Curie temperature

The material- specific Curie temperature and ( after Pierre Curie ) refers to the temperature at which point ferromagnetic and ferroelectric properties of a sample are completely gone, so they above is only paramagnetic and paraelectric.

• 2.1 Data Storage
• 2.2 thermostat in the " Magnastat " soldering iron
• 2.3 paleomagnetism
• 2.4 Geophysics
• 2.5 ferrite cores

Occurrence

The Curie temperature indicates the reversible phase transition of ferromagnetic or ferrimagnetic material in its paramagnetic form high temperature:

• Above the Curie temperature (spontaneous or directed) magnetization of crystal regions disappears
• Below this temperature the materials reach their magnetic properties back, i.e. with no external magnetic field is a spontaneous magnetization of the white areas.

Materials are considerably lower than the Curie temperature can be used as a magnetic material.

The Curie temperature of some typical magnetic materials is:

For antiferromagnetic materials, the corresponding phase change in the Néel temperature takes place.

The polarization of a permanent magnet disappears already well below the Curie temperature irreversible because a macroscopically uniform orientation of the white districts is thermodynamically unstable.

An analogous behavior also polarized and non-polarized ferroelectrics on heating and transition to the paraelectric phase. This is the cause of partially pretty low operating temperatures of ferroelectric materials for capacitors and piezoelectric actuators.

Behavior above the Curie temperature

Above the Curie temperature, the magnetic behavior can be described often by a Curie -Weiss law. The paramagnetic and dielectric susceptibility follows a good approximation of the relation

With the Curie constants.

Importance and applications

Data Storage

In magneto- optical storage media, the magnetic layer is heated by a laser point-like up to the Curie temperature to delete the existing information and to write new data. Upon cooling, the magnetization is "frozen". The heating of conventional ( non- magneto- optical ) disks over the Curie temperature ensures complete erasure of data stored on the disk surface due to remanence data. This technique is applied but mostly only with top secret data.

Thermostat in the " Magnastat " soldering iron

The soldering iron is turned on by a magnetic switch. Once the tip is hot enough, on an affixed ferromagnetic material loses its magnetism and opens the magnetic circuit, ie the magnetic field of a permanent magnet can no longer close the switch. The current remains interrupted until the material is re- ferromagnetic at cooling and closes the switch.

Paleomagnetism

Hot, emerging from the earth's interior lava lies in its temperature above the Curie temperature. If it solidifies, " freeze " crystallizing ferrous minerals a the prevailing magnetic field. Typically this is the natural magnetic field of the earth. In this way, variations and pole reversals can be detected in the course of history.

Geophysics

As soon temperatures will be reached within the earth, which are above the Curie temperature ( Curie depth) and the magnetic field of the earth can not be created by a permanent magnet in the center of the earth.

Ferrite cores

Ferrite cores, including for switching power supply transformer, show somewhat below the relatively low Curie temperatures, a large change in permeability, it initially rises to fall steeply with further increasing temperature. This temperature must therefore not be achieved in operation. Often, however, have the core loss in the range of 100 ° C a minimum, so that a further heating is limited in operation.