Vacancies and Interstitials in Antiferromagnets

In this paper static properties of two kinds of point defects in an antiferromagnet are examined. The topology of the two defects, vacancies and interstitials, is different, the first belonging to a well-defined sublattice, and the other acting symmetrically on both sublattices. The perpendicular susceptibility of the vacancy is found to be positive due to a polarization of the medium, overcompensating the lack of magnetization due to the removal of one atomic moment. The additional susceptibility due to the interaction of pairs of vacancies is investigated; its sign depends on whether the two are on the same or different sublattices. It is shown that an interstitial embedded in an antiferromagnetic crystal has a moment that is linked to a plane of easy magnetization, perpendicular to the axis of the crystal, by a pseudoanisotropy field of the order of magnitude of an exchange field. The coupling of such interstitials is shown to decrease as r−7 but could be strong enough to induce cooperative effects.