Magnetic excitations, cluster modes and disorder phenomena in Rb 2Mn xCr 1−xCl 4

Abstract

In the random quasi-two-dimensional magnetic system Rb 2Mn xCr 1−xCl 4 with competing exchange interactions, the spectrum of magnetic excitations has been studied by means of neutron inelastic scattering, for a sample with ferromagnetic order (x=0.25) and for a sample with spin glass behaviour (x=0.59) at low temperatures. This investigation has revealed in both samples dispersionless Ising cluster modes, coexisting in the same frequency range with q-dependent spin wave branches. In the ferromagnetic sample, the spin wave branch extends above and below the frequency range of the Ising cluster modes. In the spin glass sample, ferromagnetic spin waves as well as antiferromagnetic ones have been found, in addition to the Ising cluster modes, at least for q-values where the wavelength is smaller than the two-dimensional correlationlength. Such an excitation spectrum as that observed in our sample of a quasi- two-dimensional XY spin glass has not yet been reported for any other spin glass.