The magnetic structure of MnSi under an applied field

Abstract

Below TC = 29 K the weak itinerant ferromagnet MnSi becomes ordered in a left-handed spin helicalstructure as a result of the Dzyaloshinskii–Moriya (DM) interaction. We give a recipe forcalculating the orientation of the helix under an applied field. The recipe is derived on abasis of a theory recently developed for cubic magnets with DM interaction. The theoryevaluates the ground state energy and the spin wave spectrum. It is shown that inzero field the orientation of the helix depends solely on the anisotropic exchangeinteraction and cubic anisotropy. Under an applied field the helix possesses twotypes of magnetic susceptibility: one parallel and another perpendicular to theapplied magnetic field. The perpendicular susceptibility is related to the fact thatthe helical structure itself is unstable with respect to the small magnetic fieldH applied perpendicularlyto the wavevector k.The spin wave gap Δ provides the stability of the spin wave spectrum of the helix structure and its presence maybe revealed in the magnetic field behaviour. Our calculations show the essence of thefield-induced transformations of the magnetic structure related to the spin wave gap. Theexperimental data provide the evidence for its existence. On the basis of ourfindings we discuss a possible scenario for the quantum phase transition in MnSi.