Ferromagnetism

Ferromagnetism is a form of magnetism exhibited by a relatively small group of materials in which the atomic magnetic moments align spontaneously, giving rise to strong magnetization even in the absence of an external magnetic field. It is the basis of everyday magnetism and underpins a wide range of industrial and technological applications. Materials such as iron, cobalt, nickel, many of their alloys, and certain rare-earth compounds are characteristically ferromagnetic, showing both high magnetic permeability and, in many cases, substantial coercivity, which enables them to retain magnetization and act as permanent magnets.
When a ferromagnetic material is exposed to an external magnetic field, its internal magnetic domains align, producing a pronounced temporary or permanent magnetic response. A familiar example is the attraction of a steel plate to a magnet, an effect resulting from induced magnetization within the plate. Whether that plate remains magnetized after the external field is removed depends on its coercivity, which is determined by its composition and heat treatment.

Magnetic Behaviour and Related Concepts

Magnetic behaviour in materials spans several categories. Ferromagnetism and the closely related ferrimagnetism are the strongest forms and are responsible for most practical magnetic effects. Other types—paramagnetism, diamagnetism and antiferromagnetism—produce far weaker responses, usually detectable only with laboratory instruments.
Permanent magnets must be either ferromagnetic or ferrimagnetic. They are magnetized by strong external fields during manufacture, which align their crystallites or magnetic domains. The robustness of this alignment depends on material coercivity: magnetically soft materials (low coercivity) are easily magnetized and demagnetized, whereas magnetically hard materials (high coercivity) retain their alignment, making them suitable for permanent magnets. Typical hard magnetic materials include alnico alloys and ferrites.
The strength of a magnet is expressed through its magnetic moment or total magnetic flux, while local magnetic strength within a material is described by its magnetization.

Terminology and Distinction Between Magnetic Orders

Historically, the term ferromagnetism was used broadly to describe any material capable of spontaneous magnetization. In 1948, Louis Néel demonstrated that two distinct mechanisms could produce such behaviour:

• Ferromagnetism, in which all magnetic moments align parallel to one another.
• Ferrimagnetism, in which some moments oppose others but do not cancel entirely, resulting in a net moment.

A related state, antiferromagnetism, occurs when opposing magnetic moments cancel completely, producing no spontaneous magnetization.

Materials Exhibiting Ferromagnetism

Ferromagnetism is closely connected to the electronic structure and crystalline arrangement of a material. It is most commonly found in the transition metals iron, cobalt and nickel, whose electronic configurations contain unpaired electrons in their d-orbitals. Rare-earth metals can also display strong ferromagnetic behaviour due to unpaired electrons in their f-orbitals.
Several notable material classes illustrate the variety of ferromagnetic systems:

• Heusler alloys, whose constituents are not themselves ferromagnetic, yet form ferromagnetic compounds through particular atomic arrangements.
• Non-magnetic stainless steels, showing that even alloys composed predominantly of ferromagnetic elements can be non-magnetic depending on structure.
• Amorphous metallic alloys, produced by rapid cooling, which have nearly isotropic properties. These materials combine low coercivity, low hysteresis loss, high permeability and high electrical resistivity. A typical example is a transition-metal–metalloid alloy such as Metglas 2605, which exhibits high magnetization at room temperature and a comparatively low glass-transition temperature.

A major twentieth-century development was the introduction of rare-earth magnets, incorporating lanthanides with strongly localized f-electron moments. These materials are exceptionally powerful and are widely used in compact motors, high-performance magnets and advanced electronic devices.

Unusual and Specialised Ferromagnetic Systems

Although ferromagnetism is typically associated with metals, unusual examples occur in other classes of materials:

• Some actinide compounds, such as plutonium phosphide and neptunium-based materials, display ferromagnetism at low temperatures, often accompanied by structural distortions of the crystal lattice.
• In 2009, experiments demonstrated ferromagnetism in a ultra-cold lithium gas, marking the first observation of ferromagnetic behaviour within a gaseous system.
• Rare cases exist among compounds of s- and p-block elements, such as rubidium sesquioxide.
• In 2018, body-centred tetragonal ruthenium was shown to be ferromagnetic at room temperature, a surprising result given ruthenium’s conventional non-magnetic behaviour.

Electrically Induced Ferromagnetism

Recent research has produced evidence that ferromagnetic ordering can be induced by electrical means:

• Certain oxides, such as LaMnO₃ and SrCoO, have been switched from antiferromagnetic to ferromagnetic states by electric currents.
• In 2020, experiments showed that applying voltage to normally diamagnetic iron pyrite (fool’s gold) can induce ferromagnetism in thin surface layers.

These findings raise possibilities for electrically controlled magnetic devices and new forms of data storage.

Quantum Mechanical Explanation

Classical physics cannot explain ferromagnetism, as demonstrated by the Bohr–van Leeuwen theorem, which shows that classical models predict no net magnetism in thermal equilibrium. Ferromagnetism instead arises from quantum mechanical principles:

• Each electron carries an intrinsic spin-based magnetic moment.
• The Pauli exclusion principle restricts electron occupancy of atomic orbitals, leading to unpaired spins in certain configurations.
• The exchange interaction, a quantum effect, energetically favours parallel alignment of neighbouring magnetic moments in specific materials.

These quantum-derived interactions create regions of aligned spins called magnetic domains. When a ferromagnetic material is magnetized, domain walls shift, enlarge or reorient to support a uniform alignment. The degree to which this alignment persists—dependent on material structure and coercivity—determines whether the material functions as a permanent magnet.