Partial magnetic order in the itinerant-electron magnet MnSi

MnSi is an itinerant ferromagnet with a long-wavelength helical modulation of the spin structure. Macroscopic measurements suggest that the ordering temperatureT _c is reduced with increasing pressure fromT _c = 30 K atp = 0 to zero at the critical pressurep _c = 14.6 kbar. Resistivity measurements show that MnSi enters a non-Fermi liquid state atp _c, which remains to be understood. Neutron scattering techniques have been used to investigate the magnetic structure at and abovep _c, i.e. triple-axis spectrometry and small angle neutron scattering. Surprisingly, sizeable quasi-static moments were found to survive to pressures considerably abovep _c. They are, however, organized in a highly unusual way such that the magnetic Bragg reflections are sharp in the longitudinal direction but are very broad in the transverse direction, implying a partial magnetic order that was never seen before.