In this paper we present the experimental results and analysis of the critical-field determination of aluminum films and foils. We studied the thermodynamic parallel and perpendicular critical fields as a function of thickness and temperature. The measurement of the critical fields together with suitable theories allows the determination of the penetration depth $\lambda$, the coherence length $\xi$, and the surface-energy parameter $\Delta$. The thermodynamic transition of all films studied was of second order. Measurement in both magnetic field orientations offers a check on the consistency of the methods used to analyze the data. The value of the Ginzburg-Landau parameter $\kappa$ obtained from these measurements is six times larger than the value obtained from supercooling experiments. This phenomenon has been observed previously by other authors in different metals. The superconducting transition of all foil samples was of first order. In perpendicular fields the transition to the intermediate state was analyzed by means of an interpolation formula having the correct theoretical behavior in the limits of both large and small thicknesses. The value of the surface-energy parameter so obtained is in agreement with the supercooling result.

• Received 23 August 1971

DOI:https://doi.org/10.1103/PhysRevB.5.3558