Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W7: Focus Session: Molecule-Based Magnets |
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Sponsoring Units: GMAG Chair: Mark Meisel, University of Florida Room: 106 |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W7.00001: A DFT+DMFT study of spin crossover molecules Jia Chen, Chris Marianetti, Andrew Millis Recent studies have shown promise in applying dynamical mean-field theory (DMFT) based methods to molecular systems. Molecules with transition metals are an ideal target system as they may be complex enough to preclude a treatment via accurate quantum chemical methods, yet exhibit sophisticated many-body behaviour which eludes density functional theory (DFT). Here we apply DFT+DMFT to the molecular complex Fe(phen)2(NCS)2 which exhibits a spin crossover transition as a function of temperature. First, we show that DFT overestimates the stability of low spin state and hence yields an excessively large crossover temperature, in agreement with existing DFT studies. Subsequently, we present both DFT+U and DFT+DFMT to show the effect of treating local correlations. When using the standard double-counting and an on-site Coulomb repulsion computed within linear response, we show that both methods excessively favour the high-spin state. Alternate approaches to the methodology are discussed to remedy this deficiency. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W7.00002: Photoinduced Magnetism in Heterostructures of a Hofmann-like Framework and a Prussian Blue Analogue M.K. Peprah, P.A. Quintero, T.V. Brinzari, M.W. Meisel, B. Hosterman, M. Sendova, C.R. Gros, D.R. Talham Heterostructured films of the Prussian blue analogue (PBA), K${}_{k}$Ni[Cr(CN)${}_{6}$]${}_{l}$$\cdot$nH${}_{2}$O, and the 3D Hofmann-like Fe(azpy)[Pt(CN)${}_{4}$]$\cdot$mH${}_{2}$O (azpy = 4,4$^\prime$-azopyridine) spin crossover system have been studied by magnetometry and Raman spectroscopy. The magnetization of the ferromagnetic NiCr-PBA, $T_c$~$\approx$ 70~K, can be altered by white light irradiation when coupled with the photoactive Fe-Pt framework. The effect is attributed to interface strain that develops when cooling from room temperature. This lattice distortion is relaxed when irradiation causes the Fe(II) to experience a low spin $(S=0)$ to high spin $(S=2)$ transition at $T\leq 50$~K. This work extends our recent photo-controlled magnetic heterostructures studies\footnote{D. M. Pajerowski \textit{et al.}, J. Am. Chem. Soc. \textbf{132} (2010) 4058; M. F. Dumont \textit{et al.}, Inorg. Chem. \textbf{50} (2011) 4295.} to include Fe(II) spin crossover complexes. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W7.00003: Transport Properties of Films of Cobalt Iron Prussian Blue Analogues Pedro A. Quintero, Mark W. Meisel, Olivia N. Risset, Daniel R. Talham The magnetic and transport properties of films of the bistable, photomagnetic, cobalt iron Prussian blue analogue A$_j$Co$_k$[Fe(CN)$_6$] (A = Na, K, Rb)\footnote{O. Sato \emph{et al}., Science \textbf{272} (1996) 704; O. Sato \emph{et al}., J. Am. Chem. Soc. \textbf{126} (2004) 13176.} on ITO and FTO substrates as a function of temperature \mbox{(100 - 300 K)} and under white light irradition have been studied. The magnetic data show the charge transfer induced spin transition (CTIST) between \mbox{230 - 280 K.} The DC and AC transport measurements also show a transition between different conduction states in the same temperature \mbox{region.} Specifically, the AC data (1 kHz) reveals a smooth hysteresis, which is similar to the response observed in the magnetic data. Upon irradiation with white light, swithching is observed between the two conduction states at low temperature \mbox{T $\sim$ 120 K}. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W7.00004: Tuning spatial exchange and single-ion anisotropies in low-dimensional Ni(II) quantum magnets Invited Speaker: Jamie Manson Spatial exchange anisotropy is key to determining $J$'/$J$ and $T_{N}$/$J$ critical ratios and unveiling $B$/$T$ phase diagrams in low-dimensional quantum magnets. Systematic design strategies have enabled us to synthesize a series of model $S=$1 Ni(II) systems whereby this anisotropy, in combination with the single-ion anisotropy, can be tuned by adjusting the nature of the coordinating ligands. For example, the coordination polymers [Ni$L_{x}$(pyz)$_{2}$]$Y$ (pyz $=$ pyrazine; $L=$ HF$_{2}$, $x=$ 1, $Y=$ PF$_{6}$, SbF$_{6}$; $L=$ Cl, Br, I, $x=$ 2, $Y=$ nil), possess 2D [Ni(pyz)$_{2}$]$^{2+}$ square lattices that are spaced apart by bridging or non-bridging $L$ anions such that 1.7 $\le T_{N}\le $ 12 K depending on the magnitude of $J$'. Chemical substitution of pyz for other organic ligands leads to quasi-1D [Ni(HF$_{2})$(3-Clpy)$_{4}$]BF$_{4}$ (Clpy $=$ chloropyridine) and the 2D Kagome lattice [Ni(H$_{3}$F$_{4})$(3-Fpy)$_{4}$]SbF$_{6}$ (Fpy $=$ fluoropyridine) which contain HF$_{2}^{-}$ or H$_{3}$F$_{4}^{-}$ bridges, respectively. Furthermore, the inherent flexibility of strong F\textbullet \textbullet \textbullet H\textbullet \textbullet \textbullet F and O-H\textbullet \textbullet \textbullet F bonds also renders them highly sensitive to external stimuli such as high pressure. Time permitting, these examples and others will be presented. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W7.00005: Magnetism in a family of $S=$ 1 square lattice antiferromagnets, Ni$X_{2}$(pyz)$_{2}$ ($X=$ Cl, Br, NCS; pyz $=$ pyrazine) J. Liu, J.L. Manson, T.J. Woods, K.E. Carreiro, H.E. Tran, S.H. Lapidus, K.H. Stone, P.W. Stephens, Y. Kohama, J.S. Moeller, F.L. Pratt, P.J. Baker, T. Lancaster, A. Ardavan, S.J. Blundell, J. Singleton, P.A. Goddard The crystal structures of Ni$X_{2}$(pyz)$_{2}$ ($X=$ Cl, Br and NCS, henceforth Ni-Cl, Ni-Br and Ni-NCS, respectively), were determined at 298 K from synchrotron powder X-ray diffraction data. All three compounds consist of two-dimensional (2D) [Ni(pyz)$_{2}$]$^{2+}$ square lattices spaced by $X$ ligands, resulting a staggered stacking fashion of 2D layers. Long-range antiferromagnetic order occurs below 1.5 (Ni-Cl) and 1.9 K (Ni-Br and Ni-NCS) as determined by heat capacity and Muon-spin relaxation. The single-ion anisotropy and $g$ factor of Ni-Br and Ni-NCS were measured by electron spin resonance where no zero-field splitting was found. The magnetism of Ni-NCS is interpreted by the 3D simple cubic Heisenberg model with the Ni-pyz-Ni interaction $J_{pyz}=$ 0.70 K. A good overall agreement was found between the pulsed field magnetization data, magnetic susceptibility and $T_{N}$ for Ni-NCS. Ni-Cl and N-Br are characterized as quasi-2D antiferromagnets with the interlayer magnetic coupling significantly suppressed by varying the $X$ ligand. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W7.00006: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W7.00007: Evolution of magnetic interactions in a pressure-induced Jahn-Teller driven magnetic dimensionality switch Johannes Moeller, Saman Ghannadzadeh, Paul Goddard, Tom Lancaster, Fan Xiao, Stephen Blundell, Alexander Maisuradze, Rustem Khasanov, Jamie Manson, Stan Tozer, David Graf, John Schlueter Much of the research in molecular magnetism focusses on low-dimensional magnetic systems. Here we discuss the interesting possibility of controlling the magnetic dimensionality in a molecular magnet by driving the system through a quantum critical point using applied pressure. We present the results of muon-spin relaxation measurements and high-field magnetisation experiments on the coordination polymer CuF$_2$(H$_2$O)$_2$(pyrazine) in pressures up to 22.5 kbar that demonstrate a transition from a quasi-two-dimensional to a quasi-one-dimensional antiferromagnetic phase driven by a rotation of the Jahn-Teller axis at 9.1 kbar. Antiferromagnetic ordering is observed in both regimes. The dimensionality switch is accompanied by a halving of the primary magnetic exchange energy $J$ and a fivefold decrease in the ordering temperature $T_{\mathrm{N}}$. Density-functional theory calculations of the spin density and muon sites are used to complement the experimental data. Part of this work is published in S. Ghannadzadeh et al., Phys. Rev. B 87, 241102 (R). [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W7.00008: Pressure-Induced Magnetic Crossover Driven by Hydrogen Bonding in CuF$_{2}$(H$_{2}$O)$_{2}$(3-chloropyridine) Kenneth O'Neal, Tatiana Brinzaria, Joshua Wright, Janice Musfeldt, Santanab Giri, Qian Wang, Puru Jena, John Schlueter, Zhenxian Liu We combined high pressure vibrational spectroscopy with complementary dynamics calculations to investigate the 0.8 GPa pressure driven magnetic crossover in CuF$_{2}$(H$_{2}$O)$_{2}$(3-chloropyridine). Our work reveals that compression forces the 3-chloropyridine ring closer to the H$_{2}$O ligands, resulting in the formation of --Cl$^{...}$H$_2$O hydrogen bonds. These new intermolecular hydrogen bonds act as ancillary superexchange pathways between copper centers, introducing a third dimension to the hydrogen bonding network and triggering the antiferromagnetic to ferromagnetic crossover. These findings are important for understanding magnetoelastic coupling in quantum magnets and other multifunctional materials in which hydrogen bond formation facilitates magnetic switching. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W7.00009: Optical and Raman Study of $\alpha$-RuCl$_3$ Luke Sandilands, Anjan Reijnders, Yao Tian, Kemp Plumb, Jennifer Sears, Vijay Venkataraman, Hae-Young Kee, Young-June Kim, Kenneth Burch We report on the optical and Raman spectroscopy of the layered honeycomb magnet $\alpha$-RuCl$_3$. Anomalies in the phonon spectrum reveal a structural phase transition near 150K and we also also observe a series of Ru $d-d$ excitations in the range 0.1 to 3.5 eV . We compare our measurements with LDA calculations of the electronic structure and discuss the possibility of $J_{eff}$ physics in this compound. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W7.00010: Single-Crystal Growth of the Low-Dimensional Antiferromagnet NiTa$_2$O$_6$ Aaron Schye, Sueli Masunaga, J.J. Neumeier Single crystals of the low-dimensional antiferromagnet NiTa$_2$O$_6$ were prepared via the floating zone method and chemical vapor transport with TeCl$_4$ as the transport agent. X-ray powder diffraction was used to verify the sample purity and the single crystals were oriented using Laue diffraction. The results of magnetic susceptibility, specific heat, and thermal expansion measurements will be presented. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W7.00011: Transition from short-range to long-range antiferromagnetic order in single-crystalline CoSb$_2$O$_6$ Aaron B. Christian, A. Rebello, M.G. Smith, J.J. Neumeier Single-crystalline CoSb$_2$O$_6$ has been grown via chemical vapor transport. A broad peak in the magnetic susceptibility indicates short-range antiferromagnetic order. Anisotropy among the peak positions suggests a ratio of interchain coupling constants along the \textit{a}- and \textit{c}-axes of $J_a/J_c\sim 1.4$. A second order phase transition to long-range antiferromagnetic order is observed at 13.4 K as an abrupt change in slope. Heat capacity measurements suggest that local 1D magnetic order begins upon cooling below $\sim 70$ K. Thermal expansion measurements also exhibit anisotropic behavior that is affected by the short-range order. Unlike that of the \textit{a}-axis, the thermal expansion coefficient, $\mu$, for the \textit{c}-axis becomes negative below $\sim 70$ K due to anharmonic lattice vibrations resulting from the formation of short-range 1D antiferromagnetic order. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W7.00012: Growth and Transverse Field Muon Spin Rotation of Cobalt Niobate Timothy Munsie, Anna Millington, Casey Marjerrison, Teresa Medina, Murray Wilson, Edwin Kermarrec, Lian Liu, Hanna Dabkowska, Yasutomo Uemura, Travis Williams, Graeme Luke Cobalt niobate, CoNb$_{2}$O$_{6}$, is a material whose spins, when in a transverse field, act like the theoretical ideal 1D-Ising model. This occurs due to the magnetic spins aligning highly anisotropically along the Co$^{2+}$ chains. Because of this unique structure and material performance, the creation and characterization of this material is of both experimental and theoretical interest. The research we will present is a detailing of changes in the characteristics of the growth of the material utilizing the optical floating zone crystal growth method compared to previous growth parameters and an examination of this material in a moderately high transverse field using the technique of muon spin rotation ($\mu$SR). We have determined that the quality of crystals created by the floating zone are highly dependent on the growth parameters utilized (original ceramic shape and rotation rate) and dictate the speed at which the growth can be performed. Transverse Field $\mu$SR shows a gradual but significant change to the magnetic structure of the material below 5 K. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W7.00013: $\mu$SR Investigation of Transition Metal Doped IrTe$_2$ Murray Wilson, Jiaqiang Yan, David Mandrus, Timothy Munsie, Teresa Medina, Graeme Luke Materials with strong spin orbit coupling such as IrTe$_2$ frequently exhibit interesting magnetic and electronic properties. In particular, this material exhibits a structural phase transition at 270~K into a charge density wave state, which is supressed when the parent compound is doped with transition metals such as Pd, Fe, Pt, Ni, Mn, Cu, or Co. As this transition is supressed, superconductivity or magnetic order appears at low temperatures, depending on the metal dopant. These low temperature properties are of interest as the high spin-orbit coupling raises the possibility of topological superconducting states or exotic magnetic order. However, despite significant interest in recent years, the relationship between magnetism and superconductivity in this material system has not yet been well established. In this talk, we present an investigation of the properties of transition metal doped IrTe$_2$ by $\mu$SR which confirms spin glass behavior for Fe doping and probes the low temperature magnetic states for other transition metal dopants. [Preview Abstract] |
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