Anomalous spin dynamics in the isotropic ferromagnet CdCr2Se4

We studied experimentally the low-frequency uniform longitudinal susceptibi lity of a nearly isotropic cubic ferromagnet CdCr2Se4 (T-c approximate to 1 28 K) in weak magnetic fields. The measurements have been performed mostly at T = 82 K, i.e. outside the critical region near T-c. At frequencies high er than the two-magnon threshold the longitudinal susceptibility behaves as chi proportional to (i/omega)(rho), with the exponent rho (T = 82 K) appro ximate to 0.28. This exponent is found to be slowly increasing with tempera ture. Both the low-frequency crossover scale to the above dependence and th e characteristic frequency scale are determined by the internal magnetic fi eld Hi. We propose the description of chi(omega, H-i) in terms of a phenome nological scaling and attribute the observed anomalous behavior to the infl uence of dipolar forces. (C) 2000 Elsevier Science B.V. All rights reserved .