The anomalous behaviour of the spin-lattice relaxation observed for single crystals of ytterbium chloride hexahydrate at fields stronger than 5 kOe is due to the poor heat transfer in the liquid-helium bath. The thermal conduction effects can be explained by means of a thermal conduction model for the relaxation.
Flokstra, J., Gerritsma, G. J., & Blokhuis, A. C. (1979). Thermal conduction effects in spin-lattice relaxation experiments on ytterbium chloride hexahydrate. Physica B+C, 97(4), 377-379. https://doi.org/10.1016/0378-4363(79)90088-3