Coercitive force

Family of minor hysteresis loops for an iron with grains. BR
note remanence, HC, the coercitive force.
In science of materials, the coercitive force of a ferromagnetic material indicates the intensity of the magnetic field which it is necessary to apply, with a material having initially reached its saturation magnetization, to cancel the magnetization of material. The coercitive force is usually noted HC or BC.
When the coercitive force of ferromagnetic is very high, the material is described as hard. It is then very suitable like material for the manufacture of permanent magnets, for example inside electrical motors or of the supports of tape recording (hard drives, diskettes, magnetic bands, etc).
Contrary, a ferromagnetic material having a low coercitive force is qualified the soft one and can be used in the magnetic shielding, in transformers or recording heads.
Experimental measurement

Determination of the coercitive force of a ferromagnetic material
can be carried out by a simple analysis of the hysteresis loop.
The measurement of the coercitive force can be typically realized using a measurement of the hysteresis loop, for example using a magnetometer, for example a magnetometer with vibrating sample or a magnetometer with gradient of field. The coercitive force is determined by the half-width of the curve of hysteresis. If no antiferromagnetic material is present in the sample, the coercitive force is measured at the point where the curve crosses the axis of the fields. If an antiferromagnetic material is present, the effect of the coupling of exchange can induce a shift of the hysteresis loop on the axis of the fields, thus modifying the procedure of measurement.
Force coercitive of some soft and hard magnets standard.
Material Coercitive force
Permalloy, Ni81Fe19 0,5-1
Co 20
Ni 150
Ni1-xZnxFeO3 a material microwave 15-200
Alnico 1500-2000
CoPtCr 1700
NdFeB 10000
Fe48Pt52 > 12300
SmCo5 40000
During the measurement of the coercitive force, the measured numerical result depends on the scale of time over which the curve of hysteresis is measured. Thus, the magnetization of a material beforehand magnetized and subjected to a field lower than the coercitive, but opposed force signs some with the direction of initial magnetization, tends to be cancelled on the large scales of time. This decrease occurs when the movement of magnetic walls of fields is thermically activated and dominated by magnetic viscosity. Conversely, at great frequency of measurement, the coercitive force increases. It is about one of the limitations to the increase the speed of transfer in the devices of tape recording.
Renversement d’aimantation
At the time of the course of a hysteresis loop, the coercitive force corresponds to the moment when the magnetization of material is on the way to change. According to the nature of the sample, several mechanisms can take place within the sample during the inversion of magnetization. The two most frequent mechanisms include the inversion by coherent rotation the magnetization of material, regarded as a whole, turns then gradually, the magnetization read on the hysteresis loop corresponding at the time to projection of this magnetization on the direction of measurement or by propagation of walls of the fields of opposite magnetization are created within material, the magnetization of the hysteresis loop being then the algebraic sum of magnetizations of these fields.
As in any process of hysteresis, the surface drawn by the curve of magnetization corresponds to an energy dissipated at the time of the cycle. Among the current processes of dissipation, the magnetostriction and the propagation of walls of fields intervene. This loss of energy can be awkward in the macroscopic applications of soft magnetic materials. The coercitive force thus makes it possible to evaluate the material adaptation.
Conversely, square character of a cycle, that is to say the association of a strong residual magnetization, a brutal inversion of the hysteresis loop and of a high coercitive force, are advantageous properties for a permanent magnet.
Because of their crystalline properties, certain magnetic materials can not have the same properties of coercitive force in all the directions of measurement. the measurement of the hysteresis loop and thus of the surface under the cycle then makes it possible to characterize the energy of anisotropy of material.

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