Magnetic And Multiferroic Properties Of Rare Earth Based Single-Molecule Magnets

Single-molecule magnets(SMMs)are organic molecular nanomagnets characterised by magnetic bistability and slow magnetization relaxation at low temperatures.SMMs are macroscopically magnetised through a variety of magnetic relaxation processes interacting with lattice phonons and have macroscopic quantum effects.For years,researchers have aimed to enhance the effective energy barrier and magnetic blocking temperature of SMMs to harness their potential in high-density magnetic storage,quantum bits,and quantum computing.Rare-earth-based single-molecule magnets contain rare-earth ions as the central element of magnetism,and their strong spin-orbit coupling results in substantial magnetic anisotropy.This characteristic is crucial for improving the performance of single-molecule magnets.In this thesis,focusing on rare-earth-based mono/triple core single-molecule magnets,three primary works have been conducted as follows:1.The study explores slow magnetization relaxation properties of two novel rareearth Dy-based single-molecule magnets:the mono-core Dy-based single-molecule magnet(Dy-SMM)and the triple-core Dy-based single-molecule magnet(Dy3-SMM).In the molecular unit of Dy-SMM,the Dy3+ion is situated in the slightly distorted pentagonal bipyramidal structure of DyO7,with the organic ligands coordinated in its axial direction.Dy-SMM exhibits pronounced single-molecule magnet properties under the anisotropic crystal field.The magnetic blocking temperature is approximately 12 K and the low-temperature magnetization curves display hysteresis accompanied by three distinct magnetization steps of quantum tunneling,having unique macroscopic quantum magnetic properties.The molecular unit of triple-core Dy3-SMM contains three Dy3+ions arranged in equilateral triangles,and theoretical calculations indicate that the intramolecular local magnetic moments are arranged in a ring.The complex magnetic structure endows it with novel relaxation properties.AC magnetism of Dy3-SMM exhibits a complex coexistence of three relaxation processes at the same temperature.The fast relaxation processes does not show the frequency shift with temperature,which is dominated by quantum tunneling processes.While the medium and slow relaxation move toward higher frequencies with increasing temperature,which are governed by thermally excited Raman and Orbach processes.These findings offer valuable insights into the magnetization dynamics of single-molecule magnets and contribute to the design of novel SMMs.2.The multiferroic and magnetoelectric coupling effects of mono-core Dy-based single-molecule magnets(Dy-SMM)are investigated.Structural analysis reveals a nonpolar to polar transition in the space group of Dy-SMM as temperature decreases.Dielectric and pyroelectric measurements confirm the presence of ferroelectric phase transition at 255 K.This is the first reported multiferroic single-molecule magnet,opening new avenues for achieving microscale integration of magnetoelectric multifunctional materials.Significantly,we have observed both modulation of magnetism by an electric field and modulation of electric polarization by a magnetic field in this material.Both the magnetic dielectric and the magnetoelectric coupling voltage coefficient display distinct anomalous peaks or inflections at the magnetic fields corresponding to quantum tunneling.Leveraging these properties,multiferroic Dy-SMM can convert its unique magnetization quantum tunneling characteristics into detectable electrical signals via magnetoelectric coupling.This introduces a novel approach for the application of quantum magnetism of SMMs and guides the exploration of SMMs towards multiferroicity.3.The thermal expansion and magnetostriction effects of the mono-core Dy-based single-molecule magnet(Dy-SMM)and the mono-core Gd-based single-molecule magnet(Gd-SMM)are investigated.Dy-SMM shows a large magnetostriction effect,with its length perpendicular to the(0-11)plane reaching a large variation of 24,000 ppm at the magnetic field of 7 T compared to 0 T at 2 K.The Dy-SMM has both strong magnetic anisotropy and highly ductile lattice structure,which results in the significant magnetostriction effect.Moreover,due to the strong spin-lattice coupling,Dy-SMM exhibits different deformation behaviour at different stages of the magnetisation process.For Gd-SMM,despite its crystal structure being isomorphic to Dy-SMM,it does not display strong magnetostriction effect due to the isotropic 4f orbital electron density of trivalent Gd ions.