Exchange interaction via crystal-field excited states and its importance in CsCoCl₃

The low-energy effective Hamiltonian is derived for a typical quasi-one-dimensional Ising-like antiferromagnet CsCoCl₃. In contrast to previous treatments, in which only the processes within the crystal-field ground Kramers doublet are considered, second-order processes via crystal-field excited states are also taken into account in the present paper. They result in a new term, which is essentially a next-nearest neighbor ferromagnetic exchange whose anisotropy is different from the nearest-neighbor exchange. By including the new term the magnetization curve and ESR in CsCoCl₃ are analyzed to show that those experiments can be explained in a way consistent with other experiments. Related theories on CsCoCl₃ are discussed in this connection.