ferrimagnetism
/ FAIR-ih-mag-net-iz-um /
Picture a tug-of-war where the two teams pull in opposite directions, but one team has more people. The rope still moves, just not as much as if everyone pulled the same way. Ferrimagnetism is magnetism from exactly that kind of unequal opposition: neighboring moments point opposite to each other, but because they are not the same size, they do not fully cancel and a real, useful net magnetism is left over.
This happens in ceramic compounds where different metal ions sit on two different sets of lattice sites. The exchange interaction between the two sublattices is antiparallel, so ions on one sublattice point up while ions on the other point down. In magnetite, Fe3O4, the classic case, the two sublattices carry unequal iron moments, so their opposition leaves a net moment per formula unit. Externally, a ferrimagnet behaves much like a ferromagnet, it can be permanently magnetized and has a Curie temperature, but its saturation magnetization is lower because part of the alignment is spent cancelling.
Ferrimagnetism is the working principle behind the ferrites, the family of magnetic ceramics used everywhere from transformer cores to fridge magnets. Its great practical advantage over metallic iron is that these oxides are electrical insulators, so they do not waste energy to eddy currents at high frequency. The original lodestone, magnetite, is ferrimagnetic, meaning the very first magnet humans ever used was technically a ferrimagnet, not a simple ferromagnet.
In magnetite the two iron sublattices point opposite but are unequal, so about a quarter of the moments survive uncancelled to give a net magnetization.
Antiparallel but unequal moments leave a net magnetism, weaker than a ferromagnet's.
Ferrimagnets look like ferromagnets from the outside but have lower saturation because opposing sublattices partly cancel; if the two sublattices were exactly equal and opposite, the material would be antiferromagnetic with no net moment at all.