Metamagnetism of CoS2 under high pressure and in Co(SxSe1-x)(2)

The effects of external pressure and the substitution of S by Se on the ferromagnetic phase transtion in CoS2 were studied by magnetization measurements. Both operations reduced T-c, made the magnetic phase transition discontinuous and gave rise to the metamgnetic transition immediately above T-c. In terms of the phenomenological Landau theory, our experimental results are consistently interpreted on the basis of the variation of the fourth coefficient (F-4) from a small positive value through zero to a negative value by varying Se-concentration or applying pressure. The appearance of weak metamagnetic transition in CoS2 at ambient pressure also supports that F-4 is nearly equal to zero under the external field. Therefore, the anomalous critical exponents in CoS2 reflect the tricritical behavior, as in the previous argument.