A very good fit was obtained for 2-Tb yielding the parameters τ0 = 3.66 x 10 -8 s, Ueff = 406 K (282 cm -1 ), C = 1.04 K -n s -1 , n = 1.81 and τQTM = 0.37 s (Fig. ). Similarly, a good fit was achieved for 3-Tb with the following parameters: τ0 = 5.70 x 10 -6 s, Ueff = 172 K (119 cm -1 ), C = 7.56 x 10 -1 K -n s -1 , n = 2.34 and τQTM = 7.10 x 10 -3 s (Fig. ). As the application of the external field is used to suppress quantum tunnelling of the magnetization, the data obtained from measuring at 2000 Oe were fit using equation ( ) which corresponds to a combination of only Orbach and Raman type relaxation.