Ferromagnetism

Ferromagnetism

Ferromagnetism is the effect that creates permanently magnetized materials, such as ordinary horseshoe magnets. Although the effect is most common in pure iron, it can also be reproduced in iron-like substances, including compounds like barium ferrite, and heterogeneous compounds that unite iron with aluminum, nickel, cobalt, and tin.

Ferromagnetism occurs when magnetic moments of the atoms inside a material interact, leading to the overall magnetism of the object. Ferromagnetic substances will maintain their magnetism until the atoms are forced out of alignment. The easiest way to do this is to heat the substance. Above a certain temperature, the heat is enough to break the alignment and that subsequent cooling is not enough to allow realignment to take place. This temperature is known as the Curie temperature, T, named after Marie Curie's husband, Pierre Curie.

Magnetic moments of atoms are created by the contribution of individual magnetic moments of the constituent particles: the proton, the neutron, and the electron. The electron's magnetic moment, however, is approximately 1,000 times stronger than either the proton or the neutron, so electrons essentially account for all the magnetic moment of the individual atom. The magnetic moment comes from the spin of the electron.

A particle's spin is an inherent quantum mechanical property, not a result of the particle's actual angular motion around another particle. Instead, spin comes from particles behaving as if they were charge distributions spinning around an axis. Thus, each particle is like a tiny magnetic dipole. This causes each particle to have a tiny magnetic moment, contributing to the overall magnetic moment of the atom. There is evidence that an electron orbiting a nucleus does, in fact, create a magnetic moment component for the atom, but the effect is so small that it is negligible when compared to the electon's spin contribution.

It is not the just the magnetic moment of the atoms in the substance that creates a magnet; it is the interaction of these magnetic moments. There is an interaction energy between adjacent magnetic moments that causes them to correlate between themselves; this is called the exchange energy. The strength of the interaction and the alignment of the magnetic moments depends on the distance between atoms and the atoms' individual quantum states.

In a substance, there are three ways these magnetic moments can align: all in the same direction, alternating pairs of directions, or in random directions (which usually gives a small net magnetism to the substance). Those substances where all the atoms' moments are aligned in the same direction are ferromagnetic substances. Those substances with small net magnetism from an overall random effect are weak magnets. Finally, substances that have alternating pairs of directions are considered anti-ferromagnetic.