Curie Temperature

Curie temperature is named after the French physicist Pierre Curie, who in 1895 discovered that ferromagnetic materials, when heated to a certain temperature, tend to lose their magnetic properties. Pierre Curie along with his wife Marie Curie won the Nobel prize in physics in 1903 for their immense contribution to the research on radiation phenomenon. Curie temperature (T_c) can be defined as the temperature at which a ferromagnetic material loses its magnetic properties and becomes paramagnetic. This phenomenon is also known as superconductivity. For example, we know that iron is attracted by magnets, but if we heat iron to a temperature of 1043 Kelvin, it will no longer be attracted by magnets. Curie temperature is important in the study of ferromagnetic materials and electromagnetism.

In a ferromagnetic material, when temperatures are below the Curie point, the magnetic moments are aligned in a parallel fashion, whereas in case of ferrimagnetic materials, these are anti-parallel. On increasing the temperature, thermal fluctuations increase, which destroy this alignment and at Curie point the net magnetization of the ferromagnetic material becomes zero. This state is known as paramagnetism.

Curie's Law
The relationship between magnetization of a substance and the temperature is given by Curie's law. According to Curie's Law,

M =

C×B T

where M is the magnetization
B is the magnetic field, (in teslas)
T is the absolute temperature (in Kelvins)
C is a material specific Curie constant

Curie Temperature of Magnets
The effect of temperature on magnets is known to change their strength. Let us find out the Curie point for some of the known varieties of magnets.

• Ceramic or Ferrite Magnets - These magnets find their use in refrigerators and are known as refrigerator magnets. They are usually black or gray in color. The Curie point of ceramic magnets is about 734 K.
• Alnico Magnets - Alnico magnets have a color like aluminum or sometimes they are coated in red color. The Curie point for these magnets is about 1135 K.
• Samarium Cobalt Magnets - These are the more expensive magnets that are used in the DVD's etc. as a motor. The Curie point for these magnets is about 1025 K.
• Neodymium-Iron-Boron Magnets - These magnets are the brittle in nature. The Curie temperature of these magnets is about 585 K.

Curie Temperature for Piezoelectric Materials
Piezoelectric materials are the materials which accumulate a certain amount of charge in the presence of a mechanical strain. Piezoelectricity is the electricity which is derived by the application of pressure. In context of piezoelectric materials, Curie temperature is defined as the temperature above which the substance loses its piezoelectric properties.

Curie Temperature Table

Name	Curie temperature in K
Iron	1043
Cobalt	1394
Nickel	631
Gadolinium	292
Terbium	222
Dysprosium	87
Holmium	20
Erbium	32
Thulium	25

The concept of Curie temperature finds its use in thermo-magnetic motors. The principle behind these motors is that the material is heated above its Curie point and then it is allowed to cool down. Its also finds its use in the study of various magnetic heat engines and geology. In geology, it refers to the temperature at which a magnetic mineral crystal melts to lose the polarity of its magnetism.