Fig. 4. The dipolar couplings between two neighbouring spin centres with strong axial anisotropy, showing the importance of the relative orientation of the magnetic moments with their spatial locations (top), from an independent spin center (b) to a ferromagnetic (c) or antiferromagnetic (a) dimer. The solid grey lines qualitatively represent the magnetic field lines generated by the spin centre treated as a small bar magnet. The corresponding Zeeman diagrams (middle) and the typical magnetic hysteresis loops (bottom, at a relatively low temperature) are also presented. Green/grey arrows indicate the possible positions for strong/weak avoided level crossings and thus fast/slow QTM, respectively. They are further linked with the steps on the magnetic hysteresis loops by the grey dashed lines. The one-way green arrows in the FM scenario should be particularly noted, which only happens in the demagnetization process. Field-enhanced relaxation process such as the direct process is mainly responsible for the closing of the hysteresis loops at higher fields, which is marked with yellow wavy arrows.