Magnetic and thermodynamic properties of cobalt doped iron pyrite: Griffiths Phase in a magnetic semiconductor

TL;DR

This study investigates how cobalt doping affects the magnetic and thermodynamic properties of iron pyrite, revealing the emergence of Griffiths phases due to quenched disorder, with systematic changes observed across various measurements.

Contribution

It provides detailed experimental evidence of Griffiths phases in cobalt-doped iron pyrite, highlighting the role of quenched disorder in magnetic semiconductor behavior.

Findings

Paramagnetic to disordered ferromagnetic transition at x=0.007

Power law dependencies near critical concentration and Curie temperature

Non-analytic magnetic susceptibility and specific heat around H=0

Abstract

Doping of the band insulator FeS$_2$ with Co on the Fe site introduces a small density of itinerant carriers and magnetic moments. The lattice constant, AC and DC magnetic susceptibility, magnetization, and specific heat have been measured over the $0\le x\le 0.085$ range of Co concentration. The variation of the AC susceptibility with hydrostatic pressure has also been measured in a small number of our samples. All of these quantities show systematic variation with $x$ including a paramagnetic to disordered ferromagnetic transition at $x=0.007\pm 0.002$. A detailed analysis of the changes with temperature and magnetic field reveal small power law dependencies at low temperatures for samples near the critical concentration for magnetism, and just above the Curie temperature at higher $x$. In addition, the magnetic susceptibility and specific heat are non-analytic around H=0 displaying an extraordinarily sharp field dependence in this same temperature range. We interpret this behavior as due to the formation of Griffiths phases that result from the quenched disorder inherent in a doped semiconductor.