Failure of the Rayleigh hysteresis law in low magnetic fields

The Rayleigh expression for minor hysteresis loops gives the remanent flux density Bᵣ to be proportional to the square of the amplitude of the applied field H₁, i.e., Bᵣ∝H₁². By use of a phase-sensitive detector, it has been found that this is valid only over a limited range of H₁ and that, in particular, Bᵣ∝H₁ for smaller values of H₁. All measurements were made at a frequency of 10 Hz.