Intrinsic and Extrinsic Demagnetization Curves

What is a demagnetization curve?

Ferromagnetic materials such as iron, cobalt, nickel, and many alloys, including permanent magnets, show a magnetic hysteresis phenomenon. The hysteresis loop is generated by measuring the magnetization M, while changing the applied external magnetic field H (see green curve in the figure to the left).

The magnetization in a ferromagnetic material increases from 0 or ‘virgin’ state (point ⓪ as show in the figure to the left) due to an increasing applied magnetic field, to a maximum--the point where the magnetization does not increase anymore, called magnetic saturation (point ①). Upon reducing the magnetic field back to 0, permanent magnets still retain a high value of magnetization (point ②) also known as B_r. The magnetization will eventually decrease to 0 (point ③) by applying a magnetic field in the opposite direction. The magnetic saturation in the opposite direction will occur (point ④) with increasing magnetic field in that direction.

Why is the demagnetization curve important?

Important magnetic properties of permanent magnets, such as residual induction B_r, coercivity H_c, intrinsic coercivity H_ci, and maximum energy product (BH)_max, can be obtained from demagnetization curves.

B_r is the residual induction or flux density, which represents the magnetic induction corresponding to zero magnetizing field in a magnetic material after full saturation when measured in a closed magnetic circuit. The unit for magnetic induction B and residual induction B_r is G, Gauss (CGS unit) or T, Tesla (SI unit).

M_r is the remanent magnetization or remanence, which represents the retained magnetization when the external magnetic field is reduced to zero after full saturation when measured in a closed magnetic circuit.

The unit for magnetization 4πM and remanent magnetization 4πM_r is G, Gauss (CGS unit) or A/m, Ampere per meter (SI unit)...