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How far can a magnetic field penetrate into a superconducting material?

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Figure 1: Specific heat in the normal (Cen) and superconducting (Ces) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (Tc) and above Tc. — superconductivity Figure 1: Specific heat in the normal (C_en) and superconducting (C_es) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (T_c) and above T_c.

Although superconductors are characterized by the Meissner effect—the expulsion of magnetic fields from their interior—this exclusion is not absolute. A magnetic field does penetrate a superconducting material for a small distance known as the electromagnetic penetration depth.12

For superconducting elements at low temperatures, this penetration depth typically ranges from approximately 390 to 1,300 angstroms. As the temperature of the material approaches its critical temperature, this penetration depth increases significantly.1

In the case of type II superconductors, when an applied magnetic field exists between the lower and upper critical fields, the field enters the material in the form of quantized fluxoids rather than being entirely expelled.13