Effects of Two Step Annealing on Dynamic Magnetic Properties of Co- and Co-Fe-Based Amorphous Ribbons

Abstract

Co-and Co-Fe-based amorphous ribbons with general composition Co80-xFexB10Si10 have been prepared by the melt- spinning technique where Co has been substituted by Fe from 0 to 8 At%. The amorphousity of the samples has been confirmed by X-ray diffraction technique. The kinetics of crystallization of different crystalline phases of the samples has been studied by DTA. Two exothermic peaks in the curve indicate two distiiict crystallization temperatures, the exothermic reaction caused by the first crystallization process, may responsible for a absence of a clear glass transition Temperature (Tg) and the second stage of crystallization (Tx) corresponds to the another sharp exothermic peak. It is observed that, with increasing amount of Fe, the T. increases monotonically. The values of T, which are higher than those of T. remain practically constant for all the compositions. DTA traces of amorphous ribbons taken in argon atmosphere with continuous heating rate of 10 to 50OC/min at a step of 10°C. The peak temperature shifts to higher values with increasing heating rate. The crystallization activation energy determined from the Kissinger plot using DTA thermograms for two crystallization phases are found. The thermal activation energies for T. phases only slightly increase with increasing Fecontent in the amorphous ribbon. A vibrating sample magnetometer (VSM) has been used for measuring magnetization of Co-Fe-based ribbons of different composition at room temperature to look for the composition dependence of magnetization. The study of magnetization process as aflected by two step annealing time at constant annealing temperature 390°C and is interpreted internis of domain reorganization by heat treatment. The effect of annealing time on magnetization has also been measured using VSM and is slightly increased in saturation state. The complex permeability of amorphous ribbon with composition Co8oFeB1 oSi10 [x= 0, 2, 4, 6 & 8] are measured as a function of frequency in the range 1KHz to 13 MI-Iz using impedance analyzer. Two step annealing time effect on initial permeability (µi), loss factor and relative quality factor have been measured for different annealing temperature in the range 25 to 450°C. The real part of the permeability (µ') of©all the samples as affected by annealing at different temperature with two step annealing time in each case, is measured at low frequency (1 KHz) and at very low field (H= 0.11 AIm). The µ' decreases monotonously with increasing annealing temperature and with increasing annealing time. These effects of annealing lead to the ordering of magnetic atomic pairs, which increases anisotropy and decreases permeability. Our results of annealing on Co-Fe based samples, which show a decrease of µ' with step annealing time, are explained as due to the growth of the nucleation centers of the crystallites form atom during preparation. These effects contribute to an increase of the µ' but at the same time the 40. imaginary component is also influenced, particularly due to eddy current effects. It is evident from the results that annealing temperature and annealing time are both important parameters in controlling the frequency response of permeability of the samples. The best choice of these parameters depends on the desired characteristics of the material in respect of permeability value and its annealing time effects.