Non-linear magnetisation effects in the spin glass Fe0.3Mg0.7Cl₂

The authors present a study of the non-linear magnetisation in the vicinity of the freezing temperature Tf of the insulating spin glass Fe0.3Mg0.7Cl2. The magnetisation of the sample was measured in the range 1 or=1.70K and for a slow cooling rate of 0.5 mK s-1 the equilibrium magnetisation is obtained. In these experimental conditions, non-linear effects are studied and a rapid increase of the non-linear part mod M- chi 0H mod of the magnetisation is observed when decreasing the temperature down to 1.7K. The present the temperature dependence of the two first non-linear coefficients a3 and a5 (b3 and b5) in the expansion of (M- chi 0H) in function of odd powers of H( chi 0H). Coefficients b3 and b5 vary by one and two orders of magnitude in the temperature range 1.70-2.50K. Comparison with both simple superparamagnetic and typical spin glass systems is made.