This theory proposes that ferromagnetic materials are divided into small magnetic regions called domains, with each domain spontaneously magnetized in a different random direction, resulting in no net magnetization. When an external magnetic field is applied, it causes the domains aligned parallel to increase in size through the motion of domain walls or rotation of the domains. The total internal energy of the domain structure includes exchange energy, crystalline anisotropy energy, domain wall energy, and magnetostriction energy.

1. DOMAIN THEORY
This theory was proposed by Weiss 1907. It explains the hysteresis
and the properties of ferromagnetic materials.
Magnetic Domains
 A ferromagnetic material is divided into a large number of small region is called
domains.
 Each domain is spontaneously magnetized.
 The direction of magnetization various from domain to domain
and the net magnetization is zero, in the absence external mag. field.
 The two domain separates by domain wall or Block wall.
When the magnetic field is applied to the Ferromagnetic material, the
magnetizations produced by two ways.
1. By the motion of Domain walls
The weak magnetic filed is applied, the domains having dipoles parallel to the
applied magnetic field increases in area by the motion of domain walls.
2. By the rotation of Domains
If the strong magneticfield applied, the domains are rotated parallel to the field
direction
Energies involved in the domain growth
The total internal energy of the domain structure in a ferromagnetic material is
made up from the following contributions.
1. Exchange energy
2. Crystalline energy
3. Domain wall energy
4. Magnetostriction energy.

2. 1. Exchange energy is define, “ The interaction energy which
makes the adjacent dipoles align themselves”
2. Anisotropy energy : In ferromagnetic materials there are two types of
directions of Magnetization namely,
a. Easy direction b. Hard direction
In easy direction of magnetization, weak field can be
applied and in hard direction of magnetization, strong field
should be applied.
For example, in BCC iron the easy direction is [100],
the medium direction is [110], and the hard direction [111].
The energy difference between hard and easy direction to
magnetize the material is about. This energy is very important in determining
the characteristic domain boundaries.
Domain wall energy or Bloch wall energy
Based on the spin alignments, two types of Bloch walls may arise, namely
a. Thick wall :
When the spins at the boundary are misaligned
and if the direction of the spin changes gradually
as shown figure,
b. Thin wall : When the spins at the boundaries
changes abruptly, as shown in fig.
Magnetostriction energy :
When a material is magnetized, it is found that it suffer a change in
dimensions. This phenomenon is known as Magnetostriction. The work done
by the magnetic field against these elastic restoring forces is called magneto-
elastic energy or Magnetosrictive energy.