Magnetic materials are important industrial materials used for many engineering designs.
Most industrial magnetic materials are ferro- or ferrimagnetic and show large magnetizations. The most important ferromagnetic materials are based on alloys of Fe, Co, and Ni. More recently some ferromagnetic alloys have been made with some rare earth elements such as Sm. In ferromagnetic materials such as Fe, there exist regions called magnetic domains in which atomic magnetic dipole moments are aligned parallel to each other. The magnetic domain structure in a ferromagnetic material is determined by the following energies that are minimized: exchange, magnetostatic, magnetocrystalline anisotropy, domain wall, and magnetostrictive energies. When the ferromagnetic domains in a sample are at random orientations, the sample is in a demagnetized state. When a magnetic field is applied to a ferromagnetic material sample, the domains in the sample are aligned; the material becomes magnetized and remains magnetized to some extent when the field is removed. The magnetization behavior of a ferromagnetic material is recorded by a magnetic induction versus applied field graph called a hysteresis loop. When a demagnetized
ferromagnetic material is magnetized by an applied field H, its magnetic induction B eventually reaches a saturation level called the saturation induction Bs. When the applied field is removed, the magnetic induction decreases to a value called the remanent induction Br. The demagnetizing field required to reduce the magnetic induction of a magnetized ferromagnetic sample to zero is called the coercive force Hc.
A soft magnetic material is one that is easily magnetized and demagnetized. Important
magnetic properties of a soft magnetic material are high permeability, high saturation
induction, and low coercive force. When a soft ferromagnetic material is repeatedly magnetized and demagnetized, hysteresis and eddy-current energy losses occur. Examples of soft ferromagnetic materials include Fe–3 to 4% Si alloys used in motors and power transformers and generators and Ni–20 to 50% Fe alloys used primarily for high-sensitivity communications equipment.
A hard magnetic material is one that is difficult to magnetize and which remains magnetized to a great extent after the magnetizing field is removed. Important properties of a
hard magnetic material are high coercive force and high saturation induction. The power of a hard magnetic material is measured by its maximum energy product, which is the maximum value of the product of B and H in the demagnetizing quadrant of its B-H hysteresis loop. Examples of hard magnetic materials are the alnicos, which are used as permanent magnets for many electrical applications, and some rare earth alloys that are based on SmCo5 and Sm(Co, Cu)7.5 compositions. The rare earth alloys are used for small motors and other applications requiring an extremely high energy-product magnetic material.
The ferrites, which are ceramic compounds, are another type of industrially important
magnetic material. These materials are ferrimagnetic due to a net magnetic moment produced
by their ionic structure. Most magnetically soft ferrites have the basic composition MO · Fe2O3, where M is a divalent ion such as Fe2+, Mn2+ and Ni2+,. These materials have the inverse spinel structure and are used for low-signal, memory-core, audiovisual, and recording-head applications, as examples. Since these materials are insulators, they can be used for high-frequency applications where eddy currents are a problem with alternating fields. Magnetically hard ferrites with the general formula MO · 6Fe2O3, where M is usually a Ba or Sr ion, are used for applications requiring low-cost, low-density permanent magnetic materials. These materials are used for loudspeakers, telephone ringers and receivers, and for holding devices for doors, seals, and latches.
