Session S30: Theory of Ferroic Systems

New design strategy for realizing multiferroic materials

Ferroelectricity is a property that only insulating materials can exhibit. % For this reason, nearly all searches for new multiferroic compounds, those simultaneously exhibiting ferroelectric and magnetic order, have focused on \emph{insulating} magnetic oxides. % Here, we propose a different approach: Start from a \emph{conducting} oxide with broken inversion symmetry and search for routes to induce long-range magnetic order [1]. % Using density-functional and dynamical mean-field theories, we investigate the electronic properties of the polar metallic oxide LiOsO$_3$. % We show that a multiferroic state can be engineered by enclosing LiOsO$_3$ between an insulating material, LiNbO$_3$. % We predict that the 1/1 superlattice of LiOsO$_3$ and LiNbO$_3$ exhibits strong coupling between magnetic and ferroelectric degrees of freedom with a ferroelectric polarization of 41.2 $\mu$Ccm$^{-2}$, Curie temperature of 927\,K, and N\'eel temperature of 379\,K. % Our results show that one can start with polar metallic oxides to make multiferroics.\\[1em] [1] D.\ Puggioni \emph{et al}, Phys.\ Rev.\ Lett.\ \textbf{115}, 087202 (2015).   

Crystal structure and electronic properties of bulk and thin-film brownmillerite oxides

The equilibrium structure and functional properties exhibited by brownmillerite oxides (general formula $A_2B_2$O$_5$), a family of perovskite-derived structures with alternating layers of $B$O$_6$ octahedra and $B$O$_4$ tetrahedra arising from ordered arrangements of oxygen vacancies, is dependent on a variety of competing crystal-chemistry factors. Using first principles electronic structure calculations, we investigate two antiferromagnetic brownmillerite ferrites, Sr$_2$Fe$_2$O$_5$ and Ca$_2$Fe$_2$O$_5$, and find that the stability of the equilibrium ground state is governed by complex interactions among several structural descriptors, including ionic size, distortions of nominally regular oxygen octahedral, and in-plane and out-of-plane separation of tetrahedral chains. Furthermore, we find that these same effects control the preferred oxygen vacancy orientation under epitaxial strain, a tunable parameter which also strongly influences the magnitude of the electronic band gap via an asymmetric-vacancy alignment dependent response. Finally, we show that $A$-site cation ordering in these materials can lift inversion symmetry, providing a potential new route to room temperature multiferroics.   

Unveiling hidden ferrimagnetism and giant magnetoelectricity in polar magnet Fe$_{2}$Mo$_{3}$O$_{8}$

Polar magnets, belonging to the polar crystallographic symmetry groups and containing magnetic ions, can exhibit non-trivial magnetoelectric (ME) effects below magnetic ordering temperatures due to the broken time reversal and space inversion symmetries. Mono-domain polar single crystals can often be grown, and eliminate the need for any poling procedures to reveal the possible ME response. Here, we report a giant ME effect in a polar magnet Fe$_{2}$Mo$_{3}$O$_{8}$ at temperature as high as 60 K. Polarization jumps of 0.3 $\mu $C/cm$^{2}$~and repeated mutual control of ferroelectric and magnetic moments with differential ME coefficients on the order of 10$^{4\, }$ps/m are achieved. The sign of the ME coefficients can be switched by changing the direction of the applied ``bias'' magnetic field. Importantly, no electric or magnetic poling is needed, as necessary for applications. Using first principles calculations, we show that exchange striction is the leading mechanism responsible for the observed ME effect.   

Theory of colossal magnetoelectric response near spin-flop transition in Ni3TeO6

The manipulation of magnetic ordering with applied electric fields is of pressing interest for new spintronic and information storage applications. Recently, such magnetoelectric control was realized in multiferroics [1]. However, their magnetoelectric switching is often accompanied by significant hysteresis, resulting from a large barrier, separating different ferroic states. Hysteresis prevents robust switching, unless the applied field overcomes a certain value (coercive field). I will discuss the role of a switching barrier on magnetoelectric control, in particular, in a collinear antiferromagnetic and pyroelectric Ni3TeO6 [2,3]. The barrier between two magnetic states in the vicinity of a spin–flop transition is almost flat, and thus small changes in external electric/magnetic fields allow to switch the ferroic state through an intermediate state in a continuous manner, resulting in a colossal magnetoelectric response. This colossal magnetoelectric effect resembles the large piezoelectric effect at the morphotropic phase boundary in ferroelectrics. [1] T. Kimura, T. Goto, H. Shintani et al., Nature 426, 5 (2003) [2] Y.-S. Oh, S. Artyukhin J. J. Yang et al., Nature Communications 5, 3201 (2014) [3] J. W. Kim, S. Artyukhin, E.?D. Mun et al., Phys. Rev. Lett. 115, 137201 (2015)   

Multiferroic behavior at a spin state transition

Traditionally, multiferroic behavior is studied in materials with coexisting long-range orders, such as ferromagnetism and ferroelectricity. Here we present multiferroic behavior at a spin-state transition (SST). SSTs, for example, the S $=$ 1 to S $=$ 2 transition in Mn$^{\mathrm{3+}}$ can become cooperative magneto-structural phase transitions due to structural coupling between ions. SSTs are accompanied by change in the orbital occupation and hence, strongly coupled to the lattice and charge degrees of freedom. They are a dominant functionality in metal-organic materials, persisting up to room temperature in some compounds. We demonstrate that a magnetic SST can induce ferroelectricity. We study a Mn-based metal-organic system in which a three-fold degenerate dynamic Jahn-Teller effect at high temperatures vanishes when the temperature is lowered, and the system drops into a lower spin state. Application of a magnetic field restores the high spin Jahn-Teller-active state and allows the Jahn Teller distortions to order cooperatively, creating a dielectric constant change and a net electric polarization. We use high magnetic fields at the NHMFL to study the magnetic and electric behavior of this system across a significant fraction of its T-H phase space, and compare to theoretical modeling.   

Spin polarized electronic states and spin textures at the surface of oxygen-deficient SrTiO$_3$

We investigate the electronic structure and spin texture at the (001) surface of SrTiO$_3$ in the presence of oxygen vacancies by means of {\it ab initio} density functional theory (DFT) calculations of slabs. Relativistic non-magnetic DFT calculations exhibit Rashba-like spin winding with a characteristic energy scale $\sim 10$ meV. However, when surface magnetism on the Ti ions is included, bands become spin-split with an energy difference $\sim 100$ meV at the $\Gamma$ point. This energy scale is comparable to the observations in SARPES experiments performed on the two-dimensional electronic states confined near the (001) surface of SrTiO$_3$. We find the spin polarized state to be the ground state of the system, and while magnetism tends to suppress the effects of the relativistic Rashba interaction, signatures of it are still clearly visible in terms of complex spin textures.   

Electronic structure studies on competing phases of Aurivillius Bi$_4$Ti$_3$O$_{12}$ using first-principles calculations

The low temperature ferroelectric to high temperature paraelectric phase transition in bismuth titanate (Bi$_4$Ti$_3$O$_{12}$) has been experimentally observed at Curie temperature (T$_C$) around 675 $^{\circ}$C. The first-principles calculations using density functional theory as implemented in the Vienna \textit{ab initio} simulation package (VASP) with generalized gradient approximation (GGA) for the exchange-correlation interaction are performed to investigate this monoclinic (\textit{b}1\textit{a}1) to tetragonal (\textit{I}4/\textit{mmm}) crystal structural transition. We further, provide discussion of the band structure and the Ti--O orbital hybridization, in addition to the frequency dependent dielectric and optical properties of Bi$_4$Ti$_3$O$_{12}$ due to the potential applications in the electronic devices.   

Microscopic description of oxide perovskites and automated high-throughput analysis of their energy landscape

Even if ferroelectric materials like BaTiO$_3$ or KNbO$_3$ have been used for decades in a broad range of technological applications, there is still significant debate in the literature concerning their microscopic behavior. For instance, many perovskite materials display a high-temperature cubic phase with zero net polarization, but its microscopic nature is though still unclear, with some materials displaying a very complex energy landscape with multiple local minima. In order to investigate and clarify the microscopic nature of oxide perovskites, we perform a study on a set of about 50 representative ABO$_3$ systems. We use spacegroup techniques to systematically analyze all possible local displacement patterns that are compatible with a net paraelectric phase, but can provide local non-zero ferroelectric moments. The energetics and the stability of these patterns is then assessed by combining the spacegroup analysis with DFT calculations. All calculations are managed and analyzed using our high-throughput platform AiiDA (www.aiida.net) [1]. Using this technique, we are able to describe the different classes of microscopic models underlying the perovskite systems. [1] G. Pizzi et al., Comp. Mat. Sci 111, 218-230 (2016).   

Temperature-dependent phase transition of ferroelectric perovskites: A Wang-Landau-DFT approach

Since the discovery of ferroelectricity in perovskite oxides, considerable efforts have been devoted to understanding their phase transition behaviors in terms of temperature, pressure, and composition. Such materials have regularly been used in transducer and actuator applications. As our first step to make accurate predictions of the crystal phases of more complex oxides such as Pb(Zr$_{x}$Ti$_{1-x})$O$_{3}$, we have used the Wang-Landau (WL) algorithm and density functional theory (DFT) to examine the temperature-dependent phase transition of PbTiO$_{3}$, BaTiO$_{3}$, and KNbO$_{3}$. DFT was employed to evaluate the energetics of important crystal-structure candidates, which were later used as the input for WL algorithm. In addition, we examine how the choice of exchange-correlation functionals affects our predictions of the relevant phase transition temperatures.   

First Principle Studies of Electromechanical Properties in Mn-Doped BaTiO$_3$

We are performing density functional calculations to elucidate the electromechanical properties for Mn-doped BaTiO$_3$ with an oxygen vacancy, applying electric field perpendicular to polarization directions. We find that local dipole switching and lattice changes take place at 3MV/m. Spontaneous polarization along x is 0.251 C/m$^2$ and $c/a$ ratio changes from 1.021 to 1.000. This indicates that coercive field of Mn-doped BaTiO$_3$ increases as were experimentally reported since our results for pure BaTiO$_3$ exhibit onset of the switching and changes at 0.8 MV/m. We report our computational dielectric constant and strain as a function of electric field for Mn-doped BaTiO$_3$.   

Towards an understanding of antiferroelectricity in PbZrO$_3$ from first principles

For decades, PbZrO$_3$ has been referred to as the prototypical antiferroelectric. According to a recent analysis, an essential requirement for antiferroelectricity is that there is a polar phase almost degenerate with the nonpolar ground state. Indeed, as previously reported, first-principles calculations show that the polar $R3c$ structure of PbZrO$_3$ is only 1 meV per formula unit higher in energy than the nonpolar ground state $Pbam$ structure. Here, we explore the question of how these two structures, which seem to be only distantly related, can be so close in energy. Using first-principles methods we investigate the energy landscape of PbZrO$_3$. We introduce a simple structural model that both describes the relevant, low-energy, structural motifs and captures the gross energy landscape relating to both structures. We use this model (and test with direct first-principles calculations) to explore a possible switching path between the non-polar ground state and the metastable polar structure. Our results provide insight into why PbZrO$_3$ is antiferroelectric, which may prove useful in identifying new antiferroelectric materials.