Concentration Dependence of Pressure Effect in La(FexSi1-x)13 Compounds

Effects of pressure P on the magnetic moment M and the Curie temperature TC have been investigated for La(FexSi1-x)13 above and below the magnetic phase boundary concentration x=0.86, where the ferromagnetic-paramagnetic transition at TC changes from the first-order (x>= 0.86) to the second-order (x<0.86). The pressure coefficient of M exhibits a sluggish variation against concentration and no anomaly was observed at x=0.86, being consistent with the Landau expansion model. On the other hand, TC for the second-order transition has a large negative pressure coefficient dTC/dP and its magnitude increases with increasing x. Above x=0.86, the magnitude of dTC/dP for the first-order transition increases with x, contrary to the theoretical expectation. It has been revealed that the spin-wave dispersion coefficient becomes smaller when the first-order transition becomes clear by changing the concentration and also applying pressure. Consequently, it is plausible that dTC/dP above x=0.86 is enhanced by the increase of instability of the ferromagnetic state.