SQUID and magnetically modulated microwave absorption observations of the superconducting transition in single crystals of YBa2Cu3Ox

Two melt‐grown single crystals of YBa2Cu3O7−x, one (crystal A) in the shape of a nearly perfect parallelepiped of dimensions 200×200×70 μm3 and with x≂0, the other (crystal B) with somewhat less oxygen content and of less geometric perfection, with dimensions≂750×900×10 μm,3 were measured by both SQUID magnetometry and by magnetically modulated microwave absorption (MAMMA). The two methods respond to the change in magnetic properties at the superconducting transition temperature, but in different ways, giving complementary information. For crystal A both methods show a single sharp transition near 90 K. For crystal B the SQUID measurements show a broad transition near 80 K, implying a material with a range of oxygen concentrations, while the MAMMA reveals considerable structure, with details of this structure depending on the direction in which the magnetic fields are applied.