Field dependence of the magnetic quantum phase transition in MnSi

The dependence on pressure (P) and field (H) of the magnetic phase diagram of MnSi has been investigated by means of measurements of the AC susceptibility (X) up to 16 kbar and 7 T at temperatures down to 30 mK. For ambient pressure, we report on a peak in X above T-c at a characteristic temperature T-m that rises quickly with field. The pressure dependence of T-m is found to be strongly analogous to that of the zero-field transition to long-range order at T-c. Features of X in the field versus temperature (T) phase diagram may be viewed as 'fingerprint' evidence of a field-induced crossover at T-m of the itinerant magnetism from a non-polarized regime at high T and low H to a polarized regime at low T and high H. The long-wavelength spin spiral, present at low H, appears to be supported only in the polarized regime, so in the immediate vicinity of the critical pressure P-c for which T-c --> 0, a small pocket exists as a re-entrant state.