Hidden Magnetoelectric Phase Transition by Emergent Staggered Magnetic Field

Magnetic fields are among the most fundamental external fields for manipulating the physical property of magnetic materials. Uniform magnetic fields induce phase transitions to obtain a gain from Zeeman energy, resulting in a metamagnetism characterized by transitions from antiferromagnets (AFMs) to ferromagnets. However, staggered magnetic fields, whose directions are antiparallel between neighboring magnetic atoms, remain relatively unexplored. In contrast to uniform magnetic fields, staggered magnetic fields potentially induce a phase transition between two distinct AFM phases, which is expected to accompany a jump in staggered magnetization.

An interesting application of this new type of phase transition is its usage as a knob for controlling composite order parameters in multiferroics. Multiferroics are a group of materials wherein multiple degrees of freedom, such as spin, charge, and lattice, are mutually coupled. They are considered a promising platform for realizing cost-effective next-generation devices. In multiferroics, unconventional spin degrees of freedom are known to be relevant to describe their unusual behavior. Certain types of staggered magnetic moments are considered to be responsible for ferroelectricity. However, the direct application of a staggered magnetic field in an experimental setup is challenging; this hinders the development of their functionality.

In this study, we observed a magnetic field-induced phase transition from a zero-field AFM phase to another AFM phase in the multiferroic material Ba₂FeSi₂O₇. Consequently, the hidden role of the emergent staggered magnetic field was identified. A key ingredient is the Dzyaloshinskii–Moriya (DM) interaction of relativistic spin-orbit coupling origin, which exists in non-centrosymmetric materials, and facilitates effective conversion of the uniform magnetic field into a staggered magnetic field.

We measured the magnetic field dependence of magnetization and electric polarization of a single crystal Ba₂FeSi₂O₇ and observed a phase transition accompanying electric polarization changes. Using a newly established spin model, the experimental observation was reproduced through mean-field numerical simulations. The simulation results indicate the jump in staggered magnetization at the phase transition, thus validating the proposed scenario. In other words, the phase transition is induced by the emergent staggered magnetic field converted from the applied uniform magnetic field through DM interaction. The fragile nature of the zero-field AFM structure against the emergent staggered magnetic field originates from the low dimensionality of this material, which enables the competition between exchange interaction and relatively weak DM interaction. Thus, our study demonstrated the utilization of the field-induced emergent staggered field to manipulate AFM phases and a design principle of materials.

(written by Y. Watanabe on behalf of all the authors.)