hysteresis
/ his-tuh-REE-sis /
Bend a fresh paperclip and let go: it springs back. Bend it hard and it stays bent — it remembers. Magnets behave the same way. A magnet's response to a field depends not just on the field right now, but on where it has been. This lag, this memory of past fields, is hysteresis.
Plot a ferromagnet's magnetization against the field applied to it and you do not get a single line — you get a loop. Crank the field up and the magnetization rises and saturates; turn the field back to zero and some magnetization stubbornly remains. To erase it you must push the field the other way. Tracing the field up and down forever traces out a closed loop, and the loop's area is the energy lost as heat each cycle. The lag exists because domain walls get snagged on defects and do not glide back freely.
Hysteresis matters because it is engineerable, and the choice runs the technology. A wide, fat loop means a stubborn 'hard' magnet that keeps its memory — perfect for fridge magnets and hard-disk bits. A skinny loop means a 'soft' magnet that forgets instantly and wastes little heat — perfect for transformer and motor cores that flip millions of times a second. The common misconception is that hysteresis is a flaw; it is the very property that lets a magnet store information at all.
An old cassette or hard disk stores each bit as a patch left magnetized one way or the other. It survives without power precisely because of hysteresis: once magnetized, the patch holds its direction until a strong enough write field deliberately flips it.
Magnetic data storage relies on hysteresis: each bit keeps its direction with no power until rewritten.
The word hysteresis is not special to magnetism — it names any system whose state lags behind its cause, from a stretched rubber band to a thermostat. Magnets are just its most familiar example.