THE MAGNETIC PHASE-TRANSITION IN FE17DY2 AND FE23ER6 SINGLE-CRYSTALS

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