Angular dependence of the unusual first-order transition temperature in single-crystal Gd5(Si∼0.5Ge∼0.5)4

Single crystal Gd₅(Si_∼0.5Ge_∼0.5)₄ was studied using thermal expansion measurements in the vicinity of the magnetic-structural phase transition. A strain gauge was attached on the a-b plane oriented to measure the strain along the b axis and a magnetic field of H=0.8×10⁶ A/m (B=1 Tesla) was applied along this same b axis. The sample was then rotated through 45°, 90°, 120°, 165°, and 180° from the direction of the applied magnetic field and the thermal expansion measurements were repeated. It was found that there was an unexpected variation in the temperature at which the sudden discontinuity in strain occurred. The results seem to suggest that the transition temperature changes with direction of applied field relative to the crystal lattice. This surprising result is attributed to the combined effects of the first order phase transformation and the presence of an anisotropy "field," which differs with angle. This combination of factors, therefore, results in a different strength of effective field being experienced by the magnetic moments on the Gd atoms depending on their orientation. Comparing this with the previously observed field dependence of the Curie temperature in this material provides a possible explanation for this highly unusual observation.