1.5
Magnetic materials are generally divided into two categories, soft materials and hard materials. The soft magnetic materials are easy to magnetize and demagnetize; they have high permeability and low losses. The hard magnetic materials are difficult to magnetize and demagnetize. Therefore, the hard magnetic materials are generally referred to as permanent magnets.
Permanent magnets were initially made from quench-hardened steels. Some references suggest that good permanent-magnet steel was available from China as early as 500 A.D. These permanent-magnet steels, which were initially made from plain carbon steel, steadily progressed to highly alloyed cobalt steels with a carbon content as high as 1.2 percent by 1920. These permanent-magnet steels are mechanically very hard materials, with Brinell hardness values as high as 690.
High-permeability low-loss materials are made from very low carbon annealed steels. These low-alloy annealed steels are mechanically very soft materials, with Brinell hardness values as low as 130.
The permanently magnetic materials became known as hard magnetic materials, and the high-permeability low-loss materials became known as soft magnetic materials, with the terms soft and hard referring to the mechanical hardness of the material. The following sections describe the material properties for both the soft and hard materials.
The electrons in the atoms of materials circulate around the atoms in orbits, creating an orbital magnetic field. The electrons also spin on their own axes, producing a spin magnetic field. In most materials, there are other electrons which cancel these magnetic fields. However, magnetic materials such as iron, nickel, and cobalt have lone electrons which contribute a net magnetic field to each atom. Groups of atoms with the same magnetic field direction are called domains. Also, groups of atoms that form into one continuous crystal structure are called grains. In general, there are approximately 1015 atoms in a domain, 106 domains in a grain, and 102 grains in a cubic centimeter.
During magnetization of some materials, the domain boundaries (or walls) move, so that the aligned domains become larger and the misaligned domains become smaller. If the magnetic material contains alloying elements, such as in the case of the quench-hardened permanent-magnet steels, nonmagnetic carbide inclusions are formed in the grain boundaries. The motion of a domain boundary through a nonmagnetic inclusion requires more energy, and this makes it more difficult to magnetize and demagnetize the permanent-magnet steels.
In other materials, the domain walls are pinned at the grain boundaries by carbide or other intermetallic compounds. If the energy required to move the domain wall is not exceeded during magnetization, the domain wall will remain pinned, and the entire domain will rotate to align with the magnetizing field.