Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H15: Molecular Based Magnets |
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Sponsoring Units: GMAG Chair: Arthur Hebard, University of Florida Room: Colorado Convention Center Korbel 4E |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H15.00001: Molecular Magnets based on Fe and TCNE (Tetracyanoethylene, C$_{2}$(CN)$_{4}$): structures from powder diffraction Jae-Hyuk Her, Peter Stephens, Konstantin Pokhodnya, Michael Bonner, Joel Miller There are many interesting organic-based magnets for which understanding is limited because they are not available as single crystals. However, in many cases it has proved possible to determine complete crystal structures from high resolution powder diffraction data. We discuss two specific systems with Fe(II): ~Fe[TCNE]$_{2}$ (which is unexpectedly Fe[TCNE][C$_{4}$(CN)$_{8}$]$_{1/2})$, and Fe[TCNE][MeCN]$_{2}$FeCl$_{4}$. ~Both structures contain a previously unobserved configuration of $\mu_{4}$-[TCNE]$^{-}$ anion bonded to transition metal ions. High resolution powder diffraction patterns were collected at the X16C beamline, National Synchrotron Light Source, Brookhaven National Laboratory.~ Simulated annealing and direct methods programs (FOX, Topas-Academic, EXPO) were used to solve and refine the structures. ~One cannot hope to understand properties of systems such as molecular magnets without knowing their structures, and the present work demonstrates the utility of powder diffraction to obtain that information. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H15.00002: Neutron diffraction and specific-heat studies of magnetic ordering in [Fe$^{II}(\Delta )$Fe$^{II}(\Lambda )$(ox)$_{2}$(Phen)$_{2}$]$_{n}$ molecular magnet C.J. Ho, Chia Pin Sun, C.C. Yang, C.L. Huang, C.C. Chou, Lu-Lin Li, K.J. Lin, W.H. Li, J.W. Lynn, H.D. Yang The magnetic characteristics of molecular magnet [Fe$^{II}(\Delta )$Fe$^{II}(\Lambda )$(ox)$_{2}$(Phen)$_{2}$]$_{n}$ (\textbf{1}), its chemical formula is C$_{28}$H$_{16}$Fe$_{2}$N$_{4}$O$_{8}$ for unity, has been studied by neutron powder diffraction and field dependence of specific heat and magnetization. The intrinsic antiferromagnetic ordering between magnetic Fe ions and magnetic hysteresis below $T_{m}\approx $8.6 K are observed by analyzing Bragg pattern of neutron scattering and isothermal magnetization, respectively. The long-range magnetic ordering (LRMO) is also confirmed from the observation of a small $\lambda $-type anomaly at $T_{m}$ in specific heat measurement. However, the magnetic entropy due to this anomaly is estimated as 0.03R, which is much smaller then expected Rln5 (S=2 for Fe$^{II})$ indicating the spin fluctuations as short-range ordering at $T>T_{m}$. In addition, another magnetic anomaly located at 1K at zero field is increased in temperature and became broadening when applying magnetic field. It might be explained by quantum spin and Zeeman splitting phenomena. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H15.00003: High Frequency Electron Paramagnetic Resonance Studies of High Spin Co(II) Complexes Jon Lawrence, Chris Beedle, En-Che Yang, James Ma, Stephen Hill, David Hendrickson Variable-High-Frequency Electron Paramagnetic Resonance (HFEPR) data have been collected for single crystals of [Zn(hmp)(dmb)Cl]$_{4}$ (\textbf{1}) doped with a small quantity of high-spin Co(II), and an isostructural tetranuclear cobalt complex [Co$^{II}$(hmp)(dmb)Cl]$_{4}$ (\textbf{2}). Crystals of complex \textbf{2 }exhibit low temperature hysteresis, suggesting that it may be a single molecule magnet (SMM).$^{1}$ However, HFEPR data for complex \textbf{2} cannot be fit to a standard Giant Spin model, as is routinely the case for other SMMs. HFEPR data obtained for complex \textbf{1} indicate that the ground state of the Co$^{II}$ ions is an effective spin \textit{S$\prime $}~=~$^{1}$/$_{2}$ Kramers' doublet with a highly anisotropic $g$-tensor. The anisotropy is of the easy-axis type, with the individual easy axis directions tilted away from the crystallographic $c$ direction by 58$^{o}$. We will attempt to rationalize the EPR spectrum obtained for complex \textbf{2} (as well as its possible SMM behavior) in terms of a simple model involving anisotropic exchange coupling between four effective spin \textit{S$\prime $}~=~$^{1}$/$_{2}$ Co$^{II }$ions, with the local anisotropy entering only through the anisotropic g-tensor at each Co$^{II}$ site. $^{1}$ E.-C. Yang, J. Appl. Phys. \textbf{91}, 7382 (2002). [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H15.00004: Tunnelling transport through a Mn$_{12}$ molecular magnet in an external magnetic field. L. Michalak, C.M. Canali, V.G. Benza, M.R. Pederson Recent single-electron-transistor experiments with Mn$_{12}$ as a central island exhibit a puzzling behaviour in the tunnelling conductance as a function of the external magnetic field, such as the lack of magnetic hysteresis. We describe the system in terms of a phenomenological giant spin model with two charge states: the $N$-electron state (neutral molecule) and the $(N+1)$-electron state (one extra electron added). The parameters of the model, such as the total spin and the magnetic anisotropy barrier, are calculated by state-of-the-art DFT. The addition of the tunnelling electron's spin to the giant spin is represented in terms of Schwinger bosons. We compute transport by means of a quantum rate equation in the sequential tunnelling regime, which is appropriate for experimental conditions. We find that the model cannot display hysteresis in the differential conductance as a function of the magnetic field when the coupling with the leads is the only source of relaxation. Coherence effects and cotunnelling are further analyzed by means of a master equation for the full density matrix. DFT calculations can also shed light on the influence of the tunnelling electron orbital degree of freedom on the tunnelling amplitudes. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H15.00005: Exchange constants and second-order magnetic anisotropy in cyanide-bridged Fe2M2 single-molecule magnets Kyungwha Park, Stephen Holmes Electronic structure and intramolecular exchange constants are calculated for three cyanide-bridged single-molecule magnets, {[Tp$^{\star}$Fe$^{\mathrm{III}}$(CN)$_3$M$^{\mathrm{II}}$(DMF)$_4$]$_2$(OTf)$_2$}$\cdot$2DMF (M$^{\mathrm{II}}$=Mn, Co, Ni) (abbreviated as Fe$_2$Mn$_2$, Fe$_2$Co$_2$, and Fe$_2$Ni$_2$) that have been recently synthesized, within a generalized-gradient approximation in spin-polarized density-functional theory (DFT). Due to strong ligand fields present in the molecules, the Fe ions exhibit a low ground-state spin of $S=1/2$ in which the orbital angular momentum may not be quenched even without spin-orbit coupling. Based on the calculated electronic structures, the magnetic anisotropy for Fe$_2$Mn$_2$, Fe$_2$Co$_2$, and Fe$_2$Ni$_2$ is computed including single-electron spin-orbit coupling within a DFT formalism. The theoretical values of the induced orbital angular momentum and of the magnetic anisotropy parameters are compared to those for a single-molecule magnet Mn$_{12}$. The total magnetic anisotropy present in the three single-molecule magnets is due to competition between the magnetic anisotropy of the Fe and of the M ions. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H15.00006: Observation of self-assembled Mn$_{12}$-ac Molecules on Highly Ordered Pyrolytic Graphite Dongmin Seo, Winfried Teizer Thin films of the single molecule magnet Mn$_{12}$-ac have been deposited on Highly Ordered Pyrolytic Graphite (HOPG) by a solution evaporation method [1-3]. Mn$_{12}$-ac molecules in a well-ordered self-assembled triangular lattice were subsequently observed in these films by Scanning Tunneling Microscopy (STM) at room temperature under ambient conditions. STM images show typical center to center intermolecular separations of $\sim $ 5 nm. X-ray photoelectron spectroscopy shows that the self-assembled compound on the HOPG surface is consistent with Mn$_{12}$-ac and a control experiment demonstrated that it cannot be another species that may be present in the solvent. \newline [1] K. Kim et al., Appl. Phys. Lett. 85, 3872 (2004). \newline [2] D. M. Seo et al., J. Mag. Magn. Mater. 301, 31 (2006). \newline [3] D. Seo et al., J. Mag. Magn. Mater. in press (doi:10.1016/j.jmmm.2006.09.034). [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H15.00007: Dispersive Frequency Shifts in the EPR spectra of the Single-Molecule Magnet Fe$_8$ Jonathan Friedman, Mustafa Bal, Christopher Beedle, David Hendrickson High-frequency electron paramagnetic resonance (EPR) has been used to study single-molecule magnets (SMMs) for more than a decade. We observe dispersive effects in a cylindrical cavity when a single crystal of the Fe$_8$ SMM is tuned to resonance with millimeter-wave radiation. The reflected power from the cavity is measured as a function of the radiation frequency at magnetic fields from 0 to 1.5 Tesla and temperatures between 2.0 and 20.0 K. Although the sample/cavity filling factor is small, $\sim$0.1\%, we observe a substantial sample-induced frequency shift of the cavity resonance when the field brings a dipole- allowed transition near resonance with the applied radiation. At 2.0 K, the resonant frequency of the cavity ($\sim$117.5 GHz) exhibits a shift on the order of 10 MHz (comparable to the width of the cavity resonance). At the same time, we observe a reduction in both the cavity Q and the amount of power absorbed by the cavity. The data allows us to gain both dispersive and absorptive information about the material. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H15.00008: Spatial Determination of Magnetic Avalanche Ignition points Reem Jaafar, S. Mchugh, Yoko Suzuki, M.P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, G. Christou Using time-resolved measurements of local magnetization [1], we report studies of the propagation of magnetic avalanches (fast magnetization reversals) in Mn12-ac crystals triggered stochastically in response to a time-varying (swept) magnetic field. The spherical nature of the fronts produced by avalanches originating within the bulk is reflected in the time-of-arrival at an array of micro-Hall sensors placed on the surface of the sample. By treating the propagating front as a spherical bubble of radius $r\propto t$, we locate the approximate ignition points in a two-dimensional cross-section of the crystal. The trigger points are stochastically distributed (some in the bulk and some at the edges), with higher density regions that vary from sample to sample. This suggests that avalanches originate preferentially in weak regions of a crystal where the defect density is high. \newline \newline [1] Yoko Suzuki et al. Phys. Rev. Lett. 95, 147201 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H15.00009: Local Temperature Measurement of Avalanche Front in Mn12-Acetate Sean McHugh, M.P. Sarachik, Y. Suzuki, R. Jaafar, Y. Myasoedov, E. Zeldov, A. Finkler, R. Bagai, G. Christou Local magnetization measurements using micro-Hall bars have recently revealed that a magnetic avalanche propagates as a narrow front moving with subsonic speed. In analogy with chemical deflagration (or combustion), we have proposed ``magnetic deflagration,'' a thermodynamic process by which the reversing spins produce heat stimulating the reversal of neighboring spins in the crystal [1]. Based on this model, a simple calculation yields a ``flame front'' temperature of the order of 10 K or higher. Although it has been established that the average temperature exhibits a (small) increase, confirmation of this model requires local, fast measurements of the temperature of the front. We describe experimental progress using $\mu$m sized arrays of germanium thermometers. \newline \newline [1] Y. Suzuki, et. al., Phys. Rev. Lett. 95, 147201 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H15.00010: Conditions for Triggering Avalanches in Mn$_{12}$-acetate. Yoko Suzuki, S. Mchugh, R. Jaafar, M.P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, N.E. Chakov, G. Christou Recent measurements in Mn$_{12}$-acetate have shown that magnetic avalanches (corresponding to fast magnetization reversal) propagate as a narrow front with a velocity that is roughly two orders of magnitude smaller than the speed of sound. This phenomenon is closely analogous to the propagation of a flame front through a flammable chemical substance (deflagration) [1]. The conditions for nucleation of avalanches triggered in response to a time-varying (swept) magnetic field were studied for different fields and temperatures. In these crystals, avalanches happened only at low temperatures and were found to occur stochastically at fields ranging from 1.0 T to 4.5 T. There is no apparent structure in the distribution of avalanches for fields below $\approx$ 3.5 T; at higher fields we find evidence that the probability is lower at ``nonresonant'' magnetic fields where tunneling across the anisotropy barrier is suppressed. This provides evidence that lowering the barrier by quantum mechanical tunneling facilitates the ignition of avalanches. Based on these and other measurements, we suggest that avalanches are triggered below 3.5 T by defects with lower energy barriers. [1] Y. Suzuki, et al., Phys. Rev. Lett. 95, 147201 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H15.00011: Frequency Domain Magnetic Resonance Spectroscopy in Molecular Magnetism Joris van Slageren We have shown over the past years that frequency domain magnetic resonance spectroscopy (FDMRS) is excellently suited to the determination of zero-field splittings (ZFS) in molecular magnets. Among its merits are: the lack of necessity of an external magnetic field, and easy access to very large zero-field splittings. Several examples will be shown. The magnetic resonance lineshapes give information on distributions in the sample as well as excited spin state dynamics. The theoretical analysis of the origin of the cluster ZFS has shown that antisymmetric exchange interactions can play a large role. Because frequency and magnetic field are independent experimental parameters in the FDMRS technique, more sophisticated experiments can be performed. For example, we have spectroscopically studied the relaxation of the magnetization including quantum tunneling. We have also studied the dipolar interaction between single-molecule magnets using magnetic resonance measurements in solutions of various concentrations. Finally, we have shown that single molecule magnets can function as tunable radiation polarization rotators in the THz range. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H15.00012: Hydrogen Bonding and Multiphonon Structure in One- and Two-Dimensional Polymeric Magnets J.L. Musfeldt, S. Brown, J. Cao, M.M. Conner, A.C. McConnell, H.I. Southerland, J.L. Manson, J.A. Schlueter, M.D. Phillips, M.M. Turnbull, C.P. Landee We report a systematic investigation of the temperature dependent infrared vibrational spectra of a family of chemically related coordination polymeric magnets based upon two different bridging anions, fluoride (F$^-$) and bifluoride (HF$_2^-$), in copper-pyrazine complexes including Cu(HF$_2$)(pyz)$_2$BF$_4$, Cu(HF$_2$)(pyz)$_2$ClO$_4$, and CuF$_2$(H$_2$O)(pyz)). We compare our results with several one- and two-dimensional prototype materials including Cu(NO$_3$)$_2$(pyz) and Cu(ClO$_4$)(pyz) $_2$. Unusual low temperature hydrogen bonding, local structural transitions associated with stronger low-temperature hydrogen bonding, and striking multiphonon effects that derive from coupling of an infrared-active fundamental with strong Raman-active modes of the pyrazine building-block molecule are observed. Based on the spectroscopic evidence, these interactions are common to this family of coordination polymers and work to stabilize the low temperature magnetic state. Similar interactions are likely to be present in other molecule-based magnets. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H15.00013: Preliminary Studies of the Metal Organic Molecule $C_{24} H_{36} N_2 O_4 Cu$ David Wiseby, Danqin Feng, Peter Dowben, Carter Silvernail, John Belot, Anthony Caruso We have investigated the metal organic molecule $C_{24} H_{36} N_2 O_4 Cu$, (CuII) and have characterized some of its electronic and magnetic properties. The molecule is of interest because it has a small magnetic moment of 1.03$\mu _b $ per molecule, expected of a Cu spin 1/2 system. There is some preliminary evidence that vapor deposited thin films of the Cu(II) molecule on Cu(111) and Co(111) are crystalline, with some evidence of band structure ultra violet photoemission spectroscopy (UPS). There is generally good agreement between the photoemission and model calculations performed using restricted Hartree-Fock under the semiempirical PM3 methodology. This new molecule has a HOMO-LUMO gap, but is well screened in the photoemission final state. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H15.00014: Low Temperature Magnetic Behavior of Manganese Carboxylates Marshall Bremer, Shengming Liu, Bridger Anderson, Joseph Sandstrom, Doug Schulz, Anthony Caruso Antiferromagnetic ordering has proved to be very useful in producing high temperature remnant moments in organic-based compounds. A manganese carboxylate-based complex demonstrating strong antiferromagnetic coordination has been discovered and shown to exhibit exotic magnetic behavior at low temperatures. The complex is comprised of two-dimensional sheets containing 12 member, edge sharing hexagons featuring carboxylate bridged manganese. These honeycomb sheets provide the structure which supports several magnetic phase transitions. ac and dc magnetometry data support spin glass, metamagnetic and ferrimagnetic behavior at low temperatures. The interaction parameter J is estimated by comparing mean field theory models to the high temperature susceptibility data. The magnetic states and supporting evidence will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H15.00015: Phonon Bottleneck in the Single-Molecule Magnet Fe$_8$ Induced by Pulsed Millimeter-Wave Radiation M. Bal, Jonathan Friedman, Wei Chen, Mark Tuominen, Evan Rumberger, David Hendrickson We report measurements of the magnetization dynamics of the Fe$_8 $ single-molecule magnet on timescales as short as $\sim$10 ns using millimeter-wave radiation to drive transitions between the ground ($m$ = 10) and first excited ($m$ = 9) states. We find that during the radiation pulse the magnetization decreases linearly, while afterwards it decays exponentially back to its initial value with a long time constant of $\sim$10 $\mu$s. We interpret these results as evidence of a phonon bottleneck in which a non-equilibrium number of phonons resonant with the 10- to-9 transition builds up in the crystal, leading to an population increase in the m = 9 state. The time for these phonons to decay (either by escaping the crystal or through scattering) is interpreted to be the measured $\sim$10 $\mu$s. We observe that the phonon bottleneck is established in less than $\sim$15 ns, which suggests that the spin-phonon relaxation time $T_1$ is (rather unexpectedly) shorter than this value. [Preview Abstract] |
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