NéelTemperaturen

Néeltemperaturen, or the Néel temperature, is the temperature below which an antiferromagnetic material exhibits long-range magnetic order with moments aligned in an antiparallel pattern. Above this temperature thermal fluctuations destroy the ordered arrangement, and the material becomes paramagnetic. The concept is named after Louis Néel, who described antiferromagnetism in the 1940s.

In an antiferromagnet, the net magnetization can be zero even though individual spins are ordered, because neighboring spins point in opposite directions. The order parameter below the Néel temperature is the staggered magnetization. The transition at T_N is often detected by a cusp or peak in magnetic susceptibility and by anomalies in specific heat or neutron diffraction patterns, where antiferromagnetic Bragg peaks appear below T_N.

Néel temperatures depend on the strength of exchange interactions, crystal structure, and magnetic anisotropy. They can vary from a few tens of kelvin to well above room temperature, and in some oxides can approach or exceed 900–1000 K. Common examples include NiO with T_N around 523 K, MnO around 125 K, CoO near 290 K, and hematite Fe2O3 around 956 K. In many materials, additional magnetic or structural transitions can occur at lower temperatures, but the Néel temperature marks the onset of antiferromagnetic order upon cooling.

The Néel temperature is a central concept in the study of magnetism, complementing the Curie temperature (for ferromagnets) and serving as a key parameter in modeling exchange interactions and magnetic phase diagrams. It has practical relevance in spintronics and magnetic storage, where antiferromagnetic materials are used for exchange bias and high-frequency applications.