Ferromagnetism
Ferromagnetism is a form of magnetism in which the magnetic moments of neighbouring atoms spontaneously align in parallel, without requiring any applied external field, producing strong, persistent macroscopic magnetisation that survives below a material-specific critical temperature (the Curie temperature) and vanishes above it.
Curious
Most materials show only a weak, temporary magnetic response that disappears the moment an applied field is removed. Ferromagnetic materials — iron, cobalt, nickel, and a handful of others — are dramatically different: below a critical temperature, their atomic magnetic moments spontaneously lock into parallel alignment across large regions, producing a strong magnetisation that persists indefinitely with no external field required at all, which is exactly what makes a permanent magnet permanent.
This spontaneous alignment is a genuinely collective, cooperative phenomenon — no single atom "decides" to align on its own; rather, each atom's magnetic moment strongly favours aligning with its neighbours, and this local preference, repeated across an enormous number of atoms, produces the large-scale ordered magnetic state.
Exploring
| Material | Curie temperature |
|---|---|
| Iron | ~1043 K (770°C) |
| Cobalt | ~1400 K (1127°C) |
| Nickel | ~627 K (354°C) |
Deep Dive
Pierre Weiss proposed the "molecular field" hypothesis in 1907, phenomenologically modelling ferromagnetism as if each atomic magnetic moment experienced an enormous internal effective field generated collectively by its neighbours' alignment — a mathematically successful early model, though Weiss himself could not explain the physical origin of this "molecular field," which remained a purely fitted parameter for over two decades. Werner Heisenberg's 1928 quantum mechanical explanation (via the exchange interaction, see the Codex's Exchange Interaction entry) finally provided the correct microscopic origin: Weiss's molecular field turns out to be, in modern terms, a mean-field approximation to the quantum exchange interaction between neighbouring electron spins.
Hysteresis: Ferromagnetic materials characteristically display hysteresis — their magnetisation depends not just on the currently applied field but on the material's magnetic history, tracing out a characteristic loop when magnetisation is plotted against applied field strength as the field is cycled up and down. This history-dependence, arising from the energy required to reorganise magnetic domains, is directly exploited in magnetic data storage (where a stable, retained magnetisation state encodes information) and gives rise to energy losses (hysteresis losses) in AC-driven magnetic components like transformer cores, a significant practical engineering consideration.
Not all ferromagnetic-like ordering is identical: ferrimagnetism (found in materials like magnetite, the original "lodestone" that first introduced humanity to magnetism) involves antiparallel alignment between two different, unequal sublattices of magnetic moments, producing a net magnetisation smaller than true ferromagnetism but still substantial — a distinct but closely related ordered magnetic phase, often grouped with ferromagnetism in practical discussions despite its distinct microscopic origin.
Concept Relationship Map
Requires: magnetism, exchange interaction
Enables: magnetic domains [PENDING FULL ENTRY], magnetic data storage applications
Commonly confused with: assuming an unmagnetised ferromagnet has no internal order (it has domain-level order that simply cancels on average)
Sources
- Weiss, P. (1907). Journal de Physique Théorique et Appliquée, 6(1), 661–690.
- Heisenberg, W. (1928). Zeitschrift für Physik, 49(9–10), 619–636.
- Kittel, C. (2004). Introduction to Solid State Physics, 8th ed. Wiley, Ch. 12.