Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J16: Focus Session: Molecular Magnets |
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Sponsoring Units: GMAG DMP Chair: Enrique del Barco, University of Central Florida Room: Colorado Convention Center Korbel 4F |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J16.00001: Interactions Between Thin Metallic Films and Mn$_{12}$-Acetate. Joel Means, Winfried Teizer, Kim R. Dunbar Single-molecule magnets are a novel class of materials which have been extensively studied in recent years. One such molecule is Mn$_{12}$-Acetate, [Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{16}$(H$_{2}$O)$_{4}$]$\cdot $2CH$_{3}$COOH$\cdot $4H$_{2}$O. Its high-spin ground state (S=10) at low temperatures leads to many interesting phenomena. Here we explore the effect these molecules have on the electronic transport properties of a normal-conducting, metallic thin film in the temperature range from 0.2K to 1K and magnetic fields up to 3T. The magnetoresistance of Au films is measured in agreement with published results. Measurement of Au films with a few monolayers of Mn$_{12}$-Ac on the surface show a different behavior. [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J16.00002: Magnetism of Rubidium Cobalt Hexacyanoferrate Nanoparticles. D.M. Pajerowski, M.W. Meisel, F.A. Frye, D.R. Talham Although photoinduced magnetism in nanoparticles of Prussian blue analogs has been reported, these samples are superparamagnetic. We have generated and characterized nanoparticles of Rb$_{j}$Co$_{k}$[Fe(CN)$_{6}$]$_{l}\cdot $nH$_{2}$O, which exhibit photoinduced magnetism and, for the largest particles, long-range ferrimagnetism with finite coercive fields. The synthesis involves the variation of the concentration of the poly(vinylpyrrolidone), PVP, the encapsulating polymer, which controls the resulting particle sizes. From HR-TEM, the particle size distributions have been obtained for four batches of samples, with mean diameters ranging from nominally 3~nm to 13~nm. Upon irradiation with white light at 5~K, all samples exhibit photoinduced magnetism. Magnetization studies indicate that the smallest particles are superparamagnetic, while the largest ones are ferrimagnetic with long-range ordering temperatures ($T_{c}$ $\sim $ 17~K) and coercive fields ($H_{c}$ $\sim $ 250~G) varying with particle size in a manner consistent with the predictions of finite-size scaling. [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J16.00003: Fabrication of nano-gapped single-electron transistors for transport studies of individual single-molecule magnets Christopher Ramsey, John Hemderson, Enrique del Barco, Abhudaya Mishra, George Christou Three terminal single-electron transistor devices utilizing Al/Al2O3 gate electrodes were developed for the study of electron transport through individual single-molecule magnets (SMMs). These devices were patterned via multiple layers of optical and electron beam lithography. Al gate electrodes were allowed to oxidize in the ambient atmosphere overnight, creating a robust Al2O3 insulating layer. The single-electron transistor devices were then treated with O2 plasma and Mn12-(3-thiophenecarboxylate) SMMs were self-assembled on the surface. These molecules are Mn12-acetate derivatives, which have been functionalized with thiophene groups and are known to attach to Au surfaces. Self-assembly of the molecules was verified using scanning probe microscopy and XPS measurements. Nano-gapped electrodes were produced at low temperature by electromigration of the 90 nm wide Au wire, reliably yielding 1-3 nm gaps in which the SMM can be situated. We show that the nano-gap spacing can be fine tuned by adding resistance in series with the nanowire. Electron transport measurements were then performed to reveal gate dependent low level (less than 40 meV) excitations in the conductance of a single Mn12 SMM. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:27PM |
J16.00004: Will spin-relaxation times in molecular magnets permit quantum information processing? Invited Speaker: Certain computational tasks can be efficiently implemented using quantum logic, in which the information-carrying elements are permitted to exist in quantum superpositions. To achieve this in practice, a physical system that is suitable for embodying quantum bits (qubits) must be identified. Some proposed scenarios employ electron spins in the solid state, for example phosphorous donors in silicon, quantum dots, heterostructures and endohedral fullerenes, motivated by the long electron-spin relaxation times exhibited by these systems. An alternative electron-spin based proposal exploits the large number of quantum states and the non-degenerate transitions available in high spin molecular magnets. Although these advantages have stimulated vigorous research in molecular magnets, the key question of whether the intrinsic spin relaxation times are long enough has hitherto remained unaddressed. Using X-band pulsed electron spin resonance, we measure the intrinsic spin-lattice ($T_1$) and phase coherence ($T_2$) relaxation times in molecular nanomagnets for the first time. In Cr$_7M$ heterometallic wheels, with $M$ = Ni and Mn, phase coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples $T_2$ reaches 3~$\mu$s at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J16.00005: Transverse magnetization and transient oscillations in the quantum tu nneling of molecular magnets Timothy Ziman, Maxime Clusel We calculate the response of a molecular magnet subject to a time-varying magnetic field and perturbatively coupled to a heat bath. The effective model of a triangle of Heisenberg spins and weak anisotropies is as has been used to model the molecular magnets $\{V_{15}\}$ and $\{Cu_3\}$. Oscillations parallel and transverse to the field direction correspond to transient effects in quantum tunneling. We propose that observations of these oscillations, particularly those transverse to the field, may be an effective way to probe the details of level repulsion and coupling to the environment. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J16.00006: Energy relaxation between low lying tunnel split spin-states of the single molecule magnet Ni$_4$ G. de Loubens, G. D. Chaves-O'Flynn, A. D. Kent, C. Ramsey, E. del Barco, C. Beedle, D. N. Hendrickson We have developed integrated magnetic sensors to study quantum tunneling of magnetization (QTM) in single molecule magnet (SMMs) single crystals. These sensors incorporate a microstrip resonator (30~GHz) and a micro-Hall effect magnetometer. They have been used to investigate the relaxation rates between the 2 lowest lying tunnel split spin-states of the SMM Ni$_4$ ($S=4$). EPR spectroscopy at 30~GHz and 0.4~K and concurrent magnetization measurements of several Ni$_4$ single crystals are presented. EPR enables measurement of the energy splitting between the 2 lowest lying superposition states as a function of the longitudinal and transverse fields. The energy relaxation rate is determined in two ways. First, in cw microwave experiments the change in spin-population together with the microwave absorption directly gives the relaxation time from energy conservation in steady-state. Second, direct time-resolved measurements of the magnetization with pulsed microwave radiation have been performed. The relaxation time is found to vary by several orders of magnitude in different crystals, from a few seconds down to smaller than 100~$\mu$s. We discuss this and the form of the relaxation found for different crystals and pulse conditions. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J16.00007: Electrodes for Molecular Spin-Valves Bruce Hinds, Pawan Tyagi, Steve Holmes, Dongfeng Li Realization of spin devices based on the spin-state of magnetic molecules remains a difficult challenge due to the lack of a reliable molecular electrode fabrication process. We have successfully fabricated magnetic Molecular Junctions (MJ's) by having paramagnetic molecular clusters molecules span across the surface of a metal-insulator-metal tunnel junctions (MJT) [Ta/Co/NiFe/Al$_{2}$O$_{3}(\sim $2nm)/NiFe] at the exposed cross-junction pattern edge. Interestingly the current from $\sim $1$\mu $A to $\sim $1nA (RT, 100mV bias) a short time after molecular attachment presumably due to magnetic ordering. Low temperature in-plane magnetization (77 K, 0.4T) further increased magnetic ordering and decreased the junction current to $\sim $1pA level. Magnetic force microscopy (MFM) spatially showed strong antiferromagnetic coupling between the top and bottom magnetic electrodes. SQIUD- magnetometer study on an array of MJT dots (4um diameter) showed reduction in magnetization after molecular attachment consistent with antiferromagnetic coupling and the dramatic changes in magneto-junction current (uA to pA). [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J16.00008: Ferromagnetism in Organic Iron Phthalocyanine Thin Films Thomas Gredig, Ge Liu, Corneliu N. Colesniuc, Ivan K. Schuller Organic iron phthalocyanine (FePc) thin films were deposited with the planar molecule either parallel or perpendicular to the substrate. Hysteretic ferromagnetic loops are observed below 5 K, lower than the previously found 15 K temperature for short range ordering in bulk powder samples [1]. An induced molecular magnetic anisotropy is found based on ac-susceptibility measurements with the magnetic field parallel and perpendicular to the substrate. The molecular plane spacing, as determined from X-ray diffraction, is correlated with the magnetic susceptibility. This indicates that the molecular spacing, controllable by appropriate substrate and growth temperature selection, is an important parameter for the magnetic properties of FePc. Work supported by AFOSR-MURI. [1] M. Evangelisti, J. Bartolome, L. J. de Jongh, and G. Filoti, Phys. Rev. B \textbf{66}, 144410 (2002). [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J16.00009: High frequency (240 GHz) ferrimagnetic resonance (FMR) of room temperature organic based magnetic semiconductor V[TCNE]$_{x}$ (x$\sim $2) films N.P. Raju, K.I. Pokhodnya, J.van Tol, J.S. Miller, A.J. Epstein V[TCNE]$_{x}$ (x$\sim $2) is an organic based ferrimagnetic semiconducting material ($\rho _{300K}\sim $10$^{2}\Omega $.cm and activation energy, E$_{a} \quad \approx $ 0.5 eV) with an ordering temperature well above room temperature. Magnetoresistance (MR) behavior of this material has been explained on the basis of spin polarization of charge carriers in the $\pi $* electronic subbands of [TCNE]$^{-}$ forming a `half-semiconductor'.[1,2] X-band ($\sim $9 GHz) ferrimagnetic resonance (FMR) studies on V[TCNE]$_{x}$ (x$\sim $2) have been reported earlier.[3] Temperature and angular dependence of FMR spectra of V[TCNE]$_{x}$ (x$\sim $2) films, obtained using $\sim $240 GHz radiation, indicate the coexistence of long-range magnetic ordering and glassy behaviors. These results will be discussed in terms of competing interactions between V$^{2+}$ and [TCNE]$^{-}$ spins based on the local structural order. 1.V.N. Prigodin et. al., Adv. Mater. \textbf{14}, 1230 (2002). 2.N.P. Raju et. al., J. Appl. Phys., \textbf{93}, 6799 (2003). 3.R. Plachy et. al., Phys. Rev. B \textbf{70}, 064411 (2004). [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J16.00010: Exotic quantum magnetization process observed in the \{Cu$_3$\} triangular spin ring K.-Y. Choi, A. P. Reyes, P. L. Kuhns, N. S. Dalal, Y. H. Matsuda, H. Nojiri, F. Hussain, U. Kortz We present a comprehensive set of pulsed field magnetization, ESR, and NMR measurements on the triangle spin ring system [Cu$_{3}$(H$_{2}$O)$_{3}(\alpha $-XW$_{9}$O$_{33})_{2}$]$^{12- }$(X=As, Sb). We observed half step magnetization and hysteresis loops for X=As in a fast sweeping magnetic field of $\sim $10$^{4}$T/s at 0.4 K. These features become less pronounced for X=Sb. A comparative ESR study of both compounds reveals that Dzyaloshinskii-Moriya (DM) interactions are weaker in X=Sb than X=As because of the size difference between the diamagnetic heteroatom X. This leads to a reduction of an anti-level crossing gap in X=Sb compared to X=As. This is consistent with the NMR results which show an appreciable peak of the spin-lattice relaxation rate 1/T$_{1}$ at anti-level crossing fields of 2 and 4.4 T only for X=Sb. Our work suggests that the dependence of half step magnetization on X in a nanocluster system arises from a delicate balance between the adiabatic magnetization and the relaxation rate, relying on DM interactions. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J16.00011: Magnetic ordering and switching of iron porphyrin molecules on a substrate Olle Eriksson, Heiko Wende, Pooja Panchmatia, Biplab Sanyal, Peter Oppeneer, Klaus Baberschke We have studied the structural ordering and the magnetic coupling of in-situ sublimated Fe-based porphyrin molecules on epitaxially grown Ni and Co films on Cu(100) by means of X-ray absorption spectroscopy and X-ray magnetic circular dichroism at third generation synchrotron radiation facilities, in an experimental study which is combined by density functional theory (DFT). We demonstrate the necessary sensitivity to probe the magnetic properties even for sub-monolayer porphyrin coverages. We show that due to 90 degree super-exchange interaction between Fe atoms in the molecules and atoms in the substrate (Co or Ni) the paramagnetic molecules can be made to order ferro magnetically and even have their magnetisation direction switched by a magnetisation reversal of the substrate. Theory is demonstrated to reproduce the experimental observations. [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J16.00012: Spatial and Morphology Controlled Magnetic Patterns on Organic Monolayers Shahid Ahmad, Saleem Rao, Donny Magana, Geoffrey Strouse, Shahid Shaheen We report on the effect of polarity of self-assembled monolayers on magnetic properties and morphology of a deposited magnetic material. Sputtering of Permalloy (Ni79Fe21) on a patterned self-assembled monolayer (SAM) structure results in formation of ferromagnetic film on polar regions and superparamagnetic clusters on non-polar regions of the SAM. The existence of two distinct morphologies of the deposited magnetic material can be attributed to the difference in wettability of the SAM surface. These results demonstrate new opportunities for integration of controlled regions with different magnetic behavior without using conventional lithography. [Preview Abstract] |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J16.00013: Magnetic ordering of new molecule-based magnet: [Fe(TCNE)(NCMe)$_{2}$][FeCl$_{4}$]. Konstantin Pokhodnya, Michael Bonner, Jae-Hyuk Her, Peter W. Stephens, Arthur J. Epstein , Joel S. Miller The magnetic properties of [Fe(TCNE)(NCMe)$_{2}$][FeCl$_{4}$] (TCNE = tetracyanoethylene), a molecule-based magnet synthesized via reaction of FeCl$_{2}$(NCMe)$_{2}$ with TCNE in CH$_{2}$Cl$_{2}$. $M(T)$ is discussed. Both \textit{$\chi $'}($T)$ and \textit{$\chi $}''($T)$, ac susceptibilities are almost frequency independent, and exhibit a sharp peak at $\sim $90 K in accord with $T_{c}$. The zero field cooled (ZFC) and field cooled (FC) magnetic susceptibilities, \textit{$\chi $}$_{ZFC}(T)_{ }$and \textit{$\chi $}$_{FC}(T)$, at 5 Oe rise sharply below 95 K indicative of a magnetic transition. \textit{$\chi $}$_{ZFC}(T)$ reaches maximum at 88 K followed by a rapid decrease suggesting an antiferromagnetic(AFM) ground state attributed to AFM coupling between ferrimagnetically ordered [Fe[TCNE](NCMe)$_{2}$]$^{+}$ layers. In contrast, \textit{$\chi $}$_{FC}(T)$ rises upon further cooling suggesting a strong irreversibility and indicating the presence of a remanant magnetization below 90 K, which increases upon cooling. [Fe(TCNE)(NCMe)$_{2}$][FeCl$_{4}$] is the initial member of a new class of magnets. It is the first metal-TCNE magnet with direct bonding between metal ion and [TCNE]$^{- }$whose structure has been determined, and it possesses a novel planar $\mu _{4}$-[TCNE]$^{-}$ spin coupling unit. [Preview Abstract] |
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