Similarly, at a fixed temperature below the critical temperature, superconducting materials cease to remain superconducting when they are placed under an external magnetic field which is greater than a critical magnetic field (H_c). The field follows a parabolic form with temperature T and is given as

where  H₀ is a constant while T_C is the critical temperature i.e. the onset of the superconductivity. This is shown to be because the Gibbs free energy of the superconducting phase increases quadratically with the magnetic field while the free energy of the normal phase is roughly independent of the magnetic field.

The difference between type-I and type-II superconductors is that while type-I superconductors exhibit a distinct boundary between superconducting and normal state, type-II superconductors show a region of mixed normal and superconducting states. Also Type-II superconductors have much higher critical magnetic fields, as high as 40-50 Tesla than type-I superconductors, typically less than 1 Tesla.