Evolution of Helimagnetic Correlations when approaching the Quantum Critical Point of Mn$_{1-x}$Fe$_x$Si

TL;DR

This study investigates how helimagnetic correlations in Mn$_{1-x}$Fe$_x$Si evolve with doping, revealing that precursor fluctuations persist despite the loss of long-range order, likely due to increasing frustration from RKKY interactions.

Contribution

It provides new insights into the evolution of helimagnetic correlations near the quantum critical point in Mn$_{1-x}$Fe$_x$Si using neutron scattering techniques.

Findings

Precursor fluctuations persist beyond the disappearance of long-range order.

Frustration from RKKY interactions increases with Fe doping.

The precursor phase expands with increasing x.

Abstract

We present a comprehensive investigation of the evolution of helimagnetic correlations in Mn$_{1-x}$Fe$_x$Si with increasing doping. By combining polarised neutron scattering and high resolution Neutron Spin Echo spectroscopy we investigate three samples with $x$=0.09, 0.11 and 0.14, i.e. with compositions on both sides of the concentration $x^* \sim 0.11$ where the helimagnetic Bragg peaks disappear and between $x^*$ and the quantum critical concentration $x_C \sim 0.17$, where $T_C$ vanishes. We find that the abrupt disappearance of the long range helical periodicity at $x^*$, does not affect the precursor fluctuating correlations. These build up with decreasing temperature in a similar way as for the parent compound MnSi. Also the dynamics bears strong similarities to MnSi. The analysis of our results indicates that frustration, possibly due to achiral RKKY interactions, increases with increasing Fe doping. We argue that this effect explains both the expansion of the precursor phase with increasing $x$ and the abrupt disappearance of long range helimagnetic periodicity at $x^*$.