Types of magnetism: (A) paramagnetism (B) ferromagnetism (C) antiferromagnetism (D) ferrimagnetism (E) enforced ferromagnetism.
Ferromagnetism is a type of magnetism in which the magnetic moments of atoms in a solid are aligned within domains, which can in turn be aligned with each other by a weak magnetic field. The total magnetic moment of a sample of the substance is the vector sum of the magnetic moments of the component domains.
Introduction: magnetic properties of materials
Magnetic properties other than diamagnetism, which is found in all materials, come about because of the interactions of unpaired electrons. These properties are commonly found in transition elements, lanthanides, and their compounds as a result of unpaired electrons in the d and f orbitals. There are three main types of magnetic behavior (apart from diamagnetism): paramagnetism, in which the unpaired electrons are randomly arranged; ferromagnetism, in which the unpaired electrons are all aligned; and antiferromagnetism, in which the unpaired electrons line up in opposite directions to one another.
Ferromagnetic materials have an overall magnetic moment, whereas antiferromagnetic materials have a magnetic moment of zero. A substance is said to be ferrimagnetic if the electron spins are orientated antiparallel to one another but, due to an inequality in the number of spins in each orientation, there exists an overall magnetic moment. There are also enforced ferromagnetic substances (called spin-glass-like) in which antiferromagnetic materials have pockets of aligned spins.
Ferromagnetic materials can retain their magnetization when the external field is removed, as long as the temperature is below a critical value, the Curie temperature. They are characterized by a large positive magnetic susceptibility. Examples of ferromagnetic substances include iron, nickel, cobalt and some of the rare earth elements, such as gadolinium and dysprosium.
In an unmagnetized piece of ferromagnetic material the magnetic moments of the domains themselves are not aligned; when an external field is applied those domains that are aligned with the field increase in size at the expense of the others.