First Principles Studies Of Perovskite Multiferroic Materials And Ferroelectric Materials

Multiferroic materials have recently attracted great attention for their potential applications in spintronics with its combination of ferroelectric and magnetic order.Especially,coexistence of multiferroic properties(eg.Ferroelectricity and ferromagnetism)and the mechanism of their coupling effect have attracted a great attraction.As a kind of most studied material,perovskite oxides have highly variable crystal structure,unique magnetic and electrics properties.Present studies suggest that the hybrid organic-inorganic perovskite(HOIPs)is a potential multiferroic material.HOIPs shows the variable chemical and structural properties,such as its high effective light absorption capability.Understanding the mechanism of its light absorption would make a huge difference to the future development of photovoltaic devices.When we introduce hydrogen bond at the A site of ABX3 perovskite,we can cause ferroelectricity(the spatial inversion symmetry breaking effect).When the transition metal ion locates on the B site,magnetic order couples with ferroelectric order and results in additional multiferroic materials.According to first-principles calculations,the main results are concluded as follows:We interpreted the ferromagnetic and ferroelectric properties in rare earth oxide perovskite systems(eg.RMO3 and R2NiMnO6).First,we investigated the weak magnetism in RMO3,then the ferroelectrics in rare earth titanium under epitaxial strain,and discuss the multiferroic in the superlattice.We suggest a method search for the reference phase(?=0)in HOIPs.Consideration of the Berry phase method,we estimated the polarization value of 12 HOIPs ABX3(A=MA,FA;B=Sn,Pb;X= Cl,Br,I),and investigated their relation with components.We got the maximum FE polarization of 19.16?C/cm2 in MASnBr3.When the(A,B)components are fixed and halogen elements X is varied,we could find that the average FE polarization decreases as(A,B)changes as(MA,Sn)X,(MA,Pb)X,(FA,Sn)X,(FA,Pb)X.Considering functional mode,it's suggested that the hydrogen bond weakens when the same(A,B)system changes its X element from Cl,Br,and to I.We studied the Rashba effect in HOIPs.First,followed out the relevant parameters of Rashba constant with respect to the calculated GW0+SOC band;second,through introducing Pearson correlation coefficient to study the relation between Rashba parameters with ferroelectric polarization,and we found there are no significant correlation with ferroelectric polarization values,but with directions;Last,we compare our ferroelectric polarization as well as Rashba parameters with published papers.Hydroxyapatite,which is the main component of human bones and enamel,is a hypothetical non-perovskite ferroelectric biomaterial,so it is meaningful to be investigated.We estimated the ferroelectric polarization and structural transition under its separate phase and figured out the origin of its ferroelectric polarization.The biomaterials is thus better understood and we have made the point in a novel direction of searching multiferroic materials.