Theoretical Study Of Magnetic Competition Effect In Complex Magnetic Materials

Transition metal and correlated electronic materials exhibit rich magnetic properties such as magnetic properties due to their spin,charge,orbital,lattice and other degrees of freedom coupling and competitive effects.The development of new spintronics devices urgently requires excellent magnetic carrier materials,which can revolutionize the next generation of information storage and quantum computing technologies.The resistivity of a colossal magnetoresistance(CMR)material has a sensitive response to the external magnetic field and can be used for ultra-high-density magnetic storage;one spin channel of a half-metal material is metallic and the other is insulating,thus generating 100% highly efficient polarization currents that can be used for magnetic tunnel junctions(MTJ).Colossal magnetoresistance and half-metal properties are derived from complex magnetic competition effects.In the colossal magnetoresistive materials represented by Mn-based perovskites(e.g.,La1-x SrxMnO3),there is a complex competition between ferromagnetism(FM)and antiferromagnetism(AFM),which is closely related to the orbital order of Mn-3d electrons on the microscopic level.The half-metal alloy materials represented by Mn-based Heusler(e.g.,Cu2MnAl)usually show ferromagnetism similar to elemental Fe,but Heusler also has strong compound bonding properties,which can produce 100% efficient spin polarization.In this thesis,the transition metal oxide SrRuO3 and the Fe-based Heusler alloy were selected,and the first-principles method was used to study its rich magnetic properties and microscopically complex magnetic interaction mechanism.First of all,bulk SrRuO3 is a relatively rare 4d electron oxide metal.The crystal structure is a common perovskite structure.It can form a high-quality heterojunction with SrTiO3 and other substrates,and SrRuO3 heterojunction can produce interesting "metal-Insulation" phase transition.In this thesis,we will study the microscopic results that determine the competition between the SrRuO3/SrTiO3 heterojunction metal phase and the insulating phase,and discover the important role of crystal structure distortion.Microscopically explain the insulator of the single-layer SrRuO3 surface structure and the metal of single-layer SrRuO3 superlattice.Second,Full-Heusler half-metal alloys usually exhibit strong ferromagnetism.This thesis found that the Fe-based Fe2TiSb system has a more stable antiferromagnetic coupling.This thesis theoretically studies the microscopic magnetic competition mechanism and explains this "abnormal magnetic coupling" phenomenon.This thesis provides important theoretical information for understanding the magnetic properties of SrRuO3 and Fe2TiSb systems.The density functional theory of the first-principle method usually cannot reliably describe the correlation properties of d electrons of transition metals,and is widely criticized.The research in this thesis found that the latest SCAN(Strongly Constrained and Appropriately Normed)exchange correlation functional better grasps the correlation properties,and is very suitable for SrRuO3 and Fe2TiSb two types of weak magnetic systems.This is the innovation of this article in theory and method.In short,the complex and subtle magnetic competition effect of complex magnetic materials on the one hand is the difficulty of materials research,on the other hand,it brings huge space for the regulation of physical properties of materials.The theoretical study of SrRuO3 and Fe2TiSb magnetic materials in this thesis reveals several interesting physical phenomena and will provide important reference information for future related research.