Evolution of magnetism in UCoGe and UCoAl with Ru doping*

Michal Vališka, Petr Opletal, Jiří Pospíšil, Jan Prokleška and Vladimír Sechovský

Abstract

Intriguing magnetic properties of the UCo_1−xRu_xGe and UCo_1−xRu_xAl pseudoternary systems studied on single crystals are presented and discussed in terms of anisotropic 5f-ligand hybridization as the principal driving mechanism. Ferromagnetism of the ferromagnetic superconductor UCoGe is initially stabilized by Ru substitutions for Co (superconductivity suppressed already for x > 0.01) providing maximum Curie temperature (T_C), spontaneous magnetic moment (μ_s) and magnetic entropy (S_mag) for 10% Ru. Further increasing Ru concentration leads to gradual suppression of these parameters towards a ferromagnetic quantum critical point (FM QCP) at ≈0.31. Non-Fermi liquid scaling of specific heat and electrical resistivity is observed in the vicinity of FM QCP. The itinerant electron metamagnet UCoAl is transformed to a ferromagnet by only 0.5% Ru substituted for Co. The ferromagnetism is suppressed for x > 0.7. On the descending branch of the dome-like T − x magnetic phase diagram two distinctly different ferromagnetic phases are observed. The strong magnetocrystalline anisotropy leaving the hard magnetization directions Pauli paramagnetic like irrespective of magnetic ground states projected exclusively in the easy magnetization directions is accounted to the anisotropic 5f-5f wave-function overlap and 5f-ligand hybridization, and 5f-electron orbital moment