1. M. Steiner, J. Villain and CG. Windsor, Adv. in Physics 25, 87 (1976).
2. M. Steiner, K. Kakurai and J.K. Kjems, Z. Phys. B 53, 117 (1983).
3. A.P. Ramirez and W.P. Wolf, Phys. Rev. Lett. 49, 227 (1982).
4. J.P. Boucher, L.P. Regnault, J. Rossat-Mignot, J.P. Renard, J. Bouillot and W.G. Stirling, J. Appl. Phys. 52, 1956 (1982); J.P. Boucher, L.P. Regnault, A. Pires, J. Rossat-Mignot, Y. Henry, J. Bouillot, W.G. Stirling and J.P. Renard, In Magnetic Excitations and Fluctuations, ed. by S.W. Lovesey, U. Balucani, F. Borsa and V. Tognetti (Springer, Berlin, Heidelberg 1984).
5. F. Borsa, Phys. Rev. B 25, 3430 (1982); F. Borsa, M.G. Pini, A. Rettori and V. Tognetti, Phys. Rev. B 28, 5173 (1983).