The ability of a permanent magnet to support an external magnetic field is due to crystalline anisotropy within the magnetic material that "locks" small magnetic domains in place. Once the initial magnetization is established, these positions remain unchanged until a force exceeding the locked domains is applied.
The energy required to disturb the magnetic field produced by the permanent magnet varies with each material. Permanent magnets can generate extremely high coercivity (Hcj), maintaining magnetic domain alignment in the presence of high external magnetic fields.
Stability can be described as the repeated magnetic properties of a material under specified conditions during the lifetime of the magnet. Factors that affect the stability of magnets include time, temperature, changes in reluctance, adverse magnetic fields, radiation, shock, stress, and vibration.
Time has little effect on modern permanent magnets, which research shows change immediately after they are magnetized. These changes, known as "magnetic creep," occur when less stable magnetic domains are affected by fluctuations in thermal or magnetic energy, even in thermally stable environments. This variation decreases as the number of unstable regions decreases.
Rare earth magnets are less likely to experience this effect because of their extremely high coercivity. Longer time versus flux studies show that freshly magnetized permanent magnets lose a small amount of flux over time. For more than 100,000 hours, the loss of samarium cobalt material is basically zero, while the loss of alnico material with low permeability coefficient is less than 3%.
Temperature effects fall into three categories: reversible loss, irreversible but recoverable loss, irreversible and irreversible loss. Reversible losses: These are the losses that recover when the magnet returns to its original temperature, permanent magnet stabilization cannot eliminate reversible losses.
Reversible losses are described by the reversible temperature coefficient (Tc), as shown in the table below. Tc is expressed as a percentage per degree Celsius, and these numbers vary for the specific grade of each material, but are representative of the material class as a whole. This is because the temperature coefficients of Br and Hcj are significantly different, so the demagnetization curve will appear "inflection point" at high temperature.
Irreversible but recoverable losses: These losses are defined as partial demagnetization of the magnet due to exposure to high or low temperatures, these losses can only be recovered by demagnetization, the magnetism cannot be recovered when the temperature returns to its original value. These losses occur when the operating point of the magnet is below the knee of the demagnetization curve. An effective permanent magnet design should have a magnetic circuit in which the magnet operates with a permeability above the inflection point of the demagnetization curve at the expected high temperature, which will prevent performance changes at high temperature.
Irreversible and irrecoverable loss: Magnets exposed to extremely high temperatures undergo metallurgical changes that cannot be recovered by demagnetization. The following table shows the critical temperature of various materials, where: Tcurie is the Curie temperature at which the fundamental magnetic moment is randomized and the material is demagnetized; Tmax is the maximum practical operating temperature of the main material of the general class.
The different grades of each material show slightly different values than those shown here. Note that the maximum practical operating temperature depends on the operating point of the magnet in the circuit. The higher the operating point on the demagnetization curve, the higher the operating temperature of the magnet. Flexible materials are not included in this table because the binder used to make the magnet flexible breaks down before the metallurgical change in the ferrite powder that provides the magnetic properties of the flexible magnet occurs.
In conclusion
Magnets keep temperature stable by partially demagnetizing them through exposure to high temperatures in a controlled manner. A slight decrease in flux density increases the stability of the magnet, since the domains with lower alignment are the first to lose their orientation. Such a stable magnet will exhibit a constant magnetic flux when exposed to equal or lower temperatures. In addition, a stable batch of magnets will exhibit lower flux variation when compared to each other because the top of the bell curve, which is characterized by normal variation, will be closer to the flux value of the batch.