1. A. Aharoni (1996) An Introduction to the Theory of Ferromagnetism, Oxford University, London
2. A.J. Freemann and R. Wu, J. Magn. Magn. Mat.,100, 497 (1991).
3. I.M.L. Billas, A. Chatelain, W.A. de Heer, J. Magn. Magn. Mat.,168, 64 (1997).
4. W.Wernsdorfer and R. Sessoli, Science,284, 133 (1999).
5. A. Garg, Europhys. Lett.,22, 205 (1993).