Magnetic-wave-like excitations and cluster modes in randomly disordered Rb2MnxCr1−xCl4 (invited)

Abstract

The randomly disordered magnetic system Rb2MnxCr1−xCl4 with competing exchange interactions and with competing anisotropies has been studied by means of inelastic neutron scattering. We have investigated three samples of different concentrations (x=0.70, 0.25, 0.59) at various temperatures (4.2 K<T<50 K). In the samples with ferromagnetic order (x=0.25) and with antiferromagnetic order (x=0.70), a well-defined spin-wave branch has been found coexisting with a number of well-defined dispersionless excitations in the same frequency range of 0.8 THz≤ν≤2.5 THz. Of particular interest are the excitations in the sample with x=0.59 which shows no long-range order but spin-glass behavior with a freezing temperature of Tf=11 K. For T<Tf, two-dimensional ferro- and antiferromagnetic correlations are observed with the same correlation length ξ=15 Å. The excitation spectrum of this sample gives evidence for a variety of cluster modes and for two types of spin waves, namely antiferromagnetic and ferromagnetic ones, for wavelengths smaller than the correlation length. Such an excitation spectrum as that observed for our sample with x=0.59 has not yet been reported for any other spin glass.