First-principles Calculations On New Multiferroics

Multiferroic compounds in which magnetism and ferroelectricity coexist have received a new surge of attention because of the fundamental importance of magnetoelectric(ME)effect and potential technological applications.As early as in 1960s,multiferroics were syn-thesized and stuided.However only since the discovery of epitaxial BiFeO3 multiferroic thin film in 2003 by Ramesh's group,multiferroics have become a hot topic in condensed matter community.From the point of view of basic physical research,electricity and magnetism were com-bined into one common discipline and described as Maxwell equations in the 19th century.However,electric and magnetic ordering in solids are most often considered separately due to their different kinds of origination.But chances lie in multiferroics in which these orders could coexist.Nonetheless,there are still unavoidable general problems open,i.e.,why the coexistence of magnetism and ferroelectricity is so rare?Given the tremendous potential for applications,multiferroics can be utilized in broad fields,such as applications which include the ability to address magnetic memory electri-cally(without currents),the creation of new types of 4-state logic(with both up and down polarization and up and down magnetization)and magnetoelectric sensors.During the past ten years since the renaissance of multiferroics,many achievements have been gained.Despite of this,there are still challenges remaining in this field.Not to mention the intrinsic insufficiencies of multiferroics,there are rare physical mechanisms that can quantitatively analyze the experimental results.Due to its own advantages,the first-principles calculation has become one of the most important methods we can choose.It starts directly at the level of established laws of physics and does not make assumptions such as empirical model and fitting parameters,finally the results will be quantitatively accurate.In the first chapter,the background of multiferroics is introduced firstly,including the classification and some representative mechanisms.Besides this,a brief review on recen-t first-principles theoretical works on multiferroics is also given and some significant and representative works are expressed in details.This part suggests the role and importance of first-principles calculations playing in studying multiferroic physics.In the second chapter,the introduction of first-principles calculations and other related methods will be given.Starting from highlighting the Hartree-Fock approximations,densi-ty functional theory is exhibited next.The bases of density functional theory including the Kohn-Sham equation and the Hohenberg-Kohn theorem are introduced particularly.Consid-ering the techniques relevant to this thesis,the Berry-Phase method and spin-orbital coupling effect are also discussed.As a matter of fact,the first-principles calculations always play a crucial role in the mul-tiferroics field.Three major contributions are listed as follows:predicting new multiferroics,helping to explain the experiments and verifying the theoretical mechanisms.In the third chapter,the strain effect in GdN with a simple rocksalt structure is addressed.GdN is a ferromagnetic metal in its original rocksalt structure and strain effect can bring a metal-insulator transition in it while the magnetic ground state remains as ferromagnetic state.More interestingly,the ferroelectric instability which corresponds to the softening from the polar phonon mode can be induced by proper epitaxial strain in GdN,and the polarization is sensitive to the strain.The calculated phonon spectra of strained GdN also confirm the existence of ferroelectric polarization.A giant ferroelectric polarization?85?C/cm2 will exhibit in strained GdN.The present work opens up the possibility for epitaxial GdN thin films as potential multiferroics with coexistence of ferromagnetism and ferroelectricity.Besides this,combination of first-principles calculations and experiments can help solv-ing problems.Peroskite SrTiO3 is well-known as a quantum paraelectric compound and lies in the quantum critical region.Nevertheless,such a quantum paraelectric state is fragile and a ferroelectric instability may occur against even weak intrinsic or external stimulus,which has been a hot topic for intensive focus.In nonstoichiometric SrTi03,the extra Ti ions will occupy the Sr site or the otherwise.This antisitelike occupation of Ti ions will induce both magnetism and ferroelectric polarization in nonstoichiometric SrTiO3.The nonstoichiometry provides a new approach to modulate magnetism and the ferroelectric instability in traditional paraelectric materials.All the results are presented in the fourth charpter.In the fifth chapter,the electronic structures and magnetism of mixed-valent manganite SrMn7O12 in the rhombohedral symmetry as a potential multiferroic candidate are investi-gated.Our calculations show clearly that the possible magnetic ground state accommodates a particular helical spin order responsible for the insulating state in SrMn7012.The giant ferroelectric polarization is revealed,which is almost purely from the electronic contribution rather than the ionic displacements and is closely related to the magnetism.The influences of the on-site Coulomb interaction and spin-orbital coupling on the electronic structure and multiferroicity are also discussed.The present results suggest that SrMn7O12 is a promising multiferroic candidate with desirable properties.The sixth chapter is devoted to the conclusions and perspectives.