NONLINEAR AC SUSCEPTIBILITY OF FE3-XMNXS (X=1.2, 1.5, 1.8)

We have measured the temperature dependence of the nonlinear ac suscep tibility of Fe3-xMnxSi(x = 1.2, 1.5, 1.8) under different ac fields in the range 1-30 Oe. The third harmonic results suggest spin-glass-like features. The possible origin of such spin-glass features in this con centrated spin system with mixed exchange interactions is discussed. A n attempt has been made to estimate the critical exponent gamma from t he nonlinear susceptibility data as chi 3 approximate to [(T - T-f)/T- f](-gamma), and its value has been compared with other spin-glass syst ems. Nonlinear ac susceptibility experiments covering the ''re-entrant spin-glass'' regime have been emphasised.