Retentivity

Certain ferromagnetic materials stay magnetized better than others. When a substance, such as iron, is subjected to a magnetic field as intense as it can handle, say by enclosing it in a wire coil carrying a massive current, there will be some residual magnetism left when the current stops flowing in the coil. Retentivity, also sometimes called remanence, is a measure of how well the substance will “memorize” the magnetism, and thereby become a permanent magnet.
Retentivity is expressed as a percentage. If the flux density in the material is x tesla or gauss when it is subjected to the greatest possible magnetomotive force, and then goes down to y tesla or gauss when the current is removed, the retentivity is equal to 100(y/x).
As an example, suppose that a metal rod can be magnetized to 135 gauss when it is enclosed by a coil carrying an electric current. Imagine that this is the maximum possible flux density that the rod can be forced to have. For any substance, there is always such a maximum; further increasing the current in the wire will not make the rod any more magnetic. Now suppose that the current is shut off, and 19 gauss remain in the rod. Then the retentivity, B_r, is

Br = 100(19/135) = 100 × 0.14 = 14 percent

Various different substances have good retentivity; these are excellent for making permanent magnets. Other materials have poor retentivity. They might work well as electromagnets, but not as permanent magnets.
Sometimes it is desirable to have a substance with good ferromagnetic properties, but poor retentivity. This is the case when you want to have an electromagnet that will operate from dc, so that it maintains a constant polarity, but that will lose its magnetism when the current is shut off.
If a ferromagnetic substance has poor retentivity, it’s easy to make it work as the core for an ac electromagnet, because the polarity is easy to switch. If the retentivity is very high, the material is “sluggish” and will not work well for ac electromagnets.