Relaxor-like improper ferroelectricity induced by Si · Sj-type collinear spin ordering in a M-type hexaferrite PbFe6Ga6O19

M-type hexaferrites with the magnetoplumbite structure are industrially mass produced uniaxial magnets that possess the simplest crystal structure among various hexaferrites. Herein we present an intriguing phenomenon of relaxor-like ferroelectricity observed in a M-type hexaferrite PbFe6Ga6O19 (PFGO) single crystal. The unique feature of the present discovery is that the improper polarization (∼30 nC cm−2 at 5 K) with a direction parallel to hexagonal [11¯0] arises from the collinear Si · Sj-type spin exchange coupling. This collinear coupling, in turn, is triggered by selective local spin reversal of the Fe3+ magnetic moment at the octahedral 2a sublattice. The frequency-dependent dielectric data indicated relaxor-like responses of PFGO with the Vogel–Fulcher freezing temperature of ∼17 K. This freezing temperature for the polar-cluster reorientation coincided with the onset of a rapid decrease in polarization with temperature.