Crystal-structure transformations and magnetic-ordering phenomena inGdCu1−xGax

Abstract

Electrical resistivity, magnetic susceptibility, thermal expansion, x-ray diffraction, and scanning calorimetry measurements have been performed over wide temperature ranges on the pseudobinary compounds GdCu1–xGax. In GdCu, which forms in the CsCl crystal structure at room temperature when prepared from the melt, a crystal-structure transformation into a low-temperature noncubic phase occurs at 250 K. Large hysteresis effects are observed with the transformation back into the CsCl structure occurring around 600 K, probably due to a stress-relieving process. In GdCu1–xGax the CsCl crystal structure is stabilized for x ≥ 0.03. The CsCl pseudobinary compound GdCu1–xGax exhibits a kink-like anomaly in the magnetic susceptibility for 0.03 ≤ x ≤ 0.17. This is ascribed to a complicated type of long-range antiferromagnetic ordering. For 0.17 < x < 0.29 we suggest a mixed or canted low-temperature magnetic state for the ordered Gd moments. Over the whole 0.03 ≤ x < 0.29 concentration region a negative temperature coefficient is observed in the measured electrical resistivity at the ordering temperature. For x ≥ 0.29 ferromagnetic long-range order is found. A simple model with only nearest-neighbor magnetic interactions, depending on the local Cu and Ga surroundings, is proposed to describe the x dependence of the paramagnetic Curie temperature. Finally a structural and magnetic phase diagram is constructed for the GdCu1–xGax system with 0 ≤ x ≤ 0.5.