Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T18: Focus Session: Low D/Frustrated Magnetism - Molecular Magnets II |
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Sponsoring Units: GMAG DMP Chair: M. Sarachik, City College of New York Room: D172 |
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T18.00001: Recent developments in Molecular Magnetism Invited Speaker: Two hot topics in this area are highlighted namely the design of new multifunctional magnetic materials, and the study of mononuclear single-molecule magnets. In the first topic I will present the attempts to design materials exhibiting coexistence of superconductivity and magnetism using a chemical approach [1]. In the second topic I will show that single-molecule magnets based on lanthanides provides the opportunity to tune quantum tunneling effects and to use these systems as qubits in quantum computing [2]. Finally, I will highlight the relevance of these magnetic systems in molecular spintronics.\\[4pt] [1] E. Coronado, C. Mart\'i-Gastaldo, E. Navarro-Moratalla, A. Ribera, S. J. Blundell, P. J. Baker Nature Chem. 2010, 2, 1031.\\[0pt] [2] F. Luis, M. J. Mart\'inez-P\'erez, O. Montero, E. Coronado, S. Cardona-Serra, C. Mart\'i-Gastaldo, J.M. Clemente-Juan, J. Ses\'e, D. Drung, T. Schurig, Phys. Rev. B 2010, 82, 060403 R. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T18.00002: Random Exchange Antiferromagnetic Heisenberg Chains Christopher Landee, Fan Xiao, Brian Wells, Brian Koopman, Mark Turnbull, Matthias Thede, Andrey Zheludev Copper \textit{bis}pyridine dichloride (CPC) and copper \textit{bis}pyridine dibromide (CPB) are isostructural Heisenberg S = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ linear chains in which the copper atoms are bridged by two halides with the pyridine molecules in the axial sites. The exchange strengths are 27 K (CPC) and 45 K (CPB). We have prepared mixed halide versions Cu(py)$_{2}$[Cl$_{1-x}$Br$_{x}$]$_{2}$ for values of x from 0 to 1. We will report on the effect of the exchange randomness on the susceptibilities and N\'{e}el temperatures of this family of compounds, as determined by dc-magnetometry, muon spin relaxation, and low-temperature calorimetry. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T18.00003: Magnetic dilution in the cadmium-doped spin ladder compound $Cd_xCu_{1-x}(quinoxaline)Br_2$ Brian Keith, Chris Landee, Mark Turnbull Both $Cu(quinoxaline)(Br_2)$ and $Cu(quinoxaline)(Cl_2)$ are examples of molecule-based magnets where the $CuX_4$ dimers are linked into ladders by quinoxaline molecules, where X is either Cl or Br. The rung exchange occurs through the bridging halides while the rail exchange occurs through the quinoxaline rings. Introducing random rung interactions into the system [$Cu (quinoxaline)(Br_2)_{1-x}(Cl_{2})_x$] has caused the spin gap to close, in contrast with the gapped pure spin ladder parents. Crystal growth of non-magnetic-doped molecular magnets, $Cd_xCu_ {1-x}(2,3-dimethylpyrazine)Br_2$, have been performed for several values of the nominal conentration, x, and have been confirmed. The magnetizations and susceptibilities of the magnetically diluted ladder assemblage are presented along with a comparison of the effects of dilution from the pure case (x=0). [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T18.00004: Electron and Spin Transport in Mn$_{12}$ Single Molecule Magnets Bridging Gold Electrodes: How to Determine the Easy Axis Orientation Experimentally Fatemeh Rostamzadeh Renani, George Kirczenow We present a tight-binding theory of Mn$_{12}$ single molecule magnets thiol-bonded to gold electrodes. The model includes both the spatial and spin aspects of the electronic states. Spin-orbit coupling is included explicitly in the Hamiltonian and magnetic anisotropy values in agreement with experiment are obtained. We demonstrate that Mn$_{12}$ single molecule magnets strongly coupled to gold electrodes should exhibit strong spin filtering under appropriate conditions. In experiments, the orientation of the molecule's easy axis relative to leads is not controllable and it has not been feasible to measure it. Our calculations reveal the possibility of determining the easy axis orientation experimentally by means of current measurements: In the molecular junction with the easy axis parallel to leads the current is predicted to be at least two orders of magnitude larger than if the easy axis is perpendicular to the leads, for molecules thiol-bonded to the leads with similar gold-sulfur distances in the two cases. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T18.00005: Hall magnetometry measurements of the susceptibility of variants of Mn$_{12}$-ac Pradeep Subedi, A.D. Kent, B. Wen, M.P. Sarachik, Y. Yeshurun, A.J. Millis, S. Mukherjee, G. Christou The temperature dependence of the inverse magnetic susceptibility of both Mn$_{12}$-ac and Mn$_{12}$-ac-MeOH is found to give finite temperature intercepts, indicating a ferromagnetic phase at low temperature [1, 2]. A magnetic field applied transverse to the Ising axis suppresses the Curie temperature, T$_{CW}$, to a T = 0 quantum critical point. However, the decrease of T$_{CW}$ with transverse field, H$_{\bot }$, in Mn$_{12}$-ac does not agree with mean field calculations performed with a spin Hamiltonian that includes dipolar interactions between molecules and H$_{\bot }$ effects. We attribute the pronounced suppression of T$_{CW}$ of Mn$_{12}$-ac to the effect of random fields arising from a distribution of molecular easy axis tilts due to ligand disorder [1]. Mn$_{12}$-ac-MeOH is of interest in this regard because it appears to have no ligand disorder. We discuss these experiments as well as ongoing studies of Mn$_{12}$-toluate, a faster relaxing variant of Mn$_{12}$, which has a lower effective anisotropy barrier that permits the study of the susceptibility in larger H$_{\bot }$ at very low temperature. \\[0pt] [1] Wen et.al PRB \textbf{82}, 014406 (2010) \\[0pt] [2] Li et.al PRB \textbf{82}, 174405 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T18.00006: Local magnetic susceptibility study of long-range order in Mn$_{12}$-ac Bo Wen, P. Subedi, Y. Yeshurun, M.P. Sarachik, A.D. Kent, A.J. Millis, S. Mukherjee, G. Christou The magnetic susceptibility of single crystals of Mn$_{12}$-ac obeys a Curie-Weiss law, indicating a transition to a ferromagnetic phase below 1 K [1]. Measurements by Hall magnetometry have yielded different temperature intercepts ranging from 0.4 K to 0.9 K. Moreover, these values differ from those obtained for the same crystals in a SQUID-based MPMS. We have proposed two possible origins: (1) the effect of crystal's aspect ratio, reported in [2]; (2) the possibility that the local ordering temperature differs from the global average value. Here we report an ongoing study of the longitudinal magnetic susceptibility at different locations of a single crystal, performed on a Hall sensor array in both zero and finite transverse magnetic field. Preliminary results yield temperature intercepts that are lower near the end of the crystal than in the middle. \\[4pt] [1] Bo WEN et al., PRB 82, 014406 (2010). \\[0pt] [2] Shiqi LI et al., PRB 82, 174405 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T18.00007: Magneto-structural study of M(TCNE)(NCMe)]X molecule-based magnets Konstantin Pokhodnya, Chris Olson, Chris Heth, John Schlueter, Gregory Halder In M$^{II}$(TCNE)(NCMe)$_{2}$]X (M=Fe, Mn, Ni; TCNE = tetracyanoethylene; X = monovalent anion) the magnetic properties can be tuned by systematic altering of transition metal from Mn to Ni, as well as adjusting the inter-plane distances via changing the anion volume (e.g., PF$_{6}$, AsF$_{6}$ and SbF$_{6})$. The magnetic properties of the molecule-based magnets are highly responsive to structural perturbations. For the series of M[TCNE]X magnets the synchrotron-based powder diffraction experiments in combination with magnetic susceptibility, all under hydrostatic pressure, were performed revealing the correlations between metal-ligand bonding and magnetic exchange and allowing the structure-magnetic property correlations to be established. The pathways toward important conductivity and band spin polarization improvements substantial for spin-polarized current injection in microelectronic applications are discussed. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T18.00008: Experimental determination of the Weiss temperature of several variants of Mn12-ac Shiqi Li, Lin Bo, Bo Wen, P. Subedi, Y. Yeshurun, A.D. Kent, M.P. Sarachik, A.J. Millis, C. Lampropoulos, S. Mukherjee, G. Christou We report measurements of the susceptibility in the temperature range from 3.5 K to 6.0 K of a series of Mn12-ac and Mn12-ac-MeOH samples in the shape of rectangular prisms. The susceptibility obeys a Curie-Weiss Law, where the temperature intercept varies systematically with sample aspect ratio. Using published demagnetization factors, we obtain the Curie-Weiss intercept for an infinitely long sample corresponding to intrinsic ordering temperatures Tc $\sim $ 0.85 K and $\sim $ 0.74 K for Mn12-ac and Mn12-ac-MeOH, respectively [1]. The difference in Tc for the two materials suggests an additional non-dipolar (exchange) contribution to the Weiss temperature that differs in the two materials because of the difference in ligand molecules. A similar comparison will also be reported for Mn12-toluate. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T18.00009: Asymmetric Berry-Phase Interference Patterns in a Mn$_{4}$ Single-Molecule Magnet H.M. Quddusi, J. Liu, S. Singh, K. Heroux, E. del Barco, S. Hill, D. Hendrickson We present a low temperature magnetometry study of the quantum interference effect in a Mn$_{4}$ single-molecule magnet. Asymmetric modulations of the Berry phase interference patterns upon application of a transverse field are observed for $k$~$>$~0 resonances (i.e. non-zero longitudinal field), contrary to the symmetric patterns obtained at $k$~=~0. These asymmetries can be reversed by a full inversion of the total applied field. The observation of a fascinating motion of the Berry-phase minima as a function of both the magnitude and direction of the transverse field can be understood as an outcome of a competition between different intramolecular magnetic interactions. A multi-spin description using non-collinear zero-field splitting tensors and intra molecular dipolar interactions between the manganese ions is employed to explain the results. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T18.00010: Observation of double hysteresis in a MnMn$_6$(CH$_3$H$_5$O$_3$)$_3$ single-molecule magnet Klaus Gieb, Wolfgang Kroener, Paul M\"uller, Carl-Georg Freiherr von Richthofen, Thorsten Glaser We report on high field and low temperature magnetization measurements of a novel {MnMn$_6$}(CH$_3$H$_5$O$_3$)$_3$ complex. A home-made micro-Hall-probe magnetometer was used to perform the characterization at mK temperatures and fields up to 17\,T. Most 3d-ion based single-molecule magnets, known up to now, have a spin ground state well separated from the first excited state, leading to the formation of giant spin at low temperatures. In contrast to this situation, the ground state (S$=$6) of the present complex can already be exited at moderate magnetic fields. Surprisingly, magnetic hysteresis was observed for both the ground state and the first exited state leading to a double hysteresis in the low temperature magnetization measurements. The blocking temperature was found to be T$_B \approx$1.3\,K. Origin and possible consequences of this unusual behavior will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T18.00011: A Muon Spin Relaxation Study of NiTCNQ2 Adam Berlie, Ian Terry, Marek Szablewski, Sean Giblin NiTCNQ$_2$ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) is a novel metal-organic material that exhibits a magnetic transition at approximately 20~K. The material was first synthesized by Dunbar \textit{et al} (Chem. Mater. 15, 1840) who identified the low temperature magnetic phase as a glassy ferromagnet. We have investigated this magnetic transition with muon spin relaxation ($\mu$SR). We synthesised the deuterated form of the material to minimise hyperfine coupling between the muon and H nuclear moments on TCNQ. Using $\mu$SR we probed the transition region to determine whether there was a local or long range coherence of the ferromagnetism. We found that zero field measurements yielded a Lorentzian relaxation along with a nuclear component from the nuclear spins of the nitrogen atoms. A longitudinal field of 0.5~mT decoupled the nuclear component revealing a dynamical Kubo-Toyabe relaxation suggesting that there may be a coexisting static and dynamic field distribution. This may imply a long range static ordering below the transition temperature. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T18.00012: Magnetization measurements of a two-leg spin-1/2 ladder with strong leg interactions K. Ninios, Tao Hong, S.N. Herringer, M.M. Turnbull, C.P. Landee, Y. Takano, H.B. Chan We have measured the magnetization of a single crystal of the spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY), using micromechanical force magnetometry down to 30mK. DIMPY undergoes a quantum phase transition from a gapped phase with no long--range magnetic order to a gapless phase, at critical field Hc. Recent specific heat results by Hong et al. show that the gapless phase is a Tomonaga-Luttinger liquid (TLL). Low temperature magnetic susceptibility as a function of field exhibits a maximum near Hc, in consistence with the divergence of the zero temperature susceptibility at Hc expected for an ideal 1-D system. A clear minimum in the magnetization of the gapless phase as a function of temperature is observed, a minimum that marks the limit below which the TLL exists. In addition the field dependence of the TLL parameter K is studied. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T18.00013: Field-induced Tomonaga-Luttinger liquid phase of a two-leg spin-1/2 ladder with strong leg interactions Tao Hong, Y.H. Kim, C. Hotta, Y. Takano, G. Tremelling, M.M. Turnbull, C.P. Landee, H.-J. Kang, N.B. Christensen, K. Lefmann, K.P. Schmidt, G.S. Uhrig, C. Broholm We study the magnetic-field-induced quantum phase transition from a gapped quantum phase that has no magnetic long-range order into a gapless phase in the spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY) [1]. At temperatures below about 1~K, the specific heat in the gapless phase attains an asymptotic linear temperature dependence, characteristic of a Tomonaga-Luttinger liquid. Inelastic neutron scattering and the specific heat measurements in both phases are in good agreement with theoretical calculations, demonstrating that DIMPY is the first model material for an $S=1/2$ two-leg spin ladder in the strong-leg regime. \\[4pt] [1] T. Hong et al., Phys. Rev. Lett. 105, 137207 (2010) [Preview Abstract] |
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