Possible competitive coexistence of ferromagnetism and superconductivity in UGe2 -: art. no. 020510

We have measured magnetizations of the ferromagnet UGe2 which shows superconductivity in a narrow external pressure range, P(1)less than or equal toPless than or equal toP(2), in the ferromagnetic state. When P is close to P, in the superconducting phase, the ac magnetic susceptibility indicates a peak-anomaly associated with the ferromagnetic transition and an imperfect superconducting shielding effect. As P increases away from P-1, the peak anomaly becomes substantially broad and obscure, while the diamagnetic susceptibility approaches a perfect superconducting shielding. We have also observed that a saturation magnetization at low temperature shows a steep decrease with increasing P in the superconducting state. From these observations, we suggest that the superconductivity coexists in a competitive way with the ferromagnetism; as P increases, a volume fraction of the superconducting state grows over the system, while the ferromagnetic ordering possibly becomes spatially inhomogeneous.