Magnetic Proximity Effect Of Yttrium Iron Garnet: First-principles Calculations And Experimental Study

Yttrium iron garnet?YIG?,a ferromagnetic insulator,is the lowest damping material of spin wave propagation so far.The damping parameter in single crystal YIG nano-film can be as low as 10^-5 magnitude,in which the information tansmission through spin wave is highly efficient,which is promising for applications in spintronic devices.Nonmagnetic materials in closed proximity to magnetic material,such as YIG,exhibits ferromagnetic order due to the electron exchange effect at interface.This phenomenon is named magnetic proximity effect?MPE?.MPE is very important for understanding the operating mechanism of YIG based spintronic devices and to develop the new heterojunction devices.In metal/YIG heterostructures,revealing the origin of MPE is important to clarify novel several interface spin transport phenomenons,such as spin Hall magnetoresistance?SMR?,anomalous Hall effect?AHE?,and spin Seebeck effect?SSE?.On the other hand,manipulation of the magnetism and valley structrue of two dimensional?2D?materials can be realized via MPE in 2D material/YIG heterojunctions,which is highly promising to develop magnetic 2D materials.In this thesis,the mechanism,influence factor,and manipulation method of MPE in YIG based magnetic heterojunctions are studied by first-principles calculations and experimental methods.The details are list as follows.1.An accurate first-principles calculations model for YIG and doped YIG materials is determined.The structure,magnetic properties and optical properties of YIG are studied by first-principles calculations.The results show good agreements with experimental results.The charge transfer from Ce 4f orbital to Fe?tetrahedral site?3d orbital is the dominating mechanism of enhanced near infrared absorption in Ce doped YIG.In addition,the oxygen vacancies are found to be able to stabilize Ce³⁺from converting to Ce⁴⁺due to localized lattice strain.2.The MPE at Pt/YIG and Au/YIG interfaces is studied by first-principles calculations.The exchange effect between Pt 5d electrons and tetrahedral Fe 3d electrons is the origin of MPE at Pt/YIG interface according to charge desnsity distribution,spin density distribution,electron structure and magnetic moment calculations.The crystal orientation,YIG termination layers,and vacancies in YIG are found to strongly influence the MPE in Pt/YIG.In particular,the tetrahedral Fe vacancies can significantly decarese the MPE.The exchange effect of Au-Fe is much weaker than that of Pt-Fe,which results in negligible MPE at the Au/YIG interface.By introducing oxygen vacancies in YIG,the Fe³⁺converts to Fe²⁺and the MPE at Au/YIG interface is increased due to stronger orbital hybridization of Au-Fe²⁺.3.Using nonmagnetic Al³⁺substitute tetrahedral Fe³⁺in YIG and measuring the anomalous Hall effect in Pt/YIG,we experimentaly proved that the exchange effect between Pt 5d electrons and tetrahedral Fe 3d electrons is the origin of MPE at Pt/YIG interface.The Y₃Fe_5-xAl_xO₁₂?x=0,0.187,0.392,0.568,0.783?films are deposited on GGG substrates by pulsed laser deposition?PLD?method.Then Pt Hall bars are deposited on Y₃Fe_5-xAl_x O₁₂ films by magnetron sputtering method.At 5K tempature,the anomalous Hall resistance in Pt/Y₃Fe_5-xAl_xO₁₂ device decrease with incareseing Al³⁺concentration.The MPE is the dominant contribution to the anomalous Hall resistance at low temperatures?<20K?.Therefore,the experiment indicates the tetrahedral Fe vacancy significantly decrease the MPE at Pt/YIG interface,agreeing with our calculation results.4.The MPE at 2D transition metal dichalcogenides?TMDCs?/EuS interface due to superexchange interaction of Eu²⁺-S^2--Mo⁴⁺is proved by first-principles calculations,which provide a new method of valley and magnetism manipulation in 2D TMDCs.Due to the MPE at monolayer MoS₂/EuS interface,the valley degeneracy of monolayer MoS₂ can be lifted and valley Zeeman splitting is observed.In addition,the magnetism and valley Zeeman splitting of monolayer MoS₂ in this heterojunction can be modulated by an electric field.The monolayer MoS₂/YIG device is experimentally fabricated.Right-hand and left-hand circularly polarized photoluminescence spectra of monolayer MoS₂ in this heterojunctions are almost totally overlapped under a magnetic field correlated with the saturation magnetization of YIG.The Hall resistance of monolayer MoS₂/YIG device shows linear relationship with magnetic field.These experimental results indicate the MPE at monolayer MoS₂/YIG interface is negligible.