The relationship between Temperature & Magnet

The relationship between Temperature & Magnet

Summary

Not at any temperature, magnetic materials are magnetic. They have a critical temperature Tc, which is Curie temperature.

The relationship between Temperature & Magnet

What's Curie Temperature?

Curie point (Curie point) is also called Curie temperature (Curie temperature, Tc) or magnetic transition point. Refers to the temperature at which the spontaneous magnetization in magnetic materials drops to zero, and is the critical point where ferromagnetic or ferrimagnetic materials transform into paramagnetic materials. When the temperature is lower than the Curie point, the substance becomes a ferromagnet. At this time, the magnetic field related to the material is difficult to change. When the temperature is higher than the Curie point, the substance becomes a paramagnet, and the magnetic field of the magnet easily changes with the surrounding magnetic field. The magnetic sensitivity at this time is about 10-6. The Curie point is determined by the chemical composition and crystal structure of the substance.

Material
Curie Temp.

NdFeB
310~380℃

SmCo
750~820℃

AlNiCo
750~850℃

Hard Ferrite

≥450℃

Why high temperature would lead to demagnetization?

As the temperature rises, the increased thermal motion of the metal lattice will affect the ordered arrangement of the magnetic moments of the magnetic domains. When the temperature reaches a sufficient arrangement to destroy the magnetic moments of the magnetic domains, the magnetic domains are disintegrated and the average magnetic moment becomes zero , The magnetic disappearance of the ferromagnetic substance becomes a paramagnetic substance, and a series of ferromagnetic properties (such as high permeability, hysteresis loop, magnetostriction, etc.) associated with the magnetic domain all disappear, and the corresponding ferromagnetic substance The permeability is converted to the permeability of paramagnetic substances.

Application

Using this feature, people have developed many control elements. For example, the working principle of rice cooker which is using the characteristics of the curie point of magnetic materials. A permanent magnet and a soft magnet with a curie point of 103 ° C are installed in the center of the bottom of the rice cooker. When the water in the pan is dry, the temperature of the food will rise from 100 ° C. When the temperature reaches about 103 ° C, the soft magnet loses its attraction. At this time, the spring between the permanent magnet and soft magnet will separate them and drive the power to be cut, stop heating.