Field-induced incommensurate-to-commensurate phase transition in the magnetoelectric hexaferrite Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22

Using neutron diffraction, we investigated the spin structures of magnetoelectric hexaferrite, Ba0.5Sr1.5Zn2(Fe1-xAlx)(12)O-22 (x = 0.08) under a magnetic field (H perpendicular to c). When the crystal was cooled in a zero field, longitudinal spin cones were observed at low temperatures. These incommensurate phases, however, were replaced by the commensurate phase [ k(1) = (0,0,3/2)] when the ferroelectricity was induced by an external field perpendicular to the c axis. Magnetic structure refinement confirms that this commensurate order is compatible with the spin-current polarization. We argue that planar magnetic anisotropy plays an important role in determining the magnetic structure that is responsible for its magnetoelectricity.