  The microscopic magnetism in the helical, the conical and the
ferro-magnetically polarized phases in an itinerant helical magnet, MnSi, has
been studied by an extended 29Si NMR at zero field and under external magnetic
fields. The temperature dependence of staggered moment, M_Q(T), determined by
the 29Si NMR frequency, nu(T), and nuclear relaxation rate, 1/T_1(T) is in
general accord with the SCR theory for weak itinerant ferromagnetic metals and
its extension. The external field dependence of resonance frequency, nu(H),
follows a vector sum of the contributions from atomic hyperfine and macroscopic
fields with a field induced moment characteristic to the itinerant magnets. A
discontinuous jump of the resonance frequency at the critical field, H_c,
between the conical and the polarized phases has also been found that suggests
a first order like change of the electronic states at H_c.
