Critical behaviour of iron borate and iron-yttriun garnet

The Moessbauer spectra are investigated and measurements are made of the magnetization of monocrystalline samples of iron borate FeBO3 and the iron-yttrium garnet Y3Fe5O12. The measurements are performed near the Curie temperature and in external magnetic fields from 0 to 30 kOe. The temperature and field dependences of the reduced magnetizations of sublattices of both substances are obtained. The critical indices in the temperature range 7x10sup(-4)<=(T-Tsub(c))/Tsub(c)<=10sup(-2) are determined for FeBO3, β=0.37+-0.01, delta=3.9+-0.2, γ=γ'=1.2+-0.1 and for Y3Fe5O12, β=0.39+-0.01, delta=4.3+-0.2, γ=γ'=1.33+-0.03. The experimental dependence of the critical qualities on the dimension of the order parameter is compared with the results of calculations based on scaling theory