Can early lanthanide sandwich complexes be efficient single-molecule magnets?
Single-molecule magnets (SMMs) have unique properties that open new possibility in high-density data storage, spintronic devices as well as quantum computing.[1-4] Their defined feature is the slow relaxation of their magnetization in the absence of an applied field. In the so-called lanthanide sandwich complexes, the high intrinsic magnetic moment is combined with a strong axial coordination environment, which has proven to be favourable for SMMs and has led to the best reported magnetic properties so far.[4-8] State-of-the-art SMMs are based on lanthanide ions (mainly Dy3+, Er3+ and Tb3+) due to their highly anisotropic ground state and their intrinsic unquenched spin-orbit. Despite these clear advantages, the low natural abundance of late lanthanides renders SMMs based on these elements expensive and could prohibit their future industrial implementation. Conversely, the lighter elements of the series are substantially more abundant and are already used commercially as permanent magnets (NdFeB and SmCo).[10-11] However, their usage in SMMs has been so far limited as light lanthanide complexes rarely exhibit slow relaxation of magnetization in zero field. In principle, this issue could be ameliorated by careful tailoring the coordination environment of the lanthanide ion. We decided to explore bulky ring-type organic moieties as ligands of choice for the design of neutral early lanthanides based SMMs. We hypothesized that a strong axial coordination environment in combination with the absence of ligands in the equatorial plane will engender formation of a zero-field SMM. Indeed, our results show that the successfully synthesized novel organometallic complex exhibits slow magnetic relaxation without applied external field at low temperatures. Furthermore, we show that the dilution of the complex in the diamagnetic matrix can further improve the relaxation properties of this system.
In conclusion, we have leveraged the design principles for known late lanthanide sandwich complexes to synthesize a novel zero-field SMM based on a less investigated element of the lanthanide series. We demonstrate that the use of bulky ligands can tailor the coordination environment to mitigate the fast relaxation of the magnetic moment.
 A. Ardavan, O. Rival, J. J. L. Morton, S. J. Blundell, A. M. Tyryshkin, G. A. Timco, R. E. Winpenny, Phys. Rev. Lett. 2007, 98, 057201.
 N. Leuenberger, D. Loss, Nature 2001, 410, 789-793.
 L. Bogani, W. Wernsdorfer Nat. Mater. 2008, 7, 179-186.
 E. Coronado, Nat. Rev. Mater. 2020, 5, 87-104.
 F.-S. Guo, B. M. Day, Y.-C. Chen, M.-L. Tong, A. Mansikkamäki, R. Layfield, Science 2018, 362, 1400-1403.
 A. P. Goodwin, F. Ortu, D. Reta, N. F. Chilton, D. P. Mills, Nature 2017, 548, 439-442.
 F.-S. Guo, B. M. Day, Y.-C. Chen, M.-L. Tong, A. Mansikkamäki, R. A. Layfield, Angew. Chem. Int. Ed. 2017, 56, 11445-11449.
 C. A. Gould, K. R. McClain, J. M. Yu, T. J. Groshens, F. Furche, B. G. Harvey, J. R. Long, J. Am. Chem. Soc 2019, 141, 12967-12973.
 H. K. Wedepohl, Geochim. Cosmochim. Acta 1995, 59, 1217-1232.
 T. Kawai, B. M. Ma, S. G. Sankar, W. E. Wallace, J. Appl. Phys. 1990, 67, 4610-4612.
 S. Walmer, C. H. Chen, M. H. Walmer, IEEE Trans. Magn. 2000, 36, 3376-3381.