Ising limit for a pair of transition metal ions with unquenched orbital angular momenta and its relevance to single molecule magnets

We discuss the magnetic anisotropy of exchange-coupled pair of transition metal ions with unquenched orbital angular momenta in axially distorted octahedral surroundings. The case of ground orbital doublets of the metal ions is shown to give rise to a strong magnetic anisotropy and Ising type effective exchange Hamiltonian. Providing ferromagnetic exchange such Ising interaction is shown to create a magnetization reversal barrier