A study of magnetoelectric domain formation in Cr2O3

The newly developed technique of zero-held neutron polarimetry has been used to determine the imbalance in the population of 180 degrees domains in antiferromagnetic Cr2O3 produced by cooling through the Neel temperature under different conditions. The method allows the absolute spin configuration of the preferred domain to be determined. In the structure of Cr2O3 the Cr3+ ions distributed along the trigonal axes in pairs across the centres of symmetry; in the antiferromagnetic phase their moments are aligned parallel and antiparallel to this axis. it is found that although a magnetic field applied parallel to this axis during cooling is able to change the domain populations, these changes are neither reversible nor completely predictable. On the other hand, cooling with combined electric and magnetic fields along the trigonal axis leads to much more consistent results, and virtually single-domain crystals of either type can be obtained. The type of domain produced depends on whether the electric and magnetic fields are parallel or antiparallel. When the sample is cooled in parallel fields, the domain produced is that in which the magnetic moments on the Cr3+ ions of each closely spaced pair point towards one another. The different atomic perturbations which lead to a magnetoelectric effect are discussed and it is shown that the g-factor process can give the observed magnetoelectric annealing effect.