magnetic domain
/ mag-NET-ik doh-MAYN /
If iron's atoms all want to point the same way, why is a fresh nail not a magnet? The answer is that the iron quietly divides itself into many small neighborhoods, like a country split into provinces. Inside each province every atomic moment points one way — but different provinces point different ways. Each of these uniformly-aligned neighborhoods is a magnetic domain.
Why split up? A bar where every moment pointed the same way would fling its magnetic field out into the surrounding space, which costs energy. The material can lower that cost by breaking into domains whose fields loop into each other and mostly close up inside the metal. So an unmagnetized piece of iron is not disordered — each domain is perfectly ordered — but the domains are arranged so their fields cancel, and the lump looks magnetically dead from outside.
Domains matter because magnetizing something is really the story of domains changing. The common misconception is that magnetizing iron forces its atoms to flip one by one. Mostly it does something gentler: domains already pointing the right way grow at their neighbors' expense, their boundaries sweeping across the crystal, until one direction wins out.
Magnetize a piece of iron near a sensitive detector and you can hear faint clicks — the Barkhausen effect. Each click is a domain boundary jumping past a tiny defect as the domains rearrange, a literal soundtrack of magnetic order being rewritten.
The Barkhausen effect: audible clicks as domain walls jump during magnetization.
Domains exist only in materials with spontaneous magnetic order — ferromagnets and ferrimagnets. A paramagnet has no domains, because its moments never agree among themselves long enough to form the uniform neighborhoods that domains are made of.