Hidden quantum phase transition in Mn$_{1-x}$Fe$_{x}$Ge: evidence brought by small-angle neutron scattering

TL;DR

This study reveals a quantum phase transition from long-range to short-range helical magnetic order in Mn$_{1-x}$Fe$_{x}$Ge, evidenced by small-angle neutron scattering showing temperature-independent correlation lengths at certain compositions.

Contribution

It provides experimental evidence of a quantum phase transition in Mn$_{1-x}$Fe$_{x}$Ge using neutron scattering, and proposes a modified exchange interaction model to explain the quantum critical behavior.

Findings

Quantum phase transition from LRO to SRO observed at x in [0.25, 0.4]

SRO characterized by Lorentzian scattering profile, LRO by Gaussian

Correlation length remains temperature independent at low/intermediate T

Abstract

The magnetic system of the Mn$_{1-x}$Fe$_{x}$Ge solid solution is ordered in a spiral spin structure in the whole concentration range of $x \in [0 \div 1]$. The close inspection of the small-angle neutron scattering data reveals the quantum phase transition from the long-range ordered (LRO) to short range ordered (SRO) helical structure upon increase of Fe-concentration at $x \in [0.25 \div 0.4]$. The SRO of the helical structure is identified as a Lorentzian contribution, while LRO is associated with the Gaussian contribution into the scattering profile function. The scenario of the quantum phase transition with $x$ as a driving parameter is similar to the thermal phase transition in pure MnGe. The quantum nature of the SRO is proved by the temperature independent correlation length of the helical structure at low and intermediate temperature ranges with remarkable decrease above certain temperature $T_Q$. We suggest the $x$-dependent modification of the effective Ruderman-Kittel-Kasuya-Yosida exchange interaction within the Heisenberg model of magnetism to explain the quantum critical regime in Mn$_{1-x}$Fe$_{x}$Ge.