Study On Magnetic And Transport Properties Of Two-Dimensional Materials/Magnetic Semiconductor Heterojunctions

The spin and charge are the two intrinsic properties of the electrons.The processing and transmission of information mainly apply the charge of electrons.While the spin is used as the storage of information in magnetic materials.If a material can combine both the charge and spin properties of electrons,it will undoubtedly promote the rapid development of spintronic devices.Diluted magnetic semiconductors?DMSs?have both ferromagnetic and semiconducting properties,attracting widespread attention from researchers.At the same time,with the further miniaturization of electronic devices,the combination of two-dimensional materials and spintronics has been a new development field.Van der Waals heterojunctions,constructed using stacked two-dimensional metal,semiconductor,and insulator materials,can be used to fabricate a variety of electronic devices,such as spin valves,magnetic tunnel junctions,and photovoltaic devices.However,there are still some important challenges in feasible materials with high Curie temperature,high carrier mobility,and high spin polarizability.In this thesis,the first-principles calculation method is used to study the carrier mobility and Curie temperature of dilute magnetic semiconductors/graphene heterojunction.In addition,the spin valve with high spin polarizability and its transport properties were studied.The main contents and results are as follows:1.Oxide-based diluted magnetic semiconductors usually exhibit above room-temperature ferromagnetism,but low carrier mobility limits their applications in novel spintronics.In this study,using first-principles calculation,the mobility,electronic and magnetic properties of V-N codoped ZnO??V,N?ZnO?system and its composite system?V,N?ZnO/graphene are studied.Compared with?V,N?ZnO,our results show that the electron mobility of the composite system is obviously increased by about three times(from 500 cm²V^-1s^-1 to 1500 cm²V^-1s^-1).In addition,we find that the ferromagnetism of the composite system is largely enhanced than that of the only?V,N?codoped ZnO.The Curie temperature of the composite system is estimated to be about 630 K based on the mean-field approximation theory,which is nearly twice as high as the?V,N?ZnO?321 K?.This study provides an effective theoretical guide for the experimental improvement of carrier mobility and Curie temperature of oxide-based dilute magnetic semiconductors.2.We designed a new-type CrI₃/hBN/CrI₃ spin valve based on van der Waals heterojunction,and studied the electrical transport properties of the system by density functional theory combined with non-equilibrium Green's function.Our results show that the system has a strong spin filtering effect.For the parallel configuration,the spin polarizability can reach more than 75%when the applied bias voltage is in the range of 0-0.7V.For the anti-parallel configuration,there is only a spin down current in the small bias voltage range of 0-0.2V,and the spin polarizability is nearly 100%.In addition,we find that the tunneling magnetoresistance of the system exhibits a negative value?a positive value?when the bias voltage is less than 0.3V?more than 0.3V?,and a maximum of about120%between 0-1.0V can be obtained.This study provide an new theoretical guidance for the experimental fabrication of novel magnetic tunnel junctions using two-dimensional magnetic materials.