Magnetism and Matter
Why are some materials attracted to magnets while others are repelled? The answer lies within atoms themselves.
Start with the simplest version: this lesson is about Magnetism and Matter. If you can explain the core idea to a friend using everyday language, examples, and one clear reason why it matters, you have moved from memorising to understanding.
Why are some materials attracted to magnets while others are repelled? The answer lies within atoms themselves. Every atom is a tiny electromagnet—electrons orbiting the nucleus create magnetic dipoles, and unpaired electron spins add their own magnetism. This chapter explores how atomic magnetism scales up to create the bulk magnetic properties we observe in materials, from iron filing patterns to the Earth's protective magnetic field.
Building on Atomic Magnetism
From moving-charges-and-magnetism, we learned that moving charges create magnetic fields. In atoms, electrons orbit and spin—both sources of magnetism. The challenge is explaining why some materials are strongly magnetic (iron, cobalt, nickel) while others are not (copper, aluminum).
Types of Magnetic Materials
Paramagnetic Materials
Paramagnetic materials have atoms with unpaired electrons—tiny atomic magnets. Individually, these magnetic moments point randomly. When you apply an external magnetic field, they partially align with it, making the material slightly attracted to the field.
Effect: χ = 0.001 to 0.1 (positive susceptibility)
Examples: aluminum, copper (surprisingly), oxygen gas
Diamagnetic Materials
Diamagnetic materials have no unpaired electrons—their atomic magnetic moments cancel. However, when an external field is applied, the electron orbits precess slightly (like a top wobbling), creating tiny magnetic moments opposed to the field. The material is weakly repelled.
Effect: χ = -0.00001 to -0.0001 (negative susceptibility)
Examples: most organic molecules, water, bismuth
Ferromagnetic Materials
Ferromagnetic materials are special. In iron, cobalt, and nickel, the unpaired electron spins create a strong collective effect—the atoms' magnetic moments spontaneously align with each other, even without an external field. These materials can become strongly magnetized.
Effect: χ = 1 to 10,000+ (huge positive susceptibility)
The magnetic strength of ferromagnetic materials emerges from exchange interaction—a quantum mechanical effect where neighboring unpaired electron spins prefer to align parallel.
Hysteresis and Magnetization Curves
When you magnetize iron, something remarkable happens: it doesn't demagnetize completely when you remove the external field. This hysteresis shows memory—the material "remembers" being magnetized.
Retentivity: A ferromagnetic material retains magnetism after the external field is removed. Coercivity: The reverse field strength needed to fully demagnetize the material.
This hysteresis loop is the basis for permanent magnets and magnetic recording (tape, hard drives).
Domain Theory
Ferromagnetic materials contain domains—regions where atomic magnetic moments are already aligned. In an unmagnetized iron bar, domains point randomly, canceling each other. When you apply an external field:
- Domains aligned with the field grow at the expense of others
- Domains rotate to align with the field
- Eventually, most domains align, creating a strong net magnetization
When you remove the field, some alignment persists—permanent magnetism.
The Magnetic Moment and Atomic Structure
The magnetic moment of an atom comes from:
- Orbital motion: Electrons orbiting the nucleus (like a current loop)
- Electron spin: Intrinsic angular momentum of electrons
The Bohr magneton is the natural unit of atomic magnetic moment:
μ_B = eℏ/(2m_e) = 9.27 × 10⁻²⁴ J/T
Atomic magnetic moments are typically 1-5 Bohr magnetons.
Earth's Magnetism
Earth itself is a giant magnet, with a magnetic field around 25-65 microtesla at the surface. The origin is iron in Earth's outer core, kept molten by heat. This liquid iron, moving due to convection, acts like a massive electromagnet—the geodynamo effect.
Related Topics
moving-charges-and-magnetism | electromagnetic-induction | atoms
Socratic Questions
- Why does heating a permanent magnet above a certain temperature (the Curie point) cause it to lose its magnetization? What's happening to the domain alignment?
- If you could selectively align all domains in a piece of iron perfectly with an external field, then remove the field, why wouldn't the iron remain magnetized forever?
- Diamagnetic materials are repelled by both north and south poles of a magnet. Why does this work, given that the induced magnetic moment opposes the applied field?
- In paramagnetic materials, thermal motion randomizes the alignment of atomic magnetic moments. Why doesn't thermal motion equally disrupt ferromagnetic ordering? What's different?
- Could we ever create a material with a Curie point below room temperature? What technological uses might such a material have?