An ab initio cluster study of the magnetic properties of the CoO(0 0 1) surface

Abstract

The magnetic properties of bulk CoO and the CoO(0 0 1) surface were studied by means of quantum chemical ab initio calculations. Small mononuclear CoO clusters embedded in extended point charge fields were used for the calculation of the energies of the ground state and the low-lying electronically excited states of Co 2+ ions in CoO. Similarly, embedded binuclear clusters were employed to calculate the superexchange interaction between next-nearest neighbor Co 2+ ions. The calculations were performed at the complete active space SCF (CASSCF) and valence configuration interaction (VCI) levels; spin–orbit coupling and external magnetic fields were included at the VCI level. In bulk CoO, each Co 2+ ion has a 4 T 1 g ground state which is split by spin–orbit coupling into several components. The lowest excitation energy is found to be 51 meV. The tetragonal distortion at the (0 0 1) surface causes a further splitting and reduces the lowest excitation energy to 29 meV. This means that the lowest excited state will be markedly populated at the Néel temperature of 292 K. The superexchange coupling between next-nearest neighbor Co 2+ ions is antiferromagnetic and is reduced at the surface by about 30% as compared to the bulk. Recommended values for the exchange integral J 2 are −6.5 and −5.0 meV for bulk CoO and CoO(0 0 1), respectively. These values refer to effective spin quantum numbers of S=1/2 at the Co 2+ ions. g-Tensors were also calculated and exhibit a pronounced anisotropy at the surface.