Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N13: Focus Session: Multiferroics IV |
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Sponsoring Units: DMP GMAG Chair: Claude Ederer, Columbia University Room: Colorado Convention Center Korbel 4C |
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N13.00001: Multiferroic Domain Dynamics and Phase Transitions in Strained SrTiO$_3$ Films S. Denev, A. Vasuvaderao, A. Kumar, M. Biegalski, Y. Li, L.-Q. Chen, S. Trolier-McKinstry, D. Schlom, V. Gopalan SrTiO$_3$ is a material that is not normally ferroelectric or multiferroic at any temperature. However, epitaxial biaxial strain in thin film form can induce multiferroicity in strained SrTiO$_3$ (J.H.Haeni \textit{et al.},Nature \textbf{430},758 (2004)). We have demonstrated multiferroicity in strained SrTiO$_3$ films on scandate substrates, with the presence of two independent order parameters, a polar ferroelectric polarization vector, and an axial antiferrodistortive rotation vector. Using Optical Second Harmonic Generation (SHG), we have distinguished these axial and polar properties, tracked the ferroelectric and antiferrodistortive phase transitions as a function of temperature, and determined the point group symmetry of the ferroelectric and multiferroic phases. For the first time, we have shown direct imaging of ferroelectric domains and revealed the mechanism of coupled switching of ferroelectric-ferroelastic domains under electric fields using piezoelectric force microscopy combined with phase field simulations. (Phys. Rev. Lett., Accepted, in print (2006)). These studies have broader relevance to multiferroics with coupled polar and axial order parameters, such as ferroelectric antiferromagnets. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N13.00002: Broadband IR Spectroscopy of Multiferroic BiFeO$_{3}$ J. Seidel, C.L.S. Kantner, Y.-H. Chu, L. Yang, Z. Schlesinger, D. Viehland, J. Orenstein, R. Ramesh BiFeO$_{3}$ (BFO) is a multiferroic material in which both the ferroelectric and antiferromagnetic ordering is present at room temperature. In order to investigate the dynamics of the coupling between order parameters, optical spectroscopy measurements were made on both a single crystal and epitaxially grown thin film samples. Measurements were made from 3-30cm$^{-1}$ using time domain terahertz spectroscopy, and from 20-700 cm$^{-1}$ with FTIR reflectivity. We report on the spectral weight and damping of modes in BFO in the spectral region where antiferromagnetic resonance is typically observed in orthoferrites. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N13.00003: Electrodynamics of a multiferroic perovskite manganite in terahertz frequency range N. Kida, Y. Ikebe, R. Shimano, Y. Yamasaki, T. Arima, Y. Tokura There is a growing interest for the study of the magnetoelectric effect, as stimulated by the observation of a magnetic control of the ferroelectric polarization in perovskite manganites. Recently, a broad peak structure was observed in TbMnO$_3$ and GdMnO$_3$ in terahertz (THz) frequency range [1]. The spin-wave excitation driven by ac electric field, which is referred to as electromagnon, was proposed as an origin of this structure. However, detailed characteristics, especially, the role of the rare-earth ion and the relationship between the complex dielectric constant $\tilde\epsilon$ in THz and low ($\sim$kHz) frequency range were not clarified yet. Here we used the THz time-domain spectroscopy to directly extract $\tilde\epsilon(\omega)$ (1.2-- 4.5 meV) of a multiferroic perovskite manganite and discuss the origin of the ferroelectricity, as manifested by a gigantic response of the low-frequency $\tilde\epsilon(\omega)$ with temperature and magnetic fields. [1] A. Pimenov {\it et al.}, Nat. Phys. {\bf 2}, 97 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N13.00004: Magnetoelectric Emission in a Magnetic Ferroelectric Er-doped (Ba,Sr)TiO$_3$ Yoshi-Aki Shimada, Masakazu Matsubara, Yoshio Kaneko, Jing-Ping He, Yoshinori Tokura In the system where both space-inversion ($\mathcal{I}$) and time-reversal ($\mathcal{R}$) symmetries are broken simultaneously, a nonreciprocal optical phenomenon termed the optical magnetoelectric (OME) effect is expected to show up. As the crystal that has neither $\mathcal{I}$ nor $\mathcal{R}$ symmetry, we have investigated an Er$^{3+}$-doped ferroelectric (Ba,Sr)TiO$_3$ single crystal under magnetic field in which the luminescent magnetic Er$^{3+}$ ion occupies a noncentrosymmetric site. The ${\boldmath k}$-directional dichroism derived from the OME effect was verified in the $^{4}I_{13/2} {\rightarrow}^{4}I_{15/2}$ emission by the reversal of magnetic field and spontaneous polarization. The observed nonreciprocity $\Delta I/I \sim 0.5$ \% at 3000 Oe implies the possibility of the application of the OME effect to the function of an optical isolator. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N13.00005: Colossal magnon-phonon coupling in multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_3$ Rolando Valdes Aguilar, A.B. Sushkov, H.D. Drew, C. Zhang, S-W. Cheong The temperature dependence of the far infrared (IR) transmission spectra (1-30 meV) of multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_3$ has been measured. This system is chosen to correspond to TbMnO$_3$, but without the magnetism of the rare earth ion. We find a spectacular transfer of spectral weight from the lowest frequency IR active phonon to a magnetic excitation, at lower frequencies, when light is polarized parallel to the static polarization \textbf{P}$_a$. We also observe the electromagnon at a frequency of 2.5 meV, with the same selection rule. The electromagnon produces the observed increase in the dielectric constant as the system enters the ferroelectric phase. The observations of large spectral weight transfer and of the electromagnon selection rule, are not consistent with the model of the electrodynamic response of helical magnets proposed by Katsura, et al\footnote{Katsura, et al. CondMat (2006)}. We compare and contrast these results to other multiferroic manganites. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N13.00006: Giant magnetoelastic effect in multiferroic Ba$_{0.6}$Sr$_{1.4}$Zn$_{2}$Fe$_{12}$O$_{22}$ Diyar Talbayev, Richard D. Averitt, Antoinette J. Taylor, Tsuyoshi Kimura Dynamical studies of multiferroic materials help unravel the fundamental interactions between various degrees of freedom and answer technological questions such as achievable switching speeds in multiferroic-based memory elements. We report the results of the ultrafast optical study of multiferroic Ba$_{0.6}$Sr$_{1.4}$Zn$_{2}$Fe$_{12}$O$_{22}$, which reveals a giant magnetoelastic effect in the material. The compound exhibits a hexagonal crystal structure and a helical magnetic ground state below $\sim $ 330 K. In applied magnetic field, the hexaferrite undergoes a series of magnetic phase transitions and develops ferroelectric polarization. The magnetoelastic effect is detected via the measurement of the speed of sound in the crystal as a function of magnetic field. The oscillation in the optically induced transient reflectivity resulting from the propagating coherent-phonon strain pulse allows us to measure the field-induced changes in the speed of sound and the corresponding dramatic changes in the elastic stiffness. The dependence of the exchange interaction on the distance between Fe ions gives rise to the observed magnetoelasticity. Our results indicate a route towards the magnetically modulated transducers and piezoelectric devices. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N13.00007: Far-infrared transmission spectroscopy studies of HoMn$_{2}$O$_{5}$ single crystals at the commensurate-incommensurate phase transition A.A. Sirenko , S. Park , S. M. O' Malley , G. L. Carr , S-W. Cheong Spectra of the low-frequency IR-active excitations in HoMn$_{2}$O$_{5}$ multiferroic single crystals have been studied using synchrotron radiation based far-infrared transmission spectroscopy at U12IR beamline of NSLS-BNL in the frequency range between 8.5 and 105 cm$^{-1}$. Both preferable polarization of IR-active excitations along crystallographic directions of HoMn$_{2}$O$_{5}$ and temperature variation of their oscillator strength reveal strong changes at the commensurate-incommensurate phase transition at $T_{3 }$= 19 K. Transmission spectra are interpreted in terms of the electromagnon, magnon, and crystal-field splitting excitations. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N13.00008: Electromagnons in multiferroic RMn$_{2}$O$_{5}$ compounds Andrei Sushkov, Rolando Valdes Aguilar, Dennis Drew, Soonyong Park, Sang-Wook Cheong Electromagnons, or magnons with electric dipole activity, were observed so far only in some multiferroic RMnO$_{3}$ and RMn$_{2}$O$_{5}$ compounds. Electromagnons in these two systems have essentially different properties. We try to take a systematic look at electromagnons in the whole RMn$_{2}$O$_{5}$ family. In this talk, we report the results of detailed infrared study$^{\ast }$ of YMn$_{2}$O$_{5}$ and TbMn$_{2}$O$_{5 }$ as well as some results on other RMn$_{2}$O$_{5}$ compounds. We found that electromagnons in the RMn$_{2}$O$_{5}$ family is a property of the manganese sublattices. The electromagnon spectrum consists of a set of well defined far infrared (3-80 cm$^{-1})$ modes which are just slightly broader than uncoupled magnons. No obvious changes in the phonon spectrum have been observed. Polarization of electromagnons is in agreement with the dominating symmetric exchange. * A. B. Sushkov, R. Valdes Aguilar, S. Park, S-W. Cheong, and H. D. Drew, cond-mat/ 0608707. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N13.00009: Magnons and electromagnons in multiferroic materials Maxim Mostovoy The interest in studying excitations in frustrated magnets lies in their unusual nature and strong effect on frustrated ordering. The coupled spin-lattice dynamics in frustrated magnets, in which magnetic ordering breaks inversion symmetry and induces electric polarization, was recently studied in optical absorption and neutron scattering experiments. I will present a theory of magnetic excitations coupled to polar phonon modes (electromagnons) in multiferroic materials showing incommensurate magnetic orders, e.g. RMnO$_{3}$, Ni$_{3}$V$_{2}$O$_{8}$, and MnWO$_{4}$, and discuss the evolution of the excitation spectrum at the transition from the paraelectric sinusoidal to the ferroelectric spiral state. The incommensurate orders give rise to a multi-band structure of magnetic excitations, while the magnetoelectric coupling makes possible to excite magnons by oscillating electric field. Even for weak coupling the probability of electro-excitation of magnons is relatively large. Furthermore, the polarization dependence of the optical absorption makes possible to discriminate between the electromagnon and antiferromagnetic resonance peaks. I will also discuss electromagnons in a different class of multiferroics, such as the RMn$_{2}$O$_{5}$ compounds. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N13.00010: Theory of electrical control of spin waves in multiferroic materials Rogerio de Sousa, Joel Moore We consider the question of electrical generation, control, and detection of magnons in thin films of multiferroic BiFeO$_{3}$. This material possesses simultaneous ferroelectric and antiferromagnetic order, with Dzyaloshinskii-Moriya and other magnetoelectric couplings. The spectrum for the coupled spin and polarization waves is found to be extremely anisotropic, allowing the control of spin waves via electrical switching of the direction of the spontaneous polarization vector. Electrical generation and detection of spin waves is optimal at the anticrossings of the polarization and magnetic branches, where the excitations have electromagnon character. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N13.00011: Temperature-dependent Raman spectroscopy of multiferroic TbMn$_2$O$_5$ J. R. Simpson, A. R. Hight Walker, R. Valdes Aguilar, A. B. Sushkov, H. D. Drew, S. Park, S.-W. Cheong Multiferroic materials that display coupling between order parameters, \textit{e.g.}, magnetic and dielectric, stimulate fundamental interest and provide the potential for applications in novel multifunctional devices. The multiferroic manganite TbMn$_2$O$_5$ exhibits non-collinear magnetic order and a strong magnetoelectric coupling effect. The recent observation\footnote{R. Vald\'{e}s Aguilar \textit{et al.}, Phys. Rev. B \textbf{74}, 184404 (2006).} of infrared (IR) phonon modes correlated with magnetic and dielectric phase transitions suggests a complementary Raman study may provide important information regarding the nature of coupling in these systems. We present Raman measurements of single-crystal TbMn$_2$O$_5$ in a collinear backscattering configuration as a function of temperature ($4-300\,$K) and polarization along various crystallographic axes. Additionally, we compare the temperature dependence of Raman active phonons with the activation of an IR forbidden mode in the low-temperature ferroelectric state. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N13.00012: Magnetic properties and electronic structure of doped multiferroic Y$_{x}$A$_{1-x}$MnO$_{3}$(A=Ca, Sr, Ba) J. Y. Juang, C. C. Hsieh, T. Y. Cheng, J. M. Lee, J. M. Chen, J.-Y. Lin, K. H. Wu, T. M. Uen, Y. S. Gou We report the magnetic properties, X-ray absorption spectroscopy (XAS) on a series of doped multiferroic materials Y$_{x}$A$_{1-x}$MnO$_{3 }$(A=Ca, Sr, Ba). YMnO$_{3}$ when doped by alkaline-earth metal with various ionic sizes, display dramatic changes in magnetic properties as compared with the parent compound. For Ca-doped sample, the antiferromagnetic (AFM) phase transition appears to take place at a much lower temperature (30 K) as compared to that of undoped one (42 K), which could be Mn-rich. On the other hand, when doped with ions of larger size such as Ba and Sr, the AFM temperature decreased only slightly to around 38 K but with significantly smeared transition. By comparing the XAS results to standard manganese oxide powder, YMnO$_{3}$ exhibits the dominant Mn$^{+3}$ characteristics obtained from the standard Mn$_{2}$O$_{3}$ powder. Although, the undoped-YMnO$_{3}$ and Sr-, Ba-doped YMnO$_{3}$ exhibited very similar electronic structure as revealed in the XAS data, the XAS of Ca-doped sample, again, is very different from that of YMnO$_{3}$. It is surprising to observe that Ca-doping has resulted in most significant modifications in the magnetic property and electronic structure of YMnO$_{3}$, since Ca$^{+2}$ is having exactly the same ionic size as that of Y$^{+3}$ and is expected to cause minimal distortion on the lattice. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N13.00013: Multiferroic Behavior in Barium Hexaferrite Probed with Optical Second Harmonic Generation Eftihia Vlahos, Sava Denev, Venkatraman Gopalan, Tsuyoshi Kimura Barium hexaferrite $Ba_{0.5} Sr_{1.5} Zn_2 Fe_{12} O_{22} $ is a very promising material, which exhibits significant magnetoelectric (ME) effect, i.e., the generation of electric polarization/magnetization by the application of magnetic/electric) field. Optical second harmonic generation (SHG) in the reflection geometry was used to determine the magnetic point group symmetries and phase transitions of the sample versus temperature, and variable magnetic field. Simultaneous measurements of magnetocapacitance, and ME current as functions of temperature and applied magnetic field are performed and correlated with SHG measurements. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N13.00014: Observation of local non-centrosymmetry in weakly ferroelectric YCrO3 Anna Llobet, Kannadka Ramesha, Thomas Proffen, C.R. Serrao, C.N.R. Rao Using high resolution neutron powder diffraction we have determined the average and local structure of YCRO in order to explain the recently reported ferroelectric character YCRO. Unlike other ferroelectric systems, YCRO has been found to have a centrosymmetric crystal structure which is inconsistent with the development of electric polarization because it requires atomic off-centering. We have characterized the different length scales existent in YCRO and found that, although the average crystallographic structure above and below the ferroelectric transition is orthorhombic and centrosymmetric ($Pnma$), in the ferroelectric state YCRO is locally non-centrosymmetric and Cr is displaced about 0.01~{\AA} from its position along \textit{z} direction. We conclude that the local character of the Cr off-centering and the small value of the displacement observed could explain the weak ferroelectric behavior. This new concept of ``local non-centrosymmetry'' might be of great importance for the understanding of unusual properties of other multifunctional materials as well. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N13.00015: Computational study of the ferromagnetic and ferroelectric properties of (Bi$_{2}$Mn NiO$_{6})_{4}$. Leonard Kleinman, B.R. Sahu, Adrian Ciucivara Using the VASP PAW code with the GGA and including spin-orbit coupling and allowing for non collinear magnetization, we performed electronic structure calculations for the multiferroic crystal, (Bi$_{2}$Mn NiO$_{6})_{4}$. The lattice angle and lattice constants are in excellent agreement with experiment. The magnetization is 4.94 $\mu _{B}$. The polarization, for which there is no experimental value, is 16.84 $\mu $Ccm$^{-2}$. Inverting the positions of all the atoms we iterated to convergence. The magnetization did not change and the total energy was also unchanged. Thus we conclude that the polarization and magnetization are uncoupled and (Bi$_{2}$Mn NiO$_{6})_{4}$ is unlikely to have any device applications. [Preview Abstract] |
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