Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H43: Multiferroic Oxides IIFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Colin Heikes, NIST Room: 390 |
Tuesday, March 14, 2017 2:30PM - 2:42PM |
H43.00001: Multiferroic Polar Metals. Shiming Lei, Shalinee Chikara, Danilo Puggioni, Xianglin Ke, Z. Q. Mao, J. M. Rondinelli, Marcelo Jaime, John Singleton, Vivien Zapf, Venkatraman Gopalan Ca3Ru2O7 undergoes a second-order magnetic phase transition to AFM-a (ferromagnetic bilayers antiferromagnetically stack along c-axis with magnetic easy axis along a) at TN $=$ 56 K, followed by a concomitant first-order structural and magnetic phase transition to an AFM-b (antiferromagnetic with magnetic easy axis along b) at TS $=$ 48 K. For T\textless 30 K, a quasi-two-dimensional (2D) metallic state exists due to the survival of small non-nested Fermi pockets. With a proper magnetic field applied along b-axis, an additional phase of canted-AFM is induced. Here we propose a new strategy to tune the polar metal Ca3Ru2O7 into insulating state by chemical doping. In the meantime, the superexchange interaction is significantly weakened to allow the existence of a weak ferromagnetic state. Combined with its robust polar nature, we offer an experimental demonstration of a new multiferroic material. The mechanism is further discussed in the framework of hybrid improper ferroelectricity proposed by Benedek and Fennie. This new strategy proposed here may be utilized as a general approach for new multiferroics starting from a material on the verge of the Mott insulating. Here we will discuss our comprehensive magnetization and magnetostriction, and magnetic field dependent SHG study on this material [Preview Abstract] |
Tuesday, March 14, 2017 2:42PM - 2:54PM |
H43.00002: THz spectroscopy and directional dichroism in the high magnetic field AFM phase of bulk BiFeO$_3$ Toomas R{\~o}{\~o}m, J. Viirok, U. Nagel, D. Farkas, S. Bord\'acs, I. K\'ezsm\'arki, H. Engelkamp, K. Thirunavukkuarasu, J. Krzystek, R. S. Fishman, Y. Ozaki, Y. Tomioka, T. Ito Magnetic ordering can induce local electric polarization via three mechanisms in BiFeO$_3$: spin-current, exchange-striction, and single-ion anisotropy. Only the spin-current mechanism causes the observed large non-reciprocal directional dichroism (NDD) at THz frequencies in the cycloidal state [PRL, 115:127203 (2015)]. However, the static magneto-electric effect persists even in the canted AFM state [Nat. Comm., 6:5878, (2015)] where the cycloid is destroyed by high magnetic field. Single crystal samples were studied in fields up to 35\,T with $\mathbf{B}\parallel \mathbf{a}$, $\mathbf{B}\perp \mathbf{a}$ and $\mathbf{B}\parallel \mathbf{c}$. NDD was found in the canted AFM phase above 18\,T. The number of observed spin wave modes is not consistent with a current model of two AFM-coupled spin sublattices in the canted AFM phase. [Preview Abstract] |
Tuesday, March 14, 2017 2:54PM - 3:06PM |
H43.00003: Rotation and Metastability of BiFeO$_{3}$ Domains in a Magnetic Field Randy FIshman, Daniel Farkas, Sandor Bordacs, Istvan Kezmarki Earlier models for the room-temperature multiferroic BiFeO$_{3}$ assumed that the domain wavevectors \textbf{q} are restricted to the three hexagonal axis normal to the static polarization parallel to a cubic diagonal like [1,1,1]. However, increasing evidence suggests that the domain wavevectors rotate so that \textbf{q} lies perpendicular to the field orientation \textbf{m} for fields above about 9 T. We show that previously neglected hexagonal anisotropy restricts the wavevectors to lie along the hexagonal axis in zero field. For fields below B$_{c1\, }=$ 7 T, the domain with wavevector \textbf{q} along \textbf{m} remains metastable while the two wavevectors of the other domains rotate perpendicular to \textbf{m}. For B$_{c1}$ \textless B \textless B$_{c2}$, the domain with wavevector along \textbf{m} disappears as it is no longer even metastable. For fields above B$_{c2} \quad =$ 9 T, the stable domain has wavector \textbf{q} normal to \textbf{m}. These new results explain recent measurements of the critical field as a function of field orientation, small-angle neutron scattering measurements of the wavevectors, as well as spectroscopic measurements for field along [0,1,-1]. [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:18PM |
H43.00004: Successive coupled charge, magnetic, and structural transitions in Ca$_{0.5}$Bi$_{0.5}$FeO$_{3}$ Fabio Denis Romero, Yoshiteru Hosaka, Noriya Ichikawa, Takashi Saito, Graham McNally, J. Paul Attfield, Yuichi Shimakawa Stoichiometric Ca$_{0.5}$Bi$_{0.5}$FeO$_{3}$ containing high-valent Fe$^{3.5+}$ adopting the perovskite structure was prepared under a high oxygen pressure and shows two successive phase transitions on cooling at 240 K and 200 K. M{\"o}ssbauer spectroscopy and neutron powder diffraction data indicate that these transitions are associated with charge changes to relieve the instability of Fe$^{3.5+}$.The first transition is due to charge disproportionation of the iron centers while the second is due to intermetallic charge transfer between A-site Bi and B-site Fe. The transitions can be described as:\newline $(Ca^{2+}_{0.5}Bi^{3+}_{0.5})Fe^{3.5+}O_3 \rightarrow (Ca^{2+}_{0.5}Bi^{3+}_{0.5})(Fe^{(3.5-x)+}_{0.67}Fe^{(3.5+2x)+}_{0.33})O_3 \rightarrow \newline (Ca^{2+}_{0.5}Bi^{3+}_{0.25}Bi^{5+}_{0.25})Fe^{3+}O_3$ \newline In the intermediate temperature phase, one third of B-sites (Fe$^{(3.5+2x)+}$) do not contribute to the magnetic scattering while the remaining spins couple antiferromagnetically. The lowest temperature magnetic structure is simple G-type antiferromagnetic resulting from a structure containing only Fe$^{3+}$. Competing intermetallic and disproportionation charge instabilities result in a variety of electronic, magnetic, and structural ground states. [Preview Abstract] |
Tuesday, March 14, 2017 3:18PM - 3:30PM |
H43.00005: Tuning magnetic spirals beyond room temperature with chemical disorder Emmanuel Canevet, Mickael Morin, Adrien Raynaud, Marek Bartkowiak, Denis Sheptyakov, Voraksmy Ban, Michel Kenzelmann, Ekaterina Pomjakushina, Kazimierz Conder, Marisa Medarde In the past years, magnetism-driven ferroelectricity and gigantic magnetoelectric effects have been reported for a number of frustrated magnets with spiral magnetic orders. Such materials are of high current interest due to their potential for spintronics and low-power magnetoelectric devices. However, their low magnetic order temperatures (typically lower than 100K) greatly restrict their fields of application. In this talk we will show that chemical disorder is a powerful tool that can be used to stabilize magnetic spiral phases at higher temperatures. To illustrate this mechanism, we will present our recent results obtain by neutron diffraction on the perovskyte YBaFeCuCuO$_5$, where a controlled manipulation of the Cu/Fe chemical disorder was successfully used to increase the spiral order temperature from 154 to 310K. [Preview Abstract] |
Tuesday, March 14, 2017 3:30PM - 3:42PM |
H43.00006: Determination of the Magnetic Structure, Phase Transitions, and Properties of a New Family of Ferrites Timothy Ferreira, Stuart Calder, Jeongho Yeon, Hans-Conrad zur Loye The search for room temperature multiferroic materials has led to significant research on a family of ferrites that follow the stoichiometry BaO \textbullet $n$Fe$_{2}$O$_{3}$, with the M-type hexaferrite BaFe$_{12}$O$_{19}$ being the prototypical example. This motivated the synthesis of the first example of a diferrite, BaFe$_{4}$O$_{7}$, in addition to a potassium-doped analog, K$_{0.22}$Ba$_{0.89}$Fe$_{4}$O$_{7}$, as single crystals using the novel hydroflux technique. Both materials undergo a reversible phase change at high temperature mediated by oxygen loss leading to an increase in magnetization, which reverses upon exposure to open air. Neutron diffraction experiments aided by magnetometry have revealed the magnetic structure and ordering temperature (T$_{N} \quad =$ 850 K) of the pre-annealed single crystals. Presented here at the results from the neutron diffraction experiments, SQUID and electrical measurements, and synthetic considerations. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H43.00007: Ferromagnetic Resonance on Micro- and Nano-Hexaferrites at Extremely High Frequencies Konstantin Korolev, Shu Chen, Mohammed Afsar, Valencia Koomson, Vincent Harris Millimeter wave transmittance measurements have been successfully performed on micro- and nano-sized BaFe$_{\mathrm{12}}$O$_{\mathrm{19}}$ and SrFe$_{\mathrm{12}}$O$_{\mathrm{19}}$ hexaferrite powders. Broadband transmittance measurements have been performed using free space quasi-optical spectrometer, equipped with a set of high power backward wave oscillators covering the frequency range of 30 -- 120 GHz. Real and imaginary parts of dielectric permittivity for both types of micro- and nanoferrites have been calculated using analysis of recorded high precision transmittance spectra. Frequency dependences of magnetic permeability of ferrite powders, as well as saturation magnetization and anisotropy field have been determined based on Schl\"{o}emann's theory for partially magnetized ferrites. Micro- and nano-sized ferrite powders have been further investigated by DC magnetization to assess magnetic behavior and compare with millimeter wave data. Consistency of saturation magnetization determined independently by both millimeter wave absorption and DC magnetization have been found for all ferrite powders. These materials seem to be quite promising as tunable millimeter wave absorbers and filters, based on size-dependent absorption. [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H43.00008: Investigation on a giant magnetoelectric effect compound Ba$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$Mg$_{\mathrm{2}}$Fe$_{\mathrm{12}}$O$_{\mathrm{22\thinspace }}$via neutron scattering techniques Yan Wu, Huibo Cao, Kun Zhai, Young Sun, Fangwei Wang Y-type hexaferrite Ba$_{\mathrm{2}}$Mg$_{\mathrm{2}}$Fe$_{\mathrm{12}}$O$_{\mathrm{22}}^{\mathrm{\thinspace }}$has a giant magnetoelectric effect (ME). The magnetic structure consists of two groups of L- (large moment) and S- (small moment) blocks stacking along the $c$-axis. The moments align ferrimagnetically in the same block. At zero field, it displays a proper screw magnetic structure with an incommensurate wavevector \textbf{\textit{k}} along the $c$-axis below a ferrimagnetic-antiferromagnetic (FM-AFM) transition (195 K) and then transforms to a longitudinal conical phase below 50 K. Applying a small magnetic field, the material displays polarization in the conical phase. When doped with Sr, the material keeps its sensitivity to field while the ME coefficient is greatly enhanced. The transition temperatures are largely elevated and FM-AFM transition goes above room $T$. Meanwhile, temperature dependent neutron diffraction investigation shows new commensurate AFM peaks emerging at heavy Sr doped sample. However, size effect is not sufficient to explain the changes upon Sr doping. Comparison study of the crystal and magnetic structure of the doped samples is performed to identify the exquisite atomic position changes and the moment interaction picture differences with doping. [Preview Abstract] |
Tuesday, March 14, 2017 4:06PM - 4:18PM |
H43.00009: Unconventional magnetic domains in helical and ferrimagnetic phases of multiferroic Sr$_3$Co$_2$Fe$_{24}$O$_{41}$ Hiroshi Nakajima, Hiromasa Kawase, Atsuhiro Kotani, Tsuyoshi Kimura, Shigeo Mori Multiferroics, in which ferroelectricity and ferromagnetism coexist, is of great current interest in condensed matter physics. In a multiferroic hexaferrite, Sr$_3$Co$_2$Fe$_{24}$O$_{41}$, ferroelectricity is induced by applying a magnetic field at room temperature, which is due to a transverse helical spin structure. The temperature evolution of magnetic domain structures in the hexaferrite was investigated by means of in-situ Lorentz microscopy. We found that stripy domains ($\sim$ 20 nm in width) elongating perpendicular to the $c$ axis were formed in the transverse helical phase. On heating from the helical to ferrimagnetic phases, the stripy domains started to disappear. As a result, the 180° magnetic domains were formed although macroscopic magnetic domains of the helical phase remained intact. These results suggest that the stripy magnetic domains originate from the helical spin structure and have no long-ranged coherence along the $c$ axis. Furthermore, magnetic vortex structures were revealed to appear in a magnetic field of 100 mT above 480 K, which originates from the high magnetic anisotropy in the ferrimagnetic phase. Our observation suggests that the magnetic anisotropy in each phase plays an important role in the formation of the magnetic domain structures. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H43.00010: Topological defects at octahedral tilting plethora in bi-layered perovskites Sang-Wook Cheong, Fei-Ting Huang, Bin Cao, Jaewook Kim, Xuan Luo, Yazhong Wang, Ming-Won Chu, CK Chang, HS Shen Oxygen octahedral distortions including tilts/rotations, deformations and off-centering in (layered) perovskites play the key role in their numerous functional properties. Near the polar-centrosymmetric phase boundary in bi-layered perovskite Ca$_{\mathrm{\mathbf{3-x}}}$\textbf{Sr}$_{\mathrm{\mathbf{x}}}$\textbf{Ti}$_{\mathrm{\mathbf{2}}}$\textbf{O}$_{\mathrm{\mathbf{7}}}$\textbf{ with x}$\approx $\textbf{1, we found the presence of abundant topological eight-state vortex-antivortex pairs, associated with four oxygen octahedral tilts at domains and another four different oxygen octahedral tilts at domain walls. The inter-relationship between this unique structural domain topology and magnetic domains with canted magnetic moments in related compounds will be presented. In addition, we will discuss the general topological classification scheme of the connectivity of domains and domain walls.} [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H43.00011: Optical spectroscopy and band gap analysis of hybrid improper ferroelectric Ca3Ti2O7 Janice Musfeldt, Judy Cherian, Turan Birol, Nathan Harms, Bin Gao, Sang Cheong, David Vanderbilt We bring together optical absorption spectroscopy, photoconductivity, and first principles calculations to reveal the electronic structure of the room temperature ferroelectric Ca3Ti2O7. The 3.94 eV direct gap in Ca3Ti2O7 is charge transfer in nature and noticeably higher than that in CaTiO3 (3.4 eV), a finding that we attribute to dimensional confinement in the n = 2 member of the Ruddlesden-Popper series. While Sr substitution introduces disorder and broadens the gap edge slightly, oxygen deficiency reduces the gap to 3.7 eV and gives rise to a broad tail that persists to much lower energies. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H43.00012: Negative Thermal Expansion in the Homologous Series of Ruddlesden-Popper Calcium Titanate Nathan Koocher, Liang-Feng Huang, James Rondinelli Recently, it was predicted that the hybrid-improper ferroelectric compound Ca$_{3}$Ti$_{2}$O$_{7}$ ($n=2$) with the layered Ruddlesden-Popper (RP) structure ($A_{n+1}B_n$O$_{3n+1}$) exhibits a pressure-tunable negative thermal expansion (NTE) [Huang \emph{et al}., Phys.\ Rev.\ Lett.\ \textbf{117}, 115901 (2016)]. The NTE is proposed to arise from the quasi-two-dimensional nature of an acoustic mode and strong Ti--O bonding. Here, the effect of layer thickness \textit{n} on the NTE and thermodynamic properties of the homologous titanates is investigated using the self-consistent quasi-harmonic approximation (SC-QHA) method implemented within density functional theory. We show a significant dependence of the lattice dynamical modes with \emph{n}, and compare our results and associated microscopic underpinnings of the NTE responses to that proposed for Ca$_{3}$Ti$_{2}$O$_{7}$ ($n=2$). We conclude by describing a model for the anharmonic lattice properties in terms of layer dimensionality. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:06PM |
H43.00013: Irregular orthorhombic twins and non-switchable polarization in multiferroic Ca$_{\mathrm{3}}$Mn$_{\mathrm{2}}$O$_{\mathrm{7}}$ Bin Gao, Fei-Ting Huang, Yazhong Wang, Jaewook Kim, Lihai Wang, SeongJoon Lim, Sang-Wook Cheong Ca$_{\mathrm{3}}$Mn$_{\mathrm{2}}$O$_{\mathrm{7}}$ has been proposed as a prototypical magnet with hybrid improper ferroelectricity (HIF), and a significant magnetoelectric (ME) coupling effect in Ca$_{\mathrm{3}}$Mn$_{\mathrm{2}}$O$_{\mathrm{7}}$ has been reported theoretically and experimentally. However, unlike the case of Ca$_{\mathrm{3}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$, the switchable polarization has not been experimentally confirmed in Ca$_{\mathrm{3}}$Mn$_{\mathrm{2}}$O$_{\mathrm{7}}$. From the systematic investigation of single-crystalline Ca$_{\mathrm{3}}$(Mn,Ti)$_{\mathrm{2}}$O$_{\mathrm{7}}$, we found a unique irregular domain structure in Ca$_{\mathrm{3}}$Mn$_{\mathrm{2}}$O$_{\mathrm{7}}$, stemming from the random stacking of 90-degree $a$- and $b$-domains along the $c$-axis. We determined the structural transition temperatures, and also the phase diagram. We found that Ca$_{\mathrm{3}}$Mn$_{\mathrm{2}}$O$_{\mathrm{7}}$ and Ca$_{\mathrm{3}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$ go through different routes upon structural phase transition, which results in different domain stackings along the c-axis. This random stacking of twins in Ca$_{\mathrm{3}}$Mn$_{\mathrm{2}}$O$_{\mathrm{7}}$ appears to make polarization switching difficult. [Preview Abstract] |
Tuesday, March 14, 2017 5:06PM - 5:18PM |
H43.00014: Scanning Tunneling Microscopy Study on Delafossites PdCoO$_{\mathrm{2}}$ and PdCrO$_{\mathrm{2}}$. Xintong Li, Hiroyuki Inoue, Huiwen Ji, Robert J. Cava, Ali Yazdani There is a resurge of interests on Delafossite such as PdCoO$_{\mathrm{2}}$, which have layered two-dimensional electrons of Pd with exceptionally high conductivity separated by oxide layers. In this compound there are recent reports that electron transport is in the hydrodynamic regime. Another member of the Delafossite family, PdCrO$_{\mathrm{2}}$ has also been of considerable interest because of its complex spin ordering in the CrO$_{\mathrm{2}}$ layers as clarified by scattering experiments, which could be responsible for its intriguing magnetotrasnport properties. Here, we will report high-resolution scanning tunneling microscopy and spectroscopic mapping of both PdCoO$_{\mathrm{2}}$ and PdCrO$_{\mathrm{2}}$ Delafossites. On both compounds, we study scattering interference of electrons from defects and relate our results to surface band structure measurements by ARPES studies. By contrasting our results on these two compounds, we aim to determine how neighboring oxide layer with magnetism influences electrons in the two-dimensional Pd layer. [Preview Abstract] |
Tuesday, March 14, 2017 5:18PM - 5:30PM |
H43.00015: Terahertz frequency magneto-dielectric effect in Ni-doped CaBaCo$_4$O$_7$ Shukai Yu, S. Deckoff-Jones, D. Talbayev, C. Dhana Sekhar, A. Venimadhav, M.K.L. Man, J. Madeo, E. Wong, T. Harada, B.M.K. Mariserla, K.M. Dani We explore the behavior of spin and lattice excitations in high magnetic field in Ni doped magneto-electric material CaBaCo$_4$O$_7$. This material crystallizes in a polar non-centrosymmetric crystal structure and exhibits a non-collinear ferrimagnetic Co spin ordering. Additional ferroelectricity was reported due to the onset of magnetic order, as well as a strong magneto-dielectric effect of 16\%. Low-energy excitations of the material have been studied using THz spectroscopy and include spin excitations and strong lattice vibrational modes at the 1.2 and 1.6 THz frequency. Our THz magnet-spectroscopy measurements in magnetic field up to 17 T addresses the question of the origin of the magnet-dielectric effect. We find that the magneto-dielectric effect results from a significant loss of spectral weight by the lattice vibrational resonances near 1.2 and 1.6 THz frequency. We also find that the vibrational frequencies soften upon doping with Ni. [Preview Abstract] |
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