The Helical Magnet MnSi: Skyrmions and Magnons

Abstract

Since the late 1970s, MnSi has played a major role in developing the theory of helical magnets in non-centrosymmetric materials showing the Dzyaloshinsky-Moriya interaction (DMI). With a long helimagnetic pitch of 175 Å as compared to the lattice d-spacing of 4.55 Å, it was ideal for performing neutron studies, especially as large single crystals could be grown. A (B-T)-phase diagram was measured, and in these studies, under the application of a field of about 180 mT perpendicular to the scattering vector Q, a so-called A-phase in the B-T phase diagram was found and first interpreted as a re-orientation of the magnetic helix. After the surprising discovery of the skyrmion lattice in the A-phase in 2009, much interest arose due to the rigidity of the skyrmionic lattice, which is only loosely bound to the crystal lattice, and therefore only relatively small current densities can already induce a motion of this lattice. A very interesting approach to even better understand the complex structures in the phase diagram is to measure and model the spin excitations in MnSi. As the helimagnetic state is characterized by a long pitch of about 175 Å, the associated characteristic excitations form a band structure due to Umklapp scattering and can only be observed at very small Q with energies below 1 meV. Similarly, the excitations of the skyrmion lattice are very soft and low-energetic. We investigated the magnons in MnSi in the whole (B,T)-phase diagram starting in the single-k helimagnetic state by applying a small magnetic field, B = 100 mT. This way, the complexity of the magnon spectrum is significantly reduced, allowing for a detailed comparison of the data with theory, resulting in a full theoretical understanding of the spin system of MnSi in all its different magnetic phases.

Spin-disorder intervened avoidance of quantum criticality in B20 cubic Mn 1−xVx Si

“…It is reported to evince the chiral modulations caused by the competition between D ij and J ij energies, apart from exhibiting prototypical itinerant magnetism in which the same electrons are responsible for both magnetism and conduction. Such chiral modulations are manifested into the formation of hexagonal structured skyrmion lattice under the influence of small magnetic fields below the ordering temperature T hel [17][18][19][20][21]. In addition, the magnetic interactions in MnSi are long-range governed by the tricritical exponents.…”

Quantum Beams for New Aspects in Magnetic Materials and Magnetism

“…The number of publications reached more than 2000 and citations rose to some thousands per year recently. The early developments in this field are the topic of the review [3]. Interestingly, Muon spin rotation (µSR) spectra recorded for manganese silicide MnSi in [4] showed that the magnetic structures of MnSi in zero-field at a low temperature and in the conical phase near the magnetic phase transition deviate from the expected helical and conical structures.…”