High-temperature Magnetic Properties And Mechanism Of Exchange-coupling Interaction For Feco-based Nanocrystalline Alloys

High-temperature magnetic properties, the structural characteristics and high-temperature mechanism of exchange-coupling interaction for nanocrystalline Fe_78.4-xCoxSi₉B₉Nb_2.6Cu1 (x=27.4, 40, 51, 78.4) alloys were investigated. The dependence of high-temperature and room-temperature magnetic permeability on Co content and the main cause of the higher permeability of FeCo-based alloys at elevated temperature were mainly studied.XRD was used to analyze the microstructures of Co content FINEMET alloys, and the dependence between magnetic properties and annealing conditions was also discussed. Partial crystallization occurs when the alloys were isothermally treated and a two-phase nanocrystalline structure composed of the amorphous matrix and theα-FeCo(Si) nanocrystallites were obtained. The results indicate that annealing conditions have a great influence on microstructures.With annealing temperature or annealing time increasing, the volume fraction of the precipitated crystalline phase gradually increases, and the thickness of the intergranular amorphous layer decreases, however, the lattice parameter increases, but it is smaller than that of bcc-FeCo alloy.The evolution of initial permeabilityμ_i with measuring temperatures T for the alloys were described from room temperature to 800℃.The results show that partially substitution of Fe by Co in FINEMET type alloys can obviously enhance the Curie point of the crystalline phase and the amorphous phase, which results in no dropping sharply in initial permeability at high temperature. However, due to the decrease of saturation magnetic induction and the increase of magnetocstriction, the room-temperature permeability drops obviously.The initial permeability of different annealing temperature samples was investigated. It can be seen that the initial permeability of 600℃-annealed sample was not drop sharply at the Curie temperature of the residual amorphous phase, which is a new magnetic phenomenon in dual-phase nanocrystalline alloys and different from the Fe-based nanocrystalline alloys. The origin of the above phenomenon has been explored by estimating the Curie temperature of amorphous ribbons which have the same composition with the residual amorphous phase in annealed nanocrystalline alloys and the result shows that the Curie temperature of the intergranular amorphous region can be enhanced drastically up to the Curie temperature of the crystalline phase (T_C^A =T_C^α) when the exchange-field between adjacent nanograins penetrated the amorphous interphase thoroughly. Furthermore the effective exchange penetration length of FeCo-based nanocrystalline alloys (L_FeCo) was evaluated as 0.61 nm, this value is much larger than that of Fe-based nanocrystalline alloys, which may be the main cause of the higher permeability of FeCo-based alloys at elevated temperature.In order to improve the initial permeability of FeCo-based nanocrystalline alloys, we fabricated Ni-contained FeCo-based nanocrystalline alloys and studied the correlation between Ni content and basic magnetic parameters such as Saturation magnetostriction, Saturation magnetization, Curie temperature as well as the initial permeability and found that adding Ni to FeCo-based alloys can enhance the permeability but drop the Curie temperature of amorphous phase, epecially for the higher Ni content.