Magnetoelectric interactions and induced toroidal ordering in Cr 2O 3

Abstract

It is shown that magnetoelectric interactions play a significant role in the onset of antiferromagnetic ordering in crystals, the form of the linear magnetoelectric tensor fully representing the magnetic point group of the crystal under consideration. When dealing with the off-diagonal components of the magnetoelectric tensor even in the case of uniform antiferromagnetic ordering we propose to treat the situation in the framework of the appearance of toroidal moment — polar vector, changing sign under space inversion and time reversal. By group theoretical analysis it is shown that in uniaxial easy-plane antiferromagnets (due to the irreducible corepresentation of the order parameter) there may occur either toroidal or weak ferromagnetic type of ordering. Following theoretical prediction, the linear transverse magnetoelectric effect, induced by the external magnetic field exceeding the spin-flop value, was investigated in Cr 2O 3 single crystals. The electric polarization components P a and P c versus pulsed magnetic field H, oriented at an angle θ=5 o with respect to c-axis in the ac plane of the crystal in the temperature range 5–300 K were measured. The study of the linear magnetoelectric effect in Cr 2O 3 crystals in the spin-flop phase made it possible to reveal the asymmetry of the magnetoelectric tensor components α 13 and α 31 and thereby to prove the existence of a toroidal moment induced by an external magnetic field.