Structural and magnetic characterization of Co-based soft magnetic amorphous nanocomposite films

Abstract

Metal Amorphous Nanocomposites (MANCs) are an evolving class of soft magnetic materials consisting of an Fe- or Co-based crystalline phase embedded in an amorphous matrix. In thin-film form, they possess highly tunable magnetic properties, low eddy current and hysteresis losses, and are capable of performing under high-frequency magnetic field reversal. Therefore they are good candidates for a range of applications including use in power electronics, transformers, and electric motors[1, 2]. We investigated the structural, static and dynamic properties of magnetron sputtered and vacuum annealed (5 minutes and 20 minutes at 520oC) films of CoFeMnNbSiB using Transmission Electron Microscopy (TEM), Atom Probe Tomography (APT), Vibrating Sample Magnetometry (VSM) and Ferro-Magnetic Resonance (FMR). The sputtered films did not show crystallinity, however, an Mn-rich columnar structure that spanned the thickness of the film parallel to the growth direction was observed. After annealing for 5 minutes and 20 minutes, the columnar structure transformed to a partially crystallized microstructure with Co and Fe enriched crystallized phases with ~7nm grain size enclosed by an amorphous intergranular region as shown in Fig. 1. The magnetization dynamics of the films were measured using a custom-built instrument with a frequency range of 1-65 GHz and fields up to 1.6 T. The FMR spectrum of annealed samples shows two overlapping resonances (Fig. 2) due to the two phases present in the samples whereas the as-sputtered sample shows only one resonance. The effective magnetization remained constant whereas the linewidth increased from 13 to 55 Oe after annealing. The magnetic coercivity was observed to be (~26 Oe) independent of annealing while the saturation magnetization increased from 828 to 953 emu/cm3. The observed changes in magnetic properties with annealing indicate that short-time annealing can be useful in optimizing the performance of these MANCs and tailoring properties for various applications.  Fig. 1.a) TEM image of 20 minutes annealed sample b) Magnified section of a crystalline region  Fig. 2. FMR spectrum of 20 minutes annealed sample at 40 GHz reflecting two overlapping peaks due to dual-phase in the sample