Abstract
We demonstrate the preparation of ultrathin Fe-rich nickel ferrite (NFO) islands on a metal substrate. Their nucleation and growth are followed in situ by low-energy electron microscopy (LEEM). A comprehensive characterization is performed combining LEEM for structural characterization and PEEM (PhotoEmission Electron Microscopy) with synchrotron radiation for chemical and magnetic analysis via X-ray Absorption Spectroscopy and X-ray Magnetic Circular Dichroism (XAS-PEEM and XMCD-PEEM, respectively). The growth by oxygen-assisted molecular beam epitaxy takes place in two stages. First, islands with the rocksalt structure nucleate and grow until they completely cover the substrate surface. Later three-dimensional islands of spinel phase grow on top of the wetting layer. Only the spinel islands show ferromagnetic contrast, with the same domains being observed in the Fe and Ni XMCD images. The estimated magnetic moments of Fe and Ni close to the islands surface indicate a possible role of the bi-phase reconstruction. A significant out-of-plane magnetization component was detected by means of XMCD-PEEM vector maps.
Highlights
Magnetic oxides with the spinel structure[1], such as ferrites, are versatile, low-cost materials with a high electromagnetic performance over a wide frequency range[2,3]
We performed a complete spectroscopic analysis employing surface sensitive spectromicroscopy techniques based on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), which allow for a precise quantification of the composition, element-specific cationic valencies, and magnetic moments
Mixed iron-nickel oxides have been grown by oxygen-assisted molecular beam epitaxy (MBE) on Ru(0001) at high temperature
Summary
Magnetic oxides with the spinel structure[1], such as ferrites, are versatile, low-cost materials with a high electromagnetic performance over a wide frequency range[2,3]. Changing the conditions of the preparation method[5] allows to fine tune its properties, even to the point of ordering the Ni cations in octahedral sites[6,7] (the Ni cations have a strong preference towards the octahedral sites, i.e., NFO is an inverse spinel). Several pathways are proposed to achieve this goal, among them the use of special substrates such as MgGa2O420, or the control of growth conditions to provide micrometric spinel crystals on surfaces arising from a single nucleus Using the latter approach we have shown that oxygen-assisted MBE growth at high temperature can lead to spinel oxides such as magnetite or cobalt ferrite films with strongly improved properties[21,22]. We performed a complete spectroscopic analysis employing surface sensitive spectromicroscopy techniques based on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), which allow for a precise quantification of the composition, element-specific cationic valencies, and magnetic moments
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