In order to gain further insight into the magnetic states, we analyse the magnetisation isotherms measured in fields up to 7 T at 2, 5, 20, 50, 100, 200 and 300 K as shown in Figure . The Brillouin functions for a pure spin half system at the aforementioned temperatures are also plotted as reference. At 300 K the magnetisation curve is only slightly lower than the Brillouin function. As the temperature is lowered, the dfference becomes greater. This is attributed to the presence of antiferromagnetic coupling, which is consistent with the conclusion drawn from the temperature dependence in Figure . At 2 K (red circles), not only the magnetisation is much smaller as compared with the Brillouin function that saturates at 1 µ B , but also it exhibit a rapid increase in fields up to 0.25 T, then goes linear at higher fields. This is in stark contrast with the continuous field dependence of the dashed curve which is the Brillouin function at 2 K multiplied by 0.05. The initial increase of the isotherm at 2 K is much greater than that of the Brillouin function (dashed curve). This can be attributed to the existence of a positive or staggered exchange coupling which dominates at 2 K.