Knee of demagnetisation, typically denoted by H_k or H_D - for hard magnetic materials, a region or point on any non-linear demagnetisation curve (B-H, J-H, or M-H), located in the second or fourth quadrant), between the part of the curve that exhibits visibly shallower slope (comparable with the level of the remanence B_R) and the part with a much steeper slope synonymous with significant changes in the state of magnetisation of the ferromagnet lead to the coercivity H_C. ¹⁾²⁾³⁾

Depending on literature, there can be several definitions of the “knee of demagnetisation”. Colloquially, it is an imaginary point which aids in quick visual analysis of the demagnetisation curves, beyond which there is a sharp decrease of the demagnetisation curve.

A related, but quite a different definition is the knee of magnetisation, because both are related to magnetic saturation.

The position of the knee of demagnetisation (dashed circles) varies strongly with temperature for permanent magnets ⁴⁾

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

Definition of quadrants

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

The position of the knee of demagnetisation is more easily identifiable for high-energy permanent magnets such as NdFeB. Exposing the magnetic material to a load line which takes it beyond the knee of demagnetisation will irreversibly demagnetise the magnet from its nominal state.⁵⁾

A measurement method was defined for example in IEC 60404-8-1:2001, in which a value H_D5 (HD5) can be measured, and which defines a such value of the demagnetising magnetic field strength H that results in irreversible loss of magnetisation by 5 %.⁶⁾ The position of H_D5 value is immediately beyond the point of the knee of demagnetisation, namely | H_knee | < | H_D5 |.⁷⁾

The position of the knee of demagnetisation depends strongly on temperature, as illustrated in the typical graphs for VACODYM 745 HR.⁸⁾

• Knee of magnetisation
• Magnetic saturation