ferrimagnetism
/ FER-ee-mag-net-iz-um /
Picture a tug-of-war where the two teams pull in opposite directions — but one team is bigger and stronger. They cancel most of each other out, yet there is a clear winner, and the rope creeps that way. Ferrimagnetism is exactly this kind of unfair standoff between two sets of atomic moments: opposed, but unequal, so something is left over.
Like an antiferromagnet, the neighboring moments here point against each other. The difference is that the two opposing sublattices are not the same size — perhaps because they sit on different kinds of atomic sites or hold ions with different moments. The larger team's alignment only partly cancels the smaller team's, and the surplus survives as a real net magnetization. From the outside it can look just like an ordinary ferromagnet, even though the mechanism inside is opposition rather than agreement.
Ferrimagnetism matters because the original 'lodestone' that first revealed magnetism to humankind — magnetite — is a ferrimagnet, not a simple ferromagnet. So are the ferrite ceramics in countless cheap magnets, transformer cores, and antenna rods. The common confusion is to lump all permanent magnets together as 'ferromagnets'; many of the most useful ones are really ferrimagnets, where strength comes from a deliberate imbalance between two opposing teams.
Magnetite — the lodestone that ancient navigators turned into compasses — is a ferrimagnet. Iron sits on two kinds of sites whose moments oppose each other, but unequally, leaving the net magnetism that first taught humanity which way is north.
Lodestone (magnetite) is ferrimagnetic: two opposed but unequal moment teams leave a usable surplus.
Outwardly a ferrimagnet and a ferromagnet behave alike — both attract iron and stay magnetized. The distinction is internal: in a ferromagnet the moments cooperate, while in a ferrimagnet they fight to a draw that simply does not come out even.