Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F4: Focus Session: Frustrated Magnets and Spin-Orbit Coupling |
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Sponsoring Units: GMAG Chair: Seunghun Lee, University of Virginia Room: 112/110 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F4.00001: Reconstruction of Chiral Edge States in Magnetic Chern Insulators Ryo Ozawa, Masafumi Udagawa, Yutaka Akagi, Yukitoshi Motome Surface and interface properties of spin-charge coupled systems are one of the central issues not only in fundamental physics but also in application to spintronics. In particular, in magnetically-ordered insulators with topological nature, topologically protected surface states may emerge. On the other hand, the magnetic state near the surface suffer from a reconstruction due to the local symmetry breaking, which may alter the surface states. It is of great interest to clarify how such a reconstruction occurs in a microscopic way. For this purpose, we consider an example of such magnetically-ordered topological insulators, i.e., a spin scalar chiral ordered phase characterized by a nonzero Chern number, recently discovered in the classical Kondo lattice model on a triangular lattice[1]. We investigate this state numerically in finite-size systems with open edges by large scale simulation. As a result, we find that ferromagnetic spin correlations are induced near the edges. Surprisingly, at the same time, the chiral edge current is enhanced. We also clarify that the relation between penetration depth and bulk energy gap. [1] Y. Akagi and Y. Motme, J Phys. Soc. Jpn. {\bf{79}}, 083711 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F4.00002: Ultrasound Velocity Measurements in the Orbital-Degenerate Frustrated Spinel MgV$_2$O$_4$ Tadataka Watanabe, Takashi Ishikawa, Shigeo Hara, A.T.M. Nazmul Islam, Elisa M. Wheeler, Bella Lake Magnesium vanadate spinel MgV$_2$O$_4$ is a geometrically frustrated magnet with $t_{2g}$-orbital degeneracy of V$^{3+}$ (3$d^2$), which undergoes a cubic-to-tetragonal structural transition at $T_s$ = 65 K and an antiferromagnetic (AF) transition at $T_N$ = 42 K. For MgV$_2$O$_4$, it is considered that the occurrence of $t_{2g}$-orbital order at $T_s$ causes the release of frustration by the AF ordering at $T_N$ lower than $T_s$. We performed ultrasound velocity measurements in high-purity single crystal of MgV$_2$O$_4$. Temperature dependence of the tetragonal shear modulus ($C_{11}-C_{12}$)/2 exhibits huge Curie-type softening in the cubic paramagnetic (PM) phase ($T>T_s$), which should be a precursor to the cubic-to-tetragonal lattice distortion at $T_s$. The trigonal shear modulus $C_{44}(T)$ exhibits softening with an upturn curvature in the cubic PM phase, indicating a coupling of the lattice to magnetic excitations. These softenings suggest the coexistence of the dynamical Jahn-Teller effect and the dynamical magnetic state in the cubic PM phase. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F4.00003: Spin-orbital liquids in non-Kramers magnets on Kagome lattice Robert Schaffer, Subhro Bhattacharjee, Yong Baek Kim Localized magnetic moments with crystal-field doublet or pseudo-spin 1/2 may arise in correlated insulators with an even number of electrons and strong spin-orbit coupling. Such a non-Kramers pseudo-spin 1/2 is the consequence of crystalline symmetries as opposed to the Kramers doublet arising from time-reversal invariance. We investigate possible spin-orbital liquids with fermionic spinons for such non-Kramers pseudo-spin 1/2 systems on the Kagome lattice. Using the projective symmetry group analysis, we find ten new phases that are not allowed in the corresponding Kramers systems. We compute the spin-spin dynamic structure factor that shows characteristic features of these non-Kramers spin-orbital liquids arising from their unusual coupling to neutrons, which is therefore relevant for neutron scattering experiments. We also point out possible anomalous broadening of Raman scattering intensity that may serve as a signature experimental feature for gapless non-Kramers spin-orbital liquids. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F4.00004: Study of the magnon spectrum in $FeV_{2}O_{4}$ using inelastic light scattering Y. Gim, S. Gleason, T. Byrum, G.J. MacDougall, H.D. Zhou, S.L. Cooper The interplay between orbital, spin and lattice dynamics create a rich environment for the study of novel properties and phases. Transition metal oxides with a spinel structure, $AB_{2}O_{4}$ are excellent systems in which to explore the interplay among these dynamics: By substituting on the $A$ and $B$ sites with various elements, various phenomena and ground states can be explored. $FeV_{2}O_{4}$ is a special spinel with two orbital-active $A-$ and $B-$ site cations. This material exhibits interesting magnetic and structural phenomena, such as multiferroic behavior and a strong dependence of its physical properties on external stimuli such as pressure and magnetic field. In this talk, we present an inelastic light (Raman) scattering study of the temperature- and magnetic field-dependence of the magnon spectrum of $FeV_{2}O_{4}$. We compare these results to of the magnon spectrum of $MnV_{2}O_{4}$ in order to examine the role of A-site substitution on the spin dynamics. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F4.00005: Co Doping Effect on the Crystal and Magnetic Phases in the Frustrated Spinel Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$V$_{2}$O$_{4}$ Jie Ma, Tao Hong, Huibo Cao, Adam Aczel, Wei Tian, Zhiling Dun, Yiming Qiu, John Copley, H.D. Zhou, Masaaki Matsuda Co doping effect on the MnV$_{2}$O$_{4}$ spinel system has been investigated by the elastic and inelastic neutron scattering techniques. Our data present that a magnetic phase transition exists from collinear to noncollinear ferrimagnetic structure between the Mn$^{2+}$/Co$^{2+}$ and V$^{3+}$ moments and the Co doping decreases the V$^{3+}$ canting angle. The most remarkable finding is that with Co doping, the collinear to noncollinear transition, which coincides with the cubic to tetragonal structural transition related with the orbital ordering of the V$^{3+}$ ions in pure MnV$_{2}$O$_{4}$, occurs independently without the structural transition. Our results indicate that the Co doping changes the orbital nature of the V$^{3+}$ ions and enhances the magnetic coupling between the Mn$^{2+}$/Co$^{2+}$ and V$^{3+}$ moments. We discuss how the orbital and magnetic order are correlated in this system. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F4.00006: Magnetism in a new structural family of iridates James Analytis, Tess Smidt, Ross McDonald, Kim Modic, Itamar Kimchi, Ashvin Vishwanath, Radu Coldea, Alun Biffin, S.K. Choi, Julia Chan, Pilanda Watkins-Curry The physics of Mott insulators underlies diverse phenomena ranging from high temperature superconductivity to exotic magnetism. Although both the electron spin and the local orbitals play a key role in these phenomena, in most systems these are connected only indirectly --- via Pauli exclusion --- since the spin-orbit interaction is relatively weak. Iridium-based oxides (iridates) depart from this expectation, since the spin-orbit coupling dominates over other interactions, such that the Mott physics obtains a strong orbital character. In some cases this is thought to generate strongly spin-anisotropic exchange. Here we report a new family of iridates whose magnetic character shows that this material has highly spin-anisotropic interactions, a key ingredient of the exotic possibilities associated with these compounds. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F4.00007: Weak increase in ordering temperature with pressure in KCuF$_3$ Alexander Thaler, Andrew Christianson, Shi Yuan, Isaac Brodsky, Lance Cooper, Stephen Nagler, Gregory MacDougall The perovskite KCuF$_3$ has been extensively studied as a prototype for both orbital order and 1D Heisenberg antiferromagnetism. Despite decades of research, the nature of its orbital and spin order are still debated. Several interesting results have been shown recently, among them the discovery via Raman scattering of a glassy structural transition at $T_s=50$~K, well below the known Jahn-Teller transition at $T_{OO}=800$~K and argued to be a necessary precursor to the 3D Neel transition at $T_N=39$~K. Recent experiments have demonstrated that this transition can be suppressed to zero temperature with hydrostatic pressures as low as $P_c\sim7$~kbar. In order to directly probe the effect of pressure on the magnetic behavior at low-temperatures, we have followed the above Raman measurements with a neutron scattering study. Structural and magnetic properties of single-crystalline KCuF$_3$ were explored using elastic scattering of thermal neutrons under applied quasi-hydrostatic pressure. We will present data suggesting that the A-type antiferromagnetism observed at ambient pressure is slightly increased by our application of pressure well above 1~GPa, as well as showing a spin-reorientation with increasing pressure. We will discuss the results in the context of present literature. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F4.00008: Spin waves in the double orbital-order spinel, $FeV_2O_4$ G.J. MacDougall, A.A. Aczel, V.O. Garlea, G.E. Granroth, T. Hong, A.D. Christianson, S.E. Nagler, I. Brodsky, H.D. Zhou For the past several years, the spinel vanadates, $AV_2O_4$, have been central to the study of orbital degeneracy and the complex coupling of spin, charge and lattice degrees-of-freedom in frustrated antiferromagnets. One such material of recent interest is $FeV_2O_4$, which has orbital degeneracy at high temperatures on both cation sites of the spinel structure. Previous x-ray scattering and our own neutron powder diffraction results have identified three structural and two magnetic transitions in this compound, and the low-temperature non-collinear spin state has been associated with an emergent ferroelectric moment. In the past year, we have followed our initial results on powders with an elastic and inelastic neutron scattering study on large single crystals of $FeV_2O_4$. Our elastic data confirm the same structural and magnetic transitions inferred from powder measurements, with near identical transition temperatures. Our inelastic data reveal the presence of two low-energy spin-wave branches, qualitatively similar to what has been reported for the related compound, $MnV_2O_4$, but with an order of magnitude larger spin-gap. I will present these results, and discuss them in the context of $MnV_2O_4$ and the present state of the literature. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F4.00009: Magnetic excitations in spin-orbital liquid FeSc$_2$S$_4$ in zero and applied magnetic field probed by inelastic neutron scattering Alun Biffin, Radu Coldea, Christian R\"{u}egg, Oksana Zaharko, Jan Embs, Tatiana Guidi, Vladimir Tsurkan In systems where both spin and orbital frustration are present, an intriguing Spin Orbital Liquid (SOL) state is believed to occur where spin and orbital moments remain disordered down to the lowest measurable temperatures. The A-site spinel FeSc$_2$S$_4$ is believed to form such a SOL ground state, with its undistorted cubic structure and diamond lattice of Fe$^{2+}$ sites providing the ingredients for orbital and spin frustration, respectively. The system displays Curie-Weiss behaviour indicative of strong exchange between $S=2$, $L=2$ Fe$^{2+}$ ions, though it does not order down to the lowest measurable temperatures. Here I will present the results of inelastic, time-of-flight neutron scattering experiments that probe the full bandwidth of the magnetic excitations in a powder sample of FeSc$_2$S$_4$, and provide a consistent model of the observed dynamics in terms of spin-orbital excitations, in both zero-field and in-field measurements. I will discuss in particular how the application of a magnetic field elucidates the spin and orbital nature of these excitations, as the system shows behaviour drastically contrary to its spin-only analogue. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F4.00010: Quantum Spin States, Multiferroicity, Orbital Ordering, and Metal-Insulator Transition in New Layered-Perovskites Invited Speaker: Haidong Zhou The high chemical tunability of the layered-perovskites Ba$_3$BC$_2$O$_9$ (B = Co2+, Ni2+, Mn2+, and C = Nb5+, Ru5+, Ir5+) makes them idea systems to study various physical behaviors, such as quantum spin states, multiferroicity, orbital ordering, and metal-insulator transition, based on the geometrically frustrated lattices. In this talk we present several examples to discuss these intriguing properties: (i) Ba$_3$CoNb$_2$O$_9$, Ba$_3$NiNb$_2$O$_9$, and Ba$_3$MnNb$_2$O$_9$. For these samples, the only magnetic ions Co2+, Ni+, or Mn+ on the B sites form a triangular lattice in the ab plane, which makes them new triangular lattice antiferromagnets (TLAFs). The detailed magnetic and electric properties show that the samples not only exhibit successive spin state transitions under magnetic fields but also multiferroic behaviors [1]; (ii) Ba$_3$CoRu$_2$O$_9$. With Ru5+ ions occupy the face-shared bioctahedral C-sites, the system exhibits an orbital ordering for the Ru5+ orbitals which leads to complex magnetic and structural phase transitions [2]; (iii) Ba$_3$CoIr$_2$O$_9$. This system exhibits metal-insulator transition under high pressure, which is accompanied with complex magnetic behaviors. \\[4pt] [1] J. Hwang et al., Phys. Rev. Lett. 109, 257205 (2012).\\[0pt] [2] H. D. Zhou et al., Phys. Rev. B 85, 041201(R) (2012). [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F4.00011: Magnetic Field Imaging of the Spinel MnV$_{2}$O$_{4}$ Brian Wolin, Tyler Naibert, Taylor Byrum, Samuel Gleason, Haidong Zhou, S. Lance Cooper, Raffi Budakian The complex interplay of spin, orbital degeneracy, and lattice degrees of freedom result in many intriguing behaviors in condensed matter systems. Due to its simple lattice structure and extensive theoretical work, the spinel MnV$_{2}$O$_{4}$ is a prime candidate for archetypal study of these phenomena. We perform magnetic force microscopy imaging on single crystal samples of MnV$_{2}$O$_{4}$ at variable temperature and magnetic field. Our results show previously unobserved magnetic structure and behavior (including stripes and domain switching) as the phase diagram is explored. These represent the first direct imaging of the magnetic properties of a vanadium oxide spinel and inform the current debate over the low temperature magnetic phases of MnV$_{2}$O$_{4}$. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F4.00012: Magnon spectra and strong spin-lattice coupling in magnetically frustrated MnV$_2$O$_4$: Inelastic light scattering studies S.L. Gleason, T. Byrum, Y. Gim, S.L. Cooper, H.D. Zhou The spinel MnV$_2$O$_4$ exhibits a series of closely spaced magnetic and structural transitions at low temperatures, reflecting magnetic frustration and strong spin-lattice coupling. MnV$_2$O$_4$ has a canted ferrimagnetic ground state with an undetermined orbital configuration. Temperature dependent studies of magnetic and vibrational excitations in MnV$_2$O$_4$ are important for determining the role that spin-lattice coupling plays in the low temperature phase transitions of this material and setting constraints on the orbital ground state configuration. We report an inelastic light (Raman) scattering study of the temperature and magnetic field dependences of magnetic excitations in MnV$_2$O$_4$. We observe a pair of $\textbf{q}=0$ one-magnon modes at 74 cm$^{-1}$ and 81 cm$^{-1}$, which is in contrast with the single 80 cm$^{-1}$ $\textbf{q}=0$ magnon that has been reported for MnV$_2$O$_4$ from previous neutron scattering measurements and spin wave calculations. Additionally, we find that the two-magnon energy of MnV$_2$O$_4$ decreases with decreasing temperature below $T_N$, which we attribute to strong coupling between zone-boundary magnons and phonons. These results offer important clues to the orbital ground state and the nature of spin-lattice coupling in MnV$_2$O$_4$. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F4.00013: Temperature and magnetic field dependent Raman scattering study of magnetically frustrated Mn$_{3}$O$_{4}$ T. Byrum, S.L. Gleason, Y. Gim, A. Thaler, P. Abbamonte, G.J. MacDougall, S.L. Cooper The coupling between spin and lattice degrees of freedom is presumed to be responsible for many of the novel phenomena observed in the magnetically frustrated spinel Mn$_{3}$O$_{4}$. While the lattice excitations have previously been investigated by Kim $\textit{et al.}$,* the dependences of the spin excitations in Mn$_{3}$O$_{4}$ with magnetic field and temperature have not yet been reported. We perform inelastic light (Raman) scattering to study the spin excitations in Mn$_{3}$O$_{4}$ as functions of temperature and magnetic field. We observe both one- and two-magnon excitations below the magnetic transition temperature of Mn$_{3}$O$_{4}$. In this presentation, we will discuss the temperature and magnetic field dependent evolutions of these excitations. Interestingly, we conclude that some of the magnon excitations are likely associated with the frustrated B-site sublattice of the spinel (AB$_{2}$O$_{4}$) structure. These results set the stage for future studies of the coupling between spin and lattice degrees of freedom in Mn$_{3}$O$_{4}$ as functions of pressure, temperature, and magnetic field. *M. Kim, X. M. Chen, X.Wang, C. S. Nelson, R. Budakian, P. Abbamonte, and S. L. Cooper, Phys. Rev. B 84, 74424 (2011). [Preview Abstract] |
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