1. Nagamiya, T.: Solid state physics edited by Seitz, F., Turnbull, D. (eds.). New York: Academic Press 1967
2. Jensen, J., Mackintosh, A.R.: Rare earth magnetism: structures and excitations. Oxford: Oxford University Press 1991
3. Cheong, S.W., Aeppli, G., Mason, T.E., Mook, H., Hayden, S.M., Canfield, P.C., Fisk, Z.K., Clausen, N., Martinez, J.L.: Phys. Rev. Lett.67, 1791 (1991)
4. Kawamura, H.: J. Phys. Soc. Jpn.53, 2452 (1984)
5. Rastelli, E., Tassi, A., Pimpinelli, A., Sedazzari, S.: Phys. Rev. B45, 7936 (1992)