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Description
The epsilon phase of Fe$_2$O$_3$ is a ferrimagnetic polymorph that stands out for a remarkable magnetic anisotropy (K$_u$~2·10$^5$ J/m3) and sizeable remnant magnetization at room temperature (~50 kA/m)[1] making it appealing for applications in magnetic recording and electromagnetic applications in the range of millimetre waves [2]. Moreover, ε-Fe$_2$O$_3$ is also interesting from a more fundamental point of view, featuring low-temperature magnetoelectric properties [3] and three distinct zero-field magnetic orders between 10 K and its paramagnetic state above ~850 K [4]. Such a rich phase diagram has much to do with its non-centrosymmetric structure (Pna21), more complex than for other ferric oxides, with four different Fe sites in the asymmetric unit cell: three in an octahedral (O$_h$) and one in a tetrahedral (T$_d$) local symmetry [1].
Despite having attracted increasing attention over the last decades, the intriguing magnetic order in ε-Fe$_2$O$_3$ is still poorly understood. Here we present recent results of temperature-dependent X-ray and neutron diffraction, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) experiments that provide new insights into the origin of magnetic anisotropy in ε-Fe$_2$O$_3$ and -(Fe$_1$$_-$$_x$Cr$_x$)$_2$O$_3$ in the form of nanoparticles (NPs) and thin films. The Fe$^3$$^+$ with Cr$^3$$^+$ substitution is very revealing due to its expected isovalent and magnetic isotropic character. Although trivalent iron could be thought as behaving in a similar way, a nonzero orbital angular momentum m$_L$ value has been proposed for the Fe2 crystallographic site, which might act as the primary source for the magnetic anisotropy responsible of the gigantic coercive field. Thus, depending on the preferential entering sites for Cr in the ε-Fe$_2$O$_3$ structure, a systematic reduction in m$_L$ and a weakening of the spin-orbit coupling along the series may be expected. Our XMCD investigation has addressed this controversy.
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