Magnetic materials become magnetic because of tiny magnetic moments inside their atoms. In ferromagnetic materials such as iron, cobalt, and nickel, these moments can group into regions called magnetic domains. Each domain acts like a small magnet, but an unmagnetized sample often has many domains pointing in different directions.
Understanding domains explains how magnets are made, erased, strengthened, and weakened.
Key Facts
- A magnetic domain is a region where many atomic magnetic moments point in nearly the same direction.
- Unmagnetized ferromagnetic material has domains pointing in many directions, so the net magnetic field is small.
- Applying an external magnetic field can grow and rotate domains that are aligned with the field.
- Magnetization is magnetic moment per unit volume: M = m/V.
- Magnetic flux density in a simple linear material can be written as B = μH, where μ is permeability.
- Above the Curie temperature, ferromagnetic order is lost and the material becomes paramagnetic.
Vocabulary
- Magnetic domain
- A magnetic domain is a small region in a material where many atomic magnetic moments are aligned in the same direction.
- Ferromagnetism
- Ferromagnetism is the strong magnetic behavior caused by neighboring atomic moments aligning with each other even without an external field.
- Magnetization
- Magnetization is the magnetic moment per unit volume of a material.
- Curie temperature
- The Curie temperature is the temperature above which a ferromagnetic material loses its long range magnetic ordering.
- Permeability
- Permeability is a measure of how easily a material supports magnetic field inside it.
Common Mistakes to Avoid
- Thinking every iron sample is always a strong magnet. This is wrong because unmagnetized iron can have many domains pointing in different directions, giving little net magnetization.
- Confusing atomic magnetic moments with magnetic domains. A domain contains many aligned moments, so it is much larger than a single atom.
- Assuming heating a magnet only makes it slightly weaker. This is wrong because heating above the Curie temperature destroys ferromagnetic ordering and can remove magnetization.
- Treating hard magnets and soft magnets as the same. Soft magnets magnetize and demagnetize easily, while hard magnets resist changes and keep their magnetization.
Practice Questions
- 1 A small iron sample has total magnetic moment m = 0.030 A m^2 and volume V = 2.0 x 10^-6 m^3. Find its magnetization using M = m/V.
- 2 A material has permeability μ = 0.0040 T m/A and is placed in a magnetic field strength H = 250 A/m. Find the magnetic flux density using B = μH.
- 3 A steel paper clip is attracted to a magnet, but it does not remain strongly magnetized after the magnet is removed. Explain this behavior using magnetic domains and the idea of a soft magnetic material.