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Site-specific spectroscopic measurement of spin and charge in (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices
Speaker: Janice L. Musfeldt (Departments of Chemistry and Physics, University of Tennessee)
Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO3)m/(LuFe2O4)1 superlattices. A challenge to understanding the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres. Here, we combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m = 3 member. In particular, interface spectra establish how Lu-layer distortion selectively enhances the Fe2+ ? Fe3+ charge-transfer contribution in the spin-up channel, strengthens the exchange interactions and increases the Curie temperature. Comparison of predicted and measured spectra also identifies a non-polar charge ordering arrangement in the LuFe2O4 layer. This site-specific spectroscopic approach opens the door to understanding engineered materials with multiple metal centres and strong entanglement.
Crystal structure, growth pattern, and spin-projected density of states. a, Crystal structure of the (3, 1) film showing how a LuFe2O4 slab (which has an iron oxide double layer with both Fe2+ and Fe3+ between two Lu layers) is sandwiched between three layers of LuFeO3. d represents the total Lu-layer displacement. b, HAADF-STEM images of the m = 3, 7, 9 superlattices viewed along the  zone axis. Atomic number contrast shows the bright, heavy lutetium atomic rows layered with the less bright iron atomic rows. The scale bar is the same for all images. The ferrimagnetic TC and Lu-layer distortion amplitude corresponding to each sample are specified. c, Spin-projected density of states of the Fe double layer in LuFe2O4, an adjacent monolayer of LuFeO3 and the central LuFeO3 monolayer. The Fe3+ and Fe2+ states are indicated with dark and light blue, respectively. The arrows denote different types of excitations.1
Host: Kristjan Haule