Curie temperature

In physics and materials science, the Curie temperature (TC), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Curie temperature is named after Pierre Curie, who showed that magnetism was lost at a critical temperature. The force of magnetism is determined by the magnetic moment, a dipole moment within an atom which originates from the angular momentum and spin of electrons. Materials have different structures of intrinsic magnetic moments that depend on temperature; the Curie temperature is the critical point at which a material's intrinsic magnetic moments change direction. Permanent magnetism is caused by the alignment of magnetic moments and induced magnetism is created when disordered magnetic moments are forced to align in an applied magnetic field. For example, the ordered magnetic moments (ferromagnetic, Figure 1) change and become disordered (paramagnetic, Figure 2) at the Curie temperature. Higher temperatures make magnets weaker, as spontaneous magnetism only occurs below the Curie temperature. Magnetic susceptibility above the Curie temperature can be calculated from the Curie–Weiss law, which is derived from Curie's law. In analogy to ferromagnetic and paramagnetic materials, the Curie temperature can also be used to describe the phase transition between ferroelectricity and paraelectricity. In this context, the order parameter is the electric polarization that goes from a finite value to zero when the temperature is increased above the Curie temperature. Magnetic moments are permanent dipole moments within an atom that comprise electron angular momentum and spin by the relation μl = el/2me, where me is the mass of an electron, μl is the magnetic moment, and l is the angular momentum; this ratio is called the gyromagnetic ratio. The electrons in an atom contribute magnetic moments from their own angular momentum and from their orbital momentum around the nucleus. Magnetic moments from the nucleus are insignificant in contrast to the magnetic moments from the electrons. Thermal contributions result in higher energy electrons disrupting the order and the destruction of the alignment between dipoles. Ferromagnetic, paramagnetic, ferrimagnetic and antiferromagnetic materials have different intrinsic magnetic moment structures. At a material's specific Curie temperature, these properties change. The transition from antiferromagnetic to paramagnetic (or vice versa) occurs at the Néel temperature, which is analogous to Curie temperature.