Study of the Crystallization Kinetics of Amorphous Magnetic Materials

Abstract

Amorphous state of materials is a metastable state, which transforms into more stable crystalline phase while subjected to appropriate thermal treatment. Nanocrystallization of amorphous metallic magnetic ribbons by annealing above their crystallization temperature renewed enormous scientific and technological interest as a materials of state-of-the-art applications. In this research work we have focused on the study of understanding the crystallization behavior and magnetic properties of such nanocrystalline alloys with respect to annealing parameters, grain size and the crystalline volume fraction. Two series of amorphous ribbons of alloy compositions, (I) Fe73.5-xCrxNb3Cu1Si13.5B9 (x = 1, 5, 10, 12.5 and 17.5) and (II) Fe76.5-xNbxCu1Si13.5B9 (x = 1, 2, 3, 4, 5, 6 and 7) are cast by melt spinning technique and annealed at different temperatures based on thermal analysis data to prepare samples of varying nanocrystalline states. The magnetic, electrical and structural properties are investigated by VSM, Impedance Analyzer, XRD, FESEM and Mössbauer Spectrometer. The crystallization behaviors of as cast amorphous samples are studied by DSC technique. The volume fractions of amorphous and crystalline phases have been confirmed by the Mössbauer Spectrometry. Result obtained from detailed study show that crystallization kinetics of both the series of prepared samples are dependent on contents of refractory metals Cr and Nb substitution. The role of both Cr and Nb is to inhibit the process of crystallization and hinder grain growth and shift the crystallization temperature towards the higher temperature imparting enhancement of thermal stability of the amorphous alloys against crystallization. This unique behavior facilitates controlled nanocrystallization of the amorphous samples by annealing that finally give novel magnetic properties. Experimentally it is verified that Nb is more powerful in controlling the crystallization kinetics then Cr. It has been observed that amorphous alloys with Cr = 1, 5 and Nb = 3, 4 are suitable alloy compositions for better soft magnetic properties while annealed at Ta = 550oC and seems to be agree well with Herzer’s Random Anisotropy Model (RAM).