Anisotropy barrier

The structure of the Co4 molecule is shown in Fig. 10. The four Co and four oxygens from four hydroxy methyl pyridine ligands form a cubane like central C04O4 structure. The molecule has S4 symmetry. Our NRLMOL calculations confirmed the ferromagnetic ordering indicated by experiment. However, the magnetic anisotropy barrier was found to be 23 K. In the... 

When looking at all the SMMs reported in the literature, it is quite deceiving to conclude that to date Mnl2-ac (and the other Mnl2 belonging to the same family) still have the record for the value of the anisotropy barrier (50 cm ) and actually very few compounds have a value larger than 30 cm ... 

Fig. 4 (Left) Hysteresis loop recorded at 2.1 K on a single crystal of Mnu-acetate, with the magnetic field directed along the tetragonal axis. (Right) Splitting of the S = 10 ground spin state in Mnn-acetate. The lowest-lying suhstates are labelled with the Ms quantiun number. The arrows indicate the successive spin transitions required to overcome the anisotropy barrier U...

What makes SMMs unique magnetic systems, however, is the coexistence of classical and quantum mechanisms for spin reversal, hi fact, beside the above-described thermally activated process, spin reversal in SMMs may also occur by QT [46,47,54]. Whenever two states lying on opposite sides of the barrier have the same energy and are quantum-mechanically admixed, a direct underbarrier transition is in principle permitted. QT effects are indeed responsible for the stepped hysteresis loops of Mnu-acetate (Fig. 4) and explain why the effective anisotropy barrier probed by relaxation measurements (Ueff) is usually lower than U. Clearly, QT effects are of paramount importance for an accurate description of SMM behavior and their origin is among the most actively investigated topics in molecular nanomagnetism [54]. 

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