ESR study of the concentrated spin glass a-MnSi.

We have measured the ESR linewidth, W, and shift, DeltaH, in amorphous MnSi at 9.35 GHz and temperatures below 70K. Above the spin glass transition temperature T(g)=24K, we find W alpha DeltaH(0.5). Although this relationship usually is the signature of the extreme dynamic narrowing limit, the experimental values of W and DeltaH indicate that static fields are of major importance over the complete temperature range in a-MnSi. Using the Kubo- Toyabe model, we find a temperature independent random field strength of 2.6 x 10(10s-1) and a temperature dependent spin relaxation time tau which scales as tau alpha T(g)/(T-T(g)). These results show that spin relaxation in a-MnSi is dominated by the strength of the random fields, not by dynamic narrowing.