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 A good permanent magnet must produce a high magnetic field with a low mass and should be stable against the influences that the demagnetized. The desirable properties of magnets and are generally expressed in terms of persistence and the coercivity of magnetic materials.

When a ferromagnetic material is magnetized in one direction, does not relax to zero magnetization when the imposed magnetizing field is removed. The amount of magnetization remains zero in the camp called radiance. Must be reduced to zero by a field in the opposite direction, the amount of land needed to reverse the coercivity demagnetization is called its.If an alternating magnetic field applied to the material, its magnetization traces a loop called a hysteresis loop. The lack of traceability of the magnetization curve, the property called hysteresis and is related to the existence of magnetic domains in the material. When the magnetic domains are reoriented, it takes little energy to push them. This property ferrromagnetic materials are useful as a magnetic "memory". Some compositions of ferromagnetic materials to keep imposed magnetization indefinitely and are useful as "permanent magnets".

The following table provides some information about the materials used in permanent magnets. Both the coercivity and retentivity are listed on the Tesla, the basic unit of magnetic field B. The hysteresis loop is plotted in the form of the magnetization M as a function of guiding the magnetic field strength H.This practice is generally followed, because it shows the influence of external driving (H) on the horizontal axis and the response to the material (M) on the vertical axis. In addition, the coercivity and retentivity, the quality factor is the number of permanent magnets (BB0/μ0) max. High value of this quantity implies that the magnetic flux needed to provide a smaller amount of material, where the device is lighter and more compact.