The Study Of Itinerant Magnetic Compounds From First Principles

Magnetic materials can respond to magnetic field and produce a wealth of phenomena in condensed matter physics.They have been investigated in thousands of years,and the typical compass application is one of the four Chinese great inventions.With the development of science and technology as well as the progress of society,the demands for exploring new functional materials are increasing dramatically.In modern times,magnetic materials have been rapidly developed in many fields such as aviation,aerospace,communications,and medicine,for examples,speakers,magnetic resonance imaging,magnetic levitation,transformer cores,and various new magnetic storage devices.Among condensed matters,the Heusler compounds family and Ruddlesden-Popper（RP）materials contain a large number of magnetic materials,exhibiting various magnetic phenomena such as ferromagnetism,antiferromagnetism,spiral magnetism,paramagnetism,diamagnetism,etc.In this thesis,we studied the physical properties of Heusler alloy and metal oxide systems based on the first-principles calculation method of the density functional theory.The full text is divided into six chapters:In Chapter 1,we briefly introduced the half-metal materials and Heusler compounds and Ruddlesden-Popper materials and their research progress.we also discussed the itinerant electron magnetism,ferromagnetic spin wave theory,Boltzmann transport theory,together with the research motivation and main content.In Chapter 2,we introduced the theoretical and calculational methods,computer software programs used in this thesis,including the density functional theory,software packages including VASP,WIEN2k,Elk and calculation method of electrical transport properties.In Chapter 3,we explored and discovered new spintronics candidate materials by studying the ferromagnetic Co-based Heusler transition metal silicides,Co₂MnSi,Co₂FeSi and the Co₂TiSi.Our calculations results show that Co₂TiSi is a very itinerant magnet but Co₂FeSi and Co₂MnSi show local moment behavior regarding the Feand Mn,while retaining the itinerancy of the Co magnetism.These materials can therefore be described as itinerant systems with embedded local moment atoms.We also find that though Co₂FeSi is not half metallic,it has a very high transport spin polarization,which is interestingly opposite to the density of states（DOS）spin polarization.We also found that the spin excitations from our calculations,which do not yield a half-metallic state for Co₂FeSi,are nonetheless consistent with the experimental observation of a higher T_C for Co₂FeSi than for Co₂MnSi.It is important to note that the physical properties of Co₂FeSi are compatible with conventional electronics and that transition metal silicides have been extensively used as electrode materials for Si devices.Moreover,Co₂FeSi has a cubic crystal structure,and the lattice parameter is～4%larger than that of Si,which is very close to both Ge and GaAs.This,combined with the large moment,cubic structure,and high Curie temperature supports the further investigation of Co₂FeSi for spintronic applications that make use of the transport spin polarization.In Chapter 4,we tried to find a structure with lower energy and stronger magnetism than the full Heusler compound by constructing a disordered structure of weakly itinerant ferromagnetic Pd₂TiIn.First principles calculations showed that Pd₂TiIn is a paramagnetic metal,with an electronic structure showing a moderately high DOS at the Fermi level derived from Tid states.The Pd d states are nearly fully occupied in accord with existing photoemission data.This means that Pd moments are unlikely to possess magnetic moments.However,defects of which Tioccurs on either In or Pd site do lead to moment formation,as well as the polarization of other neighboring transition element atoms,parallel to the moment on the Tidefect.We also found that vacancies on the In sites could also lead to magnetic moments on Tiatoms.This means that the magnetic behavior should be influenced by many experimental conditions,for example the presence of excess Ti,deficiency of In,or synthesis temperatures and annealing conditions.Furthermore,we predicted that there exists an alternative structure for Pd₂TiIn that is very close in energy to the ordered full-Heusler structure.This cubic phase is a non-ferromagnetic metal.It will be of interest to verify this phase by experiments,with the assistance of our theoretical guidance.In Chapter 5,We investigated the magnetic competition in the quantum critical metal Sr₃Ru₂O₇ by constructing different magnetic configurations.Sr₃Ru₂O₇ was found to be a quantum critical metal that showed a metamagnetic quantum phase transition and electronic nematicity,through density functional calculations.LAPW and PAW results suggested a ferromagnetic ground state in contrast to the experimentally observed paramagnetism,raising the question of competing magnetic states and associated fluctuations that might suppress magnetic order.We identified such low-energy antiferromagnetically ordered metastable state ot have E-type stripe order.We also obtained the significant transport anisotropy in this E-type ordered state.Experimental investigation using neutron scattering in search of spin fluctuations arising from this E-type order will be of interest.Finally,in Chapter 6,we summarized all the theoretical computation results and provide perspectives for the future research directions.