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

In this paper we report magnetic relaxation data for antiferromagnetic α-Fe₂O₃ particles of 5-nm mean diameter in the temperature range 0.1–25 K. The average spin value of these particles S≃124 and the uniaxial anisotropy constant D≃1.6×10^-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 S_Z=S. However, the scaling M vs Tln(ν₀t) is broken below 5 K, suggesting the occurrence of tunneling from excited states below this temperature.