Session R37a: Dielectric and Ferroelectric Oxides VI

Tuning optical responses with strain in multiferroelectrics and ferroelectrics

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO$_3$ or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology, a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we investigate, both from experiments and first-principle techniques, if and how epitaxial strain engineering can tune the optical response of BiFeO$_3$ and PbTiO$_3$ thin films [Nat. Commun. 7, 10718(2016); Phys. Rev. Lett. 115, 267602 (2015)]. We find a very large variation of the optical index with strain in BiFeO$_3$, corresponding to an effective elasto-optic coefficient larger than that of quartz. We also observe there a concomitant strain-driven variation in light absorption, which is reminiscent of piezochromism and which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. We also predict large elasto-optic coefficients in two strain-driven monoclinic phases (of Pm and Cm symmetries) in PbTiO$_3$ films being under small tensile epitaxial strain. The origin of such large conversion between elastic and optical properties is further elucidated. Our findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses.   

Anisotropic symmetric exchange as a new mechanism formultiferroicity.

Discovering new magnetoelectric multiferroics is an exciting research area[1][2][3][4]. Very recently, a collinearantiferromagnetic spin order was found to induce a ferroelectric polarization in a highly symmetric cubicperovskite LaMn3Cr4O12 [5] . This spin-driven ferroelectricity could not be explained by any of the existingmultiferroic models[6][7][8][9][10][11]. Here, we put forward a new model, i.e., anisotropic symmetric exchange, to understand this phenomenon, which was confirmed by density functional calculations and tight-binding simulations. Furthermore,our perturbation analysis shows that the anisotropic symmetric exchange term can be even stronger than theconventional contributions in some 5d systems. Our multiferroic model can not only explain the experimental results, but also may open a new avenue for exploring exotic magnetoelectric coupling effects.   

Small band gap insulating state induced by Ionic liquid gating in Sr$_{2}$IrO$_{4}$ single crystals

The 5d iridates have attracted much interest due to the prediction of novel electronic phases driven by the interplay of spin-orbit coupling with the on-site Coulomb interaction. The compound Sr$_{2}$IrO$_{4}$, with a crystal structure similar to that of La$_{2}$CuO$_{4}$, was identified as a spin-orbital Mott insulator. It has been doped in various ways in search of a possible superconducting state, considering its similarities to the cuprates. We successfully fabricated multiple ionic liquid (DEME-TFSI) gated field effect transistors based on the cleaved ab plane surface of Sr$_{2}$IrO$_{4}$ single crystals. Due to the insulating behavior of the bulk, the surface sensitive gating effect can be characterized using transport property measurements. Upon hole doping, the in-plane electrical resistivity was observed to follow an activation law, with the band gap decreasing from 130 meV to 0.5 meV. A slight negative magneto-resistance below 5K was identified where the resistivity deviated from activated behavior. The low temperature insulating state will be studied further, and by improving the design of the device, the carrier concentration measured and the latest experimental results will be reported.   

Complex magnetic incommensurability and electronic charge transfer through the ferroelectric transition in multiferroic Co$_{\mathrm{3}}$TeO$_{\mathrm{6}}$

Polarized and unpolarized neutron diffraction has been carried out to investigate the nature of the magnetic structures and transitions in multiferroic Co$_{\mathrm{3}}$TeO$_{\mathrm{6}}$. Below T$_{\mathrm{M1}}=$26 K long range order develops which is fully incommensurate in all three crystallographic directions. Below T$_{\mathrm{M2}}=$19.5 K commensurate magnetic peaks develop in the $\Gamma_{\mathrm{4}}$ irreducible representation, along with a splitting of the ICM peaks along the $h$ direction. Below T$_{\mathrm{M3}}=$18 K this additional magnetic incommensurability disappears, ferroelectricity develops, a commensurate $\Gamma_{\mathrm{3}}$ irreducible representation appears, and the $k$ component of the ICM wave vector disappears. Synchrotron x-ray data demonstrate a significant shift of the electronic charge distribution from the Te ions, which together with the unusually small electric polarization and its strong magnetic field dependence suggest this material is an antiferroelectric. Below T$_{\mathrm{M4}}=$15 K the $k$ component of the ICM structure reappears, along with second-order ICM Bragg peaks, which polarized neutron data demonstrate are magnetic in origin with a small net magnetization. See also PRB \textbf{85}, 094431 (2012); PRB \textbf{88}, 184427 (2013).   

Electromagnon Resonance and Optical Magnetoelectric Effect in a Polar Magnet (Zn, Fe)$_{\mathrm{2}}$Mo$_{\mathrm{3}}$O$_{\mathrm{8}}$

Magnetic excitations are investigated for a hexagonal polar magnet (Zn, Fe)$_{\mathrm{2}}$Mo$_{\mathrm{3}}$O$_{\mathrm{8}}$ by terahertz spectroscopy. We observe magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spin parallel to the $c$ axis as well as the ferrimagnetic phase induced by Zn-doping. In the ferrimagnetic phase, we observe nonreciprocal optical effect, gyrotropic birefringence (GB), which is the nonreciprocal rotation of the optical fast/slow axes due to the diagonal magnetoelectric susceptibilities [1, 2]. An electric/magnetic-field active excitation resonantly enhances the GB effect, demonstrating an optical magnetoelectric functionality of electromagnon in multiferroics. [1] W. F. Brown, Jr., R. Hornreich, and S. Shtrikman, Phys. Rev. 168, 574 (1968). [2] E. B. Graham, and R. E. Raab, Phil. Mag. B 66, 269 (1992).   

Structural and ferroelectric phase evolution in [KNbO$_3$]$_{1-x}$[BaNi$_{1/2}$Nb$_{1/2}$O$_{3-\delta}$]$_x$ ($x$ = 0, 0.1)

The phase transition evolution for [KNbO$_3$]$_{1-x}$[BaNi$_{1/2}$Nb$_{1/2}$O$_{3-\delta}$]$_x$ ($x$=0, 0.1) is determined via complementary dielectric constant and Raman scattering measurements. Raman scattering by optical phonons over the range of 100-1000 cm$^{-1}$ for -190$^\circ$C $< T <$ 600$^\circ$C reveals six discernible zone-center optical phonon modes. They are assigned to structural and ferroelectric phases in the solid solution $x$ = 0.1 and compared with those for end member $x$ = 0 and with the results of temperature-dependent dielectric permittivity. Rigorous peak fitting analyses of spectra collected from the solid solution and end member indicate structural and ferroelectric phase transition temperatures that are quite close to those for the KNbO$_3$ end member. Remarkably, despite the inclusion of 5 atomic \% of ferroelectrically inactive Ni cations, the structural transition temperatures remain essentially unchanged. This is confirmed using density functional theory calculations to predict the polarization for $x$ = 0 and 0.1 establishing that the ferroelectric-paraelectric $T_c$ are comparable.   

Raman Scattering Studies of Magnons and Magnetodielectric Effects in CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$

The multiferroic spinel CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$ exhibits magnetodielectric behavior whose origin is a subject of controversy. We present a Raman spectroscopic study of the magnon spectrum of CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$ as functions of temperature, pressure, and magnetic field, with the aim of elucidating the microscopic origin of magnetodielectric behavior. Below T$_{\mathrm{C}}=$ 94 K, we observe a 16 cm$^{\mathrm{-1}}$ magnon mode with an anomalously large Raman intensity that reflects large magneto-optical coupling in CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$. The strong magnetic-field-induced suppression of the magnon Raman intensity in CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$ suggests that the magnetodielectric behavior in CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$ arises from the field-dependent suppression of magnetic fluctuations that are strongly coupled to long-wavelength phonons. Applied pressure suppresses the Raman susceptibility and field-dependence of the magnon, demonstrating that strain can be used to sensitively tune the magnetodielectric and magneto-optical properties of CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$.   

Controlling Phase of Brownmillerite -- Perovskite SrCoO$_{\mathrm{3-\delta }}$ through Ionic Liquid Interfacing

Ionic control in oxide solids is of particular interest due to the phenomenological properties that emerge and can be manipulated specifically through the stoichiometric control of oxygen. Control over oxygen stoichiometry in strontium cobaltite has been shown to allow a reversible crystal transition from a perovskite structure with the stoichiometry of SrCoO$_{\mathrm{3}}$ to a brownmillerite structure with the stoichiometry of SrCoO$_{\mathrm{2.5}}$. This crystal transition is accompanied by transitions in the magnetic and resistive properties of the material. While these crystal transitions were previously observed at high temperatures and modulating oxygen partial pressures, we present a new method of inducing oxygen ion migration that is capable of reversible control of crystal phase at room temperature. Here, an ionic liquid is interfaced with the cobaltite surface. Applying biases of \textless $+$/- 5V to the ionic liquid gate induces the formation of an electric double layer at the interface which imparts local surface charges sufficient to draw out and reinsert oxygen ions into the underlying oxide film depending on bias direction. Crystal transitions under varying bias conditions are monitored with in-situ 4-circle x-ray diffraction and show reversible transition between the brownmillerite and perovskite structures.   

Temperature Dependent Local Atomic Structure of LuFe$_{2}$O$_{4}$

The LuFe2O4 system has be studied intensively as a novel material with charge ordered driven ferroelectricity. However, the existence and origin of electric polarization and it coupling to the magnetic structure are open questions still to be addressed. Distinctly differing experiments yield different results. In this work, structural measurements on multiple length scales have been conducted over a broad range of temperatures. We have studied the correlation between the structural distortion and the electronic/magnetic properties in single-crystalline LuFe$_{2}$O$_{4}$ by using X-ray diffraction (XRD), temperature and orientation dependent Raman spectroscopy, temperature dependent X-ray pair distribution function (PDF) measurements and DFT modeling. The nature of the observed local atomic and electronic structural changes will be discussed and compared with previous work. This work is supported by DOE Grant DE-FG02-07ER46402.   

Spin-lattice coupling and thermal transport in multiferroic CuCrO$_{2}$

The co-existence and interplay of ferroelectricity and magnetism in multiferroic materials is of both fundamental and technological importance. We report extensive neutron and x-ray scattering measurements of lattice dynamics in CuCrO$_{2\, }$(7 $\le $T$\le $ 530 K) across the concomitant magnetic and ferroelectric transition (T$_{N}$ \textasciitilde 24 K). These experiments are complemented by first-principles simulations of the phonons. Our phonon dispersions and density of states measurements reveal very anisotropic vibrations of Cu atoms that are also supported by additional atomic displacement parameters from neutron diffraction. Little overall change in phonon frequencies is observed across T$_{N}$. We find that spin fluctuations persist up to $\sim $300 K, far above T$_{N}$. Furthermore, modeling of the thermal conductivity indicates that these spin fluctuations above T$_{N}$ constitute a strong source of phonon scattering, significantly suppressing thermal transport. We compare our results on CuCrO$_{2}$ with another geometrically frustrated multiferroic, YMnO$_{3}$.   

Electric field induced octahedral rotation and its controllability

So far, octahedral rotation (OR) in perovskite materials have been believed to have strong importance because of their intimate connection with the material's physical properties. For instance, crystal symmetry change driven by OR sometimes cause metal-insulator transition or exotic orbital selective phenomena. Moreover, it is widely known that OR angle usually plays a significant role to determine each material's magnetic ground state through super-exchange and Dzyloshinskii-Moriya interaction. In this sense, controlling OR angle is one of the most promising techniques to design functional materials. Here, we report a clear evidence that OR can be tuned with the variation of electric field. With the help of competition between surface electric field and screening effects coming from adsorbed K atoms, we observed electric field strength dependent OR angle variation by measuring electronic structure of Sr2RuO4. Our finding reveals that electric field is one of the prominent parameters to determine OR angle even in the non-piezoelectric Sr2RuO4.   

Simultaneous occurrence of multiferroism and short-range magnetic order in DyFeO$_{\mathrm{3}}$

We report a combined neutron scattering and magnetization study on the multiferroic DyFeO$_{\mathrm{3}}$, which shows a very strong magnetoelectric effect. Applying magnetic field along the c axis, the weak ferromagnetic order of the Fe ions is quickly recovered from a spin reorientation transition, and the long-range antiferromagnetic order of Dy becomes a short-range one. We found that the short-range order concurs with the multiferroic phase and is responsible for its sizable hysteresis. Our$ H-T $phase diagram suggests that the strong magnetoelectric effect in DyFeO$_{\mathrm{3}}$ has to be understood with not only the weak ferromagnetism of Fe but also the short-range antiferromagnetic order of Dy.   

Aberration Corrected STEM imaging of ferroelectric domain walls in $\mathbf{Ca_3Ru_{2(1-x)}Ti_xO_7}$

$\mathrm{Ca_3Ru_2O_7}$ is a layered Ruddlesden-Popper oxide which is a strongly correlated metal at room temperature and undergoes a MIT at 48K. Ti doped $\mathrm{Ca_3Ru_2O_7}$ is a Mott metal at 300K with the Mott transition temperature increasing with Ti concentration (90K at 5\% Ti), but without increase in strain due to the similarity in the size of $\mathrm{Ti^{4+}}$ and $\mathrm{Ru^{4+}}$ cations. The bulk crystals show the presence of domain walls as observed by polarized light microscopy. Aberration-corrected STEM imaging demonstrates the presence of both $\mathrm{90^{\circ}}$ and $\mathrm{180^{\circ}}$ domain walls along with domain junctions. EELS was performed at 300K and 77K to measure the Ru $\mathit{t_{2g}}$ $\longrightarrow$ O $\mathit{2p}$ hybridization in metallic and insulating ground states. The ferroelectric distortions inside the domains were measured to be isostructural to distortions in hybrid improper ferroelectric $\mathrm{Ca_3Ti_2O_7}$. Additionally,as confirmed by STEM imaging the $\mathrm{180^{\circ}}$ domain walls exist in head-to-head, head-to-tail and tail-to-tail configurations, thus leading to the intriguing possibility of competition between local ferroic dipole moments and a global shielding from the metallic ground state at room temperature.