Ferromagnetism

Ferromagnetism is a phenomenon where certain materials, like iron, nickel, and cobalt, exhibit strong, permanent magnetism. This property arises from the alignment of magnetic moments of atoms or molecules within the material, leading to a strong internal magnetic field.

Ferromagnetic materials possess regions called domains, where the magnetic moments of atoms are aligned in the same direction. When an external magnetic field is applied, these domains grow in size and align with the field, strengthening the overall magnetism. Once the external field is removed, the material retains this alignment, making it a permanent magnet. The behavior of these materials is characterized by hysteresis, which is the lag between the application and removal of the magnetic field and the response of the material.

The Curie temperature is a critical point for ferromagnetic materials; above this temperature, thermal energy disrupts the alignment of magnetic moments, causing the material to lose its ferromagnetic properties and become paramagnetic.

Ferromagnetism is utilized in many applications, including in the manufacture of permanent magnets, magnetic storage media like hard drives, and various types of sensors.

### Common Clues Across Multiple Question Stems

1. **Curie Temperature**: Ferromagnetism disappears above the Curie temperature, where materials transition from ferromagnetic to paramagnetic.

2. **Domains**: Ferromagnetic materials have domains with aligned magnetic moments that grow and align with an external magnetic field.

3. **Hysteresis**: The magnetic response of ferromagnetic materials is characterized by hysteresis, where there is a lag between the application and removal of the magnetic field and the material's magnetization.

4. **Iron, Nickel, and Cobalt**: These are common examples of ferromagnetic materials often mentioned in questions.

5. **Permanent Magnetism**: Ferromagnetism results in permanent magnetism, where materials retain their magnetic properties even after the external field is removed.

Fill-in-the-blank practice:

1. The " ___1___ criterion" must be met for ferromagnetism to occur.

2. The " ___2___ effect" demonstrates the discrete changes in magnetization in ferromagnetic materials.

3. Ferromagnetic materials exhibit "coercivity" and "___3___," which describe how strongly a material retains its magnetization and how much external magnetic field is needed to demagnetize it.

4. The " ___4___ model" is a theoretical model used to describe ferromagnetism through a lattice of spins.

5. The " ___5___ interaction" is a quantum mechanical effect responsible for the alignment of spins in ferromagnetic materials.

Answers:

1. Stoner

2. Barkhausen

3. remanence

4. Ising

5. exchange

additional info on the exchange interaction: The exchange interaction in magnetism is a quantum mechanical phenomenon where the alignment of electron spins in neighboring atoms or ions leads to either ferromagnetic or antiferromagnetic ordering. This interaction arises due to the Pauli exclusion principle and Coulomb repulsion, causing electrons with parallel spins to avoid each other and reduce their electrostatic energy, thereby influencing the overall magnetic properties of the material.