Magnetic Fluctuations, Precursor Phenomena, and Phase Transition in MnSi under a Magnetic Field

Abstract

The reference chiral helimagnet MnSi is the first system where skyrmion lattice correlations have been reported. At zero magnetic field the transition at TC to the helimagnetic state is of first order. Above TC , in a region dominated by precursor phenomena, neutron scattering shows the build up of strong chiral fluctuating correlations over the surface of a sphere with radius 2π/ , where is the pitch of the helix. It has been suggested that these fluctuating correlations drive the helical transition to first order following a scenario proposed by Brazovskii for liquid crystals. We present a comprehensive neutron scattering study under magnetic fields, which provides evidence that this is not the case. The sharp first order transition persists for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and start to concentrate along the field direction already above 0.2 T. Our results thus disconnect the first order nature of the transition from the precursor fluctuating correlations. They also show no indication for a tricritical point, where the first order transition crosses over to second order with increasing magnetic field. In this light, the nature of the first order helical transition and the precursor phenomena above TC , both of general relevance to chiral magnetism, remain an open question.

Universality of the helimagnetic transition in cubic chiral magnets: Small angle neutron scattering and neutron spin echo spectroscopy studies of FeCoSi

“…The corresponding stretching exponents are close to the values found in disordered systems such as glass-forming systems [47,48]. In addition, the associated characteristic relaxation times at zero field for Fe 0.7 Co 0.3 Si can be as long as 10 ns, and are therefore much longer than for MnSi where they do not exceed 1 ns [21,40]. These long relaxation times combined with the stretched exponential relaxation suggest the relaxation of large magnetic volumes, which are likely inhomogeneous in size and structure.…”

“…For fields exceeding 40 mT, the scattered intensity decreases until it is almost completely suppressed for fields above 60 mT. This is highly similar to the behavior of the transition in MnSi, where the scattered intensity is also suppressed by large enough magnetic fields [40].…”

“…Whereas the dynamics of MnSi can be described by a simple exponential, the dynamics of Fe 0.7 Co 0.3 Si around T C can only be accounted for by a stretched exponential relaxation [21,40]. This implies that the relaxation stretches over several orders of magnitude in time and involves a broad distribution of relaxation times.…”

Reorientations, relaxations, metastabilities, and multidomains of skyrmion lattices

“…The MnSi sample is a cubic single crystal with dimensions of about 5 × 5 × 5 mm 3 and was already used in a previous experiment [39]. The MnSi sample was also aligned with the [110] crystallographic direction vertical within 5…”

1.5 GPa compact double-wall clamp cell for SANS and NSE studies at low temperatures and high magnetic fields

“…The measurements were performed on a 110 mg single crystal of MnSi, approximately 2 × 2 × 3 mm 3 in size, originating from the same batch as the sample used in previous neutron scattering studies [1,13,14]. The sample was aligned with the [110] axis vertical, parallel to the Teflon sample can axis.…”