Abstract
Experimental results are presented on the magnetic relaxation in different systems (single domain particles, magnetic grains, and random magnets). The existence of two relaxation regimes is demonstrated. At high temperatures, the magnetic viscosity S = 1/M sub 0 delta M/ delta ln(t) is proportional to temperature in accordance with theoretical expectation for thermally activated processes. At low temperatures, the viscosity is independent of temperature, providing evidence of quantum tunneling of magnetization. Qualitative agreement between theory and experiment is found. FeSm compound is discussed.