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The effect of high-pressure oxygen (HPO) annealing on the electrical, magnetic, and magnetoelectric (ME) properties of room-temperature multiferroic BaSrCo2Fe11AlO22 ceramics was investigated. The electrical resistivity of the ceramics was found to strongly depend on the partial oxygen pressure during annealing at 1040°C. Samples annealed under ~10 atm of oxygen exhibited a resistivity of up to 1.6 × 109 Ω·cm at room temperature, more than two orders of magnitude higher than that of samples without oxygen annealing. Thermally stimulated current and complex impedance measurements suggested that the enhancement of the resistivity by the HPO annealing originated from a decrease in the amount of defects related to oxygen vacancies and an increase in the resistance of grains and interfaces. HPO annealing also affects the magnetic-field response of spiral magnetic ordering, which is ascribed to the ME properties. Furthermore, samples subjected to HPO annealing exhibited a lower contribution of the space charges trapped at the grain boundaries and/or defects to the magnitude of the measured magnetoelectrically induced electric polarization P. The present results indicate that HPO annealing is an effective method to evaluate authentic ME effects in multiferroic BaSrCo2Fe11AlO22 ceramics.

The magnetic structure and the effect of a magnetic field on its domain structure were investigated in a magnetoelectric Y-type hexaferrite, Ba1.3Sr0.7CoZnFe11AlO22, by means of mapping with a micro-focused and circularly polarized X-ray beam in the resonant X-ray diffraction. It was revealed that this hexaferrite exhibits a magnetic order characterized by two distinct antiferromagnetic components: incommensurate helical and commensurate collinear ones, which can be explained as the development of the so-called alternating longitudinal conical structure. A multi-domain state due to the handedness of the helical component, i.e., spin-chirality, is transformed into nearly a mono-domain one by using only a magnetic field. Furthermore, the sign of the spin-chirality in the mono-domain state is reversed by reversing the sign of a magnetic field. These results demonstrate that the spin-chirality in this hexaferrite can be manipulated by a magnetic field alone at room temperature.

Magnetoelectric (ME) properties were investigated for BaSrCo2−xZnxFe11AlO22 ceramics with the Y-type hexaferrite structure. This system exhibits a magnetic ordering above room temperature (RT) and high electrical resistivity exceeding 500 MΩ·cm by annealing under 10 atm O2. It is found that the samples with x ≤ 0.4 exhibit the robust ME effect with a reversal of electrical polarization induced by a low magnetic field at RT. Moreover, the electric-field-induced magnetization switching is also observed at RT. These results clearly demonstrate the mutual control of magnetization and electrical polarization at RT in the Y-type hexaferrite system.