Room-temperature and high-temperature magnetic permeability of Co-doped nanocrystalline alloys

Influence of composition and annealing temperature on structure and magnetic properties of amorphous and nanocrystalline Fe_78.4−xCo _x Si₉B₉Nb_2.6Cu₁ (x = 27.4, 40.0, 51.0, 78.4) alloys was investigated by X-ray diffraction (XRD) and the temperature dependence of permeability. According to the initial crystallization temperature (T_x1) from differential scanning calorimetry (DSC) curves of as-quenched amorphous alloys, 490–700 °C isothermal annealing was carried out to obtain the characteristic nanocrystalline structure. Furthermore, the soft magnetic properties were measured by temperature evolution of magnetic permeability to obtain the correlation between Co content, annealing temperature and magnetic permeability. The results show that, on the one hand, the annealing temperature exerts a significant effect on phase structure and initial permeability (μ_i). The higher-temperature (from 550 to 610 °C) annealed Co content nanocrystalline samples can remain higher μ_i at elevated temperature. On the other hand, partial substitution Fe by Co can improve the high-temperature magnetic stability; however, the room-temperature permeability of higher Co content alloys decreases obviously at the same time. This phenomenon was analyzed from the viewpoint of the saturation magnetic induction (B_s), magnetic anisotropy (<K>) and magnetostriction (λ_s).