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Quantum Griffiths Phase in the weak itinerant ferromagnetic alloy Ni$_{1-x}$V$_x$

We present magnetization ($M$) data of the $d$-metal alloy Ni$_{1-x}$V$_x$ at vanadium concentrations close to $x_c \approx 11.4%$ where the onset of long-range ferromagnetic (FM) order is suppressed to zero temperature. Above $x_c$, the temperature ($T$) and magnetic field ($H$) dependencies of the magnetization are best described by simple nonuniversal power laws. The exponents of $M/H \sim T^{-γ}$ and $M \sim H^α$ are related by $1-γ=α$ for wide temperature ($10K < T \leq 300K$) and field ($H \leq 5T$) ranges. $γ$ is strongly $x$ dependent, decreasing from 1 at $x\approx x_c$ to $γ< 0.1$ for x=15%. This behavior is not compatible with either classical or quantum critical behavior in a clean 3D FM. Instead it closely follows the predictions for a quantum Griffiths phase associated with a quantum phase transition in a disordered metal. Deviations at the lowest temperatures hint at a freezing of large clusters and the onset of a cluster glass phase, presumably due to RKKY interactions in this alloy.