Magnetic And Ultrafast Spin Dynamics Of Transition Metal Oxide Clusters

Stimulated by the increasing demand for high-density storage and high-speed information processing,optically-driven spin dynamics in nanoscale devices within the subpicosecond time regime has become one of the most fascinating topics in recent years.Transition metal(TM)oxide clusters have been the subject of numerous experimental and theoretical studies in the last decades due to their great potential applications in catalysis,electronics,and magnetic materials.Therefore,in this thesis,by using the first principles method,we study several properties of the tri-nuclear TM oxide clusters,including the geometric configurations,infrared spectra,multi-body electronic structures,magnetic anisotropies,and laser-induced ultrafast spin dynamics.The main contents and results are as follows:(1)The optimization of the homonuclear(Fe₃O₃,Co₃O₃⁺ and Ni₃O₃)and heteronuclear(Fe₂CoO₃⁺,Fe₂NiO₃,Co₂FeO₃,Co₂NiO₃,Ni₂FeO₃ and Ni₂CoO₃⁺)clusters are performed with the Hartree-Fock(HF)method,the stabilities of which are confirmed by the subsequent frequency calculations.The results show that all clusters exhibit planar geometries,with the Co₂NiO₃ cluster belonging to C_2v,symmetry and the rest possessing Cs symmetry.In addition,the calculated infrared spectra show that the frequency values of the corresponding vibration modes of clusters Fe₃O₃,Ni₃O₃,Fe₂NiO₃,Ni₂FeO₃ and Co₂FeO₃ agree with experiment in a reasonable range,while the values of the cationic clusters Co₃O₃⁺,Fe₂CoO₃⁺and Ni₂CoO₃⁺ are overestimated.Besides,the spectrum of the Co₂NiO₃ cluster exhibits different features due to the different symmetry of it compared to others.(2)The ground and excited many-body states of the TM oxide clusters are obtained by applying the symmetry-adapted cluster configuration interaction(SACC-CI)method.It is found that the energy levels of the Fe₃O₃ cluster is evenly distributed,while those of the clusters containing Co or Ni atoms appear several energy gaps.The more Ni atoms it contains,the larger energy gap it shows.(3)The magnetic anisotropy of the ground-state of each cluster and the spin localizations are investigated.Specifically,(i)By scanning the B-field directions within the molecular plane,it is found that the easy axes,which can be determined from the spin magnitudes or energy shiftings,of the ground states,are different;(ii)With the increase of the magnetic-field strength,the spin direction of the ground state of each cluster gradually aligns to the B-field direction;(iii)For the spin localization,the more distorted the structure is,the more spin-localized state it has,and further the richer spin functions it can achieve.(iv)In addition,even for the same cluster,due to the fact that the spin magnitudes and directions are magnetic-field dependent,the amount of spin-localized states and the possible fUnctionalities are different under different magnetic-field directions.(4)After including the spin-orbit coupling and applying a static magnetic field,the A-process-based ultrafast spin dynamics of the TM oxide clusters are explored under the influence of the well-tailored laser pulses.The results show that:(?)Ultrafast spin-flip scenarios can be realized in all these clusters.(?)Ultrafast spin-transfer scenarios are achieved in clusters Fe₃O₃,Ni₃O₃,Fe₂NiO₃,Ni₂FeO₃ and Ni₂CoO₃⁺,among which a reversible spin-transfer cycle is realized in the Fe₃O₃ cluster,while spin bifurcation or merging dynamics processes are obtained in the other four clusters with relatively weaker spin localization.(?)In addition,demagnetization or magnetization dynamics are achievable in clusters Co₃O₃⁺,Fe₂CoO₃⁺,Co₂FeO₃ and Co₂NiO₃,The results obtained in this thesis can guide the future experimental implementation of ultrafast spin dynamics in these clusters,provide valuable reference for the related larger systems and future spintronic device design,and thus promote their applications and development in high-density storage and quantum calculating.In particular,it is predicted that the Fe₃O₃ cluster has greater potential for the logical operation in the future due to its strong spin localization and uniformly-distributed spin states.