Magnetic And Orbital Correlations In Multiferroic Camn7o12 Probed By X-Ray Resonant Elastic Scattering

The quadruple perovskite CaMn7O12 is a topical multiferroic in which the hierarchy of electronic correlations driving structural distortions, modulated magnetism, and orbital order is not well known and may vary with temperature. X-ray resonant elastic scattering (XRES) provides a momentum-resolved tool to study these phenomena, even in very small single crystals, with valuable information encoded in its polarization and energy dependence. We present an application of this technique to CaMn7O12. By polarization analysis, it is possible to distinguish superstructure reflections associated with magnetic order and orbital order. Given the high momentum resolution, we resolve a previously unknown splitting of an orbital-order superstructure peak, associated with a distinct locked-in phase at low temperatures. A second set of orbital-order superstructure peaks can then be interpreted as a second-harmonic orbital signal. Surprisingly, the intensities of the first- and second-harmonic orbital signal show disparate temperature and polarization dependence. This orbital reordering may be driven by an exchange mechanism that becomes dominant over the Jahn-Teller instability at low temperature.