Magnetism of UCoGe and U3Co4Ge7

A series of UCoGe and U3Co4Ge7 polycrystalline samples has been prepared by arc melting and studied with respect to the phase composition and crystal structure, magnetization, a.c. susceptibility, electrical-resistivity and specific-heat behavior (down to 350 mK). U3Co4Ge7 has been found to exhibit a spontaneous magnetization below TC=21 K. Clear anomalies at TC typical for a ferromagnetic transition have been observed in a.c. susceptibility, electrical-resistivity and specific-heat data. No additional anomaly, which would indicate the second magnetic phase transition below TC reported in the literature, has been indicated. In all our UCoGe samples a transition to superconductivity has been revealed. On the other hand, no clear evidence of any transition to ferromagnetism with zero-field cooling down to 1.8 K has been obtained. The zero-field state is most probably governed by strong ferromagnetic spin fluctuations and seems to transform to a ferromagnetic state only when applying a magnetic field ≥10 mT. Simultaneously, an increase of the superconducting transition temperature is increasing with a magnetic field up to 10 mT and starts to decrease when increasing the field above this value. Measurements on bulk samples, which are by rule textured, indicate strong magnetocrystalline anisotropy in both investigated compounds.