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How does magnetic flux enter a type II superconductor between its critical fields?

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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.

In type II superconductors, magnetic flux enters the material when the applied magnetic field is between the lower critical field ( $H_{c 1}$ ) and the upper critical field ( $H_{c 2}$ ).12

Rather than penetrating the material uniformly, the magnetic field enters in the form of discrete quantized fluxoids, each carrying one quantum of magnetic flux. These fluxoids tend to organize themselves into regular, ordered patterns within the superconductor, which have been observed through techniques such as electron microscopy and neutron diffraction.2

If a sufficiently large electric current is applied to the superconductor, these fluxoids can move, leading to energy dissipation that may heat the material and drive it into the normal state. To prevent this in practical applications like high-field magnets, manufacturers often introduce inhomogeneities into the material to pin the fluxoids in place.2