Electron Transport Properties Of Two-dimensional Magnetic Materials And Their Heterostructures

Spintronics can overcome many difficulties faced by semiconductor integrated circuits.It has been widely studied and has developed into one of the most active fields of condensed matter physis.The development of spintronics also faces many challenges,such as the interface effects,efficiency of spin manipulation,and so on.In recent years,with development of two-dimensional materials,especially two-dimensional magnetic materials,some of the problems are expected to be solved.Two-dimensional materials have dangling-bond-free surfaces,which are not subject to the traditional constraints caused by lattice mismatch,and chemical incompatibility at the interfaces.Two-dimensional magnetic materials expand the family of two-dimensional materials,and provide a new platform for exploring spin related physical phenomena and spintronic devices,and thus are promising for the next generation of information technology.In this thesis work,we focus on electron transport properties of two-dimensional ferromagnetic metal Fe₃GeTe₂ and ferromagnetic semiconductor Cr₂Ge₂Te₆.The main results are summarized as follows:1.Fe₃GeTe₂ is a layered magnetic material exhibiting itinerant ferromagnetism,which can be exfoliated down to the monolayer.Up to now,a lot of electronic transport studies have been performed but mainly focused on the study of the anomalous Hall effect.In this work,we have carried out a systematic study on the longitudinal resistance of Fe₃GeTe₂.At low temperatures,the resistivity of the sample increases with a√-type dependence as the temperature decreases,followed by a tendency of saturation behavior at lower temperatures.Moreover,we have observed negative linear magnetoresistance which can be maintained up to at least 9 T.At high temperatures,the linear magnetoresistance may be attributed to the suppression of magnon scattering by the magnetic field.At low temperatures,the slope of magnetoresistance exhibits an anomalous increase resembling the zero-field longitudinal resistivity,and hence their microscopic origin might be correlated.These results shows that the temperature dependence at low temperature cannot be attributed to the Kondo magnetic impurity scattering or the weak localization effect.The pseudospin-related orbital two-channel Kondo effect is a possible mechanism,but it needs further investigation.2.Cr₂Ge₂Te₆,as a two-dimensional magnetic insulator,has been widely studied.However,there is still no consensus on some basic characteristics of the electronic band structure,such as the size of the energy gap and the sign of spin splitting.Using Cr₂Ge₂Te₆ as tunneling barrier,Fe₃GeTe₂/Cr₂Ge₂Te₆/graphite heterostructures are prepared by mechanical exfoliation and dry-transfer method,and their transport properties are studied in detail.At low bias voltages negative junction magnetoresistances with the magnitude as large as 35%have been observed.With increasing bias,a crossover from low-bias tunneling to Fowler-Nordheim tunneling is observed.Temperature and bias voltage dependences of transport properties of tunneling junctions indicate that the tunneling magnetoresistance can be attributed to the spin-polarized tunneling process caused by exchange splitting of Cr₂Ge₂Te₆.The negative sign of the tunneling magnetoresistance suggests that the bottom of conduction band in Cr₂Ge₂Te₆ belongs to minority-spin states.This work shows that the van der Waals heterostructures are a valuable platform to gain further insight into spin polarized tunneling transport and electronic structure of tunneling barrier.3.Spin polarization is one of the most important parameters in spintronics.The Tedrow-Meservey tunneling experiment is an effective way to detect the spin polarization of tunneling current.We have tried to detect the spin polarization of Fe₃GeTe₂ using Tedrow-Meservey method.Superconducting Al thin films with parallel critical magnetic field up to～3.9 T have been successfully prepared by thermal evaporation.Based on the Al thin films,we have fabricated Fe₃GeTe₂/h-BN/Al heterostructures by van der Waals assembly method.The relevant work is still in progress.In summary,we have systematically studied two-dimensional magnetic materials Fe₃GeTe₂and Cr₂Ge₂Te₆ with low-temperature electron transport method,which has deepened our understanding of the electronic structures and physical properties of these two materials.The work further demonstrates that the study of transport properties of two-dimensional magnetic systems,especially spin-polarized tunneling,can play an important role in further advancing the spintronics research.