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

Electrical resistivity, magnetic susceptibility, thermal expansion, x-ray diffraction, and scanning calorimetry measurements have been performed over wide temperature ranges on the pseudobinary compounds $\mathrm{Gd}{\mathrm{Cu}}_{1-x}{\mathrm{Ga}}_x$. 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 $\mathrm{Gd}{\mathrm{Cu}}_{1-x}{\mathrm{Ga}}_x$ the CsCl crystal structure is stabilized for $x\ge 0.03$. The CsCl pseudobinary compound $\mathrm{Gd}{\mathrm{Cu}}_{1-x}{\mathrm{Ga}}_x$ exhibits a kink-like anomaly in the magnetic susceptibility for $0.03\le x\le 0.17$. This is ascribed to a complicated type of long-range antiferromagnetic ordering. For $0.17<x<\mathrm{}0.29\mathrm{}$ we suggest a mixed or canted low-temperature magnetic state for the ordered Gd moments. Over the whole $0.03\le x<\mathrm{}0.29\mathrm{}$ concentration region a negative temperature coefficient is observed in the measured electrical resistivity at the ordering temperature. For $x\ge 0.29$ 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 $\mathrm{Gd}{\mathrm{Cu}}_{1-x}{\mathrm{Ga}}_x$ system with $0\le x\le 0.5$.