Critical behaviour in an insulating spin glass : field expansion and scaling

The amorphous manganese aluminosilicate with 15 at % Mn is a spin glass with a freezing temperature Tf = 2.95 K. We have precisely measured slightly above Tf its ac susceptibility in a constant applied field H and its low field dc magnetization. The field cooled magnetization displays interesting relaxation phenomena close above and below T f. The ac susceptibility data can be fitted with a series expansion in even powers of H and we determined the thermal variation of the first four terms of the series. The non linear part of the ac susceptibility can be described above Tf by a universal scaling function with two exponents : γ = 3.1 ± 0.1 and β = 1.4 ± 0.1. By means of numerical integration on this scaling function we calculated the non linear part of the equilibrium dc magnetization slightly above Tf : the agreement with our precise SQUID measurements is excellent for (T - Tf)/ Tf ≳ 10-2.