Structural and magnetic properties of hexagonal DyCo4M compounds (M=Al, Ga)

The DyCo4Al and DyCo4Ga compounds crystallize in the hexagonal CaCu5-type structure (space group P6/mmm). Al atoms exclusively occupy the 3g crystallographic site of the structure, whereas most of the Ga atoms occupy the 3g site, the rest being located at the 2c site. Both compounds are ferrimagnetic, and remarkable changes in the magnetic properties are observed with respect to DyCo5. Al or Ga substitution for Co causes a drastic reduction of the Curie temperature, from similar to970 K in DYCo5 to 479(5) K (M = Al) and 475(5) K (M = Ga). This reduction is accompanied by an important increase of the saturation magnetization at 4 K: from 0.7 muB/fu for DyCo5 up to 4.7 and 4.9 mu(B)/fu for DyCo4Ga and DyCo4Al, respectively. The compensation temperatures T-comp are close to 300 K, while T-comp equals 124 K in DyCo5. These phenomena are clearly linked to a decrease of the mean Co magnetic moment. For these two materials, thermomagnetic measurements have evidenced a spin reorientation transition at T-SR similar to 400 K. Powder neutron diffraction investigations were performed, and the Rietveld refinements led to the characterization of the magnetic phase diagrams of both compounds in the whole ordered temperature range. The Co magnetic moments are typically about 1.2 mu(B).