On the knee field definition of rare earth magnets using the parametric approach to the demagnetization curves

This study evaluates the magnetic field intensity at which irreversible demagnetization begins, traditionally referred to as knee field (H k ). Historically, the industry has defined H k as the magnitude of the opposing demagnetizing field at which the intrinsic magnetization (M) of the magnet decreases by 10% from its remanence (ψB r = 0.9B r ). Here we evaluated whether the 0.9Br limit accurately reflects the maximum geometric curvature of various NdFeB-based sintered magnet alloys, thereby shifting the definition of H k from empirical subjectivity to an analytical framework. A parametric and mathematical approach is proposed to determine the onset of the knee field of the demagnetization curve. The Savitzky-Golay parameters have been adaptively optimized for each alloy composition owing to variations in the signal-to-noise ratio across the different experimental datasets. It has been mathematically shown that ψ close to 0.9 remains a robust empirical reference for determining the knee demagnetization field in high squareness factor (SF > 0.5) NdFeB magnets, whereas low-squareness loops exhibit large deviations from this convention.