Hysteresis
Trace hysteresis loop. Builds on Dia/Para/Ferromagnetism.
You magnetize a steel nail by rubbing a strong magnet along it 50 times. The nail can now pick up paper clips. You want to completely demagnetize the nail.
What is the most effective way to demagnetize the nail?
Heat it above the Curie temperature! When a ferromagnet is heated past its Curie point, thermal energy overwhelms the exchange interaction that keeps domains aligned. The material becomes paramagnetic — the domains randomize and all permanent magnetization is destroyed. When it cools back down, it returns to being ferromagnetic but with randomly oriented domains, so zero net magnetization.
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Apply a field to the material and watch the B-H curve trace out the hysteresis loop. Notice the remanence (magnetization at H = 0) and the coercivity (field needed to demagnetize). Try heating the material past the Curie temperature to see the loop collapse!
B = μ₀(H + M)
The relationship between B and H is NOT a straight line for ferromagnets — it’s a loop. This means the material has memory: its current magnetization depends on its history, not just the current field.
Data is stored as magnetized regions on a disk. The high coercivity of the recording medium means the bits stay put — they need a strong, deliberate field to flip. That’s the B-H loop at work: the wider the loop, the more stable the data.
Transformers use soft iron cores with narrow hysteresis loops — minimal energy is wasted as heat per AC cycle. A wide-loop material would waste enormous energy: the area of the loop times the frequency.
Neodymium (NdFeB) magnets have extremely wide hysteresis loops: high remanence (strong field) and high coercivity (hard to demagnetize). This is why they’re so powerful and stay magnetized for decades.
Some fire safety systems use magnets that lose their magnetism at a specific Curie temperature. When a fire heats the magnet, it releases a latch, triggering sprinklers or alarms.
“Hysteresis is magnetic memory. Every hard drive bit, every permanent magnet, every transformer core is governed by this loop. The shape of the loop determines whether a material remembers (hard magnet) or forgets (soft magnet) its magnetic past.”
Hysteresis Lab
Trace hysteresis loops for different materials by cycling the applied field. Compare soft iron (narrow loop, easy to magnetize/demagnetize) with hard steel (wide loop, permanent magnet material). Heat materials past the Curie temperature and watch domains randomize.
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Start with soft iron. Cycle H from +max to -max and back to trace the loop. Notice how narrow it is — easy to magnetize and demagnetize. Now switch to hard steel — the loop is MUCH wider. Measure the remanence (M at H = 0) and coercivity (H where M = 0). Now try the temperature slider: increase T past the Curie point and watch the hysteresis loop shrink to nothing as the material becomes paramagnetic.
The hysteresis loop shape tells you everything about a magnetic material. Narrow loop = good for transformers (low energy loss). Wide loop = good for permanent magnets (high remanence). The area of the loop is the energy wasted as heat per cycle — which is why transformer cores use the softest magnetic materials possible.