Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C43: Multiferroic Oxides IFocus
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Sponsoring Units: GMAG DMP DCOMP Chair: Steven Disseler, NIST Room: 390 |
Monday, March 13, 2017 2:30PM - 3:06PM |
C43.00001: Magnon-phonon hybridization and enhanced anharmonicity in noncollinear magnets (Y/Lu)MnO$_{\mathrm{3}}$ Invited Speaker: Joosung Oh Anharmonicity in phonons, magnons, and their coupling plays a crucial role in diverse thermodynamic behavior and zero temperature damping of these quasi particles. Although such magnon-phonon coupling and damping can be in principle examined by spectroscopic techniques, actual observations are rare for real magnetic materials with collinear spin structures. In contrast, much stronger effects are expected for noncollinear magnets. We report the observation of such magnon-phonon hybridization and damping in noncollinear magnets (Y/Lu)MnO$_{\mathrm{3}}$ using an inelastic neutron scattering technique [1]. In addition to the usual spin wave signals seen below 20 meV, weaker extra peaks are observed at higher energies. We could then reproduce these additional peaks using Hamiltonian with a magnon-phonon coupling of an exchange striction type. With this analysis, the additional intensities can be attributed to magneto-elastic excitations: which are caused by a direct magnon-phonon coupling originating from noncollinear spin structure. Such magneto-elastic excitations are also found to show large linewidth broadening near certain reciprocal points, which is qualitatively reproduced by anharmonic spin waves calculations. We will also discuss possible exchange striction terms required and how the magnon-phonon coupling can enhance such damping. [1] J. Oh et al$.$, Nat. Comms. 7, 13146 (2016). [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C43.00002: Hybrid magnon-phonons in the paraelectric antiferromagnet EuTiO$_{3}$ Huibo Cao, Olivier Delaire, Jiawang Hong, Steven Hahn, Songxue Chi, George Ehlers, Douglas Abernathy, Andrew Christianson, Jaime Fernandez-Baca, Bryan Chakoumakos, Jiaqiang Yan, Brian Sales Magnetic perovskite titanate EuTiO$_{3}$ has attracted a lot of attentions for its large spin-lattice coupling. It has a number of similarities with the well-studied quantum paraelectric SrTiO$_{3}$ but a much higher cubic-tetragonal lattice transition at 290 K and an extra magnetic order at 5.5 K. The large difference between EuTiO$_{3}$ and SrTiO$_{3}$ has been attributed to the magnetic ion Eu$^{2+}$, that couples with the structural properties. However the spin-lattice coupling mechanism has not been fully understood yet although many theoretical models have been proposed. We grew a large, high-quality isotopically-enriched EuTiO$_{3}$ crystal for neutron scattering.~ The crystal and magnetic structures were calibrated with neutron diffraction at HB-3A at HFIR at ORNL at temperatures from 1.5 K to 450 K. The spin waves and phonons were measured in the temperature range of 1.5-400 K with HB-3 at HFIR, CNCS and ARCS at SNS at ORNL. I will report our new discovery of interaction between the soft ferroelectric phonon mode and likely (para)magnon mode, that is responsible for a giant magnetoelectric coupling in EuTiO$_{3}$. The magnetic excitation and atomic displacements will be discussed. [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C43.00003: Dynamics of the Ho$^{+3}$ magnetism in the multiferroic compound h-HoMnO$_3$ investigated via time domain terahertz spectroscopy N.P. Armitage, Nicholas Laurita, Rongwei Hu, Meixia Wu, Seongshik Oh The hexagonal rare-earth manganites REMnO$_3$ display a diverse array of magnetism as many contain both magnetically active rare-earth and Mn moments. Of this class h-HoMnO$_3$ (HMO) is of particular interest as Ho ions possess the largest rare-earth magnetic moment making HMO a protype material for studying magnetic exchange in these systems. However, few experiments have been performed on HMO at low temperatures, when both Ho and Mn sublattices are ordered, and therefore little is known regarding the role of Ho moments in the magnetic response. In this work we present a systematic study of the far infra-red spectra of HMO as a function of both temperature and magnetic field. A splitting of the spectra is observed at low temperatures which we attribute to exchange between Ho and Mn moments. The corresponding field dependence is studied and shown to reveal further evidence of exceptional Ho-Mn exchange including a g-factor that is enhanced by a factor of 2 at the Ho ordering transition. [Preview Abstract] |
Monday, March 13, 2017 3:30PM - 3:42PM |
C43.00004: Far infrared non-reciprocal directional dichroism reveals ferrotoroidic order in LiCoPO$_4$ Urmas Nagel, T. R\~o\~om, I. K\'ezsm\'arki, S. Bord\'acs, V. Kocsis, Y. Tokunaga, Y. Taguchi, Y. Tokura Non-reciprocal dichroism means that counter-propagating light beams experience different indexes of refraction. This phenomenon, which has been exclusively observed in non-centrosymmetric (polar or chiral) materials with finite magnetization, is the consequence of the dynamic magnetoelectric effect in materials with simultaneously broken time reversal and spatial inversion symmetries. We show that LiCoPO$_4$ with a fully compensated antiferromagnetic ground state, i.e. with zero net magnetization, can also exhibit unidirectional transmission. Following an appropriate magnetoelectric poling in crossed electric and magnetic fields, we succeeded to realize the unidirectional transmission as a remnant effect in this compound. The unidirectional transmission likely originates from a ferrotoroidic order which can be viewed as the cross-product of antiferroelectricity and antiferromagnetism coexisting in LiCoPO$_4$. The sign of the magnetoelectric effect can be set by the poling electric and magnetic fields via the establishment of mono-domain ferrotoroidic states. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 3:54PM |
C43.00005: THz spectroscopy of spin waves in multiferroic LiNiPO$_{4}$ in high magnetic fields Laur Peedu, T. R{\~o\~o}m, J. Viirok, U. Nagel, D. Szaller, S. Bord\'acs, I. K\'ezsm\'arki, D. L. Kamenskyi, V. Kocsis, Y. Tokunaga, Y. Taguchi, Y. Tokura LiNiPO$_{4}$ belongs to the family of multiferroic lithium-ortho-phosphates where correlations between magnetic and electric dipoles allow the magnetic control of the electric polarization and electric control of magnetization. LiNiPO$_{4}$ exhibits a very rich phase diagram because of competing magnetic interactions that produce step-like magnetization similar to 2D Ising AFM compounds. We have done THz absorption spectroscopy measurements of LiNiPO$_{4}$ single crystals below 4\,K and in magnetic fields up to 33\,T. In zero magnetic field we have determined selection rules of magnon excitations by looking at different orientations of the electric and magnetic field components of THz radiation, revealing three magnetic- and three electric-dipole active magnons, and electric-dipole active double-magnon. Between 0 to 33\,T along the easy axis we have identified four different phases where magnon modes at phase boundaries are discontinued. [Preview Abstract] |
Monday, March 13, 2017 3:54PM - 4:06PM |
C43.00006: Pressure effect on ferroelectricity of multiferroic Ho$_{0.5}$Nd$_{0.5}$Fe$_{3}$(BO$_{3}$)$_{4}$ Narayan Poudel, Melissa Gooch, Bernd Lorenz, L. N. Bezmaternykh, V. L. Temerov, C. W. Chu Ho$_{0.5}$Nd$_{0.5}$Fe$_{3}$(BO$_{3}$)$_{4}$ becomes multiferroic below 33 K where it enters into the AFM1 phase and gives rise to a ferroelectric polarization along the a-axis. At 9.5 K, the polarization drops sharply and remains finite value of $\sim$ 40 $\mu$C/m$^2$. This is due to the spin rotation from the a-b plane into the c-axis and gives rise to the AFM2 phase. The application of pressure suppresses the AFM2 phase and moves the spin rotation transition from 9.5 K to 4.8 K up to pressure of 18.8 kbar which is observed in both dielectric and pyroelectric measurements. The change in magnetic anisotropy of rare-earth moments and Fe ions under pressure drives the spin rotation transition of rare-earth at lower temperature. [Preview Abstract] |
Monday, March 13, 2017 4:06PM - 4:18PM |
C43.00007: Magnetic ground states and magnetodielectric effect of $R$Cr(BO$_3$)$_2$ ($R$ = Y and Ho) Ryan Sinclair, Haidong Zhou, Minseong Lee, Eun Sang Choi, Tao Hong, Stuart Calder The magnetic, electric, and structural properties of polycrystalline $R$Cr(BO$_3$)$_2$ ($R$ = Y and Ho) samples were studied using AC/DC susceptibility measurements, dielectric constant measurements, and neutron scattering experiments. Both samples' Cr$^{3+}$ ions order in a noncollinear antiferromagnetic ground state with a transition temperature $T_N$ $\sim$8 K while the Ho$^{3+}$ ions do not order down to $T$ $\sim$ 2K. When a critical magnetic field is applied below $T_{N}$, the Cr$^{3+}$ and Ho$^{3+}$ ions both adopt a canted ferromagnetic ground state. Using inelastic neutron scattering, we estimated the $R$ = Y sample's intralayer and interlayer exchange strengths, J$_{intra}$ = -4.80 meV and J$_{inter}$ = 0.215 meV. The magnetodielectric effects in this system appear to depend on these exchanges. When the nonmagnetic $R$ = Y$^{3+}$ ions are replaced by magnetic Ho$^{3+}$ ions, the system exhibits stronger magnetodielectric responses near the critical field value. Our data suggests that this behavior results from an increased magnetostriction which is dependent on the Ho$^{3+}$ ions' ordering. [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C43.00008: Magnetoelectric Coupling through the Spin Flop Transition in Ni3TeO6 Michael Yokosuk, Amal al-Wahish, Sergey Artyukhin, Kenneth O'Neal, Dipanjan Mazumdar, Peng Chen, Junjie Yang, Yoon Seok Oh, Stephen McGill, Kristjan Haule, Sang-Wook Cheong, David Vanderbilt, Janice Musfeldt We combined high field optical spectroscopy and first principles calculations to analyze the electronic structure of Ni3TeO6 across the 53 K and 9 T magnetic transitions, both of which are accompanied by large changes in electric polarization. The color properties are sensitive to magnetic order due to field-induced changes in the crystal field environment, with those around Ni1 and Ni2 most affected. These findings advance the understanding of magnetoelectric coupling in materials in which magnetic 3d centers coexist with nonmagnetic heavy chalcogenide cations. [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C43.00009: Multiferroic properties of a frustrated quantum spin chain system linarite Yaoxuan Feng, Kirill Yu. Povarov, Andrey Zheludev Dielectric measurements were performed across the strongly anisotropic phase diagram of the frustrated S=1/2 spin chain compound $\mathrm{PbCuSO_4(OH)_2}$, also know as linarite. In particular, electric polarization was measured on single crystals of the titled material in 6 different geometric configurations. At least two of the magnetic phases for H$||$b-axis are revealed to be also ferroelectric \footnote{K. Yu. Povarov, Y. Feng, A. Zheludev, Multiferroic phases of the frustrated quantum spin chain compound linarite, arXive: 1609.06087.}. The observed orientation of dielectric polarization suggests that one of the previously proposed phase-coexistence regions is actually a proper thermodynamic phase, possibly with a multi-q magnetic structure. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C43.00010: Multiferroics by design with frustrated molecular magnets Yoshitomo Kamiya, Cristian Batista Geometric frustration in Mott insulators permits perturbative electron fluctuations controlled by local spin configurations [1]. An equilateral triangle (``trimer'') of spins with S $=$ 1/2 is the simplest example, in which low-energy degrees of freedom consist of built-in magnetic and electric dipoles arising from the frustrated exchange interaction. Such trimers can be weakly coupled to make multiferroics by design [2]. An organic molecular magnet known as TNN, with three S $=$ 1/2 nitronyl nitroxide radicals in a perfect C$_{\mathrm{3}}$ symmetric arrangement, is an ideal building block as demonstrated by recent experiments on a single crystal comprising TNN and CH$_{\mathrm{3}}$CN. The fascinating thermodynamic phase diagram of this molecular crystal, TNN\textbullet CH$_{\mathrm{3}}$CN, is in excellent agreement with our theory, which predicts multiferroic behavior and strong magnetoelectric effects arising from an interplay between magnetic and orbital degrees of freedom. Our study thus opens up new avenues for designing multiferroic materials using frustrated molecular magnets. References: [1] L. N. Bulaevskii \textit{et al}., PRB \textbf{78}, 024402 (2008). [2] Y. Kamiya and C. D. Batista, PRL \textbf{108}, 097202 (2012). [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C43.00011: Spin lattice coupling in multiferroic [(CH$_{\mathrm{3}})_{\mathrm{2}}$NH$_{\mathrm{2}}$]Mn(HCOO)$_{\mathrm{3}}$ Kendall Hughey, Amanda Clune, Amal Al-Wahish, Michael Yokosuk, Shiyu Fan, Nandita Abhyankar, Hongjun Xiang, Naresh Dalal, Zhiqiang Li, Janice Musfeldt Multiferroic metal-organic framework [(CH$_{\mathrm{3}})_{\mathrm{2}}$NH$_{\mathrm{2}}$]Mn(HCOO)$_{\mathrm{3}}$ is a superior platform for investigating magnetically driven transitions and the magnetoelastic effect because the low energy scales and soft organic linkers are easily influenced by external stimuli. By analyzing the vibrational properties under temperature and magnetic field, we unravel the microscopic details of the 185 K order/disorder transition and determine that ferroelectricity stems from a combination of the ordering of a counterion and a distortion of the formate framework. We also reveal that spin-lattice coupling in [(CH$_{\mathrm{3}})_{\mathrm{2}}$NH$_{\mathrm{2}}$]Mn(HCOO)$_{\mathrm{3\thinspace }}$is different from rare earth manganites and more analogous to behavior in quantum magnets where few local lattice distortions stabilize the fully polarized state. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C43.00012: Multiferroic properties of the geometrically frustrated molecular spin-trimer compound TNN.CH3CN Yasumasa Takano, Christopher Aoyama, Kosuke Takada, Hironori Yamaguchi, Toshio Ono, Yasuyuki Shimura, Toshiro Sakakibara, Minseong Lee, Eun Sang Choi, Yoshitomo Kamiya, Hiroki Nakano, Cristian Batista, Yuko Hosokoshi A new route to multiferroicity has been proposed, which requires neither a Dzyaloshinskii-Moriya interaction nor lattice distortion [1]. In this scenario, electric dipole moments emerge from lifting of ground-state degeneracy in a spin triangle embedded in a frustrated geometry. Well known spin triangles such as \textbraceleft Cu\textbraceright 3 suffer from Jahn-Teller distortion, which preempts the scenario, whereas organic spin trimers with perfect C3 symmetry are promising candidates. We have investigated the new organic compound TNN.CH3CN, which consists of triangular-lattice layers of the spin trimer TNN---tris[4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl]amine---incorporating CH3CN. The magnetic-field-temperature phase diagram constructed from magnetization, ac dielectric constant, specific heat, and magnetic torque consists of several multiferroically ordered phases, which are in excellent agreement with theory. [1] Y. Kamiya and C. Batista, Phys. Rev. Lett. 108, 097202 (2012). [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C43.00013: Developing rich H vs. T diagrams of molecule-based multiferroics Amanda Clune, Kendall Hughey, Nandita Abhyankar, Shalinee Chikara, Wei Tian, Jaime Fernandez-Baca, Vivien Zapf, Mike Whangbo, Naresh Dalal, John Singleton, Janice Musfeldt The magnetization of two molecule-based multiferroics, (CH$_{\mathrm{3}})_{\mathrm{2}}$NH$_{\mathrm{2}}$]Mn(HCOO)$_{\mathrm{3}}$ and (NH$_{\mathrm{4}})_{\mathrm{2}}$FeCl$_{\mathrm{5}}$H$_{\mathrm{2}}$O, was measured using pulsed magnetic fields of up to 60 T and temperatures down to 0.5 K, to reveal their H vs. T phase diagrams.~ The results were compared with spin-density calculations, polarization measurements, neutron scattering, and magneto-infrared spectroscopy to understand the spin behavior of the two compounds. The ability of these experimental techniques to completely parameterize the quantum magnetism of such materials, combined with the use of molecular architecture to adjust bond lengths, potentials and interaction strengths with great subtlety, promises to yield significant progress in the field of multiferroics. [Preview Abstract] |
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