Insights into magnetoelectric coupling mechanism of the room-temperature multiferroic Sr3Co2Fe24O41 from domain observation

Abstract

The mechanism of a magnetoelectric coupling in a room-temperature multiferroic, $\mathrm{S}{\mathrm{r}}_{3}\mathrm{C}{\mathrm{o}}_{2}\mathrm{F}{\mathrm{e}}_{24}{\mathrm{O}}_{41}$, with the Z-type hexaferrite structure, is examined by three approaches: observations of domain structures and their field responses, measurements of magnetic-field effect on electric polarization, i.e., magnetoelectric effect, and phenomenological discussions on the interplay among coexisting order parameters. With use of a resonant soft x-ray microdiffraction technique, we visualized magnetic-field responses of two types of magnetic domains ascribed to ferrimagnetic and spiral components inherent in a transverse conical magnetic structure of the hexaferrite. A simultaneous inversion of these magnetic domains by a magnetic-field reversal was observed, meaning that the process of a magnetization reversal corresponds to a 180\ifmmode^\circ\else\textdegree\fi{} rotation of the cone axis. The reversal process of the magnetic structure, together with experimental results of the magnetoelectric effect, leads us to the conclusion that the magnetoelectricity in the Z-type hexaferrite originates mainly from the spin-dependent metal-ligand orbital hybridization, with minor contribution from the asymmetric spin-exchange interaction. Furthermore, such a mechanism is discussed by the symmetry analysis based on the Landau theory and is well described in terms of couplings among the coexisting order parameters included in the free energy. Thus, observations on field responses of multiple domains in multiferroics provide insights into underlying microscopic magnetoelectric coupling mechanisms.