The magnetic phase transition in Fe17Dy2 and Fe23Er6 single crystals

The magnetic phase transition in Fe17Dy2 and Fe23Er6 single crystals is investigated from detailed magnetisation measurements performed in the critical region. Two sets of measurements were performed while keeping the direction of the applied magnetic field along the easy plane as well as hard directions. Values of the Curie temperature (T-C) and the critical exponents corresponding to the spontaneous magnetisation (beta), initial susceptibility (gamma',gamma) and the magnetisation isotherm at the Curie point (delta) are deduced from the analysis based on several methods: modified Arrott plots, the Kouvel-Fisher method, scaling plots and ln J versus ln(mu(0)H) plots. Very good agreement has been observed between the exponent values obtained by the various methods. The values of beta, gamma and delta for Fe17Dy2 are close to those predicted theoretically for a three-dimensional Heisenberg ferromagnet, while those for Fe23Er6 do not belong to any known universality class. The exponents deduced from the measurements in the easy plane and hard directions vary only within our error limits.