The determination of zero-field splittings from measurements of spin-lattice relaxation times A.M. Vasson, A. Vasson, A. Gavaix, T.S. Yalcin, P. Steggles, W.S. Moore et C.A. Bates J. Physique Lett., 35 5 (1974) 73-76 DOI: 10.1051/jphyslet:0197400350507300