磁區，在數量上也是平行於表面的，所以在氧化鎳/鎳在低溫所表現出的磁性現象不同於氧化鎳/鈷。

關鍵字

薄膜 ； 氧化鎳 ； 磁性

並列摘要

Combinations of CoNi alloying and oxygen exposure are used in order to study the spin reorientation transition (SRT) in Ni/Cu(001). These modifications on the volume layers or on the surface layers may result in an increase or decrease of the critical thickness (tc) of the SRT, respectively.
For films with a thickness below tc, a certain amount of oxygen exposure drives the SRT, whereas a large amount of oxygen demolishes the magnetization. For films with a thickness above tc, the out-of-plane magnetization persists under a large amount of oxygen exposure. It was confirmed that the surface contained Ni oxide with an amorphous atomic arrangement. The coercivity is substantially enhanced without shifting of the hysteresis loop after field cooling. This could indicate that the NiO is randomly antiferromagnetic.
Before the oxidization, the magnetization of the films in the thickness of 11 to 20 monolayers (ML) is in the in-plane direction at the temperature ranging from 140 K to 300 K. After the oxidization, the magnetizations of the films are in the in-plane direction at the temperature above 200 K, but transit to magnetization demolishment, in-plane-and-out-of-plane co-existence, spin reorientation transition, and coercivity enhancement, for different ML of films, respectively.
The blocking temperature of this film is also 200 K, which implies the transitions might be driven by the ordering of the antiferromagnetic surface oxides. The various magnetizations provide a model system for manipulating the magnetization direction, as well as a spin valve device by combination of the oxidized films.
The surface NiO, which is generated through the exposure of a Ni/Cu(001) surface to oxygen, is taken as a model system on which to perform the estimation. Since no exchange bias is found in the surface NiO/Ni/Cu(001), we have built a sandwich structure of NiO/n ML Ni/10 ML Co/Cu(001) to measure the n dependence of exchange bias. With 3≦n≦6, a smaller exchange bias field is found with a blocking temperature of 190 K. This implies that the thickness of NiO is, at most, 3 ML. Discovering the thickness and ordering temperature of the surface NiO allows us to propose a mechanism explaining the magnetic transitions occurring in ferromagnetic films covered by surface NiO.
Surface NiO and the underlying Ni (NixCo1-x) were found to be AF and FM by analyzing the gray scale of XMLD-PEEM and XMCD-PEEM images; this indicates the spin orientation with respect to the polarization of the incident X-ray.
The thickness of the surface NiO that performs the surface passivation is estimated to be 2 ML or 3 ML. The tb in the surface NiO/Ni/Co system is 190 K. Therefore, the magnetic transitions that occur near 200 K in such surface NiO/FM films systems could be attributed to the formation of the AF ordering in the surface NiO/Ni.
We found both collinear and perpendicular alignment of the FM and AF spins.
The FM films is driven SRT by the AF layer NiO below the Néel temperature and can applied in the feature.

並列關鍵字

film ； magnetic ； NiO

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