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Register a new userQuadratic magnetooptic Kerr effect spectroscopy of Fe epitaxial films on MgO(001) substrates
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The magnetooptic Kerr effect (MOKE) is a well known and handy tool to characterize ferro-, ferri- and antiferromagnetic materials. Many of the MOKE techniques employ effects solely linear in magnetization $bm{M}$. Nevertheless, a higher-order term being proportional to $bm{M}^2$ and called quadratic MOKE (QMOKE) can additionally contribute to the experimental data. Here, we present detailed QMOKE spectroscopy measurements in the range of 0.8 -- 5.5,eV based on a modified 8-directional method applied on ferromagnetic bcc Fe thin films grown on MgO substrates. From the measured QMOKE spectra, two further complex spectra of the QMOKE parameters $G_s$ and $2G_{44}$ are yielded. The difference between those two parameters, known as $Delta G$, denotes the strength of the QMOKE anisotropy. Those QMOKE parameters give rise to the QMOKE tensor $bm{G}$, fully describing the perturbation of the permittivity tensor in the second order in $bm{M}$ for cubic crystal structures. We further present experimental measurements of ellipsometry and linear MOKE spectra, wherefrom permittivity in the zeroth and the first order in $bm{M}$ are obtained, respectively. Finally, all those spectra are described by ab-initio calculations.
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Whether {alpha}double prime-Fe16N2 possesses a giant saturation magnetization (Ms) has been a daunting problem among magnetic researchers for almost 40 years, mainly due to the unshakable faith of famous Slater-Pauling (SP) curve and poor consistency on evaluating its Ms. Here we demonstrate that, using epitaxy and mis-fit strain imposed by an underlying substrate, the in-plane lattice constant of Fe16N2 thin films can be fine tuned to create favorable conditions for exceptionally large saturation magnetization. Combined study using polarized neutron reflectometry and X-ray diffraction shows that with increasing strain at the interface the Ms of these film can be changed over a broad range, from ~2.1T (non-high Ms) up to ~3.1T (high Ms). We suggest that the equilibrium in-plane lattice constant of Fe16N2 sits in the vicinity of the spin crossover point, in which a transition between low spin to high spin configuration of Fe sites can be realized with sensitive adjustment of crystal structure.
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We study the underlying chemical, electronic and magnetic properties of a number of magnetite based thin films. The main focus is placed onto NiO/Fe$_3$O$_4$(001) bilayers grown on MgO(001) and Nb-SrTiO$_3$(001) substrates. We compare the results with those obtained on pure Fe$_3$O$_4$(001) thin films. It is found that the magnetite layers are oxidized and Fe$^{3+}$ dominates at the surfaces due to maghemite ($gamma$-Fe$_2$O$_3$) formation, which decreases with increasing magnetite layer thickness. From a layer thickness of around 20 nm on the cationic distribution is close to that of stoichiometric Fe$_3$O$_4$. At the interface between NiO and Fe$_3$O$_4$ we find the Ni to be in a divalent valence state, with unambiguous spectral features in the Ni 2p core level x-ray photoelectron spectra typical for NiO. The formation of a significant NiFe$_2$O$_4$ interlayer can be excluded by means of XMCD. Magneto optical Kerr effect measurements reveal significant higher coercive fields compared to magnetite thin films grown on MgO(001), and a 45$^{circ}$ rotated magnetic easy axis. We discuss the spin magnetic moments of the magnetite layers and find that the moment increases with increasing thin film thickness. At low thickness the NiO/Fe$_3$O$_4$ films grown on Nb-SrTiO$_3$ exhibits a significantly decreased spin magnetic moments. A thickness of 20 nm or above leads to spin magnetic moments close to that of bulk magnetite.
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