Properties and collapse of the ferromagnetism in UCo_1-xRu_xAl studied in single crystals

We have investigated the ferromagnetic (FM) phase which suddenly develops in UCo_1-xRu_xAl and is isolated by paramagnetic regions on both sides from the parent UCoAl and URuAl. For that purpose we have grown high quality single crystals with x = 0.62, 0.70, 0.74, 0.75, and 0.78. The properties of the FM phase have been investigated by microscopic and macroscopic methods. Polarized neutron diffraction on a single crystal with x = 0.62 revealed the gradual growth of the hybridization between U and T-site in the U-T plane with increasing x. Hybridization works here as a mediator of the strong indirect interaction, while the delocalized character of the 5 f states is still conserved. As a result very weak spontaneous magnetic moments are observed for all alloys with magnitude nearly in proportion to the TC for x < 0.62, while an enormous disproportion exists between them near x_crit. The magnetization, specific heat, electrical resistivity, and Hall effect measurements confirmed that the FM transition is suppressed continuously at the critical concentration x_crit ≈ 0.77. Two quantum critical points are then expected on both sides of the FM dome. We propose a scenario that the order of the FM=PM transition differs at opposite boundaries of the FM dome. We conclude that both criticalities are influenced by disorder. Criticality on the UCoAl side has the character of a clean FM metal, while on the Ru rich side it has the character of a magnetically inhomogeneous system involving a Griffiths phase.