Spin waves and local modes in the one-dimensional mixed antiferro-ferromagnet CsMn0.89Fe0.11Br3

CsMn0.89Fe0.11Br3 is a quasi-one dimensional magnetic material with sites occupied randomly by 89% Mn+2 ions and 11% Fe+2 ions. Between any pair of nearest neighbours, there is Heisenberg exchange which is antiferromagnetic except when two iron ions are nearest neighbours in which case the interaction is ferromagnetic. We have measured the magnetic neutron inelastic scattering from this material at 18 K where there is no long range order but short range correlations are high. Intensity is peaked in two excitation branches, one of which resembles the pure crystal's spin wave branch while the other is dispersionless at the mid-band energy of 4.5 ± 0.3 meV. Two suggestions as to the nature of this dispersionless mode are made.