Iron-based multiferroics such as BiFeO3 and LuFe2O4 exhibit the highest magnetic and ferroelectric ordering temperatures among known multiferroics. LuFe2O4 is a frustrated system with several phase transitions that result in electronically driven multiferroicity. To understand how this peculiar multiferroic mechanism correlates with magnetism, we studied electronic excitations by optical spectroscopy and other complementary techniques. We show that the charge order, which determines the dielectric properties, is due to the ”order by fluctuation” mechanism, evidenced by the onset of charge fluctuation well below the charge ordering transition. We also find a low temperature monoclinic distortion driven by both temperature and magnetic field, indicating strong coupling between structure, magnetism and charge order. BiFeO3 is the only known single phase multiferroics with room temperature magnetism and ferroelectricity. To investigate the spin-charge coupling, we measured the optical properties of BiFeO3. We find that the absorption onset occurs due to on-site Fe3+ excitations at 1.41 and 1.90 eV. Temperature and magnetic-field-induced spectral changes reveal complex interactions between on-site crystal-field and magnetic excitations in the form of magnon sidebands. The sensitivity of the magnon sidebands allows us to map out the magnetic-field temperature phase diagram which demonstrates optical evidence for spin spiral quenching above 20 T and suggests a spin domain reorientation near 10 T.


Xiaoshan Xu is now a staff research scientist at Oak Ridge National Laboratory. He graduated from Nanjing University with a master’s degree in 2000，the year when he entered Georgia Institute of Technology to study the magnetism of free metal clusters. In 2007, he got his Ph.D. and began to hold the postdoctoral position in the University of Tennessee. Xu is the author of the book “The magnetism of free cobalt clusters”, and he has published nine (1 Science, 6 PRL & 2 Nano Letters) papers in top academic journals. In 2010, Dr. Xu received the award of Eugene P. Wigner Fellowship in Oak Ridge National Lab. His research interests include multiferroics, organic photovoltacics and metallic clusters.