Magnetism of Ho1-xTbxAl2 alloys: Critical dependence of a first-order transition on Tb concentration

HoAl2 exhibits a first-order spin reorientation transition at 20 K, which is manifested as a sharp peak in the heat capacity. When Ho is partially replaced by only 5% of Tb, the sharp heat-capacity peak in Ho1-xTbxAl2 (x = 0.05) disappears, and then reappears again for x >= 0.07. For x = 0.05, the anomaly corresponding to the spin reorientation transition is barely seen in the heat capacity, but as x exceeds 0.07 the weak anomaly transforms to a sharp peak. The spin reorientation transition temperature increases to 29 K for x = 0.05, and as x increases further the transition shifts to lower temperature and returns to similar to 20 K for x = 0.25. The transition is no longer observed when x exceeds 0.60. Temperature-dependent x-ray powder-diffraction data confirm the first-order nature of the spin reorientation transition for the alloy with x = 0.40, and indicate that the compound retains the room-temperature cubic structure within the sensitivity of the technique. Experimental observations are discussed considering the easy magnetization directions of HoAl2 and TbAl2.