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This threshold temperature below which a material is ferromagnetic is called the Curie temperature and is different for each material. The Curie–Weiss law describes the changes in a material's magnetic susceptibility , χ {\displaystyle \chi } , near its Curie temperature.
As temperature is inversely proportional to magnetic susceptibility, when T approaches T C the denominator tends to zero and the magnetic susceptibility approaches infinity allowing magnetism to occur. This is a spontaneous magnetism which is a property of ferromagnetic and ferrimagnetic materials.
Below that temperature, there is a spontaneous symmetry breaking and magnetic moments become aligned with their neighbors. The Curie temperature itself is a critical point, where the magnetic susceptibility is theoretically infinite and, although there is no net magnetization, domain-like spin correlations fluctuate at all length scales.
There are two other measures of susceptibility, the molar magnetic susceptibility (χ m) with unit m 3 /mol, and the mass magnetic susceptibility (χ ρ) with unit m 3 /kg that are defined below, where ρ is the density with unit kg/m 3 and M is molar mass with unit kg/mol: =; = =.
> is the (volume) magnetic susceptibility, is the magnitude of the resulting magnetization (A/m), is the magnitude of the applied magnetic field (A/m), is absolute temperature , is a material-specific Curie constant (K).
The magnetic susceptibility of an antiferromagnetic material typically shows a maximum at the Néel temperature. In contrast, at the transition between the ferromagnetic to the paramagnetic phases the susceptibility will diverge. In the antiferromagnetic case, a divergence is observed in the staggered susceptibility.
Compounds at temperatures below the Curie temperature exhibit long-range magnetic order in the form of ferromagnetism. Another critical temperature is the Néel temperature, below which antiferromagnetism occurs. The hexahydrate of nickel chloride, NiCl 2 ·6H 2 O, has a Néel temperature of 8.3 K. The susceptibility is a maximum at this ...
Superparamagnetism is a form of magnetism which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time between two flips is called the Néel relaxation time. In the absence of an external magnetic field ...