Abstract

Ground-state fluctuations reduce the zero-temperature magnetic moments of the spins in a quantum antiferromagnet. In the neighborhood of surfaces, interfaces, and other defects which break translational symmetry, these fluctuations are not uniform. Because of this, the magnetic moments of up and down spins do not exactly compensate each other-as they do in a bulk antiferromagnet. At a surface or interface this leads to a small magnetic dipole density. The corresponding dipole field can account for the magnitude of observed exchange anisotropies. At finite temperatures localized surface (interface) excitations are populated and change the dipole density, making the exchange field temperature dependent. In a pure antiferromagnet fluctuations may induce a net surface magnetization. This should be observable in clean surfaces, by means of surface-sensitive magnetic probes such as the magneto-optic Kerr effect. © 2006 The American Physical Society.