Domain Theory of Magnetism

Magnetism is a phenomenon present in many materials, and understanding it is crucial for a wide range of modern applications. The Domain Theory of Magnetism explains how magnetization occurs at the microscopic level in materials, especially in ferromagnetic substances like iron, cobalt, and nickel. This theory reveals how magnetic domains within a material interact with external magnetic fields, resulting in magnetization.

What Are Magnetic Domains?

Magnetic domains are small regions within a ferromagnetic material where the magnetic moments of atoms (tiny atomic magnets) are aligned in the same direction. These regions behave like tiny individual magnets. Key Features of Magnetic Domains:

Aligned Moments:

Inside each domain, the magnetic moments of atoms are aligned in one direction, creating a strong localized magnetic field.

Random Orientation:

In an unmagnetized material, these domains are randomly oriented, which cancels out the overall magnetic effect.

Domain Walls:

The boundaries between different domains are called domain walls, where the direction of magnetic moments gradually shifts.

What do you mean by domain theory?

In nonmagnetic materials, the net magnetic field is effectively zero since the magnetic fields due to the atoms of the material oppose each other. In magnetic materials such as iron and steel, however, the magnetic fields of groups of atoms numbering in the order of $10^{12}$ are aligned, forming very small bar magnets. This group of magnetically aligned atoms is called a domain. Each domain is a separate entity; that is, each domain is independent of the surrounding domains. For an unmagnetized sample of magnetic material, these domains appear in a random manner, such as shown in [Fig. 1(a)]. The net magnetic field in any one direction is zero.

How Magnetization Occurs

When an external magnetic field is applied, the magnetic domains within the material start to realign with the field. The process occurs in two main ways: domain growth and domain rotation.

Domain Growth

Domains that are aligned with the external field grow by absorbing adjacent misaligned domains. This happens as domain walls move, allowing the favorable domains to expand.

Domain Rotation

Domains that are not aligned may rotate to match the direction of the external magnetic field. While domain wall movement is easier, domain rotation becomes necessary for complete alignment. Eventually, if a sufficiently strong field is applied, all of the domains will have the orientation of the applied magnetizing force, and any further increase in external field will not increase the strength of the magnetic flux through the core-a condition referred to as saturation. The elasticity of the above is evidenced by the fact that when the magnetizing force is removed, the alignment will be lost to some measure, and the flux density will drop to $B_R$. In other words, the removal of the magnetizing force will result in the return of a number of misaligned domains within the core. The continued alignment of a number of the domains, however, accounts for our ability to create permanent magnets.

Saturation Magnetization and Remanence

When all the magnetic domains in a material are fully aligned with the external magnetic field, the material reaches saturation magnetization. In this state, the magnetization cannot increase further, even with a stronger external field. Once the external field is removed, some domains remain aligned, leaving the material with remanent magnetization (or remanence). This residual magnetization explains why certain materials, like permanent magnets, continue to exhibit magnetism even without an external field. At a point just before saturation, the opposing unaligned domains are reduced to small cylinders of various shapes referred to as bubbles. These bubbles can be moved within the magnetic sample through the application of a controlling magnetic field. These magnetic bubbles form the basis of the recently designed bubble memory system for computers.

Coercivity and Hysteresis Loop

To fully demagnetize a material, the external field must be reversed. The strength of the reversed field required to reduce the magnetization to zero is called the coercive force or coercivity. This behavior can be visualized using a hysteresis loop, which shows the relationship between the external magnetic field and the material's magnetization. The loop demonstrates how a material becomes magnetized and demagnetized, revealing important properties like coercivity and remanence.

Types of Magnetic Materials and Domain Behavior

Different materials exhibit different magnetic behaviors based on how their magnetic domains interact with external fields:

Ferromagnetic Materials:

These materials have strongly aligned domains in the presence of an external field and retain magnetization. Examples include iron, cobalt, and nickel.

Ferrimagnetic Materials:

In these materials, adjacent magnetic moments are aligned in opposite directions but with unequal strengths, leading to net magnetization.

Antiferromagnetic Materials:

Here, adjacent magnetic moments are aligned in opposite directions with equal strength, resulting in no net magnetization.

Applications of Domain Theory

The Domain Theory of Magnetism has numerous practical applications, such as:

Data Storage:

Magnetic domains are used to store data in devices like hard drives, where the direction of the domains represents binary information (0s and 1s).

Transformers and Electromagnets:

Efficient magnetic materials with low coercivity are used to reduce energy loss in transformers. Electromagnets, created by realigning domains with electric currents, are essential in industries.

Permanent Magnets:

Permanent magnets are produced by permanently aligning the domains, leading to a strong and persistent magnetic field.