Magnetic phase diagram of the U(${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Cu}}_{\mathit{x}}$${)}_2$${\mathrm{Ge}}_2$ system studied by ac-susceptibility measurements and neutron diffraction of polycrystalline samples

Polycrystalline U(${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Cu}}_{\mathit{x}}$ ${)}_2$ ${\mathrm{Ge}}_2$ solid solutions, investigated by ac-susceptibility measurements and neutron diffraction, exhibit the ${\mathrm{ThCr}}_2$ ${\mathrm{Si}}_2$-type crystallographic structure and magnetic ordering of the uranium sublattice, with moments along the tetragonal axis: antiferromagnetic type-I (AF-I) [k=(0,0,1)] for x=0, 0.25, and 0.50; ferromagnetic below ${\mathit{T}}_{\mathit{C}}$, becoming at ${\mathit{T}}_0$ (<${\mathit{T}}_{\mathit{C}}$): AF-I, ferrimagnetic [k=(0,0,2/3)], and AF-IA [k=(0,0,1/2)] for x=0.75, 0.90, and 0.95, respectively; and ferromagnetic for x=1. The magnetic phase diagram (temperature versus composition) of the U(${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Cu}}_{\mathit{x}}$ ${)}_2$ ${\mathrm{Ge}}_2$ system is proposed, and is discussed in relation to the variation in the number of conduction electrons (via Ruderman-Kittel-Kasuya-Yosida-type interactions).