Ferromagnetism: Causes And Examples

What is Ferromagnetism?

Ferromagnetism is a form of magnetism in which certain materials become magnetised and retain their magnetisation after the external magnetic field is removed. It is one of the most common and well-documented forms of magnetism, observed in materials such as iron, cobalt and nickel. Ferromagnetic materials interact significantly with magnetic fields; consequently, they can exhibit a permanent magnetic state.

Causes of Ferromagnetism

The primary cause of ferromagnetism lies in the spins and magnetic moments of electrons. Each electron produces a very small magnetic field due to its spin. In most materials, these magnetic moments cancel because the spins are randomly directed. However, in ferromagnetic materials the electron spins tend to align within specific regions, known as domains, thereby generating a net magnetic moment for the material.

Key factors contributing to ferromagnetism include:

1. The alignment of electron spins: The fundamental factor in ferromagnetism is the alignment of electron spins within the material. When these spins align, their combined magnetic fields produce a strong overall magnetic field.

2. The exchange interaction: The exchange interaction is a quantum mechanical effect that causes adjacent electrons to align their spins in the same direction. This interaction arises from the Pauli exclusion principle and the Coulomb force between electrons.

3. Magnetic domains: Ferromagnetic materials are divided into small regions called magnetic domains. Within each domain, the atomic magnetic moments align uniformly; however, the domains themselves may have different orientations. When the material is magnetised, the domains align, leading to a net magnetisation.

4. The Curie temperature: Every ferromagnetic material has a specific temperature, known as the Curie temperature, above which it loses its ferromagnetic properties. Above this temperature, thermal energy disrupts the alignment of electron spins, rendering the material paramagnetic.

Examples of Ferromagnetic Materials

Ferromagnetic materials are extensively utilised in both everyday life and industrial applications. Some of the well-known ferromagnetic materials include:

1. Iron (Fe): This is the most common ferromagnetic material. It is used in the manufacture of permanent magnets, electric transformers and magnetic storage media.

2. Cobalt (Co): Noted for its high magnetic permeability, cobalt is frequently employed in high-performance permanent magnets and in the production of specialised alloys.

3. Nickel (Ni): This widely used ferromagnetic metal is common in alloys and magnetic devices.

4. Alloys (e.g. Alnico): Alloys such as Alnico (a combination of aluminium, nickel and cobalt) are often used for permanent magnets, especially in applications requiring strong magnetic fields.

5. Rare-earth metals: Certain rare-earth elements, such as Neodymium (Nd), are used in high-strength magnets (Neodymium Magnets), which are found in motors, loudspeakers and medical devices.

6. Iron oxides: Iron oxide (Fe₃O₄), also known as magnetite, is a naturally occurring ferromagnetic material that is employed in magnetic applications such as data storage.

Applications of Ferromagnetic Materials

Ferromagnetic materials play an important role in modern technology and various industrial applications. Notable applications include:

1. Permanent magnets: These materials are used to manufacture permanent magnets, which serve as key components in devices such as motors, loudspeakers and electric generators.

2. Electric motors and transformers: Ferromagnetic cores are used in electric motors and transformers to consolidate and strengthen magnetic fields, thereby enhancing performance and efficiency.

3. Data storage: Hard drives and magnetic tapes utilise the magnetic properties of ferromagnetic materials to store data reliably.

4. Magnetic resonance imaging (MRI): Ferromagnetic materials are incorporated into MRI devices to produce the strong magnetic fields necessary for medical imaging.

5. Magnetic sensors: These materials are used in sensors designed to detect magnetic fields in applications such as compasses and automotive systems.

6. Magnetic levitation: Ferromagnetic materials are employed in magnetic levitation systems, for instance in high-speed trains (maglev trains) that use strong magnetic fields to operate above the tracks.

Types of Magnetism

Magnetism can be categorised into different types, each with specific characteristics:

1. Ferromagnetism: As described above, this type occurs when materials such as iron, cobalt and nickel exhibit strong magnetic properties and maintain magnetisation after the removal of the external field.

2. Paramagnetism: These materials have a weak, positive magnetic susceptibility and become magnetised only when subjected to an external magnetic field. Examples include Aluminium and Platinum.

3. Diamagnetism: Materials in this category display a weak, negative magnetic response when an external magnetic field is applied. Examples include copper and graphite.

4. Antiferromagnetism: In these materials, neighbouring atoms or ions have opposing spins, resulting in a net magnetic moment of zero. Manganese oxide (MnO) is an example.

5. Ferrimagnetism: Whilst similar to antiferromagnetism, the opposing spins in these materials are unequal, producing a net magnetic moment. Magnetite (Fe₃O₄) serves as an example of a ferrimagnetic material.

(6)Additional magnetic fundamentals can be found at Stanford Advanced Materials (SAM).

Frequently Asked Questions (FAQs)

What is Ferromagnetism?

Ferromagnetism refers to a form of magnetism whereby certain materials become magnetised and retain this property even after an external magnetic field is removed. This phenomenon is caused by the alignment of electron spins within the material.

Why do ferromagnetic materials retain their magnetisation?

Ferromagnetic materials retain their magnetisation because the magnetic moments of electrons align to form magnetic domains. When these domains align, the material exhibits permanent magnetic characteristics.

What is the Curie temperature?

The Curie temperature is the specific temperature at which a ferromagnetic material loses its ferromagnetic properties and becomes paramagnetic. This occurs because thermal energy disrupts the alignment of the electron spins.

Can ferromagnetic materials lose their magnetisation?

Yes, ferromagnetic materials can lose their magnetisation when they are exposed to high temperatures, strong external magnetic fields or physical impacts that disturb the alignment of the magnetic domains.

What are some common applications of ferromagnetic materials?

Ferromagnetic materials are utilised in various applications including permanent magnets, electric motors, transformers, data storage devices (e.g. hard drives) and medical devices such as MRI machines.