Exploring quantum phase transition in Pd_{1-x}Ni_x nanoalloys

Abstract

Pd$_{1-x}$Ni$_x$ alloy system is an established ideal transition metal system possessing a composition induced paramagnetic to ferromagnetic quantum phase transition (QPT) at the critical concentration $x_c \sim$ 0.026 in bulk. A low-temperature non-Fermi liquid (NFL) behaviour around $x_c$ usually indicates the presence of quantum criticality (QC) in this system. In this work, we explore the existence of such a QPT in nanoparticles of this alloy system. We synthesized single-phase, polydispersed and 40-50 nm mean diameter crystalline nanoparticles of Pd$_{1-x}$Ni$_x$ alloys, with $x$ near $x_c$ and beyond, by a chemical reflux method. In addition to the determination of the size, composition, phase and crystallinity of the alloys by microscopic and spectroscopic techniques, the existence of a possible QPT was explored by resistivity and DC magnetization measurements. A dip in the value of the exponent $n$ near $x_c$, and a concomitant peak in the constant $A$, of the $AT^n$ dependence of the low temperature ($T$) resistivity indicate the presence of a quantum-like phase transition in the system. The minimum value of $n$, however, remains within the Fermi liquid regime ($n >$ 2). The DC magnetization results suggest an anticipatory presence of a superparamagnetic to ferromagnetic QPT in the mean-sized nanoparticles. The observation of a possible quantum critical NFL behaviour ($n <$ 2) through resistivity is argued to be inhibited by the electron-magnon scatterings present in the smaller nanoparticles.