Magnetic and transport properties ofPb1−xMnxTespin-glass

Abstract

Magnetic properties of ${\mathrm{Pb}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ are reported for Mn concentrations $x\lesssim 0.2$. Below 1 K, a spin-glass behavior is observed, characterized by a thermoremanent magnetization which has been studied as a function of time and of the magnetic field. The freezing temperature $T_g$ defined by a cusp in the reversible part of the magnetic susceptibility is found to be associated with the occurrence of a spin-glass state rather than a clustering effect as in the other manganese compound ${\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$. We attribute this result to the existence of long-range interband exchange interactions arising from the fact that ${\mathrm{Pb}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ is a small-gap semiconductor. For all Mn concentrations, antiferromagnetic interactions are shown to dominate, in opposition with previous theoretical predictions. Above $T_g$, the material is in a superparamagnetic state due to short-range antiferromagnetic superexchange interactions. It is shown that a mean-field approximation accounts very well for the experimental results, above a temperature $T_M\sim {2T}_g$ up to a magnetic field $H\sim 10$ kG, but breaks down at $T<\mathrm{}{T}_M$. This effect is related to the pathological behavior of the functional dependence of the magnetization on the magnetic field above the de Almeida—Thouless instability temperature. All the samples were $p$ type. The transport experiments show that the valence band is significantly perturbed upon introduction of Mn in the matrix. In particular, a crossing between valleys of the valence band with extrema at the $L$ points of the Brillouin zone and the valleys with extrema along the $\Sigma$ (symmetry) directions, is inferred at $x\sim 0.114$ at low temperatures. At high magnetic field, the magnetoresistance associated with the diffusion of holes by the spin fluctuations is shown to be positive, although a negative magnetoresistance can be observed in the low-field limit on some samples.