ON THE LINEAR MAGNETOELECTRIC EFFECT

A complete analysis of the linear magnetoelectric (ME) effect is carried out which exploits the PT symmetry of the ME susceptibility tensor alpha . The existing (or 'old') theory in effect considers only the time-reversal antisymmetry of alpha, with the result that it restricts the effect to magnetic crystals. It is shown that this presents only half the 'picture', and that consideration of the space-inversion antisymmetry of the tensor reveals, in accord with relativistic considerations, that the effect should also occur in certain paramagnetic electric (ferroelectric and antiferroelectric) crystals, which constitute the other half of the picture. A new classification of the eligible ME crystals is proposed. Among these crystals are BaCoF4 and Ti2O3, which, although prohibited by the old theory, have nonetheless been found to exhibit the effect. Other predicted candidates for the effect are presented.