Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q6: Focus Session: Emergent Properties in Bulk Complex Oxides: Fe-Oxide and Hexaferrite |
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Sponsoring Units: GMAG DMP Chair: M. Khan, Miami University, Ohio Room: 108 |
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q6.00001: Theory of magnetoelectric effects in field-induced canted affetiferromagnetic state in BiFeO$_{3}$ Nobuo Furukawa, Shin Miyahara We investigate static and dynamical magnetoelectric feffects in the field-induced antiferromagnetic state of the multiferroic compound BiFeO$_{3}$ with a distorted perovskite structure, based on calculations for a Heisenberg model with Dzyaloshinsky-Moriya interactions under external magnetic fields. Due to the distorted crystal structure, spins couple to electric polarizations via the spin-dependent metal-ligand hybridization mechanism. Due to the coupling, magnitude and direction of electric polarization depends on the external magnetic fields. Moreover, there exists a magnon which is both magneo- and electro-active due to magnetoelectric couplings, i.e., so-called a toroidalmagnon. As a result, the resonance of the toroidalmagnon shows non-reciprocal directional dichroism, i.e., the absorption intensity depends on the sign of the light propagating directions. We propose experimental conditions to clarify the existence of the couplings. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q6.00002: Microscopic Model for the Spectroscopic Modes of Multiferroic BiFeO$_{3}$ in a Magnetic Field Randy Fishman The zone-center modes measured by THz spectroscopy [1] provide the most detailed information available about the very small microscopic interactions responsible for the cycloid in multiferroic BiFeO$_{3}$. While a Dzaloshinskii-Moriya (DM) interaction perpendicular to the electric polarization \textbf{P} produces the cyloidal period, a DM interaction along \textbf{P} produces the small tilt in the cycloid, which leads to the weak ferromagnetic moment of the canted phase above a critical field of about 18 T. A microscopic model that includes both DM interactions as well as easy-axis anisotropy along \textbf{P} quantitatively predicts the field dependence of the spectroscopic frequencies [2]. Comparison with the measured frequencies indicates that only one of the three cycloidal domains survives above about 6 T.\\[4pt] [1] U. Nagel, R.S. Fishman, T. Katuwal, H. Engelkamp, D. Talbayev, H.T. Yi, S.-W. Cheong, and T. Room, \textit{Phys. Rev. Lett.} \textbf{110}, 257201 (2013)\\[0pt] [2] R.S. Fishman, \textit{Phys. Rev. B} \textbf{87}, 224419 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q6.00003: THz spectroscopy of spin waves in multiferroic BiFeO$_3$ in high magnetic fields Urmas Nagel, T. Katuwal, T. R\~o\~om, H. Engelkamp, D. Talbayev, H.T. Yi, S.-W. Cheong, Randy S. Fishman We have studied the magnetic field dependence of far-infrared active magnetic modes in a single ferroelectric domain BiFeO$_3$ crystal at low temperature. The modes soften close to the critical field of 18.8\,T along the [001] (pseudocubic) axis, where the cycloidal structure changes to the homogeneous canted antiferromagnetic state and a new strong mode with linear field dependence appears that persists at least up to 31\,T. A microscopic model that includes two Dzyaloshinskii-Moriya interactions and easy-axis anisotropy describes closely both the zero-field spectroscopic modes as well as their splitting and evolution in a magnetic field. The good agreement of theory with experiment suggests that the proposed model provides the foundation for future technological applications of this multiferroic material. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q6.00004: Static and Dynamic Magnetoelectric Effects in Multiferroic Hexaferrites Invited Speaker: Sae Hwan Chun Multiferroics, wherein magnetism and ferroelectricity coexist, are of great interest for the prospect of new multifunctional devices by utilizing cross-coupling between the electric and magnetic properties. In most multiferroics currently known, however, the magnetoelectric (ME) coupling does not reach the level enough for the practical applications and the cross control of electric polarization by magnetic field or magnetization by electric field has been realized only at low temperature. Hence, for use in the ME devices, it is essential to increase both the ME sensitivity and the operating temperature. From investigation of multiferroic hexaferrites, we discover a chemical route to effectively tailor the critical magnetic field inducing electric polarization in (Ba,Sr)$_{2}$Zn$_{2}$Fe$_{12}$O$_{22}$ (Zn$_{2}$Y-type) by Al-substitution, yielding a giant magnetoelectric susceptibility [1]. In (Ba,Sr)$_{3}$Co$_{2}$Fe$_{24}$O$_{41}$ (Co$_{2}$Z-type) hexaferrite single crystals, we realize the control of magnetization by electric field at room temperature [2]. In addition to those static ME properties, a dynamic ME effect, electric-dipole-active magnon resonance in THz frequency range, is also found in the Co$_{2}$Z-type hexaferrite, exhibiting the spectral weight even at room temperature [3]. The unprecedented supreme static and dynamic ME phenomena in the hexaferrites may provide a pathway to overcome the challenge in application of multiferroics for the real devices. \\[4pt] [1] S. H. Chun et al., Phys. Rev. Lett. 104, 037204 (2010). \\[0pt] [2] S. H. Chun et al., Phys. Rev. Lett. 108, 177201 (2012).\\[0pt] [3] S. H. Chun et al., in preparation. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q6.00005: A study for magnetic properties enhencement of strontium hexaferrite by first principles calculations Vivek Dikshit, Chandani Nandadasa, Seong-Gon Kim Owing to high magnetic anisotropy and saturation magnetization Strontium Hexaferrite is one of the most commonly used materials for hard magnets. In order to further improve the magnetic properties of the material we investigated the substitution different elements at Fe atom sites. Our calculation (using quantum mechanical DFT package, VASP) shows that both the properties: magnetization as well as magnetic anisotropy energy can be improved by a proper substitution. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q6.00006: Novel multi-Q spiral spin texture in SrFeO$_{3}$ S. Ishiwata, J. -H. Kim, D. S. Inosov, Y. Tokunaga, S. Seki, N. Kanazawa, R. Georgii, K. Seemann, G. Brandl, J. White, N. Egetenmeyer, J. Gavilano, Y.W. Long, Y. Kaneko, Y. Taguchi, T. Arima, B. Keimer, Y. Tokura A magnetic skyrmion discovered recently in chiral and cubic helimagnet such as MnSi [1] is of great interest for novel spintronic functions. SrFeO$_{3}$ has been known as a rare cubic perovskite showing both helimagnetic transition and metallic conduction. While the magnetic ground state has been believed to have a simple proper-screw-type helimagnetic order below, we found that SrFeO$_{3}$ hosts a rich variety of helimagnetic phases potentially containing novel skyrmion phases [2]. In the low magnetic field phases, a large topological Hall effect suggesting the formation of skyrmion lattice was observed. In this talk, based on the polarized and unpolarized neutron scattering studies, we will discuss the possible formation of novel type of three-dimensional skyrmion crystals, which can be characterized by quadruple-Q vectors along \textless 111\textgreater equivalents in a cubic lattice.\\[4pt] [1] S. M\"{u}hlbauer et al., Science 323, 915 (2009).\\[0pt] [2] S. Ishiwata et al., Phys. Rev. B 84, 054427 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q6.00007: Spin-Charge Coupling and Charge Order Phases in LuFe$_{2}$O$_{4}$ Chang-Jong Kang, B.I. Min Possible charge order phases in mixed-valent multiferroic LuFe$_{2}$O$_{4}$ are studied based on the first principles density functional theory. We have considered two different charge order phases of LuFe$_{2}$O$_{4}$ suggested by Angst et al. [1] and de Groot et al. [2], and investigated their electronic and magnetic properties systematically to determine the correct charge order phase that is consistent with the experiment. The systematic comparison of physical properties between two charge order phases will be discussed, and the corresponding spin-charge coupling effect will be examined. We have found that the spin-charge coupling effect is an essential ingredient in LuFe$_{2}$O$_{4}$.\\[4pt] [1] M. Angst et al., Phys. Rev. Lett. 101, 227601 (2008).\\[0pt] [2] de Groot et al., Phys. Rev. Lett. 108, 187601 (2012). [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q6.00008: Optical properties of ferrites through their magnetic ordering and spin-reorientation temperatures Brian Holinsworth, Charles Brooks, Julia Mundy, Judy Cherian, Stephen McGill, Darrell Schlom, Janice Musfeldt Iron oxides have attracted a great deal of attention due to their high magnetic ordering temperatures and semiconducting band gaps, the combination of which is very attractive for applications. In this work, we investigate band gap behavior in charge-ordered LuFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ and its cousin LuFeO$_{\mathrm{3}}$, showing that the latter is robust with temperature through the 130 K spin-reorientation and 440 K Neel transitions, and compare our findings with electronic structure calculations. Time permitting, we will discuss the spin polarized nature of the bands that form the leading edges of the gaps. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q6.00009: Electric field induced Verwey transition in single magnetite nanoparticles Qian Yu, Alireza Mottaghizadeh, Hongyue Wang, Christian Ulysse, Valentina Rebuttini, Nicola Pinna, Alexandre Zimmers, Herve Aubin In 1939, E.J.W. Verwey discovered that in magnetite (Fe3O4) electrons localize below a temperature T $\sim$ 120 K. He suggested that charge transport is due to electron exchange between ferric (Fe3$+)$ and ferrous (Fe2$+$) sites and the metal to insulator transition is due to the ordering of Fe cations into alternating layers of Fe3$+$ and Fe2$+$ ions. Using a method recently developed to fabricate single nanoparticle circuits, we trapped single nanoparticles of magnetite between nanometer-spaced electrodes that we used to study the electronic spectrum of the nanoparticles as function of temperature across the Verwey transition. In this tunnelling spectrum, we find the signature of polarons states. As function of temperature, one can observe that the density of states decreases to zero as the temperature approach the Verwey transition. Below the Verwey temperature, a clear gap is observed in the tunnelling spectrum. Above the Verwey temperature, no gap is observed. The absence of this gap indicates that electronic transport in the normal state of magnetite is due to polaron hopping, in contrast to the alternative scenario of activated band-like electronic transport. This work was supported by the French ANR grants 10-BLAN-0409-01 and 09-BLAN-0388-01. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q6.00010: Electronic structure and magnetic ground state properties of SrCoO$_{2.5}$ Chandrima Mitra, Randy S. Fishman, Satoshi Okamoto, Ho Nyung Lee, Fernando A. Reboredo ABO$_{3-\delta}$ type perovskite oxides are potential candidates for solid oxide fuel cells. The ones that crystallize in the orthorhombic brownmillerite-phase (ABO$_{2.5})$, such as SrCoO$_{2.5}$, are particularly interesting due to their crystal structure which contains ordered channels of oxygen vacancies. In this work we investigate theoretically the ground state electronic structure and magnetic properties of the brownmillerite phase of SrCoO$_{2.5}$. Strong correlations of the Co d electrons are treated within the local spin density approximations of Density Functional theory (DFT) with Hubbard U corrections (LSDA$+$U). The results are compared with the Heyd Scuzeria Ernzerhof (HSE) functional. The parameters computed with a U value of 7.5 eV are found to match closely to those computed within the HSE functional. Consistent with experimental observation a G-type antiferromagnetic structure is found to be the most stable one. From a Heisenberg Hamiltonian we compute the magnetic exchange interaction parameters, J, between the Co atoms which are then used to compute the spin-wave frequencies and inelastic neutron scattering intensities. The system has four spin-wave branches. The lowest energy mode was found to have the largest scattering intensity at the magnetic zone center. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q6.00011: Magnetically inhomogeneous ground-state below the first order valence transition in (Pr$_{1-y}$Y$_y$)$_{0.7}$Ca$_{0.3}$CoO$_{3-\delta}$ Daniel Phelan, Kanwal Preet Bhatti, Matthew Taylor, Shun Wang, Christopher Leighton Certain Pr-based cobaltites undergo an intriguing first-order metal-insulator transition on cooling, thought to be a manifestation of an abrupt valence transition (VT) where electron occupancy shifts from Pr to Co ions and thus depletes the hole concentration and inhibits ferromagnetic (FM) ordering. We discuss the inhomogenous nature of the magnetic ground-state (g.s.) of one such series of compounds, (Pr$_{1-y}$Y$_y$)$_{0.7}$Ca$_{0.3}$CoO$_{3-\delta}$. Although $y$=0 has a metallic, FM g.s., magnetization and transport studies indicate that the VT is stabilized upon Y substitution. Small angle neutron scattering measurements evidence short-range FM clusters of mean size $\sim$ 45 $\AA$, co-existing with low-density regions of long-range FM order in the g.s of $y$=0.075, which undergoes the VT at $\sim$ 80 K. These clusters are observed in magnetotransport, where a negative magnetoresistance and strong field-cooling effects occur as a result of inter-cluster tunneling. The results can be interpreted in the context of the system being driven into a phase-separated regime by the VT; however, the g.s. is significantly more inhomogeneous than in simpler cobaltites (e.g. La$_{1-x}$Sr$_x$CoO$_3$), likely due to inhomogeneities in the distributions of A-site ions and O vacancies. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q6.00012: Local Spin State Measurements in Critically Doped $La_{0.83} Sr_{0.17} CO_{3} $ Ahmet Gulec, Robert F. Klie Strontium doped $LaCO_{3} $ has fascinating magnetic phases which are believed to be directly related to Co spin states. Critically doped $La_{0.83} Sr_{0.17} CO_{3} $ undergoes a simultaneous Insulator to Metal Transition (IMT) and ferromagnetic (FM) order transition. In this work, we will utilized atomic-resolution Z-contrast imaging, annular bright field (ABF) imaging and electron energy-loss spectroscopy in the aberration-corrected JEOL JEM-ARM200CF in combination with cooling experiments to examine the local magnetic and spin-state transitions in critically doped $La_{0.83} Sr_{0.17} CO_{3} $ between 80 K and 300 K. Our energy-loss magnetic circular dichroism (EMCD) experiments confirm the non-localized increase in the dichromatic signal at low temperature, associated with a change in the co-ion spin state. On the other hand, by using he ABF imaging, a distortion of the $CO_{6} $ octahedral and the changes in the Co-O bond lengths within the same unit cell are observed. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q6.00013: Ferromagnetism and unconventional impurity effects in Rh- and Ga- doped LaCoO$_{3}$ Shinichiro Asai, Ryuji Okazaki, Ichiro Terasaki, Yukio Yasui The perovskite oxide LaCoO$_{3}$ has been long investigated because of a dramatic change of its spin state for Co$^{3+}$ ions with temperature variation. The Co$^{3+}$ ions (3$d^{6}$) in LaCoO$_{3}$ takes the non-magnetic low-spin state of \textit{t}$_{2g}^{6}$ at low temperature. The spin state of the Co$^{3+}$ ions in this system is sensitive to the chemical substitutions; we have found a weak ferromagnetism in a solid solution of LaCoO$_{3}$ and LaRhO$_{3}$ [S. Asai \textit{et al}., JPSJ 80, 104705 (2011).]. Since the two oxides are non-magnetic at low temperature, our finding is an example of ``order by disorder,'' where a non-magnetic impurity makes a non-magnetic oxide ferromagnetic. We have further investigated the magnetization and x-ray diffraction of LaCo$_{0.8-y}$Rh$_{0.2}$M$_{y}$O$_{3}$ (M = Rh, Ga) [S. Asai \textit{et al}., PRB 86, 014421 (2012).]. The magnetization decreases by the Ga$^{3+}$ substitution much more drastically than by the Rh$^{3+}$ substitution. It indicates that at least two kinds of Co$^{3+}$ ions exist in LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$; one is nonmagnetic, and the other is magnetic. In this talk, we will also discuss the change of the lattice volume with the Rh$^{3+}$ and Ga$^{3+}$ substitution [S. Asai \textit{et al}., JPSJ 82, 114606 (2013).]. [Preview Abstract] |
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