Ferro-magnetism is a phenomenon exhibited by materials such as Iron, that tend to become magnetized in the presence of an external magnetic field. The reed blades in a Reed Switch are Ferro-magnetic, but due to the special annealing process that they undergo, they tend to hold only minimum residual magnetism that does not interfere with the operation of the Reed Switch.
Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagnetic materials are familiar metals that are noticeably attracted to a magnet, a consequence of their substantial magnetic permeability. Magnetic permeability describes the induced magnetization of a material due to the presence of an external magnetic field. This temporarily induced magnetization, for example, inside a steel plate, accounts for its attraction to the permanent magnet. Whether or not that steel plate acquires a permanent magnetization itself depends not only on the strength of the applied field but on the so-called coercivity of the ferromagnetic material, which can vary greatly.
In physics, several different types of material magnetism have been distinguished. Ferromagnetism (along with the similar effect ferrimagnetism) is the strongest type and is responsible for the common phenomenon of magnetism in magnets encountered in everyday life. Substances respond weakly to magnetic fields with three other types of magnetism—paramagnetism, diamagnetism, and antiferromagnetism—but the forces are usually so weak that they can be detected only by sensitive instruments in a laboratory. An everyday example of a permanent magnet formed from a ferromagnetic material is a refrigerator magnet, such as those used to hold paper on a refrigerator door. The attraction between a magnet and a ferromagnetic material like iron has been described as "the quality of magnetism first apparent to the ancient world and to us today".
Permanent magnets (materials that can be magnetized by an external magnetic field and remain magnetized after the external field is removed) are either ferromagnetic or ferrimagnetic, as are the materials that are attracted to them. Relatively few materials are ferromagnetic and are typically pure forms, alloys, or compounds of iron, cobalt, nickel, and certain rare-earth metals. Beyond its chemical composition, a material's ferromagnetic properties (or lack thereof) are affected by its crystal structure. Ferromagnetism is vital in industrial applications and modern technologies, as it is the basis for many electrical and electromechanical devices, such as electromagnets; electric motors; generators; transformers; magnetic storage, including tape recorders and hard disks; and nondestructive testing of ferrous materials.
Ferromagnetic materials can be divided into magnetically soft materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically hard materials, which do. Permanent magnets are made from hard ferromagnetic materials, such as alnico, and ferrimagnetic materials, such as ferrite, that are subjected to special processing in a strong magnetic field during manufacturing to align their internal microcrystalline structure, making them difficult to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on the coercivity of the respective material. Hard materials have high coercivity, whereas soft materials have low coercivity. The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization.
