X-ray Magnetic Circular Dichroism Investigation of Fe Valence Ordering in Multiferroic LuFe$_2$O$_4$

A new mechanism of ferroelctricity that is based on the iron valence ordering in a charge frustrated lattice has been reported for LuFe$_2$O$_4$. In this compound, a ferroelectric transition occurs at 330 K and ferrimagnetic order develops below 250 K. The ferroelectric polarization shows a sharp increase at the ferrimagnetic ordering temperature suggesting that the two order parameters are coupled. X-ray magnetic circular dichroism (XMCD) at the Fe K edge and at Lu L$_{2,3}$ edges has been measured in LuFe$_2$O$_4$ using 4-ID-D beamline at Advanced Photon Source. Two clear peaks are seen in the Fe K-edge XMCD suggesting that the magnetism of Fe is associated with two types of Fe sites. Fe K edge XMCD probes the 4p shell, thus it is sensitive to different charge states and gives an indirect measure of the Fe magnetism through 3d-4p hybridization. The observed double peak structure in the XMCD is an indication of charge ordering of Fe$^{2+}$ and Fe$^{3+}$ in the ferrimagnetic state. XMCD is also observed at Lu L$_{2,3} $ edges suggesting a small induced Lu 5d moment. Funded by US Dept. of Energy.