Irreversible two-step transition of helix reorientation in MnSi

Abstract MnSi, a typical skyrmion-hosting helimagnet, has drawn wide attention due to its perspective in novel spintronic devices. Although the features of the sophisticated skyrmion state has been widely studied, the field-induced helix reorientation as a basis of the skyrmion formation has not reached a consensus. Here we investigate the helix reorientation process in a single crystal MnSi via precise ac susceptibility measurements. In the presence of a magnetic field along direction, the susceptibility results clearly indicate a two-step transition feature in the whole temperature range below the Curie temperature TC ∼29 K. The two steps of the helix reorientation suggest the presence of two energetically degenerate pairs of helices with respect to the magnetic field direction. Moreover, the helix reorientation process displays a strong field history dependence, manifested by a single-step transition after field cycling. The field history-dependent feature implies irreversibility of the helical-conical transition, which may be ascribed to the pinning effect of the topological defects presumably formed during the field cycling. The identification of the two-step transition and the history dependence renovate the magnetic phase diagram of MnSi and may initiate a revisiting of the helical to conical transition in other similar skyrmion-hosting systems.