Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G4: Focus Session: New Frustrated Materials |
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Sponsoring Units: GMAG Chair: Jaime Fernandez-Baca, Oak Ridge National Laboratories Room: 112/110 |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G4.00001: Gapless Spin Liquid Behaviour in the S $=$ 1/2 Vanadium Oxyfluoride Kagome Antiferromagnet [NH$_{4}$]$_{2}$[C$_{7}$H$_{14}$N][V$_{7}$O$_{6}$F$_{18}$] Invited Speaker: Lucy Clark The ionothermal synthesis of the magnetic bilayer compound diammonium quinuclidinium vanadium oxyfluoride, [NH$_{4}$]$_{2}$[C$_{7}$H$_{14}$N][V$_{7}$O6F$_{18}$], or DQVOF was recently reported [1]. Its structure contains two crystallographically distinct vanadium sites. On one site sit V$^{4+}$ d$^{1}$ cations, which form a geometrically frustrated kagome network of S $=$ 1/2 spins. At the second site, between these kagome layers, reside V$^{3+}$ d$^{2}$ S $=$ 1 cations to give the kagome bilayer-type units. Here, I will show that DQVOF can be considered as an experimental realization of an S $=$ 1/2 kagome antiferromagnet, with non-interacting S $=$ 1/2 kagome planes, as evidenced by the paramagnetic behaviour of the interlayer S $=$ 1 spins of the V$^{3+}$ cations in our low temperature magnetization and specific heat data. Furthermore, I will show that the combination of strong geometrical frustration and quantum effects within the kagome planes results in exotic magnetic behaviour, with significant experimental evidence in the form of specific heat and muon spin relaxation measurements pointing towards a gapless quantum spin liquid ground state in DQVOF [2]. \\[4pt] [1] F. H. Aidoudi et al. Nat. Chem. 3, 801 (2011).\\[0pt] [2] L. Clark et al. Phys. Rev. Lett. 110, 207208 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G4.00002: Spin Waves in the FCC Kagome Lattice Martin LeBlanc, Byron Southern, Martin Plumer, John Whitehead The impact of an effective local cubic anisotropy [1] on the spin wave excitations and inelastic neutron scattering intensity peaks of the Heisenberg model on the 3D fcc kagome lattice are examined through a linear spin wave theory. Previous Monte Carlo simulations revealed that the addition of anisotropy to the fcc kagome lattice changes the order of the phase transition from weakly first order to continuous and restricts the $T=0$ spin configuration to a number of discrete ground states, removing the continuous degeneracy [2]. It is shown that the addition of anisotropy removes the number of zero energy modes in the excitation spectrum associated with the removed degeneracies. These results are relevant to Ir-Mn alloys which have been widely used by the magnetic storage industry in thin-film form as the antiferromagnetic pinning layer in GMR and TMR spin valves [2]. \\ {[1]} L. Szunyogh, B. Lazarovits, L. Udvardi, J. Jackson, and U. Nowak, Phys. Rev. B \textbf{79}, 020403(R) (2009). \\ {[2]} M.D. LeBlanc, M.L. Plumer, J.P. Whitehead, and B.W. Southern, Phys. Rev. B \textbf{88}, 094406 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G4.00003: Spin frustration and magnetic ordering in the Mott insulating fcc-Cs$_3$C$_{60}$ Yuichi Kasahara, Yuki Takeuchi, Tatsuaki Itou, Yoshihiro Iwasa, Denis Arcon, Matthew Rosseinsky, Kosmas Prassides The low-temperature magnetic state at ambient pressure has been investigated by specific heat and nuclear magnetic resonance (NMR) measurements in face-centered-cubic (fcc-) Cs$_3$C$_{60}$, which is characterized by a Mott insulating state with $S=1/2$ spins in C$_{60}^{3-}$ anions and a geometrical spin frustration inherent in the fcc lattice. Specific heat exhibited no sharp anomaly down to 0.4~K, but both magnetic specific heat and NMR relaxation rate revealed a broad peak around 2.5~K, indicating that the reported antiferromagnetic ordering is accompanied by a gradual freezing of electronic spins with distributed transition temperatures. These results are unexpected in the conventional fcc antiferromagnets. Interplay of geometrical frustration, orientational disorder of C$_{60}$ molecules, and weak Mottness gives rise to the unique magnetic ground state in fcc-Cs$_3$C$_{60}$. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:51PM |
G4.00004: A$_{3}$TeX$_{3}$Z$_{2}$O$_{14}$ (A$=$Ba, Pb, K; X$=$Co, Mn, Fe; Z$=$V, P): Understanding symmetry loss in langasites Invited Speaker: Harlyn Silverstein Ba$_{3}$NbFe$_{3}$Si$_{2}$O$_{14}$ (BNFS) is a langasite that displays simultaneous antiferromagnetic ordering and ferroelectric polarization below T$_{\mathrm{N}} =$ 26 K. But many langasites, including BNFS, crystallize in the nonpyroelectric space group P321. It has been postulated that ferroelectric domains in BNFS and related systems may arise from either the Dzyaloshinskii-Moriya interaction or through symmetry loss to either the P3 or C2 (or lower) space groups from magnetoelastic distortions. Indirect experimental evidence for symmetry loss to C2 exists, but such a distortion is too small to detect with synchrotron X-ray diffraction and implies polarization along the wrong axis. Here, we present another route to understanding symmetry loss in langasites. Rather than focusing on BNFS, where the observed small structural distortions are clouded by experimental uncertainties, we instead turn our attention to alternative chemical systems that are more prone to structural distortions. Unlike BNFS, these distortions can be directly detected using X-ray diffraction. In particular, emphasis is placed on Pb-containing langasites that distort away from P321 symmetry and the impact of this symmetry loss on the magnetism observed in each system.\\[4pt] Work done in collaboration with Arzoo Sharma, Kanisha Cruz-Kan, Avichai Stoller, University of Winnipeg; Haidong Zhou, University of Tennessee-Knoxville; Ashfia Huq, Oak Ridge National Laboratory; Pascal Manuel, ISIS - Rutherford Appleton Laboratory; Roxana Flacau, National Research Council; and Christopher Wiebe, University of Winnipeg. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G4.00005: Magnetothermal observables of geometrically frustrated systems: The case of Fe$_{2}$P-like layered structures Juan Manuel Florez, Oscar Andres Negrete, Patricio Vargas, Caroline A. Ross We study a Fe$_{2}$P-like structured material composed by alternating layers of distorted-Kagome and segmented-triangular lattices. The system is modeled as a stacked Heisenberg structure of mixed AFM/FM couplings, and the magnetothermal properties are calculated by using a Monte Carlo simulations framework. We focus on the question of whether the system could present or not a double-transition-like behavior as a consequence of an intermediate ordered state, which gives rise to a thermal delaying of the spin disorder after the planar 120$^{\circ}$ ordering of the Kagome layers is already broken. This double transition could be observed, e.g., in Fe$_{2}$P-like Iron-pnictides if the Fe-triangles behave like an effective spin center. In a more general case however, FM and AFM intra-triangular interactions lead to different magnetic specific heat sceneries, where the observable peaks evidence an opposed behavior as the strength of the couplings increases: FM promotes the shrinking of the paramagnetic-like zone of the phase-diagram; AFM boosts the global spin disorder but also triggers a competition between the canted orderings of the triangular and the Kagome lattices, which is evidenced through a doubly-bifurcated phase-diagram. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G4.00006: $^{31}$P-NMR Study of the Effect of Pressure on the Magnetic Properties of the 2d Frustrated Square-Lattice Compound BaCdVO(PO$_{4})_{2}$ at Low Temperatures Beas Roy, Ramesh Nath, David C. Johnston, Yuji Furukawa BaCdVO(PO$_{4})_{2}$ is a spin $S =$ 1/2 frustrated square-lattice compound with a nearest-neighbor exchange coupling $J_{1} = -$3.62 K and a next-nearest-neighbor exchange coupling $J_{2} =$ 3.18 K yielding \textbar $J_{2}$/$J_{1}$\textbar $=$ 0.88. A transition to an antiferromagnetic (AFM) ground state occurs below a temperature $T_{\mathrm{N}} =$ 1.0 K under ambient pressure $p$. Based on the $J_{2}$/$J_{1}$ ratio, the system is located close to the disordered ground state (``nematic state'') regime of the phase diagram. We carried out $^{31}$P-NMR measurements under high $p$, ranging from 0.74 GPa to 1.88 GPa, and at low temperatures $T$ down to 100 mK, to investigate the effects of $p$ on the magnetic properties of the system. With increasing $p$, the $T_{\mathrm{N}}$ does not change much, but the magnetization saturation field $H_{\mathrm{S}}$ is significantly suppressed from $H_{\mathrm{S}} =$ 4.2 T at ambient $p$ to $H_{\mathrm{S}} =$ 0.55 T at $p =$ 1.88 GPa. Our $^{31}$P-NMR spectra and spin-lattice relaxation rate (1/$T_{1})$ data establish the first $H$--$p$--$T$ phase diagram for this system. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G4.00007: A Monte Carlo Study of Magnetically Frustrated Chromium Andrew Macdonald, Sarah Burke, Doug Bonn, Yan Pennec As the thickness of engineered films decreases exotic magnetic configurations can appear because of boundary conditions, alloying, or metastable crystallographic phases. Characterizing the ground state of such films is imperative to building new and better magnetic devices and gaining a fundamental understanding of magnetic materials. Spin-polarized scanning tunnelling microscopy experiments have recently revealed unexpected types of magnetic order in a thin film of chromium grown epitaxially on gold. In this talk, I will discuss the characterization of the magnetic ground states of this film via classical Monte Carlo simulations. By modelling the film as an Ising system with a variable degree of lattice distortion and diluting with non-magnetic sites the simulations replicate the complexity of the magnetic ordering observed in the experimental data. Comparing the results of simulation and experiment we conclude that the observed magnetic order is a result of substantial gold inter-alloying combined with geometric frustration, making the ordered state highly sensitive to the degree of lattice distortion. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G4.00008: Magnetic short and long range order in a disordered perovskite Shravani Chillal, Severian Gvasaliya, Andrey Zheludev, Fred J. Litterst, Dennis Schr\"oder, Mathias Kraken, Sergey Lushnikov, Tatiana Shaplygina PbFe$_{1/2}$Ta$_{1/2}$O$_{3}$ (PFT) belongs to the family of PbB$'_{x}$B$''_{1-x}$O$_{3}$ perovskites with inherent chemical disorder at the B-site. We have studied the magnetic phase diagram of PFT through macroscopic techniques, neutron scattering and M\"ossbauer spectroscopy. We show that PFT undergoes two phase transitions: paramagnetic to antiferromagnetic transition at T$_{N}\sim$153K and a spinglass transition at T$_{SG}\sim$10K. Below T$_{SG}$, antiferromagnetism coexists with spinglass on microscopic scale. We suggest that the arrangement of magnetic moments in this unusual ground state of PFT is a speromagnet-like, similar to the one proposed for PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$.\footnote{S. Chillal, M. Thede, F. J. Litterst, S. N. Gvasaliya, T. A. Shaplygina, S. G. Lushnikov, and A. Zheludev, Phys. Rev. B 87, 220403 (2013).} [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G4.00009: Muon Spin Relaxation Studies of the Magnetically Frustrated Double Perovskite Ba2CaOsO6 J.P. Carlo, C. Thompson, T. Medina, T.J.S. Munsie, J. Munevar, Y.J. Uemura, J.E. Greedan The double perovskite structure A$_2$BB'O$_6$, in which antiferromagnetically-correlated magnetic B$'$ cations form an edge-sharing tetrahedral network, is an ideal laboratory for geometric magnetic frustration. The versatility of the perovskite structure enables systematic studies as a function of lattice distortion and moment size, and with 4d and 5d cations, spin-orbit coupling (SOC). Systems with large moments (d$^3$) tend toward antiferromagnetic order, albeit at T $<$ $|\Theta_{CW}|$. Systems with small moments (d$^1$) tend toward disorder, including glassy and singlet ground states. d$^2$ systems form a ``middle ground'' in which a variety of ground states are observed, and theory indicates a wealth of accessible behavior in systems with sizable SOC. Here we report on muon spin relaxation experiments of the 5d$^2$ system Ba$_2$CaOsO$_6$, which exhibits an undistorted cubic structure down to low temperatures, and in which long-lived muon spin precession is observed below 50K. These results are compared to related compounds, including the isostructural Ba$_2$YReO$_6$, an isoelectronic 5d$^2$ system exhibiting glassy behavior, and Ba$_2$YRuO$_6$, a 4d$^3$ system exhibiting commensurate antiferromagnetic order. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G4.00010: Spin Ordering Studies of Edge-sharing Iridates Tess Smidt, Itamar Kimchi, Min Gyu Kim, Zahir Islam, Robert J. Birgeneau, Ashvin Vishwanath, Jeffrey B. Neaton, James G. Analytis We have synthesized a material that is related to the layered honeycomb iridates. The magnetic order shows that this material has highly spin-anisotropic interactions, a key ingredient of the exotic possibilities associated with these compounds. We present X-ray studies of the spin ordering and lattice parameters, which aid in understanding the origin of the magnetic anisotropy and assess the possible proximity to a spin-liquid state. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G4.00011: Antiferromagnetic Exchange, Hunds Coupling and the Origin of the Charge Gap in LaMnPO Daniel McNally, J.W. Simonson, G.J. Smith, V. Leyva, C. Marques, M.C. Aronson, K.W. Post, D.N. Basov, Z.P. Yin, M. Pezzoli, G. Kotliar, Y. Zhao, J.W. Lynn, L. DeBeer-Schmidt, A.I. Kolesnikov LaMnPO is an antiferromagnetic insulator with an ordering temperature T$_{N}$ = 375 K, ordered moment of 3.2 $\mu_{B}$/Mn and a charge gap $\Delta$ = 1 eV. We present inelastic neutron scattering and magnetization data that are well described by a Heisenberg model of magnetic interactions with nearest neighbour exchange SJ$_{1}$$\sim$39 meV and next-nearest exchange SJ$_{2}$$\sim$12 meV. These measurements also show magnetic correlations persist up to T$_{max}$$\sim$700 K, significantly larger than T$_{N}$ due to the effectively decoupled MnP layers. High temperature optical transmission measurements show the charge gap has decreased by $\approx$ 10\% by T$_{max}$ suggesting the rather small exchange interactions J $\ll$ $\Delta$ have only a small effect on the gap. Density functional theory and dynamical mean field theory DFT+DMFT reproduce the observed gap in the paramagnetic state of LaMnPO only in the presence of strong Hunds coupling J$_{H}$, as well as onsite Coulomb interactions U. In light of these experimental and theoretical results, LaMnPO should be considered a Mott-Hunds insulator. [Preview Abstract] |
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