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Domains are regions in the material in which all of the dipoles are aligned in a certain direction. In a
material that has never been exposed to a magnetic field, the individual domains have a random
orientation. Because of this, the net magnetization in the virgin ferromagnetic or ferrimagnetic
material as a whole is zero.
Boundaries, called Bloch walls, separate the individual magnetic domains. The Bloch walls are narrow zones in which the direction of the magnetic moment gradually and continuously
changes from that of one domain to that of the next. The domains are typically very small, about 0.005 cm or less, while the Bloch walls are about 100 nm thick.
Figure taken from :The Science and Engineering of Materials by Donald R. Askeland, Pradeep P. Fulay, Wendelin J. Wright (Very good Book)
Effect of Applying Magnetic Field:
When a magnetic field is imposed on the material, domains that are nearly lined up with the
field grow at the expense of unaligned domains. In order for the domains to grow, the
Bloch walls must move; the field provides the force required for this movement.
Initially, the domains grow with difficulty, and relatively large increases in the field are
required to produce even a little magnetization. As the field increases in strength, favorably oriented domains grow more easily, with permeability increasing as well.
Eventually, the unfavorably oriented domains disappear, and rotation completes the
alignment of the domains with the field. The saturation magnetization, produced when all of the domains are oriented along with the magnetic field, is the greatest amount of
magnetization that the material can obtain. Under these conditions, the permeability
of these materials becomes quite small.
Effect of Removing the Field:
When the field is removed, the resistance offered by the domain walls prevents regrowth of the domains into random orientations. As a result, many of the domains remain oriented near the direction of the original field and a residual magnetization, known as the remanance (Mr) is present in the material. The value of Br (usually in Tesla) is known as the retentivity of the magnetic material. The material acts as a permanent magnet.The magnetic field needed to bring the induced magnetization to zero is the coercivity of the material. This is a microstructure-sensitive property.
The elongated shape of magnetic particles leads to higher coercivity (Hc). The dependence of coercivity on the shape of a particle or grain is known as magnetic shape anisotropy.
Note that while the coercivity is a strongly microstructure-sensitive property, the saturation magnetization is constant (i.e., it is not microstructure dependent) for a material of a given composition. This is similar to the way the yield strength of metallic materials is strongly dependent on the microstructure, while the Young’s modulus is not.
The coercivity of single crystals depends strongly on crystallographic directions. There
are certain directions along which it is easy to align the magnetic domains. There are other
directions along which the coercivity is much higher. Coercivity of magnetic particles also
depends upon shape of the particles. This is why in magnetic recording media we use acicular
and not spherical particles.
Domains are regions in the material in which all of the dipoles are aligned in a certain direction. In a
material that has never been exposed to a magnetic field, the individual domains have a random
orientation. Because of this, the net magnetization in the virgin ferromagnetic or ferrimagnetic
material as a whole is zero.
Boundaries, called Bloch walls, separate the individual magnetic domains. The Bloch walls are narrow zones in which the direction of the magnetic moment gradually and continuously
changes from that of one domain to that of the next. The domains are typically very small, about 0.005 cm or less, while the Bloch walls are about 100 nm thick.
Effect of Applying Magnetic Field:
When a magnetic field is imposed on the material, domains that are nearly lined up with the
field grow at the expense of unaligned domains. In order for the domains to grow, the
Bloch walls must move; the field provides the force required for this movement.
Initially, the domains grow with difficulty, and relatively large increases in the field are
required to produce even a little magnetization. As the field increases in strength, favorably oriented domains grow more easily, with permeability increasing as well.
Eventually, the unfavorably oriented domains disappear, and rotation completes the
alignment of the domains with the field. The saturation magnetization, produced when all of the domains are oriented along with the magnetic field, is the greatest amount of
magnetization that the material can obtain. Under these conditions, the permeability
of these materials becomes quite small.
Effect of Removing the Field:
When the field is removed, the resistance offered by the domain walls prevents regrowth of the domains into random orientations. As a result, many of the domains remain oriented near the direction of the original field and a residual magnetization, known as the remanance (Mr) is present in the material. The value of Br (usually in Tesla) is known as the retentivity of the magnetic material. The material acts as a permanent magnet.The magnetic field needed to bring the induced magnetization to zero is the coercivity of the material. This is a microstructure-sensitive property.
The elongated shape of magnetic particles leads to higher coercivity (Hc). The dependence of coercivity on the shape of a particle or grain is known as magnetic shape anisotropy.
Note that while the coercivity is a strongly microstructure-sensitive property, the saturation magnetization is constant (i.e., it is not microstructure dependent) for a material of a given composition. This is similar to the way the yield strength of metallic materials is strongly dependent on the microstructure, while the Young’s modulus is not.
The coercivity of single crystals depends strongly on crystallographic directions. There
are certain directions along which it is easy to align the magnetic domains. There are other
directions along which the coercivity is much higher. Coercivity of magnetic particles also
depends upon shape of the particles. This is why in magnetic recording media we use acicular
and not spherical particles.
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