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Schematic of the Birkeland or Field-Aligned Currents and the ionospheric current systems they connect to, Pedersen and Hall currents. [1]A Birkeland current (also known as field-aligned current, FAC) is a set of electrical currents that flow along geomagnetic field lines connecting the Earth's magnetosphere to the Earth's high latitude ionosphere.
[1]: 117 The formula above is known as the Langevin paramagnetic equation. Pierre Curie found an approximation to this law that applies to the relatively high temperatures and low magnetic fields used in his experiments. As temperature increases and magnetic field decreases, the argument of the hyperbolic tangent decreases.
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 ...
Metals. Above the Curie temperature, the atoms are excited, and the spin orientations become randomized [9] but can be realigned by an applied field, i.e., the material becomes paramagnetic. Below the Curie temperature, the intrinsic structure has undergone a phase transition, [16] the atoms are ordered, and the material is ferromagnetic. [12]
The table below shows some of the parameters of common superconductors. X:Y means material X doped with element Y, T C is the highest reported transition temperature in kelvins and H C is a critical magnetic field in tesla. "BCS" means whether or not the superconductivity is explained within the BCS theory.
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, , near its Curie temperature. The magnetic susceptibility is the ratio between the material's magnetization and the applied ...
ref T c (K) T c (°C) P c (MPa) P c (other) V c (cm 3 /mol) ρ c (g/cm 3) ; 1 H hydrogen; use: 32.97: −240.18: 1.293: CRC.a: 32.97: −240.18: 1.293: 65: KAL: 33.2: 1.297: 65.0: SMI: −239.9: 13.2 kgf/cm 2: 0.0310 1 H hydrogen (equilibrium)
When the temperature rises beyond a certain point, called the Curie temperature, there is a second-order phase transition and the system can no longer maintain a spontaneous magnetization, so its ability to be magnetized or attracted to a magnet disappears, although it still responds paramagnetically to an external field.