Study of Nanocrystal Formation in Finemet Metallic Glasses and Their Magnetic Properties

Abstract

This thesis is based on the experimental investigation of nanocrystal formation from Finemet type of amorphous ribbons subjected to controlled thermal treatment and evolution of unique soft magnetic properties that are m icrostructure dependent. The samples with nominal compositions Fe75.5Cu1 Nb1Si1.35B9 (sample-A) and Fe74 Cu0,5Nb3Si Si1.35B9 (sample-B) have been prepared by rapid solidification technique and their amorphous nature has been confirmed by X-ray diffraction (XRD). The crystallization behavior and the nanocrystal formation have been studied by Differential Thermal Analysis (DTA) and XRD. Magnetic permeability and magnetization measurements have been carried out using inductance analyzer and vibrating sample magnetometer (VSM). Crystallization onset temperature Txլ corresponding to primary crystallization of bce Fe(Si) due to which the excellent soft magnetic properties develops is higher for the sample-B (504°C) than that for sample-A (470°C) and the temperature difference between the primary and secondary crystallization peak is higher for sample-B. Therefore sample-B obviously show higher thermal stability. The crystallization onset temperatures determined by XRD are in good agreement with DTA results. The grain sizes determined on annealed samples are from 1 3 nm to 22 nm for sample-B compared with 17 tim to 27 nm for sample-A. Magnetic permeability sensitively depends on the annealing temperature for both the samples, which increases sharply with the increase of annealing temperature having substantial higher value for sample-B. Maximum permeability corresponding to optimal annealing temperature were observed at Ta = 425°C for sample-A and Ta 575°C for sample-B. This is due to more refined grain structure of 13-22 nm for sample-B. A sharp deterioration of permeability at higher Ta is observed for both the samples and is due to bonicle phase formation which has been confirmed by µ՝ vs. T and XRD experiments. Saturation magnetization Ms, increases with Ta for both the samples and finally decreases for annealing at a temperature much higher than peak crystallization temperature. The Curie temperature Tc is higher for sample-A (421°C) than that of sample-B (360°C). The Curie temperature of sample-A is found to increase gradually with Ta up to initiation of crystallization beyond which it decreases. Further increase of Ta, an enhancement of T is noticed which may be attributed to the compositional variance resulting in a distribution of Curie temperatures. The high permeability of the optimum nanocrystallized sample in both the cases has been achieved due to drastic decrease of effective anisotropy resulting from nanometric grain size effect and strong magnetic coupling. The results show that amount of Cu and Nb is very important for the soft magnetic properties of Finemet alloys.