Multiferroic Ba$_{0.5}$Sr$_{1.5}$Zn$_{2}$(Fe$_{1-X}$Al$_{X}$)$_{12}$O$_{22}$ investigated by $^{57}$Fe nuclear magnetic resonance

Abstract

Our series of works on Y-type hexaferrite Ba$_{0.5}$Sr$_{1.5}$Zn$_{2}$(Fe$_{1-x}$Al$_{x}$)$_{12}$O$_{22}$ revealed that how multiferroic properties are improved by thermal annealing and Al substitution, and the microscopic origin of electrically induced magnetization. It was clarified that the spin-canting angle of Fe$^{3+}$ ions at off-centered octahedra is fixed and Al$^{3+}$ ions substitute for Fe$^{3+}$ ions at those sites by using $^{57}$Fe nuclear magnetic resonance peak assignment.By analyzing the enhancement factor change of $^{57}$Fe nuclear magnetic resonance signals, we found that the magnetic moments of the spin blocks are rotated by a magnetic field in such a way as to increase the net magnetic moment of a magnetic unit cell, even after the field is removed.Al substitution makes multiferroic property arise easily by suppression of the easy-plane anisotropy. The effect of thermal annealing is to stabilize the multiferroic state by reducing the number of pinning sites and the electron spin fluctuation. The transverse conic structure gradually changes to the alternating longitudinal conic structure where spins strongly fluctuate.It was found that an electric field makes $M(H)$ curve shifted like exchange bias. The enhancement factor analysis indicated that the main origin of the huge magnetization induced by an electric field is due to the domain configuration change; an electric field changes the area of magnetic domains like a magnetic field due to the strong clamping between magnetic and ferroelectric orders.