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

TL;DR

This study investigates the magnetic properties of Ni$_{1-x}$V$_x$ alloys near the critical vanadium concentration, revealing quantum Griffiths phase behavior and deviations suggestive of cluster glass formation at low temperatures.

Contribution

It provides experimental evidence for quantum Griffiths phase in a disordered itinerant ferromagnet, highlighting nonuniversal power-law behavior and cluster glass phenomena.

Findings

Power-law temperature and field dependence of magnetization.

Strong x-dependence of the critical exponent γ.

Deviations at low temperatures indicating cluster glass formation.

Abstract

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 $M/H \sim T^{-\gamma}$ and $M \sim H^\alpha$ are related by $1-\gamma=\alpha$ for wide temperature ($10K < T \leq 300K$) and field ($H \leq 5T$) ranges. $\gamma$ is strongly $x$ dependent, decreasing from 1 at $x\approx x_c$ to $\gamma < 0.1$ 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.