Critical Magnetic Field

critical magnetic field

[′krid·ə·kəl mag′ned·ik ′fēld]
(solid-state physics)
The field below which a superconductive material is superconducting and above which the material is normal, at a specified temperature and in the absence of current.

Critical Magnetic Field

 

(in superconductors), the specific value of magnetic field strength Hc above which the magnetic field partially or completely penetrates the superconductor. When H < Hc the magnetic field does not penetrate the superconductor, because it is shielded by the surface superconducting current (the Meissner effect).

In type I (soft) superconductors, which include most pure metals, the substance passes into the normal, nonsuperconducting state when H > Hc (phase transition of the first kind). The critical magnetic field Hc corresponding to this transition is related to the difference between the free energies of the normal phase (Fn) and the superconducting phase (Fsc) by the formula FnFsc = Hc2/8π.

The highest value of Hc reaches hundreds of oersteds for pure metals. If the magnetic field is equal to Hc only at some points of the surface on a superconductor of the second kind, an intermediate state develops in it (alternating superconducting and normal phases).

In type II (hard) superconductors (mostly alloys), the penetration of the magnetic field begins with the formation of vortex filaments, in whose cores the magnetic field is largely concentrated. The substance has not yet lost its superconducting properties, and the currents flowing in it are partially shielding it from the external field. For these substances the critical magnetic field Hc,1 that corresponds to the beginning of penetration is smaller than the thermodynamic critical field Hc. Complete penetration of the magnetic field occurs in superconductors at Hc,2, which may be both smaller and larger than Hc. In the hard superconductors, the best known of which are alloys based on niobium, the critical magnetic field Hc,2Hc,1 and reaches hundreds of thousands of oersteds. Second-order phase transitions take place at field values Hc1 and Hc,2.

S. V. IORDANSKII

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