UC San Diego

The properties of UCoGe and URu₂Si₂ have been investigated by substitution of Ni into the Co site and Fe and Os into the Ru site. Polycrystalline samples of UCo₁₋xNixGe (0 <̲ x <̲ 1), URu₂₋xFexSi₂ (0 <̲ x <̲ 2), and URu₂₋xOsxSi₂ (0 <̲ x <̲ 1.2) were prepared by arc-melting and studied by x-ray diffraction, energy dispersive x-ray spectroscopy, electrical resistivity, specific heat, and magnetization measurements. In the UCo₁₋xNixGe system, the material evolves from the ferromagnetic state to the antiferromagnetic state with increasing Ni concentration. The superconducting state is destroyed by small amounts of Ni substitution and does not appear to re-emerge in any other portion of the phase diagram. Measurements on URu₂₋xFexSi₂ reveal a two-fold enhancement of the "hidden order" (HO)/large moment antiferromagnetic (LMAFM) phase boundary T₀(x). The T₀(Pch) curve, obtained by converting x to "chemical pressure" Pch, is strikingly similar to the T₀(Pch) curve, where P is applied pressure, for URu₂Si₂ -- both exhibit a "kink" at 1.5 GPa and a maximum at ̃ 7 GPa. This similarity suggests that the HO-LMAFM transition at 1.5 GPa in URu₂Si₂ occurs at x ̃ 0.2 (Pch ̃ 1.5 GPa) in URu₂₋xFexSi₂. The transition temperature in URu₂₋xOsxSi₂ is enhanced from 17.5 K at x = 0 to 50 K at x = 1. At x ̃ 0.2, the samples seem to evolve to another ordered phase as indicated by the decreasing size of the HO gap