Unravelling the Reason for Transition from the Field-Induced Single-Molecule Magnet to Zero-Field One in a Group of...

Single-molecule magnets (SMM) hold the potential of increasing the information-storage density in a revolutionary way. However, the necessity of applying an external magnetic field in most SMMs is detrimental. Thus, the transition from field-induced SMMs to zero-field ones deserves intensive study. Here, we report an ab initio study of three dysprosium complexes, Str1, Str2, and Str3. Although having the same coordination pocket, Str1 and Str2 are field-induced SMMs, but Str3 changes to be a zero-field one. Without applying a field, the quantum tunnelling of magnetization (QTM) rate, i.e., the lowest relaxation rate, of Str1 and Str2 is predicted to be higher than the upper limit of the experimental apparatus by at least 1 order of magnitude. Thus, the experimental apparatus is hardly capable of recording the SMM behaviors of Str1 and Str2 under zero field. The zero-field QTM rate of Str3 lies within the experimental detective range, and thus applying a field is not necessary. This difference arises mainly from their different magnetic axialities, of which the highest one comes from Str3. Covalent interaction is usually assumed to be insignificant in lanthanide complexes. However, our analysis based on the crystal field model and perturbation theory indicates the crucial role of covalent interaction here. When the covalent effect is excluded via replacing all the ligand atoms by atomic charges, ab initio calculations indicate that the zero-field SMM will change to be Str1 and Str2 rather than Str3. The change of the QTM rate, given by the covalent effect, could be as much as 3 to 4 orders of magnitude.