What is the coercivity of a magnet?

The coercive magnetic field strength (Hc) denotes the magnetic field strength required to completely demagnetize a charged magnet.

What does coercivity mean?

Magnetic coercivity is the magnetic field strength that must be applied to demagnetize a magnetic product such as a ferrite magnet , pre-charged to its saturation flux density.

Demagnetization means that the total flux or local flux density is zero. This happens when a permanent magnet is in a reverse polarised magnetic field of coercive force Hc. If a magnet is exposed to an opposite field, it depends on its coercive force to maintain its magnetization, depending on its quality. The rule applies: the higher the coercive force of a magnet, the better a magnet can retain its magnetization.

Differentiation of coercive force HcB and HcJ

Coercive field strength is distinguished between the coercive force (HcB) of the magnetic flux density and the coercive force (HcJ) of the magnetic polarisation:

A permanent magnet loses its magnetic flux density when exposed to the field strength HcB but remains magnetic when removed from the field.

The flux density generated is opposite, but the same size as the flux density of the demagnetizing field, so that the two sides cancel each other out and feel no effect. When the external opposite field is turned off again or removed: The magnetic forces still leave the magnet, as a remnant, of the magnet.

Only an HcJ field strength leads to polarisation and thus the magnet loses its magnetization completely and permanently. In this case, it is no longer magnetic but becomes magnetized again. In the same way, the coercive electric field strength can be defined.

Measurement of coercive field strength

All magnetic fields are measured in the unit A/m (ampere per meter).

Occasionally, you may still find the unit of measurement Oe (Oersted). As a conversion aid: 1 Oe corresponds to approximately 80 A / m.

You can measure magnetic coercivity with a device, called a coercimeter. This coercimeter measures the induction polarisation in a moving coil as a function of the external magnetic field strength.

To measure an electric coercive force, solid electrodes are vapor-deposited in a plate capacitor arrangement on the material to be measured.

From the recharge current and the measured voltage, the charge on the plate can be determined, together with the measurement of the electric field strength and the electric displacement.

Different materials have different coercive magnetic field strengths, measured in A/m, while technically pure iron has a value of 10 to 200 A/m, nickel (50% nickel) has a value of 3 to 16, and neodymium iron a value of (0.87 to 2.75) x 10 6. 6.

It can be seen very quickly how different the materials are in terms of their coercivities.

Reasons for determining coercivity

Why is it important to measure and know the coercivity?

Measuring the coercive field strength helps in particular in the non-destructive testing of ferromagnetic materials and materials such as iron or steel as construction materials. Here it is important to verify and know the microstructural properties, any heat treatment, or even previous plastic deformations. The mechanical hardness corresponds here to the coercive force, i.e. the magnetic hardness.

Magnetic coercivities

The determined values of the magnetic coercive field strength of ferromagnetic materials vary in some cases clearly with similar or even the same composition. The field strength depends not only on the composition but also on factors such as the crystalline structure and size, the mixed phases occurring in the alloy, and the residual stress state. The residual stress state describes whether a material has been hardened, cold-worked, or annealed.

Remanence

Remanence was mentioned above: if ferromagnetic substances are exposed to a magnetic field, a residual magnetism remains even after the removal of the magnetic field. This residual magnetism is also called remanence.

Ferromagnetic substances differ from ferromagnetic materials not in the arrangement of the so-called white districts in the crystal structure, but in the magnetic arrangement of their elementary magnets, which are produced by an energetically favorable orientation.

The magnetizations of two adjacent elementary magnets partially cancel each other out, so that the white areas are more weakly magnetized. The macroscopic behavior is therefore a weaker form of ferromagnetism.

The strength of the coercive electric field is necessary to cancel the remaining polarisation of a ferroelectric. Again, the higher the field strength, the better the material will retain its polarisation. Field strength also influences piezoelectricity. Piezoelectricity describes the change in electrical polarization and the occurrence of electrical strain when elastically deformed.

Hysteresis

Hysteresis is a secondary effect, i.e. a delayed change of effect after a change of a cause. This side effect occurs especially in the magnetization of magnets and characterizes a delayed behavioral variant of the affected output which has reached its maximum or minimum.

A well-known phenomenon is the hysteresis behavior of a ferromagnet in a magnetic field. An unmagnetized ferromagnet, which has been exposed to an external magnetic field and then switched off again, maintains a positive or negative magnetization depending on the polarity of the external magnetic field.