The Effective Magnetic Anisotropy And The Magnetostriction For FeCo-based Nanocrystalline Alloys

The magnetic properties and microstructures of FeCo-based Fe_39.4-xCo₄₀Si₉B₉Nb_2.6Cu_x and Fe_78.4-yCo_ySi₉B₉Nb_2.6Cu₁ nanocrystalline alloys annealed at different temperatures were investigated. The results show that the gap between first and second crystallizations temperatures is enlarged with Cu(Co) content increasing, which is benefit to gain a single crystalline phaseα-FeCo(Si) in a broad temperature range. But, the magnetic properties of FeCo-based alloys are quite harder than FINEMET at room temperature.Herzer deems that the good magnetic properties for nanocrystalline alloys are obtained because of their extreme fine crystalline grain size which induces the magnetocrystalline anisotropy being largely averaged out. We measured the effect magnetic anisotropy of the nanocrystalline Fe_39.4-xCo₄₀Si₉B₉Nb_2.6Cu_x and Fe_78.4-yCo_ySi₉B₉Nb_2.6Cu₁ alloys by means of the law of approach to saturation. It is shown that the effect magnetic anisotropy of FeCo-based alloys is equivalent with that of FINEMET in order of magnitude(about 70-240J/m³); the disaccord between changes of the effect magnetic anisotropy and the initial permeability with Cu(Co) content for FeCo-based alloys was observed.In the random anisotropy model for double phases, Hernando pointed out that the effect of the magnetostriction on magnetic properties should be considered. We deduce that the magnetostriction is positive and large, which controls the initial permeability of FeCo-based alloys. The saturation magnetostrictions for nanocrystalline Fe_39.4-xCo₄₀Si₉B₉Nb_2.6Cu_x and Fe_78.4-yCo_ySi₉B₉Nb_2.6Cu₁ alloys were evaluated by means of small-angle magnetization rotation. The changes of the saturation magnetostrictions(about 20-90ppm) with annealing temperatures for FeCo-based alloys are different with Fe-based alloys. The saturation magnetostrictions of crystalline phaseα-FeCo(Si) is positive, besides, it is larger than amorphous phase. The saturation magnetostrictions increase with annealing temperature increasing, as a result of the increasing volume of crystalline phase with annealing temperature. Larger magnetostrictions result in the coercivity of FeCo-based alloys is much greater than that of Fe-based alloys; meanwhile the initial permeability is much lower. The random anisotropy model for double phases well demonstrates the reason why the magnetic properties of FeCo-based alloys are quite harder than FINEMET at room temperature.