We investigated the temperature dependence of the trapped magnetic field and levitation force for c-axis-oriented single-grain Gd-Ba-Cu-O bulk superconductors 48 mm in diameter. The trapped magnetic fields of the samples were 2.1-2.2 T at 77 K and increased with decreasing temperature, reaching 4.1 T at 70 K. However, the samples fractured during the measurements at lower temperatures due to a large electromagnetic force. Reinforcement using a metal ring was effective in improving the mechanical strength. The sample encapsulated in an Al ring could trap a very high magnetic field of 9.0 T at 50 K. In liquid O2(90.2 K), the Gd-Ba-Cu-O bulk superconductor exhibited a trapped magnetic field of 0.4 T and a levitation force about half the value of that in liquid N2.
