Magnetoelectric Transport Of Fe₃GeTe₂-based Van Der Waals Heterostructure

As the size of transistors continues to scale down,Moore’s Law is gradually failing.Spintronic devices have become a research hotspot in the post-Moore era.The discovery of two-dimensional(2D)magnetic materials provides huge possibilities for developing low-dimensional spintronic devices.2D magnetic materials exfoliated to few-/mono-layers provide an ideal platform for studying fundamental magnetic properties in the 2D limit.At the same time,the high-quality interface in the heterostructure provides new opportunities to explore the novel physical phenomena such as skyrmions,Majorana zero energy mode.Therefore,the study of 2D magnetic materials and their heterostructures is of great significance to the frontier fields of spintronics,materials science,and condensed matter physics.At present,the research on 2D intrinsic magnetic materials is still in its infancy.Whether the physical phenomena in traditional material systems,such as exchange bias effect in antiferromagnetic/ferromagnetic,magnetic proximity effect between ferromagnetic/nonmagnetic materials,superconducting spin triplet state at the superconducting/ferromagnetic interface exist in the 2D van der Waals magnetic heterostructure,which is rarely reported.In this dissertation,by utilizing the 2D metallic ferromagnet Fe3GeTe2 as the research object,we construct antiferromagnetic/ferromagnetic MnPS3/Fe3GeTe2,superconducting/ferromagnetic NbSe2/Fe3GeTe2 and NbSe2/Fe3GeTe2/NbSe2 van der Waals heterostructures,and explore the magnetoelectric properties of Fe3GeTe2,exchange bias effect in MnPS3/Fe3GeTe2,the superconductivity/ferromagnetism in NbSe2/Fe3GeTe2 and Josephson effect in NbSe2/Fe3GeTe2/NbSe2 mainly through low temperature magnetoelectric transport technology.Specifically,this dissertation can be divided into the following parts:The first chapter introduces the research status of 2D magnetic materials in detail,including the types of 2D magnetic materials,the magnetic detection,and the magnetic regulation,the research progress of the van der Waals heterostructures based on 2D magnetic materials.Subsequently,the research of spintronics is introduced.The phenomena in spintronics are mainly expounded,including magnetoresistance effect,Hall effect,spin-orbit coupling effect,superconducting spintronics,etc.Finally,the research ideas and research contents of this paper are put forward.In the second chapter,the synthesis of high-quality Fe3GeTe2 single crystal and the study of its magnetoelectric properties are introduced.High-quality Fe3GeTe2 single crystal materials were synthesized by chemical vapor transport,and the bulk Fe3GeTe2 samples were confirmed to have strong perpendicular magnetic anisotropy with a Curie temperature of 183 K by magnetic characterization.The Fe3GeTe2 is exfoliated down to few layers,and transferred onto a Hall device.The longitudinal magnetoresistance of few-layer Fe3GeTe2 shows a symmetrical butterfly-shaped curve,and the anomalous Hall curve shows a nearly square hysteresis loop.Fe3GeTe2 nanoflakes are metallic ferromagnets with large coercivity,high Curie temperature,and strong perpendicular magnetic anisotropy,which has a great application potential in spintronic devices such as tunneling magnetoresistance and spin-orbit torque structures.In the third chapter,the phenomenon of exchange bias and antisymmetric magnetoresistance in antiferromagnetic/ferromagnetic MnPS3/Fe3GeTe2 van der Waals heterostructures are systematically studied.The MnPS3/Fe3GeTe2 heterostructures are fabricated by mechanical exfoliation and dry transfer.The longitudinal resistance and anomalous Hall resistance were studied by magnetoelectric transport measurement.The exchange bias phenomenon appeared in the anomalous Hall resistance when the temperature was lower than the Neel temperature of MnPS3.More interestingly,an unusual antisymmetric magnetoresistance was observed in longitudinal resistance.A series of experimental results confirm that this antisymmetric magnetoresistance is an asynchronous magnetization switching between the Fe3GeTe2 interface and the bulk phase caused by the exchange coupling at the antiferromagnetic/ferromagnetic interface.The different magnetic moment configurations in Fe3GeTe2 can be parallel and anti-parallel with the spin polarization direction in the spin current,resulting in different scattering of electrons.The results of this study suggest that artificially stacked van der Waals heterostructure devices have the potential to explore the novel physical phenomena and spintronic devices.In the fourth chapter,superconductivity and ferromagnetism in bilayer superconducting/ferromagnetic NbSe2/Fe3GeTe2 van der Waals heterostructures are systematically investigated.Superconductivity is induced in the few-layer-thick ferromagnetic Fe3GeTe2 by the proximity effect.The superconductivity of the Fe3GeTe2 is confirmed by the zero-resistance behavior in electrical transport measurements and the characteristic oscillatory behavior of superconducting Josephson in the NbSe2/spacer/Fe3GeTe2 tunneling structure.Furthermore,anomalous Hall resistance measurements and magnetic force microscopy characterization revealed that the superconducting Fe3GeTe2 layer still remains weak ferromagnetism,but at present the weak ferromagnetism and superconductivity in the FGT are spatially isolated from each other or coexist,which is still impossible to make accurate judgments in experiments.We need more effective in-situ observation methods to further confirm,such as STM or ARPES.Our work provides a new platform to explore spin triplet states,and the interaction between superconductivity and ferromagnetism,which provides a new opportunity for non-dissipative spintronics and topological quantum computing.In the fifth chapter,NbSe2/Fe3GeTe2/NbSe2 Josephson junctions with planar and vertical structures are designed,and the Josephson effect in the van der Waals heterostructure is studied.The results show that Josephson junction with vertical structures has a stronger Josephson current.The distribution of superconducting current in different directions in the Josephson junction is studied by measuring the response of superconducting critical current to the application of magnetic fields B in different directions.The results show that the superconducting current in Fe3GeTe2 is mainly distributed at the top and bottom surfaces.This finding provides an opportunity to explore unconventional physical properties such as spin triplet states and topological superconducting states.The sixth chapter summarizes our research work in the previous chapters,and puts forward the possible future research directions on 2D magnetic materials.We think that the future research work can be carried out from the following aspects:the exploration of new 2D magnetic materials,the improvement of the experimental methods and detection technology,the construction of new van der Waals magnetic heterosturcture and the application of low-dimensional spintronic devices.