Introduction to magnetic hysteresis, including hysteresis power loss and Steinmetz's law
Key FactsGyroscopic Couple: The rate of change of angular momentum () = (In the limit).
- = Moment of Inertia.
- = Angular velocity
- = Angular velocity of precession.
Key facts Magnetic hysteresis is a characteristic of ferromagnetic materials, consisting of the lack of retraceability of the initial magnetization curve when the magnetic field is relaxed. The hysteresis power loss of iron is given by:
where is a constant, is the frequency of operation, and is the magnetic flux density. This equation is also known as Steinmetz's law.
Magnetic hysteresis is a property which affects ferromagnetic materials. In order to better understand hysteresis, consider an unmagnetized sample found at the origin of a magnetic flux density - magnetic field strength plot (point in Figure 1). As the magnetic field strength is increased in the positive direction, the magnetic flux density begins to grow non-linearly until the sample will be found at point on the curve. This path is called the initial magnetization curve. If the magnetic field is then relaxed, instead of following back on the initial magnetization curve, the magnetic flux density falls more slowly. In fact, even when the applied magnetic field is returned to zero, there will still be a remanent magnetic flux density (point in Figure 1). This lack of retraceability of the initial magnetization curve is called hysteresis. In order to drive the magnetic flux density back to zero (point in Figure 1), the applied magnetic field must be reversed. If the reversed magnetic field is further increased, the magnetic flux density will continue to decrease (point in Figure 1). If the magnetic field is then again increased in the positive direction, the magnetic flux density will pass through zero (point in Figure 1), and then return to the value corresponding to point in Figure 1. The resulting loop is called a hysteresis loop. Hysteresis is a property of ferromagnetic materials with great applicability as a magnetic memory device. Indeed, as shown in Figure 1, when the driving magnetic field drops to zero (path ), the ferromagnetic material retains an important degree of magnetization. Certain compositions of ferromagnetic materials will actually retain an imposed magnetization indefinitely and can be used as permanent magnets. A widely used application of the magnetic memory (usually that of iron and chromium oxides) is for the magnetic storage of data on computer disks.