Curie temperature

The material- specific Curie temperature or (according to Pierre Curie ) denotes the temperature at which the ferromagnetic or ferroelectric properties of a material have completely disappeared, so that above they are only paramagnetic or paraelectric . ${\displaystyle T_{\rm {C}}}$ ${\displaystyle \vartheta _{\rm {C}}}$

Occur

The Curie temperature marks the reversible phase transition of ferromagnetic or ferrimagnetic materials into their paramagnetic high-temperature form:

• Above the Curie temperature, the (spontaneous or directional) magnetization of crystal areas disappears .
• Below this temperature, the materials regain their magnetic properties. H. Without an external magnetic field , there is a spontaneous magnetization of the Weiss areas .

Materials can only be used as magnetic materials well below their Curie temperature .

The Curie temperature of some typical magnet materials is:

Magnetic material	${\displaystyle \vartheta _{\rm {C}}}$	${\displaystyle T_{\rm {C}}}$
Cobalt	1150 ° C	1423  K
iron	768 ° C	1041 K
nickel	360 ° C	633 K
Gadolinium	19.3 ° C	292.5 K
Ferrites (depending on the composition)	100 ... 460 ° C	370… 730 K

In the case of antiferromagnetic substances, the corresponding phase transition takes place at the Néel temperature . ${\displaystyle T_{\text{N}}}$

The polarization of a permanent magnet disappears irreversibly well below the Curie temperature , since a macroscopically uniform orientation of the Weiss domains is thermodynamically unstable.

Polarized and unpolarized ferroelectrics also show a similar behavior when heated and when they transition to the paraelectric phase . This is the reason for the sometimes quite low operating temperatures of ferroelectric materials for capacitors and piezo actuators .

Behavior above the Curie temperature

The magnetic susceptibility follows the Curie-Weiss law to a good approximation above the Curie temperature :

${\displaystyle \chi ={\frac {C}{T-T_{\mathrm {C} }}}}$

with the Curie constant . An analogous relationship also applies to the electrical susceptibility in ferroelectrics. ${\displaystyle C}$

Importance and uses

Data storage

In magneto-optical storage media , the magnetic layer is heated up to the Curie temperature by a laser in order to erase the existing information and to write new data. The magnetization is “frozen” when it cools down.
Heating conventional ( non- magneto-optical) hard disks above the Curie temperature ensures complete erasure of the data stored on the disk surface through remanence . However, this technique is mostly only used for top-secret data.

Thermostat in the "Magnastat" soldering iron

Some types of soldering iron from the manufacturer Weller Tools are equipped with a so-called Magnastat temperature controller. A sensor made from a ferromagnetic alloy is attached to the soldering tip . As long as the soldering tip is not hot enough, the sensor is ferromagnetic. This closes a switch operated by a permanent magnet and switches on the heating element. As soon as the soldering tip is hot enough, the sensor loses its ferromagnetic properties, causing the switch to open. The current remains interrupted until the sensor on the soldering tip becomes ferromagnetic again due to cooling, attracts the permanent magnet and thus closes the switch. Temperatures can be selected by changing the soldering tips or a soldering tip adapter whose sensors are made from different alloys; There are five different temperatures between 260 ° C and 480 ° C to choose from.

Paleomagnetism

→ Main article : Paleomagnetism

Hot lava emerging from the interior of the earth has a temperature above the Curie temperature. When it solidifies, crystallizing iron- containing minerals “freeze” the prevailing magnetic field. Usually this is the earth's natural magnetic field . In this way, fluctuations and pole reversals in the course of the earth's history can be detected.

geophysics

→ Main article : Curie depth

Since, with increasing depth in the interior of the earth, temperatures are soon reached that are above the Curie temperatures, the earth's magnetic field can not be created by a permanent magnet in the center of the earth. The corresponding depth under the earth's surface is called the Curie depth. In the continental crust , depending on the composition, this temperature is reached at a depth of around 20 km, in the oceanic crust at a much lower depth.

Ferrite cores

Ferrite cores , etc. a. for switching power supply - exchanger , just below the rather low Curie temperatures show a strong change of the permeability ; it rises first, only to fall steeply as the temperature rises. This temperature must therefore not be reached during operation. However, core losses often have a minimum in the region of 100 ° C, so that further heating during operation is limited.

literature

• Horst Stöcker: Pocket book of physics . 4th edition. Verlag Harry Deutsch, Frankfurt am Main 2000, ISBN 3-8171-1628-4
• Hans Fischer: Materials in electrical engineering . 2nd Edition. Carl Hanser Verlag, Munich / Vienna 1982 ISBN 3-446-13553-7
• Werner Schröter, Karl-Heinz Lautenschläger, Hildegard Bibrack: Pocket book of chemistry . 9th edition. Verlag Harry Deutsch, Frankfurt am Main 1981, ISBN 3-87144-308-5

Individual evidence

1. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1682.
2. ↑ C. Rau, S. Eichner: Phys. Rev. B 34, 6347-6350 (1986).
3. ↑ Instructions for use Weller Magnastat soldering iron page 1.1 (in the PDF page 4) ( Memento from February 4, 2016 in the Internet Archive ).
4. ↑ Soldering tips> PT> Adapter , online catalog from Weller Tools
5. ^ Lexicon of Geosciences , accessed on September 28, 2016.