Helimagnetic and skyrmionic correlations close to the Quantum Critical Points of MnSi under pressure and of Mn_1-xFe_xSi.

MnSi is home to chiral skyrmions but also to phenomena arising from the interplay between localised and itinerant magnetism. Under mechanical or chemical ­pressure in the form of Fe doping in Mn_1-xFe_xSi, a non-Fermi liquid behavior sets-in which has been attributed to a chiral spin liquid and quantum fluctuations. Here we present a comprehensive investigation of these phenomena by combining small angle neutron scattering and neutron spin echo spectroscopy. 

Under mechanical pressure conical and skyrmionic scattering appear under magnetic fields even even above the critical pressure p_C, where long range helimagnetism dissapears, and where a  non-Fermi liquid and topological Hall effect behavior has been reported. We attribute this behaviour to a softening of the magnetic moment due to an enhancement of the itinerant electron character of the magnetism with increasing pressure. We argue that this enhancement could drive the destabilization of the long-range helical order at p_C [1].

Further analysis of our results shows that the effect of pressure on the helical pitch is very different from that of doping. Thus, in contrast to MnSi under pressure, frustration possibly due to RKKY interactions is important in Mn_1‑xFe_xSi. This frustration, which would increase with increasing doping,  would explain both the expansion of the precursor phase with increasing x in Mn_1-xFe_xSi and the abrupt disappearance of long range helimagnetic periodicity .

We further discuss the destabilisation of long range helical periodicity in both MnSi under pressure and Mn_1-xFe_xSi, which is driven by the modification of the electronic state but with different specific microscopic mechanisms [2].

[1] Bannenberg, L. J. et al. PRB 100, 054447 (2019). [2] C. Pappas, C. et al. PRR. 3, 013019 (2021).