Effect of La impurities on the phase transitions in PrFe₄P ₁₂

The effect of La impurities on the low-field (A phase) and high-field (B phase) ordered states in PrFe₄P₁₂ has been studied using dc magnetization measurements on a series of Pr_1-xLa _xFe₄P₁₂ (x = 0, 0.01, 0.05, and 0.15). It is found that the long-range A phase order evolves to a short-range one at a very small La concentration between x = 0.01 and 0.05. We also find clear evidence for the existence of ferromagnetism in the sample with x ≥ 0:05. Remarkably, the ferromagnetic ordering temperature T_C increases from 1.05 K for x = 0.05 to 1.45 K for x = 0.15, although the Weiss temperature decreases with increasing x. These observations indicate the competition between the A phase and ferromagnetic state, and the role of nesting on the A phase transition is emphasized.