The superconductivity of gallium in porous glasses with various pore sizes and in opals is studied using a superconducting quantum interference device magnetometer. The single and double superconducting phase transitions are observed for different samples. Magnetization hysteresis loops are also measured and found to be dependent on pore sizes and geometry. The changes in magnetization below about 6.4 K are treated within the framework of models for granular superconductors, while the alterations in magnetization near 7.1 K are treated as a result of the superconducting phase transition in a coexistent structural modification of confined gallium. X-ray diffraction measurements are performed to confirm the presence of such an additional gallium modification. The results obtained allow us to suggest that studies of magnetization at low temperatures can be used to get information about the geometry of the pore network and distribution of Josephson links in porous composite materials.

• Received 17 November 1997

DOI:https://doi.org/10.1103/PhysRevB.58.467