Structure and magnetocrystalline anisotropy of interlayer modified ultrathin epitaxial magnetite films on MgO(001)

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Title: Structure and magnetocrystalline anisotropy of interlayer modified ultrathin epitaxial magnetite films on MgO(001)
Authors: Schemme, Tobias
Thesis advisor: Prof. Dr. Joachim Wollschläger
Thesis referee: Prof. Dr. Heinz-Jürgen Steinhoff
Abstract: In this thesis the influence of different growth conditions on the structural and the magnetic properties of magnetite were analyzed. Therefore, ultrathin Fe3O4 films were grown on MgO(001) substrates, on NiO, and on Fe pre-covered MgO(001) substrates. In the first part of this thesis magnetite films with different film thicknesses were deposited directly on MgO by RMBE to investigate the thickness dependence of the anomalous strain and the in-plane magnetic anisotropy. Surface sensitive methods like XPS and LEED have shown that all films in the investigated thickness range are stoichiometric and epitactic magnetite. Bulk sensitive XRD experiments at the specular rod point to well-ordered films with homogenous film thickness indicated by the distinct Laue oscillations. However, the vertical layer distances are smaller than expected even for strained magnetite. Raman measurements were carried out to clarify this contradiction between surface sensitive and bulk sensitive measurements. While the 20 nm and 30 nm films exhibit the typical bands for magnetite, no distinct bands can be observed for the 7.6 nm film. Due to this results we first assume a partial formation of a thin maghemite layer on top of the uncapped magnetite film under ambient conditions. Nevertheless, additional XPS measurement exclude the complete oxidation of magnetite to maghemite since there is no significantly increased Fe3+-signal visible. Thus, the low vertical layer distance can be attributed to the presence of APBs causing an anomalous strain relaxation as reported in literature. Although all films feature ferromagnetic behavior there are differences in the characteristic of the magnetic in-plane anisotropy. The 7.6 nm film has an in-plane magnetic isotropy while the 20 nm and 30 nm film have an in-plane fourfold magnetic anisotropy. Here, the fourfold magnetic anisotropy is stronger for the 20 nm magnetite film than for the 30 nm film. The critical film thickness for the transition from magnetic isotropy to magnetic fourfold anisotropy may be influenced by film thickness and lattice strain induced by the substrate. The second part of this thesis features the thickness dependence of the structural quality of Fe3O4/NiO bilayers. Each film of the Fe3O4/NiO bilayer on MgO(001) have been successfully grown by RMBE. LEED and XPS experiments have proven that the surface near regions of the distinct films have high structural and stoichiometric properties. Here, too, the detailed ’bulk’ structural characterization of Fe3O4/NiO bilayers were carried out using XRD. It was shown that the Fe3O4 films grow homogeneously and smoothly on NiO films if the NiO film thickness is below 24 nm. Above this NiO film thickness the structural quality of the magnetite films gets distinctly worse. This behavior can be attributed to the fact that the interface roughness between NiO and Fe3O4 depends on the NiO film thickness. The roughness of the 3 nm NiO film is rather small and it is rising obviously with increasing NiO film thickness. Thus, the structural quality of the magnetite films grown on 30 nm NiO films is constantly reduced with increasing magnetite film thickness since the quality of the Fe3O4 films is influenced by the quality of the Fe3O4/NiO interface. In the third study the influence of initial iron buffer layers on the magnetic properties of magnetite grown on MgO(001) substrates has been investigated. In situ XPS and LEED indicate that the structural and stoichiometric properties of the surface near region of the magnetite films are not influenced by the iron buffer layer. However, the structural and magnetic properties of the whole film have changed compared to magnetite grown directly on MgO as shown by XRD and MOKE. The crystalline quality is poor and the magnetic easy axis of the magnetic in-plane anisotropy is rotated by 45◦ compared to magnetite films grown directly on MgO. Both crystalline quality and the rotation of the magnetic in-plane anisotropy show no dependence on the film thickness. However, the strength of the magnetocrystalline anisotropy decreases with increasing film thickness. XPS and XRD measurements have indicated that the iron buffer layer is completely oxidized during the second growth stage of the magnetite. The small Kerr rotation in the MOKE experiments of the samples with film thicknesses up to 28 nm confirms this result since a remaining iron film would cause a higher Kerr rotation. In the last part of this thesis the structural and magnetic properties of a partially oxidized, a completely oxidized and a metallic iron film are analyzed. The partially oxidized iron film is a bilayer with a metallic iron film and an iron oxide film. The surface near stoichiometry of both oxidized iron films correspond to magnetite as proven by XPS. The structural analysis by XRD reveals that though these magnetite films are crystalline, they have an inhomogeneous thickness. The magnetization curves of the partially oxidized film (bilayer) measured by MOKE exhibit a magnetic saturation which is comparable to the magnetic saturation of the single metallic iron film. However, the coercive field is higher compared to the single metallic iron film due to the high interface roughnesses. Considering the coercive field as a function of the sample rotation α the bilayer exhibits a fourfold anisotropy with eight maxima. However, the angular dependence of the magnetic remanence features a simple fourfold anisotropy with easy axes in h110i directions of MgO(001). Vector MOKE analysis displays that the magnetic reversal processes of the bilayer are similar to single metallic iron films. Thus, the bilayer exhibits mostly the magnetic properties of a single iron film. The bilayer has the same magnetic easy axis and a similar magnetic saturation. The magnetic easy axis of the magnetic in-plane anisotropy of the completely oxidized iron film is rotated by 45◦ compared to magnetite films grown directly on MgO as already observed in Ref. [148]. The completely oxidized iron film exhibits also a significantly increased coercive field due to high surface roughness. A magneto-dynamic investigation of the exchange-coupling of the Fe3O4/Fe bilayer system was carried out to yield all relevant magnetic parameters, such as anisotropies, as well as the coupling constant J1. Here, also a complex fourfold anisotropy was observed, which might be due to a coupling of a perpendicular spin-wave mode in the magnetite layer with the acoustical coupling mode. We have also successfully calculated the angular dependence of the resonance field of all films using the eighth order of the magnetocrystalline anisotropy. Although we have applied many measurement methods, we have found no explanation for the complex fourfold angular dependence of the coercive field of the bilayer. Nevertheless, this bilayer provides interesting properties for application in MTJs due to its enhanced magnetic properties like complex fourfold magnetic anisotropy and higher coercive field and remanence. All in all, we have shown that the structural and magnetic properties of magnetite films are strongly influenced by interlayers between film and substrate. While magnetite films directly deposited on MgO exhibit a homogeneous film thickness, both NiO interlayers with a thickness above 24 nm and initially grown iron films deteriorate the structural quality of the on top grown magnetite films. In addition, the magnetic fourfold anisotropy is rotated by 45◦ in comparison to magnetite films grown directly on MgO for the structurally disturbed magnetite on iron pre-covered MgO substrates. Here, further investigations are necessary to understand why this anisotropy rotation occurs and how we can improve the structural quality of magnetite on iron and NiO. The growth of magnetite by oxidizing previously deposited iron films leads to crystalline magnetite films but with inhomogeneous film thicknesses. In case of a Fe3O4/Fe bilayer, the magnetic properties are drastically changed due to the magnetic coupling between the iron and the magnetite film.
Subject Keywords: structure; magnetic anisotropy
Issue Date: 27-Jan-2017
License name: Namensnennung - Weitergabe unter gleichen Bedingungen 3.0 Unported
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Type of publication: Dissertation oder Habilitation [doctoralThesis]
Appears in Collections:FB06 - E-Dissertationen

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