Ordinary bulk magnetism of a material, called ferromagnetism, arises from minute interacting forces between the electrons of neighboring atoms within that material. These forces encourage the magnetism of neighboring atoms (produced by their electron spins) to be in the same direction - to be parallel. This tendency for alignment of magnetic force at the atomic level within the material competes with thermal fluctuations brought on by heating the material that randomize the directions of atomic magnets. Thus in iron and most magnetic materials, magnetism decreases with increasing temperature and increases with decreasing temperature.

Some magnetic materials, called ferrimagnets, are more complex. Not all the atomic magnets point in the same direction, and there are several different interatomic magnetic forces. In these materials, the variation of magnetism with temperature is more complicated than with ferromagnets.