Curie point

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The Curie point (T_c), or Curie temperature, is a term in physics and materials science, named after Pierre Curie (1859-1906), and refers to a characteristic property of a ferromagnetic or piezoelectric material.

[edit] Curie point in ferromagnetic materials

The Curie point of a ferromagnetic material is the temperature above which it loses its characteristic ferromagnetic ability. At temperatures below the Curie point the magnetic moments are partially aligned within magnetic domains in ferromagnetic materials. As the temperature is increased from below the Curie point, thermal fluctuations increasingly destroy this alignment, until the net magnetization becomes zero at and above the Curie point. Above the Curie point, the material is purely paramagnetic.

At temperatures below the Curie point, an applied magnetic field has a paramagnetic effect on the magnetization, but the combination of paramagnetism with ferromagnetism leads to the magnetization following a hysteresis curve with the applied field strength. The destruction of magnetization at the Curie temperature is a second-order phase transition and a critical point where the magnetic susceptibility is theoretically infinite.

See ferromagnetism for a list of ferromagnetic materials and their Curie temperatures.

The effect is primarily used in magneto-optical storage media, where it is used for erasing and writing of new data. Famous examples include the Sony Minidisc format, as well as the defunct CD-MO format.

Other uses include temperature control in soldering irons such as the Weller WTCPT and, in general, where a temperature-controlled magnetization is desirable.

[edit] Curie temperature in piezoelectric materials

In analogy to ferromagnetic materials, the Curie temperature is also used in piezoelectric materials to describe the temperature above which the material loses its spontaneous polarization and piezoelectric characteristics. In lead zirconate titanate (PZT), the material is tetrahedral below T_c and the unit cell contains a displaced central cation and hence a net dipole moment. Above T_c, the material is cubic and the central cation is no longer displaced from the centre of the unit cell. Hence, there is no net dipole moment and no spontaneous polarization.