Anomalies in the Relaxation of Small Magnetic Particles at Very Low   Temperatures
  The magnetization relaxation rate of small gamma-Fe2O3 particles dispersed in a silica matrix has been measured from 60 mK to 5 K. It shows a minimum around 150 mK, that can be discussed in terms of either thermal or quantum relaxation regime. 
R. Sappey, E. Vincent, J. Hammann, F. Chaput, J.P. Boilot, D. Zins