Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V43: Low Dimensional Magnetism: Clusters & Molecules |
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Sponsoring Units: GMAG Chair: V. Kiryukhin, Rutgers University Room: LACC 150C |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V43.00001: Neutron Magnetic Excitation Study of the Giant Magnetic Molecule {\{}Mo$_{72}$Fe$_{30}${\}} V.O. Garlea, S.E. Nagler, J.L. Zarestky, C. Stassis, D. Vaknin, P. Kogerler, D.F. McMorrow, C. Niedermayer, Y. Qiu, D.A. Tennant, B. Lake, M. Exler, J. Schnack, M. Luban We report cold-neutron inelastic scattering measurements on deuterated samples of {\{}Mo$_{72}$Fe$_{30}${\}} Keplerate. The nanocluster {\{}Mo$_{72}$Fe$_{30}${\}} is a molecular quasi-sphere in which 30 Fe$^{3+}$ ions (S = 5/2) occupy the vertices of an icosidodecahedron. The spins interact via an isotropic AF exchange coupling between nearest-neighbors. The measurements reveal a magnetic excitation spectrum that can be interpreted within the context of an effective three-sublattice Hamiltonian model. At mK temperatures, an excitation at E $\sim $ 0.63 meV is observed that can be attributed to the gap between the two lowest rotational-bands predicted by the model. Applied magnetic fields in the 0 -- 8.5 Tesla range confirm the magnetic origin of the spectrum. The temperature and field dependence of the energy spectra are discussed. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V43.00002: Mn clusters: a nanoscale magnetic transition Sudha Srinivas, Koblar A. Jackson, Petia Bobadova-Parvanova, Mihai Horoi Small Mn clusters exhibit remarkable magnetic behavior. Early ESR experiments[1] found the smallest clusters (n=2-5) to be ferromagnetic (FM), while later Stern-Gerlach measurements[2] found larger clusters (n$>$12) to have very small net moments. Our calculations show that these data reflect a transition in magnetic \textit{ordering} as a function of cluster size, occurring at n=7 atoms. Specifically, the FM arrangements of the atomic spins favored in smaller clusters give way to antiferromagnetic (AF) arrangements in larger clusters. We find that the FM $\to $ AF transition occurs at n=7, in agreement with experimental data, and is driven by a large change in the relative energies of the FM and AF structures. We present results for the structures and magnetic properties of Mn$_{n}$ (n = 2-13), focusing on correlations between the structural, electronic and magnetic properties of the clusters and discuss the effect of substitutional impurities on the magnetic properties of the clusters. \begin{enumerate} \item C. A. Baumann et al., J. Chem. Phys. \textbf{78}, 190 (1983). \item M. B. Knickelbein, Phys. Rev. Lett. \textbf{86}, 5255 (2001). \end{enumerate} [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V43.00003: Study of Local Magnetization during Avalanches in Single Crystals of Mn12-acetate Yoko Suzuki, S. Mchugh, R. Gonzalez-Rubio, D. Graybill, M.P. Sarachik, N. Avraham, Y. Myasoedov, H. Shtrikman, E. Zeldov, E.M. Rumberger, D.N. Hendrickson, N.E. Chakov, G. Christou The temporal and spatial evolution of the local magnetization has been measured during and following magnetic avalanches that occur in Mn$_{12}$-acetate during magnetic field sweeps at low temperatures. Single crystals of various sizes and shapes were mounted on an array of micro-Hall detectors, and signals from several Hall sensors were amplified and recorded simultaneously by a data acquisition card. The local magnetization was investigated at different temperatures above 0.3 K for length scales of the order of 10 to 50 microns. We will report the approximate position where the avalanches are triggered, and the estimated speed (or set a lower limit) for propagation of the magnetization avalanches through the crystal. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V43.00004: Effects of microwave on spin tunneling in single-molecule magnets Gwang-Hee Kim, Tae-Suk Kim We study theoretically the effects of the irradiated microwave on the magnetization in single-molecule magnets (SMMs) like V$_{15}$ and Fe$_8$. We find that the shape of magnetization depends on the microwave intensity as well as the microwave polarization. The applied microwave field enhances the tunneling probability. The linearly polarized microwaves induce the suppression of magnetization at both positive and negative magnetic fields. The circularly polarized microwaves are absorbed either at one direction of magnetic field or at both directions of magnetic fields, depending on the polarization directions with respect to the direction of longitudinal magnetic field. The generic features we found will be compared with the recent experimental results. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V43.00005: A high-frequency EPR study of a new S = 10 Mn12 single-molecule magnet Norm Anderson, Anthony Wilson, Jon Lawrence, Sheng-Chiang Lee, Stephen Hill, Muralee Murugesu, George Christou We will present a detailed angle-resolved high-frequency EPR study of a recently discovered analog of the Mn$_{12}${\-}acetate single-molecule magnet (SMM). Like the acetate, the new complex [Mn$_{12}$O$_{12}$(O$_{2}$CCH$_{2}$Bu$^{t})_{16}$(CH$_{3}$OH)$_{4}$]$\cdot $CH$_{3}$OH (Mn$_{12}${\-}tBuAc), possesses a spin $S$~=~10 ground state and $S_{4}$ site symmetry. Magnetic measurements also reveal the usual resonant magnetization tunneling steps in the low temperature hysteresis loops. However, we show that the solvent-disorder-induced anomalies reported in the EPR spectra for Mn$_{12}$-acetate$^{1}$ are absent for Mn$_{12}${\-}tBuAc. This suggests that Mn$_{12}$-tBuAc is intrinsically cleaner, and that detailed studies of this compound may reveal important new information concerning the quantum dynamics of large spins. Indeed, our analysis of the EPR line widths suggest that they are close to the intrinsic lifetime broadened limit, which may make it possible to extract information concerning electronic relaxation times (T$_{1}$ and T$_{2})$. $^{1}$S. Takahashi et al., Phys. Rev. B \textbf{70}, 094429 (2004) [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V43.00006: Electronic Structure, Magnetic Interactions, and the Role of Ligands in Mn$_n$($n$=4, 12) Single-Molecule Magnets Myung Joon Han, Taisuke Ozaki, Jaejun Yu We report our first-principles calculation studies of electronic structure and magnetic properties of Mn$_n$ ($n$=4,12) single-molecule magnets. For the calculations, we used the linear combination of localized pseudo-atomic orbital (LCPAO) method based on the density functional theory within local density approximation (LDA) and LDA+U. To investigate the role of ligands and its contribution to the determination of magnetic properties, we calculated the electronic structures of Mn$_n$ clusters with different ligand configurations. Detailed analysis reveals an important contribution of the bridging carbon atoms, connecting the Mn-O core and the outer ligand complex, to the magnetic ground states of the magnetic molecules. In addition, we calculated the effective exchange-coupling constants among Mn-ions by applying the rigid spin approximation in the non-collinear magnetic perturbation theory within the non-orthogonal LCPAO basis set. The results are in reasonable agreement with the experimental values and the former theoretical ones. We also consider the effect of on-site Coulomb interactions in determination of the magnetic properties of single-moloecule magnets. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V43.00007: Magnetic nanodomains in small Mn$_n$ clusters: a non collinearab initio study Aldo Humberto Romero, Jose Mejia-Lopez, Jose Luis Moran-Lopez, Martin Garcia Small manganese clusters show an intriguing magnetic behavior[1-3]. with signatures of super paramagnetism and magnetic moments smaller than 1.5$\mu_B$ per atom. This fact has called the attention of scientists due to the possibility of using Mn clusters as molecular magnets. Here we discuss the magnetic properties of small magnetic clusters by considering different topologies. Our calculation, a localized orbital pseudopotential, we include non collinear magnetism to describe the electronic properties. We find that only few clusters show noncolliner effects and the majority are collinear. We can assign a exchange constant that we find is antiferromagnetic to small distances and ferromagnetic to large distances, with this exchange we can use a Heisenberg Hamiltonian with dipolar interaction and we find very close results to the ab initio calculations, except that the noncollinear effect is much larger. Perspectives or our observations to larger clusters will be discussed. [1] M. B. Knickelbein, Phys. Rev. Lett {\bf 86} 5255 (2001). [2] K. M. Mertes {\it et al} Sol. Stat. Comm. {\bf 127} 131 (2003). [3] R. J. Van Zee {\it et al} J. Chem Phys. {\bf 76} 5636 (1982). [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V43.00008: Spin-enhanced Magnetocaloric Effect in Molecular Nanomagnets Marco Evangelisti, Andrea Candini, Alberto Ghirri, Marco Affronte, Euan K. Brechin, Eric J.L. McInnes Unprecedentedly large magnetocaloric effect for the temperature region below 10~K is found for the Fe$_{14}$ molecular nanomagnet [Angew. Chem. Int. Ed. {\bf 42}, 3781 (2003)]. We show that this comes out from a combination of several features, such as the spin ground-state that amounting to $S=25 $ is amongst the highest ever reported, and the highly symmetric cluster core that results in small cluster magnetic anisotropy. The latter enables the occurrence of long-range magnetic order below $T_{N}=1.87$~K, probably of antiferromagnetic nature. We also show that low-lying excited $S$ states additionally enhance the MCE of Fe$_{14}$. For these reasons, Fe$_{14}$ has therefore high potentiality to work as low-temperature magnetic refrigerant. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V43.00009: ESR Study of Spin and Charge Order in Organic Spin Chains Michael Dumm, Belal Salameh, Martin Dressel, Lawrence K. Montgomery Interactions between spin, charge, and lattice degrees of freedom result in the extraordinary rich phase diagram of organic spin chain compounds with a whole sequence of ground states like charge order, spin-Peierls, or antiferromagnetism. We studied the quasi one-dimensional organic charge-transfer salts (TMTTF)$_2X$($X$=PF$_6$, AsF$_6$, SbF$_6$, ClO$_4$, BF$_4$, Br and SCN) by X-Band ESR experiments in the temperature range from 4 to 300 K or 500 K. At moderate and high temperatures, the magnetic susceptibility of these compounds can be described by a spin 1/2 antiferromagnetic Heisenberg chain. In this regime, all salts show a linear increase of the linewidth with temperature. The phase transitions into spin- and/or charge-ordered ground states lead to significant changes in spin susceptibility and ESR linewidth. From a detailed analysis of the temperature and angular dependence of our ESR data we extracted important information on the ground states properties. For example, our angular dependent measurements well below the charge-order transition uncover characteristic changes in the anisotropy of the linewidth which can be related to the charge-order patterns. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V43.00010: Origin of the fast magnetization tunneling in [Ni(hmp)(tBuEtOH)Cl]4 Jon Lawrence, Cem Kirman, Stephen Hill, En-Che Yang, David Hendrickson High-frequency (40-360~GHz), angle-dependent EPR data have been collected for single-crystals of [Ni(hmp)(dmb)Cl]$_{4}$, and [Ni$_{0.02}$Zn$_{0.98}$(hmp)(dmb)Cl]$_{4}$. The all-nickel complex behaves as a single-molecule magnet (SMM) at low temperatures, displaying hysteresis and magnetic quantum tunneling. However, in spite of its high symmetry (S$_{4})$, the relaxation is found to be very fast. We show that the origin of this behavior is related to a 4$^{th}$-order transverse crystal-field interaction, $B_{4}^{4}(S_{+}^{4}$~+~$S_{-}^{4})$, which produces a significant tunnel-splitting ($\sim $10~MHz) of the $m_{s}$~=~$\pm $4 ground state of this $S$~=~4 SMM. The fourth-order ($B_{4}^{4})$ and uniaxial ($D)$ crystal-field strengths can be related to the directionality and magnitude of the single-ion interactions ($D_{i}$ and $E_{i})$ at the individual Ni$^{II}$ sites, as determined for the doped crystals. Variable-temperature EPR measurements also reveal the locations of excited spin states ($S$~=~3, 2, etc..), enabling estimates of intra-molecular exchange coupling strengths. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V43.00011: Neutron Scattering from a Coordination Polymer Spin-1/2 Ladder Tao Hong, M.M Turnbull, C.P. Landee, K.P. Schmidt, G.S. Uhrig, Y. Qiu, C. Broholm, D.H. Reich Charge and spin dynamics in Heisenberg spin 1/2 ladders have attracted much attention because of their possible relevance to high-T$_C$ superconductivity. Coordination polymer magnets are excellent systems in which to explore quantum magnetism. However, several previous coordination polymers thought to be spin ladders have turned out to contain alternating spin chains or dimers coupled in two or three dimensions. Here we investigate another possible spin ladder system $\rm Cu(Quinoxaline)Br_2$, in which neutral $\rm Cu_2Br_4$ dimers appear linked to adjacent dimers by bridging quinoxaline molecules along the monoclinic b axis. Inelastic neutron scattering measurements were carried out on a powder sample. The singular onset of magnetic scattering above a finite gap, $\Delta=1.9$ meV, in the spectrum indicates that the material is magnetically one-dimensional. Consideration of the crystal structure suggests that $\rm Cu(Quinoxaline)Br_2$ in that case should be a spin ladder system. We also calculated the one-triplon contribution to the inelastic magnetic scattering by the method of continuous unitary transformation. The excellent agreement with the experimental data supports that conclusion. Final confirmation that $\rm Cu(Quinoxaline)Br_2$ is a spin ladder will require single crystal neutron scattering experiments. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V43.00012: Quantum freezing and re-entrant melting in a quantum spin liquid Collin Broholm, Matthew Stone, Igor Zaliznyak, Daniel Reich, Peter Vorderwisch, Neil Harrison Exchange interactions in piperazinium hexachlorodicuprate (PHCC) produce a frustrated bilayer antiferromagnet. We report the field-temperature phase diagram of this system as determined via high field (up to H = 50 T) susceptibility and neutron scattering (H = 14.2 T) experiments. There are two quantum critical points: Hc1=7.6 T separates the singlet phase from a three dimensional spin-ordered state while Hc2=37 T marks the onset of saturated ferromagnetism. The long range ordered phase is embedded in a gapless quasi-two dimensional paramagnetic regime with short range spin correlations. Close to the low field quantum critical point, a reentrant phase transition between long range order and the singlet phase indicates that weak interactions with lattice or nuclear spin degrees of freedom become important. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V43.00013: Field-Induced 2D XY behavior in Molecular-Based Antiferromagnets Christopher Landee, Qing Xu, Mark Turnbull Two-dimensional XY behavior has long been observed in layered superfluids and superconductors but has never convincingly been detected in magnetic materials. Recent theoretical work$^{1}$ predicts that an external field can cause a well-isolated 2D Heisenberg antiferromagnet to crossover to 2D XY behavior at low temperatures, with characteristic behavior in the magnetization and specific heat. We report on the field-dependent magnetization M(H,T) of Cu(pz)$_{2}$(ClO$_{4})_{2}$ (pz = pyrazine), a well-isolated, molecular-based S=1/2 Heisenberg antiferromagnet (J/k = 17 K), with behavior strongly resembling that predicted by Cuccoli. $^{1}$ A. Cuccoli et al, Phys. Rev. B vol. 68, 060402 (2003). [Preview Abstract] |
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V43.00014: Magnetic Observables for Small Antiferromagnetic $S = 1$ and $S = \frac{3}{2}$ Spin Clusters J.T. Haraldsen, T. Barnes, J.L. Musfeldt Assuming near-neighbor Heisenberg interactions for S=1 and S=3/2 antiferromagnetic spin clusters, we predict the inelastic neutron scattering structure factors for several geometries. Several unusual symmetry-breaking effects are anticipated. Our results are applied to real materials. [Preview Abstract] |
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V43.00015: Magnetic Field Dependent Far-infrared Studies of Manganese-based Single-molecule Magnets Jiufeng Tu, Yoko Suzuki, S. McHugh, M.P. Sarachik, L. Mihaly, G.L. Carr, N.E. Chakov, G. Christou Far-infrared transmission studies of Mn12 single crystals (both aligned crystal assemblies and randomly oriented samples) have been carried out as a function of magnetic field below and above the blocking temperature. In these measurements the complete field-frequency map of the allowed magnetic dipole transitions can be determined as opposed to fixed-frequency cuts generated by standard EPR studies. The m$_s$=10 to 9 absorption lines (10 cm$^{-1}$) for the randomly oriented powder-like Mn12-acetate and Mn12-bromoacetate samples have similar line-widths at 0T, indicating that the disorder associated with the acetic acid crystallization does not dominate the line-width at zero field. Various contributions to the line-width will be discussed, such as: the spin-phonon interaction, dipolar fields, hyper-fine fields, and distributions in the anisotropy field. Interestingly, the 10 to 9 absorption line for the aligned crystal assemblies depends strongly on the magnetic history of the sample below the blocking temperature. The simple matrix element analysis is not adequate to explain this phenomenon, suggesting that it is due to some non-linear optical effects. [Preview Abstract] |
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