Spin-Orbit Coupling and Ion Displacements in Multiferroic TbMnO3

The electronic and magnetic properties of TbMnO3 leading to its ferroelectric (FE) polarization were investigated on the basis of relativistic density functional theory (DFT) calculations. In agreement with experiment, we show that the spin-spiral plane of TbMnO3 can be either the bc- or ab-plane, but not the ac-plane. As for the mechanism of FE polarization, our work reveals that the "pure electronic" model by Katsura, Nagaosa and Balatsky (KNB) is inadequate in predicting the absolute direction of FE polarization. For the ab-plane spin-spiral state of TbMnO3, the direction of FE polarization predicted by the KNB model is opposite to that predicted by DFT calculations. In determining the magnitude and the absolute direction of FE polarization in spin-spiral states, it is found crucial to consider the displacements of the ions from their ecntrosymmetric positions

Mechanism of Lattice-Distortion-Induced Electric-Polarization Flop in the Multiferroic Perovskite Manganites

Magnetoelectric phase diagrams of the perovskite manganites, Eu1-xYxMnO3 and Gd1-xTbxMnO3, are theoretically studied. We first construct a microscopic model, and then analyze the model using the Monte-Carlo method. We reproduce the diagrams, which contain two different multiferroic states, i.e., the ab-plane spin cycloid with electric polarization P//a and the bc-plane spin cycloid with P//c. We reveal that their competition originates from a conflict between the single-ion anisotropy and the Dzyaloshinsky-Moriya interaction, which is controlled by the second-neighbor spin exchanges enhanced by the GdFeO3-type distortion. This leads to a P flop from a to c with increasing x in agreement with the experiments.Comment: 5 pages, 5 figures. Recalculated results after correcting errors in the assignment of DM vectors. The conclusion is not affecte

A beginner's guide to the modern theory of polarization

The so-called {\it Modern Theory of Polarization}, which rigorously defines the spontaneous polarization of a period solid and provides a route for its computation in electronic structure codes through the Berry phase, is introduced in a simple qualitative discussion

Quantum Fluctuations of Chirality in One-Dimensional Spin-1/2 Multiferroics: Gapless Dielectric Response from Phasons and Chiral Solitons

We present a quantum theory for one-dimensional spin-1/2 multiferroics, where the vector spin chirality couples with the electric polarization. Based on exact diagonalization and bosonization, it is shown that quantum fluctuations appreciably reduce the chiral ordering amplitude and the associated ferroelectric polarization. This yields nearly collinear spin correlations in short-range scales, in qualitative agreement with recent neutron scattering experiments. There appear gapless chirality excitations described by phasons and new solitons, which can be experimentally verified from the low-energy dielectric response.Comment: 5 pages, 6 figures. v3: The analysis of the chirality dynamics at q=pi added. v4: Final version. To be published in J. Phys. Soc. Jp

Far-infrared optical excitations in multiferroic TbMnO_3

We provide a detailed study of the reflectivity of multiferroic TbMnO_3 for wave numbers from 40 cm^{-1} to 1000 cm^{-1} and temperatures 5 K < T < 300 K. Excitations are studied for polarization directions E || a, the polarization where electromagnons are observed, and for E || c, the direction of the spontaneous polarization in this material. The temperature dependencies of eigenfrequencies, damping constants and polar strengths of all modes are studied and analyzed. For E || a and below the spiral ordering temperature of about 27 K we observe a transfer of optical weight from phonon excitations to electromagnons, which mainly involves low-frequency phonons. For E || c an unusual increase of the total polar strength and hence of the dielectric constant is observed indicating significant transfer of dynamic charge probably within manganese-oxygen bonds on decreasing temperatures.Comment: 8 pages, 7 figures, accepted for submission in European Physical Journal

Double-exchange model study of multiferroic RRMnO3_3 perovskites

In this proceeding, recent theoretical investigations by the authors on the multiferroic $R$MnO$_3$ perovskites are briefly reviewed at first. Using the double-exchange model, the realistic spiral spin order in undoped manganites such as TbMnO$_3$ and DyMnO$_3$ is well reproduced by incorporating a weak next-nearest neighbor superexchange ($\sim10$ of nearest neighbor superexchange) and moderate Jahn-Teller distortion. The phase transitions from the A-type antiferromagnet (as in LaMnO$_3$), to the spiral phase (as in TbMnO$_3$), and finally to the E-type antiferromagnet (as in HoMnO$_3$), with decreasing size of the $R$ ions, were also explained. Moreover, new results of phase diagram of the three-dimensional lattice are also included. The ferromagnetic tendency recently discovered in the LaMnO$_3$ and TbMnO$_3$ thin films is explained by considering the substrate stress. Finally, the relationship between our double-exchange model and a previously used $J_1$-$J_2$-$J_3$ model is further discussed from the perspective of spin wave excitations.Comment: 6 pages, 3 figures; Proceeding of the Workshop on Magnetoelectric Interaction Phenomena in Crystals (MEIPIC-6); To be appeared in European Physical Journal

The Magnetoelectric Effect in Transition Metal Oxides: Insights and the Rational Design of New Materials from First Principles

The search for materials displaying a large magnetoelectric effect has occupied researchers for many decades. The rewards could include not only advanced electronics technologies, but also fundamental insights concerning the dielectric and magnetic properties of condensed matter. In this article, we focus on the magnetoelectric effect in transition metal oxides and review the manner in which first-principles calculations have helped guide the search for (and increasingly, predicted) new materials and shed light on the microscopic mechanisms responsible for magnetoelectric phenomena.Comment: 24 pages, 12 figure

Phase Composition Stability of ZrN/SiNх Multilayered Coatings under Air Annealing: the Role of the Thicknesses Ratio of the Elementary Layers

Секция 5. Влияние излучений на структуру и свойства покрытий = Section 5. Radiation Influence on Coatings Structure and PropertiesIn the present work the phase composition stability and oxidation resistance of ~300 nm thick ZrN/SiNx nanoscale multilayers at the air annealing procedure were studied. Coatings were prepared by reactive magnetron sputter-deposition on Si wafers under Ar+N2 plasma discharges. ZrN/SiNx multilayers with ZrN and SiNx layer thickness varying from 2 to 10 nm were synthesized by sequential sputtering from elemental Zr and Si3N4 targets at T dep =300 °C. According to X-ray diffraction (XRD) analysis the multilayered films consist of nanocrystalline (002)-oriented ZrN and amorphous SiNx layers. The oxidation resistance under air was studied using in situ XRD in the temperature range from 400 to 950 °C, as well as by scanning electron microscopy (SEM) and wavelength dispersive X-ray spectrometry (WDS) after air annealing procedure. While the reference ZrN film starts to oxidize at Tox. = 550 °C, a much higher oxidation resistance is found for multilayered films, till Tox. = 860–950 °C for ZrN/SiNx coatings with the elementary layer thickness ratio of 5 nm/10 nm and 2 nm/5 nm

Phase Composition Stability of ZrN/SiNх Multilayered Coatings under Air Annealing: the Role of the Thicknesses Ratio of the Elementary Layers

Секция 5. Влияние излучений на структуру и свойства покрытий = Section 5. Radiation Influence on Coatings Structure and PropertiesIn the present work the phase composition stability and oxidation resistance of ~300 nm thick ZrN/SiNx nanoscale multilayers at the air annealing procedure were studied. Coatings were prepared by reactive magnetron sputter-deposition on Si wafers under Ar+N2 plasma discharges. ZrN/SiNx multilayers with ZrN and SiNx layer thickness varying from 2 to 10 nm were synthesized by sequential sputtering from elemental Zr and Si3N4 targets at T dep =300 °C. According to X-ray diffraction (XRD) analysis the multilayered films consist of nanocrystalline (002)-oriented ZrN and amorphous SiNx layers. The oxidation resistance under air was studied using in situ XRD in the temperature range from 400 to 950 °C, as well as by scanning electron microscopy (SEM) and wavelength dispersive X-ray spectrometry (WDS) after air annealing procedure. While the reference ZrN film starts to oxidize at Tox. = 550 °C, a much higher oxidation resistance is found for multilayered films, till Tox. = 860–950 °C for ZrN/SiNx coatings with the elementary layer thickness ratio of 5 nm/10 nm and 2 nm/5 nm