High temperature behaviour of stress-annealed nanocrystalline Fe_73.5Cu₁Nb₃Si_13.5B₉

Abstract
The thermomagnetic properties of stress-annealed Fe_73.5Cu₁Nb₃Si_13.5B₉ were measured. The samples were annealed at 813 °K for 60 min with several values of tensile stress. The saturated hysteresis loops were raised from room temperature up to the Curie point of crystallites. The curves of the thermal dependence of coercivity for various stresses recover perfectly. This may indicate that the shape anisotropy of crystallites is negligible. Since the anisotropy of the stress-annealed sample is uniaxial and transverse over a wide range of temperature, the induced anisotropy can be deduced from hysteresis loops within this range. A variation of Herzer's model of creep-induced anisotropy is foreseen for samples annealed under stresses below 100 MPa ; however, a linearly-decreasing relationship of induced anisotropy versus measurement temperature is observed between 300 and 600 K for all samples. It is proposed herein that this behavior is related to the differential dilatation of the two phases. The different shape in squareness curves above the ferromagnetic Curie point of the amorphous phase serves to suggest a significant magnetoelastic contribution, even in the superferromagnetic state.