Identification and control of domain wall order in spinel ferrimagnets*

Spinel antiferromagnets have long been at the center of research into strong spin-lattice coupling and orbital effects. Among other properties, these materials frequently demonstrate concomitant magnetic and structural phase transitions, heightened magneto-elastic or dielectric response functions, and low-temperature multiferroism. There is very little agreement on the microscopic picture to be associated with these effects, but recent work has shown that mesoscale inhomogeneity can play a key role in raising the susceptibilities of complex materials to external perturbations.

In this talk, I will discuss recent work at the University of Illinois which establishes the importance of mesoscale heterogeneity in determining bulk magnetic properties of spinel ferrimagnets Mn₃O₄ and MnV₂O₄. This will first include a review of Raman and x-ray scattering results which reveal the existence of low-temperature magnetostructural transitions and magnetic force microscopy data which show the existence of stripe-like magnetic domains, before turning to our more recent neutron scattering and muon spin rotation (μSR) measurements. Our μSR work associates stripe-like domains in Mn₃O₄ with a real space separation into magnetically ordered and disordered volumes, and further shows that the ordering fraction can be grown with the application of moderate-sized fields. With small angle neutron scattering, we observe Bragg signatures of domain wall order in the bulk of both materials, with wall separations of ~200nm. Lastly, I will present data that demonstrates how domain wall motion can be used to drive these systems out of their low-temperature ferrimagnetic states, with an H_c which is highly sensitive to the applied field direction. I will discuss these results in the context of “colossal” control of magnetic properties, and implications for materials beyond the specific case of spinel antiferromagnets.