Determining the Curie temperature by using the frequency tracker device of a resonant inverter

An experimental setup designed to determine the Curie temperature Tc of solid materials is presented. The main idea is based on the experimental frequency tracking of a resonant inverter circuit, which is controlled by a phase lock loop (PLL) device. When a ferromagnetic metallic piece is placed inside the resonant coil, the effective impedance is modified due to its magnetic permeability variations caused by heating. Hence, the PLL frequency and the temperature of the sample are simultaneously recorded to determine the magnetic transition point. Later, the Curie temperature of powdered and solid pieces of pure nickel, stainless steel and MnZn-ferrite are measured. In addition, a sample of magnetite nanoparticles is analysed. Discrepancies lower than 2.3% of the known Tc values are observed for the powdered samples, and similar results are obtained with the solid samples. The ferrite did not completely reach magnetic transition, differing by up to 30% from the reference value.