Evolution of helimagnetic correlations when approaching the quantum critical point of Mn1−xFexSi

C Pappas,P Fouquet,L J Bannenberg,A O Leonov,F Weber,T Wolf

doi:10.1103/physrevresearch.3.013019

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 $x^*$.

Highlights

We present a comprehensive investigation of the evolution of helimagnetic correlations in Mn1−xFexSi with increasing 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∗
For each sample we determined the polarized neutron scattering and the neutron spin echo (NSE) spectra at the respective maxima of the magnetic scattering intensity. These occur at the scattering vector values Q = τ = 0.58, 0.68 and 0.88 nm−1 leading to helical pitches = 10.8, 9.2, and 7.1 nm, for x = 0.09, 0.11, and 0.14 respectively, in good agreement with our previous SANS results [32]

Summary

Introduction

The physics of the chiral magnet MnSi touches several fundamental questions in condensed-matter physics, from the stabilization of exotic states like chiral skyrmions [1] to the interplay between localized and itinerant magnetism [2,3,4,5,6,7,8] as well as to non-Fermi-liquid behavior [9,10,11,12,13,14] and quantum fluctuations [15,16,17,18,19,20,21,22,23]. In the region of the temperature-pressure phase diagram where p pC and TC = 0 K, long range range spiral and skyrmion correlations are restored under magnetic fields [7], a result that has been attributed to a softening of the magnetic moment. In the absence of quantum critical point (QCP) at pC [19], the origin and nature of the highly debated non-Fermi-liquid phase [16,17,18,20,26,27] remains an open question. It was suggested that this phase, out of which magnetic fields induce long range spiral correlations, is fluctuating and possibly of quantum nature [19].

Results

Discussion

Conclusion