Theoretical Study On Magnetic Anisotropy Of Rare-earth Single-molecule Magnets

With the development of informationization and digitalization of the society,the surge ofinformation constantly improves the requirements for storage technology and the storage devices moves toward smaller sizes,higher capacities and faster speeds.Nowadays,magnetic storage occupying an important position in the area of storage,it is necessary to reduce the size of a single magnetic unit to increase its storage density.The emergence of single-molecule magnets has successfully reduced the magnetic unit to the nanometer size.In recent years,great progress has been made in the field of single-molecule magnets,and a series of compounds with high magnetic anisotropy barrier and blocking temperature have been reported.Lanthanide ions have stronger spin-orbit coupling than transition metal ions due to the large unquenched orbital angular momentum in the ground state.Therefore,lanthanide-based single-molecule magnets have received extensive attention from experimental and theoretical workers in various countries.In this paper,the complete active space self-consistent field(CASSCF)method was used to study the magnetic properties of the sandwich and pentagonal bipyramid lanthanide single-molecule magnets.Firstly,the magnetic properties of three kinds of sandwich single-molecule magnets were studied.The larger aromatic ring(eight-membered carbon rings:COT₂)ligands provide a larger coordination field on the equatorial plane,which makes the lanthanide single-molecule magnets with the prolate 4f electron density of m _J=J ground state produce the larger energy level splitting.On the other hand,the higher axial symmetry of molecular structure can effectively depress the quantum tunneling of magnetization.Therefore,this kind of ligand structure helps single-molecule magnets with prolate ions to exhibit higher magnetic anisotropy.In addition,the component of the coordination field on the equatorial plane is enhanced when the ligand ring is enlarged(eight-and nine-membered carbon ring:COTCnt),thereby increasing the magnetic anisotropy barriers of single-molecule magnets with prolate 4f electron density.On the contrary,the energy barrier of the single-molecule magnets with oblate 4f electron density are decreased.The valence of the lanthanide ion is divalent when it coordinates with nine-membered carbon rings(Cnt₂).It was found that divalent complexes also have the magnetism,but they are slightly weaker than the trivalent lanthanide single-molecule magnets.Secondly,the magneto-structural correlations of the pentagonal bipyramid Dy^III single-molecule magnet was studied.It was found that the energy barriers can be significantly enhanced by reducing the number of ligands or increasing the bond length between ligand atoms and Dy^III on the equatorial plane to weaken the equatorial coordination field;reducing the bond length between ligand atoms and Dy^III or using negatively charged ligands along the axial to enhance the axial coordination field.The above structural changes can enhance the crystal field energy level splitting and decrease the probability of quantum tunneling,thereby increasing the energy barriers.In addition,the Orbach process dominates for the much more rigid compound by analysing the vibration modes of different molecular structures,leading to a higher blocking temperature.In recent years,single-molecule magnets have developed rapidly and made great progress.However,there are great obstacles in the application of single-molecule magnets,because the magnetic blocking temperature is far lower from the room temperature,and its magnetic relaxation mechanism is difficult to modulate.In future work,we will continue to study how to improve the energy barrier and blocking temperature of single-molecule magnets and contribute to the development of single-molecule magnets.