Coercive force
The magnetic coercive field strength ( H for the magnetic field strength and c for coercivity, from the Latin coercere = tame, to hold together) is the name given to the magnetic field strength that is necessary to completely demagnetize a ferromagnetic substance that has previously been charged to saturation flux density , so that the resulting total flux or the local flux density is zero. The higher the coercive field strength, the better a magnet retains its magnetization when exposed to an opposing field. The SI unit is, as in all magnetic field strengths A / m. Occasionally the outdated unit Oe (Oersted: 1 Oe corresponds to almost 80 A / m) is still used.
If ferromagnetic or ferrimagnetic materials are exposed to a magnetic field, a residual magnetism, the remanence , remains even after the field has been removed . This also applies to spin glasses , albeit in a weakened form.
Analogous to this, the electrical field strength that is necessary to cancel the remanent dielectric shift ( polarization ) of a ferroelectric or electret is called electrical coercive field strength . The higher it is, the better the material retains its polarization. It influences the piezoelectric properties ( piezoelectricity ).
For the magnetic coercive field strength the imprecise and now obsolete term coercive force was used in the past .
application
A distinction is made between the coercive field strength ( H cB ) of the magnetic flux density and the coercive field strength ( H cJ ) of the magnetic polarization . If a permanent magnet is exposed to a field strength H cB , the magnetic flux density in the magnet disappears . However, it is still magnetic after being removed from the field. Only with a demagnetizing field strength H cJ does the permanent magnet completely lose its magnetic polarization and thus its magnetization, that is to say the applied field strength H causes a magnetic flux density of the size μ 0 · H without offset.
The electric coercive field strength can also be defined in the same way.
The strong temperature dependence of these field strengths should be noted, which is also to be found in the temperature dependence of ferromagnetism or ferroelectricity .
Measurement
The magnetic coercive field strength is measured with a so-called coercimeter , which measures the polarization via induction in a moving coil as a function of an applied external magnetic field strength. Since the magnetizability and thus also the remanence or the coercive field strength depends on the structure of the material, knowledge about the material structure (e.g. degree of deformation) can be derived from the magnetic properties.
In order to measure the electrical coercive field strength, solid electrodes are vapor-deposited or liquid electrodes are applied to the substance to be examined. The arrangement then corresponds to that of a plate capacitor. The charge on the plate and, together with the dimensions, the electrical field strength and the dielectric displacement can be determined from the charge reversal and the measured voltage . The electrical coercive field strength can be determined by recording the complete hysteresis curve .
Typical values and dependencies
The values of the magnetic coercive field strength of ferromagnetic and ferrimagnetic materials sometimes vary considerably with similar or the same composition. In addition to the substance or the mixture of substances, it depends on the following additional parameters:
- Crystallite structure and size
- mixed phases occurring in alloys
- Residual stress state ( hardened , cold-formed , tempered , annealed )
The measurement of the coercive field strength therefore also serves in particular for the non-destructive testing of ferromagnetic materials (especially iron and steel as construction material ) with regard to their structural properties, their thermal pretreatment and any preceding plastic deformations. The mechanical hardness corresponds to the magnetic hardness, i.e. the coercive force.
| material | magnetic coercive field strength H cJ (A / m) |
|---|---|
|
Iron (technically pure) |
10 ... 200 |
| Dynamo sheet I | 200 |
| Dynamo sheet IV | 25 ... 60 |
| cold rolled sheets | 20… 35 |
| Nickel iron (50% Ni) |
3… 16 |
|
Mu metal (76−80% Ni, 15−16% Fe, 4−5% Cu, 2−3% Cr) |
0.8 ... 5 |
| Neodymium-iron-boron | (0.87 ... 2.75) 10 6 |
literature
- Horst Kuchling: Pocket book of physics. 4th edition. Harry Deutsch publishing house, Frankfurt am Main, 1982
- Günter Springer: Expertise in electrical engineering. 18th edition. Verlag - Europa - Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9
- Hans Fischer: Materials in electrical engineering. 2nd Edition. Carl Hanser Verlag, Munich / Vienna, 1982 ISBN 3-446-13553-7
- Horst Stöcker: Pocket book of physics. 4th edition. Verlag Harry Deutsch, Frankfurt am Main, 2000, ISBN 3-8171-1628-4
Web links
- Applet on the origin of coercivity in the macrospin model ( Memento from March 3, 2016 in the Internet Archive )
Individual evidence
- ↑ See also Wilhelm H. Westphal: Physik . Springer-Verlag, Berlin / Heidelberg / New York. While the term coercive force was still used in the 21st edition of 1953 , it is used in the 25th and 26th edition . Edition of 1970 in the text and subject index has been replaced by coercive field strength , and in the text a "(not good: coercive force)" was placed after the definition of the coercive field strength.