Magnetoelectric interactions and induced toroidal ordering in Cr2O3

It is shown that magnetoelectric interactions play a significant role in th e onset of antiferromagnetic ordering in crystals, the form of the linear m agnetoelectric tensor fully representing the magnetic point group of the cr ystal under consideration. When dealing with the off-diagonal components of the magnetoelectric tensor even in the case of uniform antiferromagnetic o rdering we propose to treat the Situation in the framework of the appearanc e of toroidal moment - polar vector, changing sign under space inversion an d time reversal. By group theoretical analysis it is shown that in uniaxial easy-plane antiferromagnets (due to the irreducible co representation of t he order parameter) there may occur either toroidal or weak ferromagnetic t ype of ordering. Following theoretical prediction, the linear transverse ma gnetoelectric k-effect, induced by the external magnetic field exceeding th e spin-flop value, was investigated in Cr2O3 single crystals. The electric polarization components P-a and P-e versus pulsed magnetic field H, oriente d at an angle theta = 5 degrees with respect to c-axis in the ac plane of t he crystal in the temperature range 5-300K were measured. The study of the linear magnetoelectric effect in Cr2O3 crystals in the spin-flop phase made it possible to reveal the asymmetry of the magnetoelectric tensor componen ts alpha (13) and alpha (31) and thereby to prove the existence of a toroid al moment induced by an external magnetic field. (C) 2001 Elsevier Science BN. All rights reserved.