The determination of Curie temperature of amorphous Fe-Nb-B alloys by DSC under magnetic field

Differential scanning calorimetry (DSC) is a convenient and accurate method for measuring magnetic Curies temperature (Tc) in terms of the observation that the specific heat has a visible variation in the transition. Already known is the fact that when heat is added to the magnetic alloys, it partially contributes to the lattice and the electronic specific heat. The remainder disorders the spins, which contributes to magnetic specific heat. The number of disordered spins per degree of temperature rise increases with the temperature in accordance with the appropriate Brillouin function, and reaches a maximum value in the vicinity of the Curie temperature, where the magnetization is decreasing precipitously. The changes in magnetization accompanying the changes in magnetic enthalpy give rise to a specific heat anomaly around the Curie point, and the magnetic enthalpy ΔHm is schematically shown by the hatched area. Some alloys, e.g., amorphous (FeO.9Nb0.1)80B20 (nominal composition), have a magnetic transition involving a very small enthalpy change which is difficult to detect from DSC measurement. An improved method presented in this paper is to apply an axial magnetic field on samples during the DSC scanning process; the magnetic enthalpy of the Curie transition is amplified and a sharp Tc transition is easily determined