Calorimetric study of the magnetic order in Ho(Co1-xRhx)2 alloys

Calorimetric measurements have been performed on a series of cubic Laves-phase compounds Ho(Co1-xRhx)2 for x=0.02, 0.05, 0.08, 0.12, 0.16, and 0.25. The heat-capacity data show first-order-type peaks at the ferromagnetic transition temperature TC for concentrations with x values between 0.02 and 0.08. For larger concentrations, the peaks at TC are much broader and more suggestive of a second-order transition. Utilizing the s-d model in conjunction with the specific-heat data suggests that the change from first to second order in the ferromagnetic transition for x 0.10 is due to a reduction in the magnetic exchange interactions between the Ho 4f moments and the cobalt 3d electrons. In addition, the calorimetric data for x 0.16 show another first-order peak at a lower temperature Tsf that can be associated with a change in the easy-magnetization direction of the Ho moments.