Measurements of electron paramagnetic resonance (EPR) and of susceptibility with a superconducting quantum interference device (SQUID) have been performed on Pb0.995Gd0.005Te crystals from 4.2 to 300 K. The EPR spectra consist of two components, one showing the fine structure of Gd3+ ions in a cubic environment and the other a broad line, which we attribute to clusters of interacting Gd ions. The resonant field of the cluster line is almost isotropic under sample rotation in the [Formula: see text] crystallographic plane, while the line width varies by a factor of 2.5 between [100] and 30° from it. This result suggests that an exchange-narrowing mechanism is responsible for the cluster line. The ratio of cluster-line intensity to fine-structure intensity increases with decreasing temperature down to 10 K and then decreases. This decrease may be related to the spin freeze-out seen in ac susceptibility. The line-width broadening of the cluster and the single-ion lines are similar below 20 K, with a higher rate for the cluster line. The Curie–Weiss temperature is negative and small for both clustered and single spins, reflecting a weak antiferromagnetic interaction.
