Pressure dependence of the magnetic structure of the itinerant electron magnet MnSi

Elastic neutron scattering measurements have been performed under hydrostatic pressure on single-crystalline MnSi. The scaling between the staggered moment and the transition temperature. of the helical magnetic structure does not follow predictions from spin-fluctuation theories. For pressures below P* approximate to 12-13 kbar, where the transition is second order, the length of the helix decreases with increasing temperature. Such a behaviour is not expected for a ferromagnetic Dzyaloshinsky-Moriya instability. With increasing pressure, the length of the helix also decreases, but there is a lock-in to an incommensurate temperature-independent value above p*, where the phase transition is first order.