Design Of Novel Magnetic Half-metals And Semiconductors And Their First-principles Calculations Study

In spintronics,controlling the directions of spins in materials by external conditions,such as magnetic field and electric field,is very desirable in practical applications.Half-metals and bipolar magnetic semiconductors are potential materials to realize such a function.They could provide 100%polarized spin current and reversibly control the direction of spins by external conditions.Seeking and designing these kinds of spintronic materials with high Curie temperatures are very demanding.On the other hand,traditional magnetic materials are composed of d/f electron metals.While,non-magnetic elements,e.g.C,H,O,can carry a magnetic moment and be used to design new magnetic materials.In this dissertation,by first-principles calculations,structural design and searching,we designed three kinds of magnetic materials,including a ternary p-electron ferromagnetic compound,half-metals with large spin gap and high Curie temperatures,and bipolar magnetic semiconductors with tunable spin polarization of carriers by electric field,and systematically studied their properties.The main research results are summarized as follows:1.Molecular oxygen resembles 3d/4f metals in exhibiting long-range spin ordering and strong electron correlation behaviors in compounds.However,the ferromagnetic ordering is not well observed in this kind of compounds,and the understanding of the formation mechanism of long-range ferromagnetic ordering is still limited.We firstly analyzed the interactions of magnetic moments of O₂ dimers in AO₂(A=K,Rb,Cs),and put forward an idea that the magnetism can be regulated through adjusting the arrangement of O₂ dimers between layers.And then using this method,it would induce long-range ferromagnetic ordering.By a structural search,we identified that?-Ba Na O₄is a ferromagnetic half-metal with Curie temperature 120 K.Our charge analysis shows that the average valence of O₂ dimer is-1.5,indicating that the?^*orbitals are partially occupied and the magnetic moment originates from the unpaired?^*electrons.O₂ dimers in?-Ba Na O₄ are toward c axis and head-to-head arrangement.Due to this unique arrangement,O₂ dimers couple ferromagnetically between layers through direct interactions,and induce long-range ferromagnetic ordering.This work deepens the understanding of the mechanism of O₂ dimer induced ferromagnetism and provides new clues to design new p-electron ferromagnetic materials.2.We choose N₂ dimer and 3d transition metals as building blocks and designed a serial of half-metals MN₄(M=Mn,Fe,Co).Through first-principles calculations,we found that they own large spin gaps?5 e V and high Curie temperatures?10³ K.We proposed that the self-doping phenomena occurs in M-3d(M=Mn,Fe,Co)orbitals.This model can well explain their magnetic moment and half-metallicity.The large spin gap originates from the large energy difference between bonding?and antibonding?^*of N₂ dimers.This study not only uncovers the role of the electronegativity and chemical bonding played in designing large-spin-gap and high-Curie-temperature half-metals,but also enlarges the structural type of half-metals and provides clues to explore new half-metals.3.Bipolar magnetic semiconductors(BMSs)are a class of compounds possessing different valence band maximum(VBM)and conduction band minimum(CBM)in terms of electron spins.Through gate voltage and chemical doping,the direction of spin polarization can be reversibly tuned.It meets the requirements of bipolar manipulating the spin current,and provides ideal materials for designing next-generation spintronic devices.At present,this kind of materials is still limited,and the searching and designing this kind of materials is still difficult.By first-principles calculations,we predict that monolayer Cr PS₄ and Cr PSe₄ are bipolar magnetic semiconductors with Curie temperature 58 K and 82 K,respectively.We predict that they can be easily cleaved from their A-type antiferromagnetic bulk form.A gate voltage can drive Cr PS₄and Cr PSe₄ into half-metals and the spin polarization of carriers is different for electron and hole doping.It provides the possibility of reversibly tuning the direction of spin current.We found that Cr PSe₄ has suitable electron affinity and ionic potential,which is easy for electron and hole doping.Further analysis found that the spin exchange splitting and crystal field splitting play a key role in the formation of bipolar magnetic semiconductor.We also predict that monolayer CrPTe₄ can realize quantum anomalous hall effect.Our work provides clues for searching and designing new bipolar magnetic semiconductors.