The magnetic properties of the glassy ferromagnets Fe81.5B14.5Si4 and Fe40Ni40B20

The galssy ferromagnets Fe_81.5B_14.5Si₄ and Fe₄₀Ni₄₀B₂₀ have been studied by Mössbauer spectroucopy from 77.5 up to 650 and 680K, respectively, In the glassy state, the average magnetic hyperfine field decreases with increasing temperature due to a distribution of exchange inter-actions. At low temperatureH(T)/H(0) has a temperature dependence (1 -BT ^/32 -CT ^/52) whereB andC are constants, indicative of spin-wave excitations in glassy ferromagnets. The value of B_/32) = 0.40 = 0.05 is almost four times larger than those of crystalline iron and nickel. On the other hand, fluctuations of the exchange interaction constant are found to decrease with increasing temperature. The Curie temperaturesT _c - 608 K for the glassy Fe_81.5B_14.5Si₄ andT _c = 583K for the glassy Fe₄₀Ni₄₀B₂₀ are found to be well defined. AtT close toT _c H(T) behaves as a power law with critical exponent = 0.3. The crystallizationT _cr glassy Fe_81.5B_14.5Si₄ was found to be 645K. The crystalline material obtained contains three different phases, namely -Fe, Fe₂B and Fe-8% Si.