Curie Temperature

The Curie temperature, also known as the Curie point, is the temperature above which certain materials lose their permanent magnetic properties, becoming paramagnetic. This transition is characterized by a sharp decrease in magnetization as the material's internal magnetic domains lose their ordered alignment due to thermal agitation. The Curie temperature is a material-specific property and is named after physicist Pierre Curie.

Here's a more detailed explanation:

Magnetic Domains:

Ferromagnetic materials, like iron, nickel, and cobalt, exhibit strong magnetism due to the alignment of electron spins within microscopic regions called magnetic domains.

Thermal Energy:

As temperature increases, the thermal energy of atoms and electrons also increases, causing them to vibrate and move more vigorously.

Loss of Alignment:

At the Curie temperature, the thermal energy overcomes the forces that maintain the alignment of magnetic domains, causing them to become randomly oriented.

Paramagnetism:

Once the magnetic domains are disordered, the material transitions from being ferromagnetic to paramagnetic. Paramagnetic materials are weakly attracted to magnetic fields, but they do not retain magnetism when the external field is removed.

Examples:

Iron has a Curie temperature of 769 °C, cobalt is 1127 °C, and nickel is 358 °C, according to supermagnete.de.

Practical Applications:

Understanding Curie temperature is crucial in various applications, including the design of electric motors, transformers, and other magnetic devices. It's also a key factor in the study of paleomagnetism, where the magnetic properties of rocks are used to understand Earth's past magnetic field.

The illustration schematically shows the alignment of the electron spins in a magnetised ferromagnet as the temperature rises. Initially, the electron spins in the Weiss domains remain aligned in parallel. Above the Curie temperature, however, the thermal energy of the spins overcomes the exchange interaction, and the magnetisation is lost.

Behaviour above the Curie temperature

In a paramagnet, the electron spins are statically orientated as long as no external magnetic field is present. The magnetised material will demagnetise again immediately after the external field is switched off.

For paramagnets, the magnetic susceptibility χ of the material, and thus also the magnetic permeability µ, continues to be highly temperature-dependent above the Curie temperature. The higher the temperature, the harder it is for the spins to be aligned by the external field, and the less the external magnetic field is amplified by the paramagnetic material.

The dependence of the magnetic susceptibility χ on the temperature T can be described above the Curie temperature T_C, i.e. for T > T_C, by the Curie-Weiss law.

The Curie-Weiss law reads:

χ = T - T ^{C} C

whereby C is the so-called Curie constant. The Curie constant is also material-specific (i.e. based on the type of material). This law was first formulated by the physicist Pierre Curie in 1896 and then further developed by the French physicist Pierre-Ernest Weiss in 1907.

Curie temperatures of certain ferromagnetic materials

Table: Overview of the Curie temperature of various ferromagnetic and ferrimagnetic materials according to sources [1]-[4].

Material	Chemical Formula	Curie temp. (K)	Curie temp. (°C)	Magnetism
Cobalt	Co	1388	1115	Ferromagnetic
Iron	Fe	1043	770	Ferromagnetic
Iron(III) oxide	Fe₂O₃	948	675	Ferrimagnetic
Nickel iron oxide	NiOFe₂O₃	858	585	Ferrimagnetic
Copper iron oxide	CuOFe₂O₃	728	455	Ferrimagnetic
Magnesium iron oxide	MgOFe₂O₃	713	440	Ferrimagnetic
Manganese bismuth	MnBi	630	357	Ferromagnetic
Nickel	Ni	627	354	Ferromagnetic
Neodymium-iron-boron	Nd₂Fe₁₄B	593	320	Ferromagnetic
Manganese antimonide	MnSb	587	314	Ferromagnetic
Manganese iron oxide	MnOFe₂O₃	573	300	Ferrimagnetic
Yttrium iron garnet	Y₃Fe₅O₁₂	560	287	Ferrimagnetic
Chromium(IV) oxide	CrO₂	386	113	Ferrimagnetic
Manganese arsenide	MnAs	318	45	Ferromagnetic
Gadolinium	Gd	292	19	Ferromagnetic
Terbium	Tb	219	-54	Ferromagnetic
Dysprosium	Dy	88	-185	Ferromagnetic
Europium(II) oxide	EuO	69	-204	Ferromagnetic

Sources:
[1] A. F. Holleman, E. Wiberg, N. Wiberg: Lehrbuch der Anorganischen Chemie. 102. Auflage. [Textbook of Inorganic Chemistry. 102nd edition] Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1, p. 1682.
[2] C. Rau, S. Eichner: Evidence for ferromagnetic order at gadolinium surfaces above the bulk Curie temperature. In: Physical Review B. Volume 34, No. 9, November 1986, p. 6347–6350, doi:10.1103/PhysRevB.34.6347
[3] C. Kittel: Introduction to Solid State Physics (sixth ed.). John Wiley and Sons, 1986. ISBN 0-471-87474-4.
[4] M. Jackson: Wherefore Gadolinium? Magnetism of the Rare Earths (PDF). IRM Quarterly. Institute for Rock Magnetism. 10 (3), 2000

The table shows a selection of materials that have various interesting applications due to their interesting magnetic properties. Neodymium-iron-boron, for example, is frequently used for permanent magnets and has a Curie temperature of 320 °C. All the materials mentioned are only ferromagnetic or ferrimagnetic below the Curie temperature; above this temperature, the materials become paramagnetic, as the exchange interaction of the electron spins is cancelled out by the thermal motion.

For many materials, the exact magnetic properties depend precisely on the specific composition and manufacturing conditions. MnAs, for example, is known for its phase transitions and associated magnetic property changes, making it an interesting candidate for thermal storage applications and sensors. The exact magnetic properties of MnAs, including its Curie temperature, depend strongly on the crystal structure and microstructure of the material.