Tuning Of Magneto-dynamics By Magnetic Heterstructure Interfaces

The synthesis of nanostructured composites with new properties to achieve"tailoring"and optimization of material properties has become an important way and a fundamental trend for the creation of new nanomaterials.An artificially modulated nanocomposite film formed by the composite of two films is called a heterostructure,which exhibits novel properties different from those of conventional single materials,and has important research values in basic research and technological applications in the field of materials.As an amorphous material with excellent soft magnetic properties,CoFeB thin film shows high saturation magnetization,excellent soft magnetic properties.Besides,its easy magnetization direction can be regulated by interface and thickness,making it currently one of the most popular magnetic materials for spintronics device research.In this thesis,CoFeB thin film is chosen as the magnetic layer,combining with light rare-earth Nd thin film with strong spin-orbit coupling and magnetic two-dimensional van der Waals crystal Fe₃GeTe₂ to prepare magnetic heterostructures.Modulations of the magnetic anisotropy as well as the magneto-dynamic properties of the magnetic heterostructure by the magnetic proximity effect are systematically investigated as follows:（1）Amorphous Co₆₀Fe₂₀B₂₀ films with large in-plane uniaxial magnetic anisotropy of up to 8.0 m T are successfully fabricated on Ga As substrates.The large in-plane uniaxial magnetic anisotropy field is accompanied by a large coercivity field₀₍（8）（3 m T）.The electrical contact type between the Co₆₀Fe₂₀B₂₀ film and the III-V semiconductor Ga As is measured to be a Schottky barrier contact with a barrier height of 0.72 e V,which promotes the spin injection efficiency.By comparing the uniaxial magnetic anisotropy field of CoFeB films with different components,it is found that the uniaxial anisotropy field decreases with the decrease of Fe element content.Thus,Co₄₀Fe₄₀B₂₀ has the largest uniaxial magnetic anisotropy field,the lowest magneto-dynamic damping factor and the highest saturation magnetization.（2）Single-crystal van der Waals material Fe₃GeTe₂ is successfully grown by the chemical vapor transport（CVT）technique.Al₂O₃-assisted exfoliation technique is used to obtain a large area of Fe₃GeTe₂ thin layers.The anomalous Hall effect showed that the thinner layer has a larger coercivity and anomalous Hall resistance,and the coercivity decreases gradually with increasing temperature.The anomalous Hall resistance and magnetoresistance are very small at18 K and reach the maximum at about 50 K,instead of at the lowest temperature of 5 K,which is due to the Kondo effect affecting the longitudinal resistance.The magnetoresistance transport measurements show that Fe₃GeTe₂ possesses a negative magnetoresistance effect,and in addition,Fe₃GeTe₂ exhibits an AMR effect with four-fold symmetry at 5 K and 10 K.We conjecture that it may be induced by the four-fold symmetric magnetocrystalline anisotropy of the cross section parallel to the magnetic field in the Fe₃GeTe₂ sample.（3）The CoFeB/Nd heterostructure is successfully constructed to achieve the enhancement of the magneto-dynamic damping factor of the CoFeB film,and the spin mixing conductivity of the interface is found to be three times higher than that of the CoFeB/Pt heterostructure,with a high spin transparancy of 82%at the interface.The mechanism of the high spin transparancy originates from the matched resistivity between CoFeB and Nd at the interface.In addition,the magneto-dynamic damping enhancement at the CoFeB/Nd heterostructure interface is almost unchanged by inserting a Cu layer of 1-2 nm,but the damping enhancement is suppressed when the Cu layer is thicker than 3 nm,which is related to the Cu resistivity and its thickness dependence.The spin-orbit torques（SOT）efficiency of the CoFeB/Nd/Pt system with different Nd insertion thicknesses is measured.By utilizing the high spin transparancy at the interface of the CoFeB/Nd heterostructure,the SOT efficiency of the CoFeB/Nd/Pt system is effectively enhanced compared with CoFeB/Pt,owing to the improved interfacial spin current transmission efficiency.The SOT efficiency is significantly increased with the increase of the rare-earth layer thickness up to 60%in a certain thickness range.Fixing the Nd intercalation layer thickness to1 nm,the SOT efficiency of CoFeB/Nd/Pt also tends to increase with the increase of Pt thickness due to the gradual enhancement of Pt bulk spin Hall effect.（4）Large-area Fe₃GeTe₂monolayer,bilayer and few-layer 2D nanoflakes are obtained by Al₂O₃ and Au-assisted exfoliation techniques,and CoFeB/Fe₃GeTe₂ and Ni/Fe₃GeTe₂heterostructures are successfully fabricated.It is found that below the Fe₃GeTe₂ Curie temperature,the interfaces of CoFeB/Fe₃GeTe₂ heterostructures formed a non-parallel magnetic moment arrangement.The saturation magnetization of the CoFeB/Fe₃GeTe₂heterostructures decreases compared to CoFeB films.While at room temperature 300 K,the saturation magnetization of the heterostructure and CoFeB single film is similar,indicating that the magnetic proximity effect leads to magnetic exchange coupling at low temperatures.The perpendicular magnetic anisotropy（PMA）constants of CoFeB/Fe₃GeTe₂ and Ni/Fe₃GeTe₂heterostructures are both enhanced due to the d-d orbital hybridization at the interface.Besides,we observe a significant increase in the magneto-dynamic damping factor due to the spin-pumping effect.At 150 K,the PMA of the CoFeB/Fe₃GeTe₂ heterostructure increases by 4.0%,whereas it increases by 20.2%at 300 K,indicating that the magnetic coupling caused by the magnetic proximity effect at the interface of CoFeB and Fe₃GeTe₂ plays a smaller contribution.The modulation of PMA and magneto-dynamic damping factor is more significant in the heterostructure of bilayer Fe₃GeTe₂ and Ni than in the heterostructure of monolayer Fe₃GeTe₂and Ni,indicating a strong dependence of the thickness of Fe₃GeTe₂ on the interfacial modulation,and further providing evidence for the mechanism of interfacial spin pumping effect.By investigating the temperature dependence,we find that,in the Ni/Fe₃GeTe₂heterostructure,the enhancements of PMA and magneto-dynamic damping factor are easier to be achieved at low temperatures.Since this temperature dependence is studied above the Fe₃GeTe₂ Curie temperature,its origin mechanism needs further research.