Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S23: Multiferroic and Magnetoelectric OxidesFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Guru Bahadur Khalsa, Cornell University Room: LACC 402B |
Thursday, March 8, 2018 11:15AM - 11:51AM |
S23.00001: Multiferroic oxides with multiple magnetic order parameters and domain structures Invited Speaker: Tsuyoshi Kimura In this presentation, we show selective observations of multiple magnetic order parameters and domain structures in multiferroic oxides with peculiar incommensurate spiral magnetic ordered states accompanying commensurate (anti)ferromagnetic ordered ones. Examples of such multiferroic oxides are hexaferrites (e.g., Ba1.3Sr0.7CoZnFe11AlO22) showing the so-called alternating longitudinal conical structure [Ueda et al., Appl. Phys. Lett. 109, 182902 (2016)] and olivine-type manganese germanate Mn2GeO4 showing canted antiferromagnetic conical spin chains [Honda, White et al., Nat. Commun. 8, 15457 (2017)]. For the observations, we adopted single-crystal measurements of scanning resonant x-ray microdiffraction for the hexaferrites or (un)polarized neutron scattering for the manganese germanate. These techniques clarify multiple magnetic order parameters and domain structures and their manipulations by external stimuli such as magnetic and electric fields in these peculiar multiferroic oxides. Furthermore, we show our recent research activity on exploration for new magnetoelectrics. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S23.00002: Electric field control of magnetization in Y-type hexaferrite single crystals close to room-temperature Vilmos Kocsis, Taro Nakajima, Masaaki Matsuda, Akiko Kikkawa, Yoshio Kaneko, Junya Takashima, Kazuhisa Kakurai, Taka-hisa Arima, Yusuke Tokunaga, Yasujiro Taguchi, Yoshinori Tokura Y-type hexaferrites with trigonal structure offer a rich variety of non-colinear magnetic phases, among them the most interesting is the ferrimagnetic and ferroelectric FE3 phase. In the Al-doped Y-type hexaferrite, (Ba,Sr)2Co2Fe11AlO22, the FE3 phase can be stabilized as a metastable state close to room temperature, as shown by neutron diffraction and magnetic field induced polarization measurements. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S23.00003: Neutron scattering investigation on a giant magnetoelectric effect compound Ba1-xSrxMg2Fe12O22 Yan Wu, Kun Zhai, Shipeng Shen, Wei Tian, Bryan Chakoumakos, Yi-Sheng Chai, Liqin Yan, Dashan Shang, Fangwei Wang, Huibo Cao, Young Sun
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Thursday, March 8, 2018 12:15PM - 12:27PM |
S23.00004: Optimized magnetoelectric coupling at room temperature via the control of spin anisotropy and phase competition in the Co2Y-type hexaferrite single crystals Chang Bae Park, Kwang Woo Shin, Sae Hwan Chun, Steven Disseler, William Ratcliff, Ju-Young Park, Kee Kim Even though the Co2Y-type hexaferrites have been known to have great potential for realizing the large magnetoelectric (ME) coupling at room temperature, there are no consensus how to to optimize their magnitude. Not only to optimize the ME coupling but to seek for the physical origin of having larger ME coupling near room temperature, we have grown Co2Y-type hexaferrites single crystals Ba2-xSrxCo2(Fe1-yAly)12O22 with systematic variation of Ba/Sr and Fe/Al ratio. We have observed that, among the grown crystals, the nominal composition with x = 1.8 and y = 0.04 has the largest ME coupling coefficient, dP/dH, and magnetic controllable electric polarization, ΔPmax, in the various temperature range. Moreover, electric phase diagram and magnetic structures are established by bulk magnetization, dielectric constant and neutron scattering studies. We suggest that optimization of spin anisotropy and phase competition between transverse cone and alternating longitudinal cone, which are driven by Sr and Al substitution, result in the largest ME coupling in the x = 1.8 and y = 0.04 composition. The role of Sr, Co and Al ions in the spin frustration and magnetic anisotropy will be also discussed. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S23.00005: Asymmetric splitting of the antiferromagnetic resonance in multiferroic hexagonal HoMnO3 Nicholas Laurita, Yi Luo, Rongwei Hu, Meixia Wu, Sang-Wook Cheong, Oleg Tchernyshyov, Peter Armitage The high precision time domain terahertz spectroscopy has uncovered exceptionally strong and unconventional Ho-Mn spin interactions in hexagonal HoMnO3, manifesting as the mysterious asymmetrical splitting of antiferromagnetic resonance modes under weak external magnetic field along c-axis. The asymmetry indicates substantially different g-factors for the high and low energy branches of this excitation. To understand this, we studied the spin excitations on the Mn sublattice, focusing on the interaction between Mn ions and their nearest neighbor Ho layers. We propose a scenario including the simultaneous presence of Dzyaloshinskii-Moriya interaction between Mn ions and 2a Ho ions, and a quartic term possibly originating from high-order interaction. The significant renormalization of g-factors observed near the Ho ordering temperature is also explained. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S23.00006: Tracking a hysteretic and disorder-broadened phase transition via the electromagnon response in improper ferroelectrics Connor Mosley, Dharmalingham Prabhakaran, James Lloyd-Hughes We demonstrate that electromagnons can be used to directly probe the nature of a phase transition between magnetically ordered phases in an improper ferroelectric. The antiferromagnetic/paraelectric to antiferromagnetic/ferroelectric phase transition in Cu1-xZnxO (x=0, 0.05) alloys was tracked via the electromagnon response using terahertz time-domain spectroscopy, on heating and cooling through the phase transition. The transition was found to exhibit thermal hysteresis, confirming its first-order nature, and to broaden under the influence of spin-disorder upon Zn substitution. The energy of the electromagnon increases upon alloying, as a result of the non-magnetic ions modifying the magnetic interactions that give rise to the multiferroic phase and electromagnons. We describe our findings in the context of recent theoretical work that examined improper ferroelectricity and electromagnons in CuO from phenomenological and first-principles approaches. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S23.00007: High-field magnetization and magnetic phase diagram of α-Cu2V2O7 Ganatee Gitgeatpong, Malliga Suewattana, Shiwei Zhaang, Atsushi Miyake, Masashi Tokunaga, Purintorn Chanlert, Nobuyuki Kurita, Hidekazu Tanaka, Taku Sato, Yang Zhao, Kit Matan High-field magnetization of the spin-1/2 antiferromagnet α-Cu2V2O7 was measured in pulsed magnetic fields of up to 56 T in order to study its magnetic phase diagram [1]. When the field was applied along a-axis, two distinct transitions were observed at Hc1 = 6.5 T and Hc2 = 18.0 T. The former is a spin-flop transition typical for a collinear antiferromagnet and the latter is believed to be a spin-flip transition of canted moments. The canted moments, which are induced by the Dzyaloshinskii-Moriya interactions, anti-align for Hc1 < H < Hc2 due to the anisotropic exchange interaction that favors the antiferromagnetic arrangement along the a-axis. Above Hc2, the Zeeman energy of the applied field overcomes the antiferromagnetic anisotropic interaction and the canted moments are aligned along the field direction. In addition, elastic neutron scattering under the applied magnetic fields of up to 10 T reveals the incommensurate helical spin structure in the spin-flop state. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S23.00008: Nonreciprocal Magnons and Symmetry-Breaking in the Noncentrosymmetric Antiferromagnet α-Cu2V2O7 Yang Zhao, Kit Matan, Ganatee Gitgeatpong, Pharit Piyawongwatthana, Taku Sato, Yiming Qiu, Leland Harriger, Nicholas Butch Symmetry plays a crucial role in condensed matter physics. While the magnetic ordering breaks the time-reversal symmetry, the lack of the spatial inversion symmetry in magnetic materials gives rise the antisymmetric coupling. In nearly all cases, this antisymmetric coupling can be described as Dzyaloshinskii–Moriya (DM) interaction. However, in α-Cu2V2O7, a large splitting of the nonreciprocal magnons has been observed in the first time. The detailed inelastic neutron scattering study clearly demonstrated the splitting related to the opposite magnon circular [1]. The spin wave calculation shows an extremely large antisymmetric coupling (~ 2.79 meV). This value is comparable to exchange J and is 1-2 order of magnitude larger than normal DM interaction. This is indicative of either an enormously large spin-orbital coupling (Δg / g ~ 1), or is a new type of the antisymmetric magnetic interaction which its true origin is still unknown. The last but not the least, the result is the first reported direct observation of the magnon circular birefringence. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S23.00009: The multiferroic and cycloidal AF5 phase of Co-doped MnWO4 Randy Fishman, Feng Ye, Jinchen Wang, Yiming Qiu, Jaime Fernandez-Baca, Bernd Lorenz, Paul C. W. Chu One of the great mysteries about MnWO4 is how two different multiferroic and cycloidal phases, labeled AF2 and AF5, can be produced by Co or Zn doping. An analysis of inelastic neutron-scattering data is used to determine the underlying exchange interactions and anisotropies for the AF5 phase of 12% Co-doped MnWO4. Although the ordering wavevector Q = (0.23, 0.5, -0.46) of the AF5 phase is incommensurate in two dimensions, a new technique reduces the computation of the spin-wave frequencies and intensities to a one-dimensional problem. Excellent fits to the inelastic spectra are obtained by including the 11 competing exchange interactions and easy-plane anisotropy without the Dzyaloshinskii-Moriya interactions that are believed to be important for the cycloidal AF2 phase. Remarkably, the exchange parameters of the AF5 phase of Co-doped MnWO4 are relatively close to those of the collinear AF1 phase of pure MnWO4, which has also been reconsidered in this work. So the primary effect of Co doping is to rotate the single-ion anisotropy from easy axis to easy plane. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S23.00010: Magnetoelectric memory with optical readout: LiCoPO4 Urmas Nagel, Vilmos Kocsis, Toomas Room, Karlo Penc, Jakub Vit, Judit Romhanyi, Yusuke Tokunaga, Yasujiro Taguchi, Yoshinori Tokura, Istvan Kezsmarki, Sandor Bordäcs One of the goals of multiferroic research is the development of new devices that take advantage of the coupling between magnetization and electric polarization. For instance in magnetoelectric (ME) memory devices the magnetic bits can be controlled by electric fields without the need to apply electric currents. We demonstrate the optical readout of ME memory states in the antiferromagnetic (AFM) and antiferroelectric (AFE) LiCoPO4, based on the strong absorption difference of THz radiation between its two types of ME domains. This unusual contrast is attributed to the dynamic ME effect of the spin-wave excitations, as confirmed by our microscopic model. Our proof-of-principle study demonstrates the control and the optical readout of ME domains in LiCoPO4 and layes the foundation for future ME devices based on AFE-AFM insulators. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S23.00011: Non-Reciprocal Directional Dichroism of THz Radiation in Multiferroics in the Paramagnetic State Toomas Room, Johan Viirok, Urmas Nagel, Daniel Farkas, David Szaller, Istvan Kezsmarki, Peter Balla, Karlo Penc, Vilmos Kocsis, Yusuke Tokunaga, Yasujiro Taguchi, Yoshinori Tokura Magneto-electric (ME) effect in multiferroic (MF) materials is the cornerstone of new electronic devices allowing the electric field control of magnetization. Collective spin excitations, coupled to electric polarization, absorb THz radiation and can be used as diodes of THz radiation controlled by electric and magnetic fields [I. Kezsmarki, et al., Phys. Rev. Lett. 106, 057403 (2011)]. ME effect appears below the magnetic ordering temperature in type-II MFs. Surprisingly, in MF Sr2CoSi2O7 the dc ME effect re-appears above the magnetic ordering temperature [M. Akaki, et al., Phys. Rev. B 86, 060413 (2012)] when strong magnetic fields align the spins and create macroscopic electric polarization through the on-site ME mechanism, specific to Sr2CoSi2O7 and other compounds of this family. In this contribution we present the results of THz spectroscopy study of non-reciprocal directional dichroism (NDD) in Sr2CoSi2O7 above the Neel temperature in strong magnetic fields as a function of temperature. We show that NDD is driven by and can be linked quantitatively to the static quadratic ME effect. |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S23.00012: High-field magnetostriction and magneto-elastic properties of UO2+x single crystals Krzysztof Gofryk, Marcelo Jaime, Keshav Shrestha, Daniel Antonio, Andres Saul, Myron Salamon, Kenneth Mcclellan, Dagmar Weickert, Tomasz Durakiewicz, David Andersson, Christopher Stanek, James Smith The thermal and magnetic properties of uranium dioxide, a prime nuclear fuel and thoroughly studied actinide material, remain a long-standing puzzle, a result of strong coupling between magnetism and lattice vibrations. The magnetic state of this cubic material is characterized by a 3k non-collinear antiferromagnetic order, that breaks time-reversal symmetry in a non-trivial way, and dynamic Jahn-Teller interactions. It has been shown recently that single crystals of uranium dioxide subjected to strong magnetic fields along threefold axes in the magnetic state exhibit the abrupt appearance of positive linear magnetostriction, leading to a trigonal distortion. Upon reversal of the field the linear term also reverses sign, a hallmark of piezomagnetism [M. Jaime et al., Nature Communications 8, 99 (2017)]. Here we show our recent studies on high-field magnetostriction of oriented (along <111>) UO2+x (x = 0, 0.033, and 0.11) single crystals. The influence of excess oxygen on magneto-elastic properties and its relationship to piezomagnetism in UO2 is examined. We will discuss implications of these new results. |
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