Influence of isoelectronic substitutions on the magnetism of UCoAl

UCoAl has a paramagnetic ground state but exhibits a susceptibility maximum and a metamagnetic transition at low magnetic fields applied along the hexagonal c axis. This behavior is easily changed by small substitutions in the nonmagnetic Co sublattice with suitable elements to ferromagnetism (Mn, Fe, Ru, Rh, or Ir) or to conventional paramagnetism (Ni, Cu, Pd, or Pt). Here the influence on magnetism from the simultaneous substitution of Co by equal amounts of Ni and Fe, UCo _1-z(Ni _0.5Fe _0.5) _zAl has been studied by magnetization measurements at ambient and high pressures on single crystals with z=0.1 and 1. This substitution was chosen so that it maintained the average number of d electrons constant. However, despite the constant number of 3d electrons with increasing z, stabilization of ferromagnetism is observed. This is attributed to the anisotropic change in lattice parameters upon substitution, which is consistent with results of the magnetostriction and uniaxial-pressure magnetization measurements of UCoAl. The ferromagnetic ground state of UNi _0.50Fe _0.50Al (z=1) is corroborated by first-principles electronic-structure calculations in the framework of the density functional theory.