Localised magnetic excitations of coupled impurities in a transverse   Ising ferromagnet
  A Green's function formalism is used to calculate the spectrum of excitations of two neighboring impurities implanted in a semi-infinite ferromagnetic. The equations of motion for the Green's functions are determined in the framework of the Ising model in a transverse field and results are given for the effect of the exchange coupling, position and orientation of the impurities on the spectra of localized spin wave modes. 
R. V. Leite, J. Milton Pereira Jr., and R. N. Costa Filho