Stability of multiferroic phase and magnetization-polarization coupling in Y-type hexaferrite crystals

Most of the Y-type hexaferrite materials family hosts a variety of magnetic structures as a ground state, and a multiferroic phase, termed the FE3 phase, can be stabilized by applying a magnetic (H) field. This phase has recently been found to persist even after removing the H field. The magnetoelectric properties of Y-type hexaferrites are dominated mainly by the FE3 phase via the spin-driven electric polarization (P). In the present study, the stability of the competing magnetic phases was investigated in Y-type hexaferrite compounds Ba2-ySryCo2Fe12-xAlxO22 (x=0.9) with Sr-doping levels of y=0.8, 1.0, and 1.2. Combining the measurements of magnetization (M), P, and neutron diffraction, we revealed the H-T magnetic phase diagrams. It was found that the stability of the multiferroic FE3 phase is greatly improved in the Sr-rich compound. At room temperature, the FE3 phase in the Ba-rich compound is fragile against the removal of the H field, while it is robust in the Sr-rich compound, even for zero-field cooling. We also investigated the interplay between P and M in the FE3 phase in the presence of both the high electric (E) and H fields, and we found that the coupling between P and M depends on the energy barrier separating the two magnetoelectric states. The energy barrier gradually decreases as the temperature is increased, leading to a reduction of the P-M coupling.