Advanced structure research methods of amorphous Co69Fe4Cr4Si12B11 microwires with giant magnetoimpedance effect: Part 2 – Microstructural evolution and electrical resistivity change during DC Joule heating

Abstract

This Part 2 focuses on the processes of atomic regions ordering of the amorphous material and continues study of the processes occurring in Part 1. The formation of pre-crystals / clusters and nanocrystallization of Co-based amorphous microwires during DC Joule heating were investigated, as well as the influence of these processes on the electrical resistance of the samples and the effect of giant magnetoimpedance (GMI). In this paper we investigated the dependence of the change in the maximum response of the GMI diagonal component at different temperatures and holding times in the DC Joule heating process, and discuss the effect of the ductile-brittle transition on the electrical resistance and the GMI effect. Using high-resolution transmission electron microscopy (HR-TEM) and the inverse Fourier transform technique, it was shown that at long exposure during the DC Joule heating, significantly below the crystallization temperature (~ 400 °C), the formation and growth of pre-crystals / clusters occur in various sizes (1 – 8 nm) and shapes. At the initial stage of crystallization, the microwire has three distinct regions of the clusters and nanocrystals formation (surface, pre-surface, and bulk). Moreover, pre-crystals / clusters growth and further bulk formation of nanocrystals (of 10 nm and more) leads to a sharp drop in the electrical resistance and degradation of the GMI microwire diagonal component response. The approach of controlled long-term holding during the DC Joule heating applied in the study allows for creation of various nanoscale structures in an amorphous matrix and provides an opportunity to grow of a given structure in amorphous alloys.