Magnetic irreversibility in type-I superconductors: Review and representative experimental study

The intermediate state of type-I superconductors remains a fertile area of research because even in ultraclean materials, magnetization curves frequently exhibit hysteresis. In this work, we review magnetic irreversibility in type-I superconductors, emphasizing intrinsic mechanisms associated with the geometrical barrier and topological transitions in the intermediate state. These concepts are illustrated through a representative experimental study on a high-purity tantalum cylinder. Isothermal magnetization curves were measured at different temperatures and magnetic field orientations and analyzed using reduced representations normalized to the thermodynamic critical field. For magnetic fields applied parallel to the cylinder axis, irreversible behavior is observed above a reduced field h1, corresponding to the transition from the Meissner state to the intermediate state, whereas the response remains reversible in the perpendicular geometry. The results demonstrate that magnetic irreversibility in tantalum is governed by geometry- and topology-driven mechanisms and is largely independent of temperature when expressed in reduced variables, supporting a unified interpretation of magnetic irreversibility in type-I superconductors.