Annealing and thickness optimisation in CoZrNb and FeSiBNbCu thin films for low-frequency magnetic flux concentrators

To develop high-performance soft magnetic flux concentrator (MFC) materials, which are crucial for amplifying magnetic fields through the enhancement of local magnetic flux density, two ferromagnetic alloys, Co₈₈Zr₄Nb₈ (CZN) and Fe_73.5Si_15.5B₇Nb₃Cu₁ (Finemet^®), were investigated. Thin films with thicknesses ranging from 100 to 1200 nm were deposited by magnetron sputtering and annealed at various temperatures. CZN demonstrated optimal soft magnetic properties in its as-grown amorphous configuration. In contrast, annealing significantly improved Finemet's performance between 200°C and 500°C, where its nanocrystalline structure yielded ultra- soft behaviour. Additionally, thickness dependence revealed distinct trends in magnetic behaviour for both materials. For CZN, magnetic softness was best achieved at a larger thickness (1200 nm), while for Finemet, an optimum was observed at an intermediate thickness (800 nm). Thus, CZN is suited for cases where annealing is impractical, such as CMOS-compatible or MEMS-integrated sensors, as its properties are optimised without further processing, particularly in thicker films. Finemet, however, requires controlled annealing to obtain soft magnetic behaviour, making it suitable for applications that demand maximum permeability and can tolerate higher temperatures. Both materials demonstrate excellent potential for MFC integration in magnetic sensing and spintronic platforms. The optimal choice, however, depends on the thermal budget, processing conditions, and the performance metrics prioritised in the target application.