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Surface-grafted metallofullerene molecular magnets with controllable alignment of magnetic moments

Periodic Reporting for period 4 - GraM3 (Surface-grafted metallofullerene molecular magnets with controllable alignment of magnetic moments)

Reporting period: 2019-12-01 to 2020-11-30

Realization of magnetism on a molecular level is very technologically attractive since it promises a dramatic increase of possible writing densities and offers a number of spintronics applications, including memory storage and quantum computing. Successful development in this direction will result in miniaturization of the devices with concomitant decrease of their energy consumption. These applications require that the magnetic molecules (single molecule magnets, SMM, hereafter) are to be organized on the substrates and their magnetic states are controlled on the single-spin level. The practical realization of such technologies are however hampered by the low temperatures of magnetic phenomena, difficulties in the obtaining of the well-structurally defined layers of SMMs, and fundamental difficulties in “writing” and “reading” information from single molecules. The main goal of this project is to overcome these problems by utilizing unique properties of endohedral metallofullerenes (EMFs) with lanthanide ions as magnetic centers. EMFs are chemically and thermally stable, which allows their further functionalization and implementation in molecular devices. This project is hence focused on the advances in synthesis of the EMFs and development of new EMF molecules with improved magnetic properties, functionalization of the EMF molecules with chemical groups facilitating formation of the single-molecule thick layers on conducting substrates, and on the studies of the magnetic properties of EMFs and their derivatives in bulk and on substrates.
The synthesis of lanthanide-based endohedral metallofullerenes (EMFs) was the starting step, which required improvement of the efficiency and accumulation of the necessary amount of target EMFs for their further studies. We therefore optimization synthetic and separation procedures used for fullerenes and synthesized a plethora of new metallofullerenes. Assessment of the production yields revealed that the most suitable EMFs for the dedicated studies of magnetic properties are nitride clusterfullerenes, such as DySc2N@C80 and Dy2ScN@C80, and dimetallofullerenes. Unexpected but very important development was the discovery of metal-metal bonds with variable electron population in dimetallofullerenes. Realization of M–M bond between lanthanides allowed improving magnetic properties by increasing the strength of exchange interaction. We thus developed the synthesis of Ln-based dimetallofullerenes M2@C80(CH2Ph) with single-electron M–M bonds for M = Y, Gd, Tb, Dy, Ho, Er.
Comprehensive magnetic studies of the large number of EMFs synthesized in the project allowed to identify the best fullerene-based single-molecule magnets used in further functionalization and surface depositions. Besides, these studies allowed s to formulate guiding principles for best fullerenes-SMMs and establish EMFs as a unique class of SMM with excellent magnetic properties.
The EMFs combining high production yields and excellent magnetic functionalities were then functionalized with surface-anchoring groups, which facilitate formation of self-assembled monolayers on suitable substrates. Then, monolayers of magnetic fullerenes were grown on conducting substrates either by vacuum sublimation (for unfunctionalized molecules) or by self-assembly from solution (for EMF derivatives with anchor groups). Morphology and electronic properties of monolayers were studied by scanning tunnelling microscopy, and their magnetic properties were then characterized by synchrotron-based X-ray magnetic circular dichroism. These studies proved that excellent magnetic properties for EMFs in powder samples were retained when they were deposited to metallic substrates, which is the necessary prerequisite for the use of SMMs in spintronic devices.
During the first period of the project, the molecules with unprecedentedly high blocking temperatures of magnetization and unusual mechanisms of relaxation of magnetization have been found. In particular, we have discovered that dimetallofullerenes featuring single-electron metal-metal bonds have very promising magnetic properties due to the exchange strong coupling of the lanthanide magnetic moments. The studies of the surface magnetism showed that monalyers of magnetic fullerenes retain good magnetic properties on metal substrates and show the highest blocking temperarures of magnetization among all SMMs studied i monolayers. These results open the new way for creation of single molecule magnets with unusual magnetic and electronic properties.
Molecule of the fullerene Dy2ScN@C80 deposited on the gold substrate