Design And Modulation Of Spin Order In Low-dimensional System

The electron simultaneously contains charge and spin degrees of freedom.Electrical regulation of electron charge freedom is the basis of modern electronics.Based on the application of the basic attributes of charge,electronic technology is developed rapidly,which is one of the important engines of the third scientific and technological revolution.However,the application of the basic properties of charge also suffers from some unavoidable and insurmountable problems.The charge-charge interaction is generally in hundreds of microvolts,which determines the need to apply a higher external field(such as voltage)to overcome the interaction between them.High voltage will lead to large heat loss and a large waste of resources.In contrast,the spin-spin interaction of electrons is generally within several microvolts,which possesses a faster correspondence to external field and lower loss.Therefore,electronic devices based on spin degrees of freedom are the best choice for information storage and processing with higher density,higher speed and lower power consumption.Using the spin of electrons has developed a new discipline:spintronics.Spintronics is a new discipline based on the degree of freedom of spin.It mainly includes the generation,maintenance,modulation,long-range transport and detection of spin.The spin order makes the material appear macroscopic observable magnetism,but the control of spin orientation usually depends on the external magnetic field.In the future integrated circuit,the external magnetic field is not convenient and easy to make the device volume too large.At present,both theory and experiment have developed some electric-controllable methods of magnetism.However,the method completely based on the electric manner is rarely reported,so it is necessary to develop the new method to flip the spin order completely in an electric manner.Magnetic semiconductors have attracted much attention because of their semiconductor properties and magnetism at the same time.However,with the increase of temperature,the spin entropy will increase,then the spin disorder will increase.The spin order will gradually be destroyed,and the macroscopic magnetism of the system is quenched at last.Although magnetic semiconductor technology has experienced many stages of development,its Curie temperature is still low,which has become a bottleneck before the application of semiconductor magnetic materials.In 2005,science magazine raised 125 important questions in today's science,"Is there a room temperature magnetic semiconductor?" is included.Therefore,there is an internal power for semiconductor spintronics to explore new high temperature magnetic semiconductor,or to improve its Curie temperature through various physical or chemical methods.Moreover,materials with high spin polarization are ideal systems for magnetic recording media and spin injection source.However,because the spin-up electrons and the spin-down electrons are not much different in energy,the spin polarizability of traditional ferromagnetic materials is not high.At the same time,an ideal magnetic recording medium and spin injection source should also have high spin polarizability in a wide energy range.If the energy range of spin polarization is narrow,because the spin-down(minority spin)electron energy states are not far from the Fermi level,when used as practical devices,they will participate in the transport of charges due to bias.Therefore,with the smaller size of modern magnetic storage materials,the demand for high spin polarization materials,especially high spin polarization materials with a wide energy range,is becoming more and more urgent.Based on the first-principles calculation within the framework of the density functional theory and non-equilibrium Green's function,this paper designs some low-dimensional spin systems and develops some spin control methods,around three aspects:generation,maintenance and regulation of spin.The main contents of the paper include,In chapter 1,as the theoretical basis,we introduce the basic theory of ground state electronic structure calculation based on the first principle framework and the electron transport theory of non-equilibrium states.Firstly,the multibody theory based on wave function and single electron theory based on Kohn-Sham equation are introduced.Secondly,the calculation method of electron transport based on Green's function is introduced.Finally,the global search method of potential energy surface based on real reaction channel is introduced.The chapter 2 is related to spin maintenance and spin regulation in spintronics.An edge reconstructed MoS2 nanoribbon is designed theoretically.The electronic band structure calculation shows that the nanoribbon will produce charge flow under volt bias,and this charge flow will produce a magnetic field at the opposite side,thus stabilizing the one-dimensional magnetic order of this edge.Thus we have solved the problem that the one-dimensional magnetic order can not survive at any finite temperature in the 1D Ising model.According to our model,if the bias is reversed,the edge magnetic order will flip at the same time.On the basis of our model,the magnetic order can be modulated in a full electric field manner.The chapter 3 is related to magnetic semiconductors and belongs to the range of spin generation and maintenance in spintronics.Inspired by the fact that reduced symmetry may induce a spin order,a two-dimensional magnetic semiconductor phase has been found inMoS2 that is regarded as a traditionally nonmagnetic materials.By Monte Carlo simulation,the magnetic ordered state in this magnetic semiconductor can be maintained to a relatively high temperature(130 K).In addition,we also find that the Curie temperature can be higher by simply applying stress.The chapter 4 is also related to the generation and maintenance of spin in spintronics.From symmetry analysis and first-principles calculations,we find a two-dimensional 100%spin-polarized half metal material with ultrawide single spin channel(3.3 eV).The magnetic ordered state that can be maintained up to 280 K,which could be further enhanced to 780 K under moderate stress or hole doping,above room temperature.The predicted new monolayer can be chosen as magnetic storage and magnetic injection.