Phase Coexistence/Separation In Multiferroic Perovskite Manganites

The strong coupling between spin, charge, orbital and lattice degrees of freedom in perovskite manganites gives rise to rich physical properties. From the discovery of colossal magnetoresistance (CMR) in the last century to extensive exploration of the multiferroicity in manganites in this century, those strongly correlated materials have a number of potential technological applications in such as memories and spintronics, etc.Multiferroic materials are those which have two or more primary ferroic orders including ferroelectric or antiferroelectric, ferromagnetic or antiferromagnetic, and ferroelastic orders. Perovskite manganites are the typical multiferroic materials, and they simultaneously have antiferromagnetic and ferroelectric orders below certain temperature. Their spiral spin order and E-type antimagnetic order are special for they can induce electric polarizations, and the corresponding mechanisms for ferroelectricity are different. The electric polarization in spiral order is induced by the super spin-current or the inverse Dzyaloshinsky-Moriya (DM) interaction. The main spiral spin orders in manganites are the ab-cycloidal order and bc-cycloidal order. With the modulations of doping or magnetic field etc., the cycloidal plane can be flopped easily. However, the spiral spin order is difficult to control by electric field. The modulation of spiral plane and polarization of manganites are studied in detail in our work. The electric polarization in E-type antimagnetic order is induced by the symmetric magnetostriction. Though the polarization of E-type antimagnetic order is larger than that of spiral spin order, it is hard to be modulated by magnetic field. The electronic phase-separation/phase-competition is a common phenomenon in most complex oxides. Those sepatated phases can be obtained by the introduction of disorder, doping, defects and nanosize effects, etc. It is demonstrated by theoretical and experimental results that the electronic phase-separation state composed of charge-ordered (CO) and ferromagnetic (FM) phases in manganites is essential to the CMR effects. Are there are any phase-separation and phase-competition to improve the properties of magnetism and ferroelectric in the multiferroic manganites? This is another concerned issue in our work. Starting from the Mn-site doping and R-site modulation, we will study the phase-separation and phase-competition in different spin orders with different methods. The contents are organized as follows:In Chapter â… , we first give an overview of our study on multiferroic manganites. Then, we review the crystal structure, magnetic structure and the ferroelectricity. After the spiral order and E-type order are studied, the motivation and methods are introduced. Finally, Monte Carlo method used is explored in detail.In Chapter â…¡, the effect of Mn-site nonmagnetic substitution on the spiral spin ordering in multiferroic manganites is investigated by Mochizuki-Furukawa (M-F) model. It is demonstrated that the nonmagnetic substitution significantly suppresses the multiferroic phase transitions. Most interestingly, a coexistence state with both the ab-plane spiral spin order and the bc-plane spiral spin order is observed under low nonmagnetic substitution.In Chapter â…¢, the modulation of spiral spin order in multiferroic manganites by the next-nearest-neighbor (NNN) spin interaction is investigated by M-F model. There are two types of the modulations, i.e., the symmetric perturbations in which the interactions over the lattice are symmetrically modulated with an equal distribution to maintain the mean interaction invariant, and the asymmetric perturbations in which the interactions over the lattice are randomly modulated with variable mean interaction. It is shown that both the types of modulations can drive the reorientation of the spiral spin order and thus lead to the coexistence of ab-plane spiral spin phase and the bc-plane spiral spin phase. And the evidence of coexisting spin configuration is given in our simulation. Meanwhile, the simulated spin-helicity vectors well explain the experimental polarizations, and they provide another evidence of the coexisting phase.In Chapter IV, semi-quantum two-orbital exchange model is used to investigate the spiral phase induced by R-site doping. In addition to the pure RMnO3(R=Tb, Dy), the spiral phase can also be obtained by synthesize A-AFM manganites(R=Eu, Sm, Nd) and E-AFM manganites(R=Ho, Y) such as Eu1-xYxMnO3, Nd1-xYxMnO3and Sm1-xYxMnO3etc. It is revealed that the substitution results in a rich multiferroic phase diagram where the coexisting A-AFM phase and spiral spin phase, pure spiral spin phase, coexisting spiral spin phase and E-AFM phase, and pure E-AFM phase emerge in sequence. Thus, some recent experimental phenomena about spiral order and phase separation induced by doping are well explained.In chapter V, the conclusion and perspectives are given.