We report on dynamic magnetic properties at low temperatures (4.2<T<30 K) of the disordered Heisenberg frustrated antiferromagnet ${\mathrm{Cd}}_{0.6}$${\mathrm{Mn}}_{0.4}$Te over an extended range of frequencies or of corresponding observation times ${(10\mathrm{}}^{\mathrm{-}6}$<$t_{\mathrm{obs}<3\mathrm{}\times {10}^3}$ s). At high frequencies, the real (${\chi }_{\nu }^{\mathcal{’}}$) and imaginary (${\chi }_{\nu }^{\mathcal{’}\mathcal{’}}$) parts of the susceptibility are determined from accurate Faraday-rotation experiments while the dynamics for longer characteristic times are investigated in a weak field, through superconducting quantum-interference device (SQUID) measurements of the magnetization. Starting from two different dynamic criteria, (i) the significant change of the relaxation of the in-field magnetization to step variations of the temperature and (ii) the appearance of long-time thermoremanent magnetization relaxation, we deduce a value $T_{f0}$=12.9±0.1 K for the freezing temperature.

Fortunately, it also corresponds to the coalescence between the temperature of appearance of irreversibilities and that corresponding to the maximum of the ac susceptibility at very low frequencies. The Vogel-Fulcher law is unable to describe the dynamics over so large a frequency range. Thus, the spin freezing has been analyzed in terms of a critical slowing down above the static freezing temperature $T_{f0}$ using the power law τ/${\tau }_0$=A[(T-$T_{f0}$)/T${]}^{\mathrm{-}z\nu \mathcal{’}}$ with reasonable values of ${\tau }_0$=3.8×${10}^{\mathrm{-}14}$ s and of the dynamic critical exponent zν’=9.7. This exponent is consistent with simulations obtained recently for three-dimensional Ising spin-glasses. We get independently a similar estimation of $T_{f0}$ by other approaches. We also determined the (H,T) magnetic phase diagrams of ${\mathrm{Cd}}_{0.6}$${\mathrm{Mn}}_{0.4}$Te for different values of $t_{\mathrm{obs}}$ up to 100 s. The field dependence of the temperature corresponding to the onset of irreversibilities, even for long observation times, looks like a Gabay-Toulouse line, associated with a transverse spin freezing. This behavior, uncommon for classical spin-glasses, is assumed to be related to the particular frustrated structure in this Heisenberg disordered antiferromagnet.

• Received 19 January 1988

DOI:https://doi.org/10.1103/PhysRevB.37.9022