Unconventional chiral phase of MnSi in zero and finite magnetic fields

Orateur : Pierre DALMAS DE REOTIER (INAC CEA Grenoble)

Chiral magnets have attracted attention since the 1960s. In 2009, the discovery of a skyrmion lattice phase in a small pocket of the temperature-magne tic field phase diagram of MnSi has triggered a renewed interest driven by the potential use of skyrmions for the storage of information. The zero-field magnetic structure of MnSi has been described since the 1970s as a long period incommensurate phase comprising planes of ferromagnetic moments of Mn which are progressively twisted to form an helix. In field, a uniform moment component parallel to the applied field sets in, in addition to the helical component. These phases have been interpreted in terms of a Ginzburg-Landau type theory with a dominant ferromagnetic exchange and a weaker Dzyaloshinski-Moriya interaction. From a detailed interpretation of muon spin rotation spectra, we show that the aforementioned magnetic structures cannot be fitted to the data. Using representation analysis, we propose more subtle structures where an additional angle is needed to describe the orientation of the magnetic moments and where the helical component is not perpendicular to the applied field, and discuss their theoretical implications.

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