Effects of pressure P on the magnetic moment M and the Curie temperature T C have been investigated for La(Fe xSi 1-x)] 13 above and below the magnetic phase boundary concentration x = 0.86. where the ferromagnetic-paramagnetic transition at T C 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. T C for the second-order transition has a large negative pressure coefficient dT C/dP and its magnitude increases with increasing x. Above x = 0.86, the magnitude of dT C/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 dT C/dP above x = 0.86 is enhanced by the increase of instability of the ferromagnetic state.
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