Field dependence of the muon spin relaxation rate in MnSi

Muon spin rotation/relaxation measurements have been performed in the itinerant helical magnet MnSi at ambient pressure and at 8.3 kbar. We have found the following: (a) the spin-lattice relaxation rate 1/T-1 shows divergence as T1T proportional to (T-T-c)(beta) with the power beta larger than 1 near T-c; (b) 1/T-1 is strongly reduced in an applied external field B-L and the divergent behavior near T-c is completely suppressed at B-L greater than or equal to 4000 G. We discuss that (a) is consistent with the self-consistent renormalization theory and reflects a departure from mean-field behavior, while (b) indicates selective suppression of spin fluctuations of the q = 0 component by B-L.