Spin-Polarized Two Dimensional Electron Gas At Oxide Interface

The discovery of two-dimensional electron gases?2DEGs?at LaAlO₃/SrTiO₃?LAO/STO?interface has attracted much attention.Many novel physical phenomena were observed in this interface,for example the tunable Rashba spin-orbit coupling,huge thermoelectric effect,superconductivity and high efficiency spin-to-charge conversion,etc.The conductive electron of traditional GaAs based 2DEG is s,p electron.Different from the traditional 2DEG,the conductive electron at LAO/STO interface is 3d electron.Due to the strong correlated interaction between d electrons,the behavior of d electrons tends to closely relate to the magnetic properties.Researchers have tried various approaches to prove the intrinsic magnetic feature of this interface,but so far only some hints of magnetic signal have been observed,however,magnetic hysteresis behaviors were rarely observed.So the core issue in this field is how to obtain the spin-polarized 2DEGs.In this thesis,the spin-polarized2DEGs has been successfully obtained at the insulting oxide interfaces through the reasonable material design,which laid a foundation for the further development of the related oxide spintronics.The main contents and conclusions are as follows:1.We grew a series of La_7/8Sr_1/8MnO₃?LSMO?-buffered LAO/STO interface with different thicknesses of LSMO buffer layer.Though Mn ions interdiffusion,the spin-polarized two dimensional electron gases was obtained.Metallic behavior was observed by the transport measurements in LSMO-buffered LAO/STO interface when the thickness of LSMO is below 1 nm.The anomalous Hall effect was observed at the LAO/STO interface with the 1 nm LSMO buffer layer.The spin-polarized temperature for the 2DEG is promoted to 30 K while the mobility still can remain at the high values.The interface shows electrically tunable anomalous Hall effect and high conductivity.The fast carriers play an important role in the magnetotransport.When the fast carrier density is higher than 7×10¹² cm^-2,the anomalous Hall effect was observed.With the fast carrier density increasing,the anomalous Hall resistance also increases.On the contrary,the Kondo effect was observed when the fast carrier density is much lower than 7×10¹² cm^-2.The negative magnetoresistance was observed,indicating the existence of magnetic scattering at the interface.The magnetism likely originates from a gradient manganese interdiffusion into STO.2.We fabricated the 2DEGs based on Fe-doped STO that is super-paramagnetic rather than diamagnetic like STO.The interface shows spin-polarized two dimensional electron gases with high Cuire temperature.The interface shows well metallic behavior and considerable Kondo effect at the low temperature.The Kondo temperature keeps unchanged when the growth oxygen pressure varies due to the kondo scattering from Fe ion.The anomalous Hall effect was observed at this interface.The sample grown at 10^-6 mbar O₂ pressure showed the anomalous Hall effect up to room temperature.With increase in carrier density,the Curie temperature and anomalous Hall resistance also increase.The negative magnetoresistance was observed,and the magnetoresistance curves can be well fitted by the semiempirical model of Khosla and Fische.These results indicate that this interface is diluted magnetic 2DEG and the magnetic exchange between magnetic moments could be mediated by the itinerant electrons of the 2DEG.3.We found a new kind of 2DEGs at interface formed between a ferromagnetic insulator EuO and 5d electron perovskite KTaO₃?KTO?.The 2DEGs are spin-polarized and highly conducting.With increasing the growth temperature,the carrier density increase.The maximal mobility is 111.6 cm²/Vs at 2 K.The 2DEG shows strongly hysteretic magnetoresistance which is the strong evidence for ferromagnetism.The hysteretic anomalous Hall effect was also observed and showed up to 70 K.This is confirmed by the results of theoretical calculations:As a result of the Eu-5d and Ta-5d hybridization,the Ta 5d states are spin polarized.