Effects of Two Step Annealing on Complex Permeability of Fe-Cu-Nb-Si-B Nanocrystalline Soft Magnetic Materials

Abstract

The typical nanocrystalline alloy known as Finemet with the composition Fe73.5Cu jNb3Si135B9 has been studied for the soft magnetic properties. The crystalline phases that evolved after crystallization were studied by X-ray diffraction, crystallization temperature by DTA and magnetic properties by VSM and impedance analyzer. The change of magnetic properties with annealing temperature, Ta in the range 400-700°C and annealing time 1-60 minutes have been investigated. The effect of a two-step annealing on the permeability Fe735Cu)Nb3Si135B9 alloy has been studied. The grain size was determined from the line broadening of the fundamental peak using Scherrers method. Annealed at 555°C, the grain size, the solute silicon content of a-Fe(Si) grains and the improvement in magnetic softness saturate as a function of time. As compared to a nanocrystalline alloy obtained by conventional one-step annealing at around the crystallization temperature, the alloy subjected to preannealing at Ta = 400°C and subsequent high temperature annealing at Ta=555°C shows similar soft magnetic properties. An enhancement of initial permeability with increasing annealing temperature was observed until the annealing temperature TL = 475°C and corresponds to the amorphous relaxed structure. An abrupt deteroriation of permeability takes place at the beginning of crystalization beyond which permeability rises dramatically attaining a maximum value at Ta = 555°C which is attributed to the nanometri grain size effect as a result of averaging out of effective magnetic anisotropy by exchange interaction and decrease of magnetoelastic energy. A sharp decrease of permeability above 580°C is assumed to be connected with iron-boride phase at the beginning of second crystallization stage. The initial permeability for the optimum annealed sample has been found to be 20786 as compared with 284 for its amorphous counterpart. The behavior of very high permeability is explained on the basis of random anisotropy model. From the temperature dependence of magnetization the Curie temperature of the amorphous state and crystallized ferromagnetic phases like Fe(Si) and iron-boride phases and the onset of crystallization temperatures of these phases have been determined. Saturation magnetization measured on the amorphous and annealed samples at different annealing temperature gradually increases with Ta and show a maximum at the medium stage of nanocrysallization at Ta 525°C beyond which magnetization decreases. The results are interpreted on the basis of irreversible structural relaxation as well as enrichment of the nanograins with Si.