Possible competitive coexistence of ferromagnetism and superconductivity inUGe2

We have measured magnetizations of the ferromagnet ${\mathrm{UGe}}_2$ which shows superconductivity in a narrow external pressure range, $P_1<~P<~P_2,$ in the ferromagnetic state. When P is close to $P_1$ 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.