Magnetic order and spin dynamics across the ferromagnetic quantum critical point in Ni\boldmath{$_{1-x}$}Mo\boldmath{$_{x}$}

Abstract

Realizing a quantum critical point (QCP) in clean ferromagnetic (FM) metals has remained elusive due to the coupling of magnetization to the electronic soft modes that drive the transition to be of first order. However, by introducing a suitable amount of quenched disorder, one can still establish a QCP in ferromagnets. In this study, we ascertain that the itinerant ferromagnet Ni$_{1-x}$Mo$_{x}$ exhibits a FM QCP at a critical doping of $x_c \simeq 0.125$. Through magnetization and muon-spin relaxation measurements, we demonstrate that the FM ordering temperature is suppressed continuously to zero at $x_c$, while the magnetic volume fraction remains $100\%$ up to $x_c$, indicating a second-order phase transition. The QCP is accompanied by a non-Fermi liquid behavior, as evidenced by the logarithmic divergence of the specific heat and the linear temperature dependence of the low-temperature resistivity. Our findings reveal a minimal effect of disorder on the critical spin dynamics of Ni$_{1-x}$Mo$_{x}$ at $x_c$, highlighting it as one of the rare systems to exhibit a clean FM QCP.