Quantum Fluctuations and the Exchange Bias Field

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 fields. At finite temeperatures localized surface (interface) excitations are populated and change the dipole density. This gives rise to the temperature-dependence of the exchange field. We expect the fluctuation-induced surface dipole density to play a role in the magnetic properties of antiferromagnetic nano-particles, as well.