SPIN-WAVE THEORY OF IMPURITY STATES IN A HEISENBERG ANTIFERROMAGNET .2. NEGATIVE IMPURITY-HOST EXCHANGE COUPLING

In a previous paper we discussed the energy modes of a magnetic impurity with positive impurity-host exchange in an antiferromagnet. This was achieved by deriving the appropriate thermal two-time Green functions from which both the impurity states and inelastic neutron cross section were calculated. In this paper we follow essentially the same scheme to discuss the case of a negative impurity-host exchange coupling. The p-, d- and f-type modes remain the same but only one s-like mode occurs as opposed to two in the previous case. We give the energies and widths of this s mode for a wide range of the perturbation parameters. The diffuse inelastic neutron cross section is also calculated. This has exactly the same form as for an 'antiferromagnetic impurity' in a ferromagnet but does not have the same energy behaviour because of the difference in the spin-wave spectra of the ferromagnetic and antiferromagnetic hosts.