No giant two-ion anisotropy in the heavy-rare-earth metals

A new Bose-operator expansion of tensor operators is applied to the heavy-rare-earth metals. The Er data for the cone phase have been analyzed and found to reflect single-ion anisotropy and isotropic exchange interaction. The Tb data can be understood on the same basis. The previously found large two-ion anisotropy was due to an inadequate treatment of the large single-ion anisotropy leading to an incorrect expression for the spin-wave energy