Magnetic relaxation measurements of a-Fe2O3 antiferromagnetic particles below 1K

In this paper we report magnetic relaxation data for antiferromagnetic -Fe2O3 particles of 5-nm mean diameter in the temperature range 0.1–25 K. The average spin value of these particles S124 and the uniaxial anisotropy constant D1.6Å~10–2 K have been estimated from the experimental values of the blocking temperature and anisotropy field. The observed plateau in the magnetic viscosity from 3 K down to 100 mK agrees with the occurrence of spin tunneling from the ground state SZ = S. However, the scaling M vs T ln(0t) is broken below 5 K, suggesting the occurrence of tunneling from excited states below this temperature.