Curie temperature

The Curie temperature marks the loss of spontaneous magnetization in a ferromagnet. Below this temperature, exchange interactions favor parallel alignment of neighboring atomic magnetic moments, producing magnetic domains and allowing a specimen to retain magnetization after an external field is removed.¹ Above the transition, thermal motion overwhelms the long-range alignment. The material is then paramagnetic: its moments respond to an applied field, but it has no permanent magnetization when that field is withdrawn.

The change is a magnetic phase transition rather than simply a gradual weakening of a magnet. Near the transition, magnetization falls rapidly, magnetic susceptibility becomes large, and properties such as heat capacity and electrical resistivity can show anomalies. The exact behavior depends on crystal structure, composition, defects, and the measurement direction.

The Curie temperature results from competition between exchange coupling and thermal energy. Exchange coupling arises from quantum-mechanical interactions among electrons; in a ferromagnet it favors a collectively ordered state, while increasing temperature supplies fluctuations that progressively destroy that order.²

Researchers determine the transition by tracking magnetization, susceptibility, or another temperature-dependent property while heating and cooling a sample. In the paramagnetic region, susceptibility often follows the Curie–Weiss law, approximately χ = C/(T − θ), where C is the Curie constant and θ is the Curie–Weiss temperature. The latter can differ from the thermodynamic Curie temperature because it reflects interactions inferred from a high-temperature approximation, not necessarily the exact point of long-range ordering.³

Common elemental ferromagnets have distinct Curie temperatures: iron is about 1,043 K, nickel about 627 K, and cobalt about 1,394 K.¹ Alloying can shift the transition substantially, so engineered magnetic materials are characterized by composition and processing rather than by the behavior of a pure element alone. Ferrimagnetic materials can also undergo an analogous loss of long-range order, although terminology and magnetic compensation effects require care.

The temperature is useful in magnetic recording, sensors, transformers, heating devices, and nonvolatile safety mechanisms. A Curie-temperature fuse or switch can exploit a material whose magnetism changes sharply at a designed temperature. Magnetic hyperthermia research likewise studies particles whose heating and magnetic response are constrained by their transition temperature, while spacecraft and industrial systems treat the transition as a possible failure limit for permanent magnets.

The Curie temperature is not always a single perfectly sharp number in practical materials. Grain boundaries, impurities, strain, finite particle size, and compositional disorder can broaden or shift the observed transition; nanoscale particles may show a size-dependent blocking or ordering behavior rather than the bulk response.

Near the transition, magnetization follows critical behavior described by critical exponents, and fluctuations occur over increasingly large regions of the crystal. This makes the Curie temperature important to statistical physics as well as materials science.⁴ The term honors Pierre Curie, whose experiments established the temperature dependence of magnetic susceptibility and helped distinguish ferromagnetic behavior from ordinary paramagnetism. It should not be confused with the Néel temperature, which describes the loss of antiferromagnetic order.

Ferromagnetism

Magnetic ordering in which neighboring moments align cooperatively, producing spontaneous magnetization below a transition temperature.

Paramagnetism

A form of magnetism in which atomic or electronic moments align weakly with an applied field but do not retain magnetization after the field is removed.

Curie–Weiss law

An approximate relation describing the susceptibility of a paramagnetic material as a function of temperature above its ordering transition.

Néel temperature

The temperature above which an antiferromagnetic material loses its long-range antiparallel ordering.