Abstract
In this paper we report on the low-temperature magnetic relaxation of CrO2 particles of digital compact cassette tapes. The observed temperature dependence of the magnetic viscosity, S=partial derivativeM/partial derivative In t, shows three different regimes below room temperature. The most remarkable is that for T less-than-or-equal-to 5 K, S(T) remains constant, suggesting the occurrence of macroscopic quantum tunneling of magnetization.