  Magnetoelectric (ME) properties under rotating magnetic field H are
comparatively investigated in two representative hexaferrites Y-type
Ba0.5Sr1.5Zn2(Fe0.92Al0.08)12O22 and Z-type Ba0.52Sr2.48Co2Fe24O41, both of
which have exhibited a similar transverse conical spin structure and giant ME
coupling near room temperature. When the external H is rotated clockwise by
2pi, in-plane P vector is rotated clockwise by 2pi in the Y-type hexaferrite
and counterclockwise by 4pi in the Z-type hexaferrite. A symmetry-based
analysis reveals that the faster and opposite rotation of P vector in the
Z-type hexaferrite is associated with the existence of a mirror plane
perpendicular to c-axis. Moreover, such a peculiar crystal symmetry also
results in contrasting microscopic origins for the spin-driven
ferroelectricity; only the inverse DM interaction is responsible for the Y-type
hexaferrite while additional p-d hybridization becomes more important in the
Z-type hexaferrite. This work demonstrates the importance of the crystal
symmetry in the determination of ME properties in the hexaferrites and provides
a fundamental framework for understanding and applying the giant ME coupling in
various ferrites with hexagonal crystal structure.
