Calculations on magnon impurity modes of a pair defect in a face-centred cubic ferromagnet

The energy levels of the magnetic impurity modes are determined for a nearest-neighbour pair defect in a face-centred cubic ferromagnet. The coupling between the host atoms and the impurity atoms is assumed to be ferromagnetic. The calculations are performed by means of a combination of Green functions and group-theoretical methods. According to their transformation properties, eight different types of impurity mode may be distinguished. Only the energy of the modes with B2u symmetry depends on the exchange integral between the two impurity atoms. The numerical computations are restricted to the resonant modes. Particular attention is given to the resonant Ag and B2u modes, which are the only modes with energies in the lower part of the magnon band.