First-principles Study Of Magnetoelectric Coupling In Multiferroic Materials

Multiferroic materials are one of the functional materials that endow with multiple“ferro”ordering such as ferromagnetic（FM）and ferroelectric（FE）at the same time,which can realize the direct cross-control of magnetism（electrical polarization）by electric field（magnetic field）.This has led to a revolutionary development in“electrically written and magnetically read”storage technology.From the application perspective,magnetoelectric coupled storage devices can largely increase the storage density while effectively reducing energy consumption.In particular,with the rapid development of information technology and the critical need for new storage technology,how to design new strategies to construct multiferroic materials and improve their magnetoelectric coupling effect have become one of the important directions of current research.Multiferroic materials are mainly classified as type I and type II multiferroic materials.As for type I multiferroics,the origin of FM and FE are independent of each other.Single-phase materials in which ferromagnetism and ferroelectricity arise independently also exist,but are rare.Therefore,increasing attention has been turned to artificial composites such as FM/FE heterostructures（HS）.The electric control of magnetism can be realized by the coupling among charge,spin,orbitals,and lattice at the interface of the HSs.In other words,the superior FM and FE materials can be selected from a vast library of type I multiferroic materials to design and construct HSs as desired.However,it is due to the different origins of magnetism and ferroelectricity,this type of materials have the disadvantage of weak magnetoelectric coupling.On the other hand,it is known from symmetry that the spatial arrangement of the magnetic ordering can break the spatial inverse symmetry and thus induce ferroelectric polarization.We can categorize them as type II multiferroic materials.Theoretically,the regulation of magnetic ordering can accordingly control the ferroelectric polarization owing to the fact that the magnetism and electrical polarization arise dependently.Although this type of multiferroic materials possess with strong magnetoelectric coupling between FM and FE,they usually resort to special frustrated ordering（helical or complex co-linear magnetic ordering）to meet the requirement,which seriously reduces the working temperature of the materials to some extent.Another,the spin-orbit coupling and spin-lattice coupling dependent physical mechanisms are usually weak,leading to a small electrical polarization,which limits the practical applications as well.In general,these two limitations remain the current challenges for type II multiferroic materials in the future research.In this thesis,we propose methods to realize magnetoelectric coupling based on type I and type II multiferroic materials by first-principles calculations and elucidate the corresponding physical mechanisms.On the one hand,for type I multiferroics,HS was adopted as a platform to achieve the modulation of magnetism and relevant electronic structures by FE polarization,i.e.,the strong magnetoelectric coupling effect is achieved in the HS system.We have constructed BiFeO₃/BaTiO₃,bilayer-CrOBr/In₂Se₃ andα-RuCl₃/CuInP₂S₆ HSs and achieved nonvolatile electric control of magnetism by reversible FE polarization in these systems.In addition,we also realized the control of conductivity,exchange coupling parameters,DM interactions,and magnetic anisotropy of these systems with the switching of polarization.Note that each of the three systems we chose above has the advantage of easy magnetic modulation.Firstly,BaTiO₃ with large polarization value strongly induces surficial FM ordering in perovskite oxide BiFeO₃/BaTiO₃ HS.We also explain that the strong magnetoelectric coupling effect of the system is due to the polarization competition between BiFeO₃ and BaTiO₃,and the induced interfacial FM ordering by reversible polarization is caused by the change from Fe³⁺-O^2--Fe³⁺superexchange-mediated antiferromagnetism（AFM）to Fe³⁺-O^2--Fe⁴⁺double-exchange-mediated FM.Unlike the regular interfacial properties in most other HSs,the change of the magnetic properties in BiFeO₃/BaTiO₃HS are directly exposed to the external surface,which makes the device more sensitive to the detection and utilization of magnetism.In addition,due to the increasing demand for miniaturization of memory devices,we have also investigated two-dimensional（2D）multiferroic HSs with thinner atomic layers.Among them,for the bilayer-CrOBr/In₂Se₃,we make the most of the weak coupling between the layers of bilayer CrOBr which is more sensitive to magnetic response upon polarization switching.What’s more,the featureless ground states of proximate quantum spin-liquid state ofα-RuCl₃ that the FM and AFM phase are nearly degenerate inα-RuCl₃/CuInP₂S₆ HS enables it more susceptible to polarization modulation.Whenα-RuCl₃is contacted with upward CuInP₂S₆polarization,α-RuCl₃ will transform into a perpendicular FM material.That is,not only magnetism regulation,but also the transformation of magnetic anisotropy from easy plane to easy axis can be realized by the switching of polarization.Unlike the in-plane recording,magnetic recording in easy-axis（perpendicular）direction can significantly increase the storage density and facilitate consistent reading and recording of data.Also,the Curie temperature of the system is substantially increased from 17 K to 89 K by polarization switching,which is beneficial for the practical application of functional devices to a certain extent.In these aforementioned systems,we have successfully realized the electrical polarization control of magnetism in the HSs,which means that the strong magnetoelectric coupling effect is well achieved while keeping their respective edge.On the other hand,for type II multiferroics,we use the magnetic domain wall structure as another platform to break the spatial inversion symmetry.The frustracted magnetic ordering control of electrical polarization was realized in rare-earth orthoferrites AFeO₃（A=Lu,Y,Gd）systems of our research.Surprisingly,we find that the A-site ion radius of rare-earth orthoferrites has an important effect on the polarization value under a certain domain wall density.Based on the octahedral distortion and the unified polarization model,we also perform a mechanistic analysis of the ferroelectric polarization at domain walls along the b direction.The different octahedral tilt angles and the different unit cell volumes caused by A-site ions are the main reasons for the polarization difference in the three compounds.Interestingly,for the rare-earth orthoferric LuFeO₃ with the smallest ionic radius of three compounds,the polarization value at the highest domain wall density is increased by 22.67%compared to the previously reported SmFeO₃.Although there is still a long way to go to continue to improve the polarization of type II multiferroics,the magnetic ordering control of polarization and the rusults about dependence of polarization on radius of our work provide a solid theoretical basis for the subsequent experimental design.We have realized the magnetoelectric coupling effect,i.e.,electrical polarization control of magnetism and magnetic ordering control of electrical polarization,utilizing type I and type II multiferroics with HSs and domain wall as the platform,respectively.Currently,the magnetoelectric coupling effect of multiferroic materials has served as one of the key technologies in the field of information storage.With it the development and application of low-energy,miniaturized,non-volatile multi-state storage has been promoted,which lays a physical foundation for the enhancement of nonvolatile memory,sensors and other micro-electronic devices in the future.