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Curie point[′kyu̇r·ē ‚pȯint]
(Curie temperature), the temperature marking a second-order phase transition related to a discontinuous change in the symmetry properties of a substance (for example, magnetic symmetry in ferromagnetic substances, electric symmetry in ferroelectric substances, and crystallochemical symmetry in ordered alloys). The Curie point was named in honor of the French physicist P. Curie, who studied this transition in detail in ferromagnetic substances.
At a temperature T below the Curie point Ⓗ, ferromagnetic substances exhibit spontaneous magnetization and a definite magnetocrystalline symmetry. At the Curie point (T = Ⓗ), the intensity of the thermal motion of the atoms in a ferromagnetic substance is sufficient to destroy its spontaneous magnetization (“magnetic order”) and to change its symmetry, which leads to the transition of the ferromagnetic substance into a paramagnetic substance. Similarly, an antiferromagnet exhibits at T = Ⓗ (at its antiferromagnetic Curie point, or Néel point) destruction of its characteristic magnetic structure (magnetic sublat-tices) and becomes paramagnetic. In ferroelectric and antiferroelectric substances, the thermal motion of atoms at T = Ⓗ nullifies the spontaneous ordered orientation of the electric dipoles of the unit cells in the crystal lattice. At the Curie point (which is also called the Kurnakov point for alloys) in ordered alloys, the degree of long-range order in the arrangement of atoms (ions) of the alloy’s components is zero.
Thus in all cases, second-order phase transitions (of the Curie point type) are accompanied at T = Ⓗ by the disappearance of some type of atomic “ordering” (for example, ordered orientation of the magnetic or electric moments, long-range order in the arrangement of the atoms in the lattice sites in alloys). Specific changes in many physical properties (such as heat capacity, magnetic susceptibility) occur in substances near the Curie point, with the maximum being attained at T = Ⓗ, which is usually used for the accurate determination of the phase transition temperature.