The property of being strongly attracted to either pole of a magnet. Ferromagnetic materials, such as iron, contain unpaired electrons, each with a small magnetic field of its own, that align readily with each other in response to an external magnetic field. This alignment tends to persists even after the magnetic field is removed, a phenomenon called hysteresis. Ferromagnetism is important in the design of electromagnets, transformers, and many other electrical and mechanical devices, and in analyzing the history of the earth's magnetic reversals.
Guillaume 6 explains the ferromagnetism of Heusler's alloy by supposing that the naturally low critical temperature of the manganese contained in it is greatly raised by the admixture of another appropriate metal, such as aluminium or tin; thus the alloy as a whole becomes magnetizable at the ordinary temperature.
Diamagnetism, in short, is an atomic phenomenon; paramagnetism and ferromagnetism are molecular phenomena.
Ferromagnetism was explained by Ampere on the hypothesis that the magnetization of the molecule is due to an electric current constantly circulating within it.