Transition from itinerant metamagnetism to ferromagnetism in UCo_1-xOs_xAl solid solutions

The influence of substitution of a small amount of Os (< 2 %) on the Co sublattice on the magnetism of the itinerant metamagnet UCoAl is studied on single-crystalline UCo_1-xOs_xAl compounds with x = 0.002, 0.005 and 0.01. For x = 0.002, the ground state is still paramagnetic, like in UCoAl. The metamagnetic-transition field is 0.37 T, twice lower than in UCoAl. The compound with x = 0.005 is at the border between the paramagnetic and the ferromagnetic ground state. At T = 2 K, it is ferromagnetic, at elevated temperatures a magnetic field is necessary to maintain the magnetic state. In zero field, the ferromagnetic state vanishes at T = 8 K. The compound with x = 0.01 is a ferromagnet with strong uniaxial magnetic anisotropy similar to the previously studied compounds with x = 0.02-0.20.