ferromagnetism and antiferromagnetism

A single iron atom in solution is paramagnetic, weakly tugged toward a magnet and otherwise unremarkable. Yet a bar of iron can become a permanent magnet that sticks to your fridge with a force you can feel. What turns the feeble magnetism of lone atoms into something so strong? The answer is cooperation: in certain solids the atomic moments stop acting independently and start lining up together, a phenomenon called cooperative magnetism.

Ordinary paramagnetism assumes each unpaired-electron moment points wherever it likes, jostled by heat, so they only weakly align with an applied field. Cooperative magnetism is what happens when neighboring moments couple through the lattice and influence each other's direction. If the coupling makes neighboring moments point the same way, they reinforce into a large net magnetism even with no field applied — that is ferromagnetism, the behavior of iron, cobalt, and nickel metal, and the basis of permanent magnets. If instead the coupling makes neighbors point opposite ways, they cancel in pairs and the solid has little or no net moment — that is antiferromagnetism, seen in many transition-metal oxides like MnO. A close cousin, ferrimagnetism, has opposing moments of unequal size that do not fully cancel, leaving a net moment (this is the magnetism of the ancient lodestone, magnetite).

These behaviors matter wherever solid-state inorganic materials meet technology — permanent magnets, magnetic storage, the iron oxides in recording media. They are also temperature-sensitive in a telling way: heat eventually overwhelms the coupling, so a ferromagnet loses its ordering above a critical temperature (the Curie point) and an antiferromagnet above its own (the Neel point), reverting to plain paramagnetism. The honest distinction to hold onto: paramagnetism is a property of isolated moments and is the realm of the spin-only formula, whereas ferromagnetism and antiferromagnetism are collective properties of the whole lattice — they cannot exist in an isolated molecule or a dilute solution, only where many magnetic centers sit close enough to talk to each other.