Structure And High-Temperature Magnetic Properties Of Co-FINEMET Alloys

High-temperature magnetic properties and the structural characteristics of nanocrystalline (Fe_1-xCo_x)_73.5Cu₁Nb₃Si_13.5B₉ (x= 0, 0.25, 0.5, 0.75) alloys were investigated. The dependence of high-temperature magnetic properties on Co content and annealing temperature were mainly studied.XRD was used to analyze the microstructures and the results indicate that changing Co content and annealing temperature have great influence on microstructures. Partial crystallization occurs when the alloys were isothermally treated and a two-phase nanocrystalline structure composed of the amorphous matrix and theα-FeCo(Si) and (FeCo)3Si nanocrystallites were obtained. With annealing temperature increasing, the volume fraction of the precipitated crystalline phase gradually increases and the thickness of the intergranular amorphous layer decreases.The evolutions of initial permeabilityμ_i with measuring temperatures T for the alloys were described from room temperature to 780℃. .Magnetic properties and structural transformation for (Fe_1-xCo_x)_73.5Cu₁Nb₃Si_13.5B₉ (x= 0, 0.25, 0.5, 0.75) alloys were analyzed by means ofμ_i–T curves in heating-cooling cycles. The results show that partially substitution of Fe by Co in FINEMET type alloys decreases the saturation magnetic induction and initial permeability at room temperature. But the Curie point of the crystalline phase and the amorphous phase are obviously enhanced; the iμvalues of the alloys could keep stable up to quite higher temperatures and the high-temperature soft magnetic properties are greatly improved.For (Fe_1-xCo_x)_73.5Cu₁Nb₃Si_13.5B₉ (x= 0, 0.25, 0.5, 0.75) alloys, Curie temperature of amorphous precursor () increases and primary crystallization temperature (Tx1) decreases with increasing Co content . However, no further increase in was observed when x≥0.5 and a noticeable decrease ofμ_i was obtained for higher Co-content alloys (x = 0.75). The selective substitution of Fe by Co into the precipitated crystalline phase on the crystallization process was analyzed. The random magnetocrystalline anisotropy model and the exchange coupling are used to analyze the improvement of the high-temperature soft magnetic properties.