Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B27: Focus Session: Molecular Magnets I |
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Sponsoring Units: GMAG Chair: Stephen Hill, University of Florida Room: Morial Convention Center 219 |
Monday, March 10, 2008 11:15AM - 11:51AM |
B27.00001: Self-Assembled Growth and Magnetism of Ordered Cluster Arrays Invited Speaker: It is generally recognized that the fabrication of magnetic storage media with bit densities of Gigabytes per square inch and more is out of reach of currently available thin film technologies. Patterned media may therefore set off to challenge thin film media as they allow in principle for bit densities several orders of magnitude larger than what is currently feasible. In this talk I will show how nanoclusters can be fabricated on substrates directly by self-assembled growth, and how their magnetism and their lateral arrangement on the substrate can be controlled. Buffer layer assisted growth is used to form clusters of controlled density and size, in the range between a few atoms to several nanometers diameter. The clusters are randomly distributed over the bare substrate surface. The cluster nucleation on the buffer layer and their growth after making contact with the substrate was studied with variable temperature scanning tunneling microscopy, and will be discussed in the talk. The investigation of the cluster magnetism with X-ray magnetic circular dichroism revealed size and strain effects as well as mutual dipolar and cluster-substrate interactions. We found a pronounced dependence of the magnetic anisotropy on the substrate material. On Pt, for instance, the preferential magnetization direction is out-of-plane, while it is in-plane on Ag. The application of self-assembled clusters as individually addressable magnetic units requires their controlled arrangement into well-defined ordered arrays. We are therefore guiding the clusters with energetic sinks provided by periodic network structures prefabricated on the substrate. We use mechanically extremely stable, electronically insulating boron nitride nanomesh monolayers as template surfaces. Repeated cluster deposition cycles increase the cluster density on the nanomesh, eventually resulting in an densely packed, ordered cluster array with a cluster-cluster distance corresponding to the BN nanomesh periodicity of 3.2 nm. These cluster layers offer densities of magnetic elements as high as 80 x 10$^{12}$ clusters per square inch. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B27.00002: Transition linewidth of Mn12-Acetate Beth Parks, Kurt Andresen, Christopher Beedle, David Hendrickson The single-molecule magnet Mn$_{12}$-acetate forms macroscopic crystals in which quantum tunneling of the magnetic moment can be observed. This quantum tunneling occurs due to localized defects in the crystal structure. These same defects are thought to be responsible for broadening the linewidth of transitions between adjacent levels, such as m$_{s} = 10$ to m$_{s} = 9$. It is possible to test this understanding by observing the linewidth of the transition during the tunneling process. If the same defects are responsible for both effects, then as tunneling progresses, the linewidth should change. Results of this measurement of the linewidth obtained using terahertz time-domain spectroscopy will be presented. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B27.00003: Low temperature relaxation crossover in one dimensional chain-like molecular magnet [Fe$^{II}(\Delta)$Fe$^{II}(\Lambda)$(ox)$_{2}$(phen)$_{2}$]$_{n}$ J.L. Her, C.P. Sun, S. Taran, C.C. Chou, C.L. Chan, C.C. Lin, L.L. Li, K.J. Lin, H.D. Yang The frequency-dependent ac susceptibility, thermoremanent magnetization relaxation (TRM) and magnetic field dependent magnetization have performed on a hand-aligned partially orientated molecular magnet compound [Fe$^{II}(\Delta )$Fe$^{II}(\Lambda )$(ox)$_{2}$(phen)$_{2}$]$_{n}$,[1] which exhibits one dimension chain like structure. The ac susceptibility shows spin glass-like relaxation at temperatures between 7.8 and 8.2 K. In addition, the TRM results show various relaxation behaviors below T$_{m}$ $\sim $ 8.6 K, indicating that there is a remnant instability at low temperature. It might be caused by the complex interaction within and/or between the chains and the stacked layers. With slowly sweeping the magnetic field, a step-like behavior in the magnetic hysteresis loop was observed below T$_{m}$. The possible origins for these properties are discussed. \newline [1] L. L. Li, K. J. Lin, C. J. Ho, C. P. Sun, and H. D. Yang, Chem. Commun. \textbf{12}, 1286 (2006). [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B27.00004: A novel experiment using rotating magnetic fields to study the pumping spin states in molecular magnets Alberto Hernandez-Minguez, Ferran Macia, Joan Manel Hernandez, Carla Carbonell, Roger Amig\'o, Javier Tejada We report here a new experimental technique to monitor spin population dynamics in molecular magnets. This deals with a huge rotating magnetic field initially applied along the easy magnetization direction, $z$--axis, that rotates with components parallel and perpendicular to the $z$ axis. This technique allows us to probe spin relaxation on reasonably fast time scales detecting the inversion of the whole spin states. The population of spin levels depends on the frequency of the rotating magnetic field. This very new technique could help to carry out new experiments in a number of different fields, broadening substantially the scope of their use until now. A Hern\'{a}ndez-M\'{\i}nguez \textit{et al.}, Appl. Phys. Lett. \textbf{91}, 202502 (2007) [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B27.00005: Anomalous magnetic relaxation in 2D layered organic-based magnet [Fe(TCNE)(NCMe)$_2$][FeCl$_4$] Jung-Woo Yoo, V. N. Prigodin, K. I. Pokhodnya, Joel S. Miller, A. J. Epstein The magnetic relaxation of the 2D organic-based magnet [Fe(TCNE) (NCMe)$_2$][FeCl$_4$] was explored using both static and dynamic measurements. Static $M$($H$,$T$) studies showed that the ferrimagnetic order between the spins in Fe$^{2+}$ ($S$ = 2) and the spin in (TCNE)$^-$ ($S$ = 1/2) occurs principally within the plane of [Fe(TCNE)(NCMe)$_2$]$^+$ layers, with no magnetic coupling to the $S$ = 5/2 of the [FeCl$_4$]$^-$, which is located between layers [1]. The DC magnetic relaxation in ZFC states shows the memory effects similar to that observed in superparamagnetic systems. This reflects the weak magnetic coupling between the layers enabling the bistable nature between FC and ZFC states [1]. The memory effects disappear when the system is cooled in field supporting bistable nature of interlayer coupling. The dynamic susceptibility near the critical $T$ shows two relaxation processes the possible origins of which will be discussed. \newline [1] K. I. Pokhodnya, ${\it et}$ ${\it al}$. J. Am. Chem. Soc., ${\bf 118} $, 12844 (2006). [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B27.00006: Ferromagnetic resonance (FMR) of a room temperature organic based mixed valent Vanadium Hexacyanochromate magnet K$_{1.54}$V$^{II}_{0.77}$V$^{III}_{0.08}$[Cr$^{III}$(CN)$_{6}$](SO$_{4})_{0.16}$3.1H$_{2}$O. N.P. Raju, J.W. Yoo, Amber C. McConnell, William W. Shum, Kendric J. Nelson, Joel S. Miller, A.J. Epstein Recently, organic materials both magnetic and non-magnetic have been receiving significant attention for their potential applicability in spintronics devices such as spin-valves, memory devices, spin-transistors etc. Here we report magnetic and X-band FMR studies of an organic based mixed valent Vanadium Hexacyanochromate magnet K$_{1.54}$V$^{II}_{0.77}$V$^{III}_{0.08}$[Cr$^{III}$(CN)$_{6}$](SO$_{4})_{0.16}$3.1H$_{2}$O with an ordering temperature well above room temperature ($>$340K). Temperature dependencies of FMR parameters including intensity, linewidth, resonance field will be discussed in terms of the coexistence of long-range magnetic ordering and spin-glass-like behavior in this material. [1] {\O}yvind Hatlevik et. al. Adv. Mater. \textbf{11}, 914 (1999). [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B27.00007: Origin of edge magnetism in zig-zag graphene nanoribbons Jeil Jung, Allan MacDonald We explore the physical origins of edge magnetism in zig-zag terminated graphene nanoribbons addressing the role of exchange effects and the band structure in the ferromagnetic alignment of the spins along the edge, the preference of anti-ferromagnetic inter-edge ground state to the ferromagnetic one, and the microscopic physics of the spin stiffness along an edge. Our analysis of the qualitative physics will rest largely on unrestricted Hartree-Fock theory calculations for $\pi$-band model Hamiltonians with long-range Coulomb interactions. Unlike Hubbard models, or ab initio LDA calculations, this type of theory can consistently account for non-local exchange effects. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B27.00008: Magnetic Properties and Inelastic Neutron Scattering for a Spin Hexamer: Application to the V$_{6}$ Molecular Magnets J. T. Haraldsen, T. Barnes, J. Sinclair, J. Thompson, R. Sacci, J. Turner We present the study of the magnetic susceptibility and inelastic neutron scattering energies and intensities for a spin hexamer formed by two interacting spin S and S' trimers. Using an isotropic Heisenberg Hamiltonian, we conclude that, regardless of spin, the structure factors for the magnetic excitations will have a specific function form which is dependent on the symmetric parts of the hexamer being excited. This work is then compared to previous work performed on two vanadium compounds (CN$_3$H$_6$)$_4$Na$_2$[H$_4$V$_6$O$_8$(PO$_4$)$_4$((OCH$_2$)$_3$CCH$_2 $OH)$_2$]$\cdot 14$H$_2$O and Na$_6$[H$_4$V$_6$O$_8$(PO$_4$)$_4$((OCH$_2$)$_3$CCH$_2$OH)$_2$]$\cdot 18$H$_2$O, which are thought to be magnetically well described as trimers of S=1/2 V$^{4+}$ ions. We show that it is possible to that a very weak inter-trimer interaction exists and we predict the energy and momentum transfer dependence for these compounds that may be observable with inelastic neutron scattering. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B27.00009: Can large magnetic anisotropy and high spin really coexist? Claudia Loose, Eliseo Ruiz, Jordi Cirera, Joan Cano, Santiago Alvarez, Jens Kortus This theoretical study discusses the interplay of the magnetic anisotropy and magnetic exchange interaction of two Mn$_6$ complexes. Our results for two polynuclear Mn$_6$ complexes show a very strong dependence of the D value on the spin of the ground state while the energy barriers are practically constant. Thus, complex 2 with a large spin (S = 12) favoured by ferromagnetic interactions has a small D value, while the lower spin complex 1 (S = 4) has a large D value. Therefor we suggest, that a large magnetic anisotropy is not favoured by a high spin state of the ground state.\\ E.Ruiz et al. Chem. Comm. 2008, DOI: 10.1039/b714715e [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B27.00010: Time Evolution of Electric Properties of Mn$_{12}$-Acetate Film Measured with Self-assembled Tunneling Junction Lianxi Ma, Chi Chen, Glenn Agnolet we report the results of tunneling measurement of Mn$_{12}$-Ac thin film ($\sim $2 monolayer) at the time right after the film is made and after 6 months. We found that for the fresh film the differential conductance can change suddenly at bias voltage about -0.1 V and staircase form of $I-V$ curves are observed. At about 0 V bias voltage, we see the conductance changes from minimum to maximum as the initial resistance decreases. For the film stored for 6 months, however, all of these properties have been lost and high frequency oscillation on conductance is frequently observed. Also for the stored sample, clean $I-V$ curves are rarely seen. All of these results indicate that deterioration of the molecules after 6 months. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B27.00011: Bottom-up creation and adsorption of hybrid organic-inorganic magnetic molecules on metal substrates Daniel Wegner, Ryan Yamachika, Yayu Wang, Michael F. Crommie, Mark Pederson, Bart M. Bartlett, Jeffrey R. Long Charge-transfer compounds of the type M[TCNE]$_x$ (M: transition-metal, TCNE: tetracyanoethylene) form an important group of molecule-based ferromagnets with potential applications due to their high Curie temperature. Despite extensive studies the origin of magnetic coupling is not well understood due to a lack of sufficient structural characterization, largely attributed to disordered growth. Using a bottom-up approach, we have used a scanning tunneling microscope to arrange single V atoms and TCNE molecules to form charge transfer complexes of different size and geometry. By tunneling spectroscopy we prove chemical bonding as well as the emergence of magnetic properties. Special attention is given to the influence of the metallic substrate on the properties of TCNE and the implications for future organic-inorganic nanoscopic devices. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B27.00012: Temperature- and field dependent Raman spectra of the single-molecule magnet Mn$_{12}$-acetate. Robert Furstenberg, Christopher A. Kendziora, Javier Macossay Mn$_{12}$-acetate and related single-molecule magnets are promising candidates for high-density magnetic storage devices, spintronics components and even quantum computing applications. However, certain aspects of their fundamental properties are still not completely understood. For example, the importance of spin-vibron interactions in the calculation of the magnetic anisotropy of Mn$_{12}${\_}acetate has been proposed a few years ago [1]. Some experimental evidence has already been provided [2]. We present further evidence in support of this theory. A detailed Raman scattering study was performed by measuring polarization-dependent spectra in the 35-1500cm$^{-1}$ frequency region, temperature range of 6-300K and in magnetic fields of up to 9T. A symmetry analysis of the vibrational modes revealed the presence of acetic acid disorder-related isomers with C$_{1}$ symmetry. This confirms an earlier low-temperature X-ray study [3]. The observed temperature dependence of vibrational modes was interpreted in terms of spin-vibron interactions. The influence of external magnetic field on Raman spectra will also be discussed. [1] M. Pederson et al., \textit{Phys. Rev. Lett}. 89, 097202 (2002); [2] A.B. Shushkov et al., \textit{Phys. Rev.} B 66, 144430 (2002); [3] Cornia et al., \textit{Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,} 58, 371 (2002) [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B27.00013: Order from disorder in the molecular-based spin ladder Cu(Qnx)Br$_{2}$ Christopher Landee, Chris Sataline, Brian Keith, Igor Partola, Mark Turnbull Copper quinoxaline dibromide is a molecular-based antiferromagnetic spin ladder in which Cu$_{2}$Br$_{4}$ dimers are linked into ladders by the quinoxaline molecules. The rung exchange occurs through the bridging bromide ions while the rail exchange occurs through the organic molecules [1]. It is possible to introduce randomness into this ladder by replacing bromide ions by chlorides, by replacing the quinoxaline molecules by the structurally equivalent 2,3-dimethylpyrazine, or by substituting diamagnetic cadmium ions into the copper sites. In all cases, order occurs at low temperatures ( 4 to 6 K) as evidenced by FC/ZFC studies, hysteresis and remnant magnetizations, as well as sharp anomalies in the susceptibility. [1] C. P. Landee \textit{et al}, \textit{Polyhedron }\textbf{22}, 2325-2329 (2003). [Preview Abstract] |
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