Most of magnetic materials of industrial interests are ferromagenetic materials. The ferromagnetic materials can be categorized into two; one is soft magnetic materials and the other is hard magnetic materials. As shown in the magnetization curve, ferromagnetic materials with the
demagnetized state does not show magnetization although they have spontaneous
magnetization. This is because the ferromagnetic materials are divided
into many magnetic domains. Within the magnetic domains, the direction
of magnetic moment is aligened. However, the direction of magnetic moments
vary at magnetic domain walls so that it can reduce the magnetostatic energy
in the total volume. In the demagnetized state, total magnetization is
cancelled because of the random orientation of the magnetizations in magnetic
domains. When external magnetic field is applied, domain walls migrate
and disappear when all magnetic moments are aligened to the direction of
the magnetic field. When all magnetic domains are wiped away and magnetizations
are all aligned to the direction of the magnetic field, magnetization is
saturated. This magnetization is called saturation magnetization, Ms.
When domain wall can easily migrate, the ferromagnetic material can be easily magnetized at low magnetic filed. This type of ferromagnetic materials are called soft magnetic material, and is suitable for applications of magnetic cores or recording heads. Since soft magnetic materials can be demagnetized at low magnetic field, coercivity Hc is low. As they can be easily magnetized, permeability is high. For ferromagnetic materials to be soft, their magnetocrystalline anisotropy and magnetostriction constant must be low. In addition, for easy migration of magnetic domains, they must have small number of defects such as crytal grains.
When domain wall is difficult to migrate, magnetization of the ferromagnetic material occurs only when high magnetic field is applied. In other words, this type of ferromagnetic materials are difficult ot magnetize, but once magnetized, it is difficult to demagnetize. These materials are called hard magnetic materials, and are suitable for applications such as permanent magnets and magnetic recording media. Hard magnetic materials have high magnetocrystalline anisotropy. Since large magnetic field is required to demagnetize, their coercivity Hc is usually high, but coercivity is highly sensitive to the microstructurure.