Study of atomic disorder in Ni-V alloys

We present a pair distribution function (PDF) analysis from neutron diffraction data of the Ni$_{1-x}$V$_x$ alloy in the Ni-rich regime. Such structural study aims to clarify the origin of the magnetic inhomogeneities associated with the quantum Griffiths phase close to the ferromagnetic-paramagnetic quantum phase transition. The PDF analysis successfully reveals the details of the structure and chemical distribution of our Ni$_{1-x}$V$_x$ polycrystalline samples prepared with high-temperature annealing and rapid cooling protocol. This study confirms the expectations that all Ni$_{1-x}$V$_x$ samples with 0$\leq x \leq$0.15 crystallize in a single phase fcc structure with some residual strain. The increase of the lattice constant and the atomic displacement parameter with V-concentration $x$ is consistently explained by a random occupation of V and Ni-atoms on the lattice, with a radius ratio ($r_{V}/r_{Ni}$) of 1.05. Probing alternate, simple models of the local PDF, such as V-clusters or ordered structures (Ni$_8$V, Ni$_3$V) give inferior results compared to a random occupation. This investigation strongly supports that the magnetic clusters in the binary alloy Ni$_{1-x}$V$_x$ originate from Ni-rich regions created from "random" occupation rather than from chemical clusters. It reveals that Ni$_{1-x}$V$_x$ is one of the rare examples of a solid solution in a wide concentration regime (up to x=0.15) persisting down to low temperatures (T=15 K).