"Intermediate Mixed" State of Type-II Superconductors

For magnetic fields in the range (1-D)Hc1<H<Hₜ, magnetization curves of low-κ type-II Pb-In and In-Bi alloys with nonzero demagnetization coefficients D exhibit a linear dependence of the magnetic moment on applied field. Hc1 is the lower critical field and Hₜ is the field at which this linear dependence terminates. For finite D, Hₜ<Hc1. This behavior can be explained by a model of alternate regions of superconducting and mixed states. This array may be metastable. The mixed-state regions are all presumed to have the same temperature-dependent fluxoid-lattice spacing characteristic of Hc1 when $D=0$. Thus, as the magnetic field is increased isothermally, the mixed-state regions grow until the whole sample has been converted into the mixed state. This model implies the existence of an attractive interaction between flux lines in low-κ materials.