Quantum relaxation in magnetism and high-T_c superconductors

We present a comprehensive study of low-temperature magnetic relaxation in magnetic systems and high-T_c superconductors (HTSC). It is argued that relaxation experiments are ideally suited for the observation of magnetic quantum tunnelling since they automatically provide the condition of very low energy barriers. Our data demonstrate the existence of two relaxation regimes: At high temperatures, the magnetic viscosity depends on temperature in accordance with the theoretical expectation for thermally activated processes. At low temperatures the viscosity is temperature-independent, providing evidence for quantum tunnelling of the magnetization. Qualitative agreement between theory and experiment is found.