Spectrum of Magnetic Excitations of a Ferromagnet with Weakly Bound Antiferromagnetic Interstitial Impurities

Abstract

AbstractThe spectrum of impurity excitations of a Heisenberg cubic ferromagnet with randomly distributed weakly‐bound antiferromagnetic interstitials is investigated. It is shown that if the impurity concentration c is less than some characteristic concentration c0, the density of impurity states has an exponentially narrow peak at the localized‐level frequency ω0. If, however, c ≫ c0, an impurity band appears between the frequencies ω0 and ω1 = ω0, (1 ‐ c(S1/S)). Impurity excitations in this band are characterized with wave vectork with wave vector k(k ⪅ c1/3a−1), their inverse lifetime being small compared with the band width c(S′/S) ω0. Inelastic scattering of neutrons as well as light absorption are studied in the resonant region near ω0 When c > c0, the spectrum of light absorption due to impurities involves two strongly asymmetrical peaks (not one, as in the case of c < c0). The main peak width (that at ω1)is of order (c/co)2 c(S′/S) ω0 and falls with growth of the impurity concentration.