Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M7: Focus Session: Molecular Nanomagnets |
Hide Abstracts |
Sponsoring Units: GMAG Chair: Stephen Hill, Florida State University and NHMFL Room: 106 |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M7.00001: Spintronic anisotropy: proximity-induced superparamagnetism Maciej Misiorny, Michael Hell, Maarten Wegewijs Superparamagnetism of molecular magnets, i.e. the preferential alignment of their spins along an easy axis, is a useful effect for nanoscale applications as it prevents undesired spin reversals. In these systems such a stabilization of axial spin states is ensured by the magnetic anisotropy barrier stemming from intrinsic spin-orbit coupling. Here we demonstrate that any spin-isotropic high-spin quantum dot coupled to ferromagnets can in fact acquire such superparamagnetic properties in a spintronic way [1], even though spin-orbit interaction is negligible. We predict a proximity-induced spin-anisotropy barrier, which has hallmarks of a spintronic exchange-field of quadrupolar nature: it is highly localized, electrically controllable, increases with tunnel coupling and spin-polarization. Such a field is a generalization of the dipolar exchange field that relates to a current-induced spin-torque, effect well established in spintronics [1-3].\newline [1] M. Misiorny, M. Hell and M. Wegewijs, Nature Phys. advanced online publication, 6 October 2013.\newline [2] J. Martinek et al., Phys. Rev. Lett. 91, 127203 (2003); Phys. Rev. B 72, 121302 (2005).\newline [3] J. Hauptmann et al., Nature Phys. 4, 373 (2008).\newline [4] M. Gaass et al., Phys. Rev Lett. 107, 176808 (2011) [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M7.00002: Magnetic Relaxation Mechanisms in Lanthanide Single Molecule Magnets Liviu Chibotaru, Liviu Ungur, Eric McInnes, Richard Winpenny Ab initio investigation of multiplet spectrum of lanthanides in archetypal coordination geometries shows an unexpected regular structure consisting of (i) mirror symmetry of anisotropic magnetic properties of doublet states, (ii) high magnetic axiality of low-lying and high-lying doublets, comparable to complexes with ideal axial symmetry, and (iii) the strong rotation of the anisotropy axes of individual doublets [1]. The obtained high axiality of the ground doublet states explains the SMM behaviour of low-symmetry lanthanide complexes. Ab initio calculations predict that depending on the relative orientation of anisotropy axes in different doublet states, the relaxation can proceed via the first or the second excited state. Here we report new lanthanide cage complexes where two competing relaxation pathways through the first and second excited states are observed, leading to very high energy barriers for loss of magnetisation [2]. \\[4pt] [1] L. Ungur, L.F. Chibotaru, P.C.C.P., 2011, 13, 20086–20090.\\[0pt] [2] R.J. Blagg, L. Ungur, F. Tuna, D. Collison, E.J.L. McInnes L.F. Chibotaru, R.E.P. Winpenny,. Nature Chem., 2013, 5, 673-678. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M7.00003: Three-fold angular modulation of the tunnel splittings in a trigonal Mn3 single-molecule magnet James Atkinson, Ross Inglis, Euan Brechin, Enrique del Barco We report the results of magnetization studies performed on a Mn3 single-molecule magnet of trigonal site symmetry, which detail Berry phase interference phenomenon that we relate to the geometry of the individual constituent ions. We observe for the first time the three-fold modulation of the quantum tunneling probabilities expected for a system of this symmetry, including tunnel-quenching effects resulting from destructive topological interference at tunneling resonances. These effects are symmetric with respect to a full inversion of the applied field (longitudinal and transverse) as a consequence of the time reversal invariance of the spin Hamiltonian. A multi-spin interaction Hamiltonian representing the three exchange-coupled manganese ions shows good agreement with experimental results. Shifts of the Berry phase minima in the transverse field magnitude and angular modulations of the tunnel probability for the various resonances enable a complete determination of the exact three-dimensional spatial orientations of the single-ion anisotropy tensors. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M7.00004: Evidence for Geometric-Phase Interference in a Mn$_{12}$-Acetate Single-Molecule Magnet Changyun Yoo, Jonathan Friedman, Adeline Fournet, George Christou, Yuri Myaesoedov, Eli Zeldov Recent work by our group has shown evidence for geometric-phase interference between tunneling paths in the Mn$_{12}$ $^t$BuAc single-molecule magnet, the first observation of this effect in a system that has true four-fold rotational symmetry [1]. This effect was not previously observed in the bellwether Mn$_{12}$Acetate molecule, presumably because of solvent disorder inherent to the crystal. Here we report measurements on a crystal of Mn$_{12}$Acetate$\,\cdot\,$MeOH, which crystallizes without solvent disorder and therefore preserves the molecule's four-fold symmetry. The relaxation rate $\Gamma$ as a function of transverse field $H_T$ exhibits structure indicative of interference between tunneling paths similar to that found in [1]. This suggests that the solvent disorder, and not the larger dipole interactions found in Mn$_{12}$Acetate, is the most important factor in suppressing the interference effect. \\ \smallskip [1] S. T. Adams et al., Phys. Rev. Lett., {\bf 110}, 087205 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M7.00005: Raman scattering studies of the temperature- and magnetic field-dependent studies of the single molecule magnet Mn$_{12}$-acetate Shi Yuan, Yewon Gim, S.L. Cooper Single molecule magnets (SMMs) have attracted much interest since they were first reported in 1991. SMMs are a class of metal-organic compounds that show superparamagnetic behavior below a certain blocking temperature at the molecular scale. We present a study of the temperature- and magnetic-field-dependence of the single molecule magnet Mn$_{12}$-acetate using Raman scattering. Temperature-dependent measurements show an anomalous phonon behavior near 200K, indicating a lower crystal symmetry than tetragonal and supporting the inclusion of a second-order rhombic term $E(S^2_x - S^2_y)$ in the Hamiltonian, consistent with previous neutron and X-ray studies. Our field-dependent measurements near 3K show that a magnetic field oriented perpendicular to the Mn$_{12}$ magnetization direction does not affect the phonon vibrational energies. However, when the magnetic field is oriented along the easy-axis direction, there is a clear phonon mode splitting at 540 cm$^{-1}$, indicating a strong spin-phonon coupling associated with this phonon mode and the existence of a fourth-order anisotropy term in the Hamiltonian for Mn$_{12}$ acetate. The field-induced nonzero local transverse term may be responsible for a small tilt of the anisotropy axis and the odd resonance tunneling. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M7.00006: The Possibility of Quantum Deflagration in the Fe8 Nano-Magnet Tom Leviant, Eli Zeldov, Yuri Myasoedov, Amit Keren We report spatially resolved, time-dependent, magnetization reversal measurements of Fe8 single crystals using a microscopic Hall bar array. We found that in some samples the molecules reverse their spin direction at the resonance field in the form of deflagration. The deflagration front velocity is on the order of 1 m/sec and sensitive to field gradients and sweep rates. We discuss the possibility that this slow deflagration can be explained by flipping rates determined by the tunnel splitting only, with no over-the-barrier motion. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M7.00007: Pressure tuning of anisotropy barrier in Fe$_8$ SMMs probed using high frequency EPR Komalavalli Thirunavukkuarasu, Christopher Beedle, Stanley Tozer, Stephen Hill Single-molecule magnets (SMMs) are spin systems with large spin ground state where quantum phenomena such as tunneling of magnetization via a considerable anisotropy barrier manifest. One such SMM that has been extensively studied is [Fe$_8$O$_2$(OH)$_{12}$(tacn)$_6$]Br$_8$.9H$_2$O, also known as Fe$_8$, with a giant spin ground state of S=10. The eight Fe atoms bridged by the ligands form a butterfly structure where six Fe atoms have spins up and two spins down in the simplest model. This structure in fact gives rise to geometrical spin frustration effects within the cluster. By varying the interaction between the spins, manipulation of quantum tunneling in SMMs may be achieved. Typically, the manipulation of spin interactions is realized using chemical methods. As an alternative approach, we employ high pressure to induce changes in the ligand-field environment of the Fe atoms. In this presentation, the pressure-dependent changes in the anisotropy barrier in single crystal Fe$_8$ SMMs investigated by high frequency electron paramagnetic resonance measurements will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M7.00008: High-Frequency Electron Paramagnetic Resonance (HFEPR) Studies on Supramolecular Aggregates of Exchange-Biased Single-Molecule Magnets M. Shiddiq, T.N. Nguyen, T. Ghosh, K.A. Abboud, G. Christou, S. Hill Single-Molecule magnets (SMMs) have potential applications in molecular memory and spintronics devices. For these applications, coupling two or more SMMs either to each other or to other components of a device is essential. However, the interaction should be relatively weak in order to maintain the intrinsic properties of each SMM. We have performed comprehensive HFEPR studies on supramolecular aggregates of triangular Mn$_{3}$ SMMs: [Mn$^{\mathrm{III}}_{6}$O$_{2}$(O$_{2}$CMe)$_{6}$(dpd)$_{3}$](I$_{3})_{2}$ ([Mn$_{3}$]$_{2})$ and [Mn$_{12}$O$_{4}$(O$_{2}$CMe)$_{12}$(pdpd)$_{6})$](ClO$_{4})_{4}$([Mn$_{3}$]$_{4})$. Single-crystal [Mn$_{3}$]$_{2}$ spectra shows additional spectral features that are lacking in spectra of the isolated Mn$_{3}$ units, which can be attributed to the exchange coupling within the dimers. Furthermore powder and solution spectra are essentially identical, indicating that the supramolecular dimers remain intact in solution and thus their unique properties survive outside of a crystal. Analysis of the powder spectra of [Mn$_{3}$]$_{4}$ suggests that the four Mn$_{3}$ building blocks are too weakly coupled to be detected by EPR. The results on [Mn$_{3}$]$_{4}$ support the DC susceptibility and micro-SQUID measurements [1].\\[4pt] [1] T. N. Nguyen, et al., JACS 133, 20688-20691 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M7.00009: Spin Hamiltonian Analysis of the SMM V15 Using High Field ESR Mathew Martens, Hans van Tol, Sylvain Bertaina, Bernard Barbara, Achim Muller, Irinel Chiorescu We have studied molecular magnets using high field / high frequency Electron Spin Resonance. Such molecular structures contain many quantum spins linked by exchange interactions and consequently their energy structure is often complex and require a good understanding of the molecular spin Hamiltonian. In particular, we studied the V15 molecule [1], comprised of 15 spins 1/2 and a total spin 1/2, which is a system that recently showed quantum Rabi oscillations of its total quantum spin [2]. This type of molecule is an essential system for advancing molecular structures into quantum computing. We used high frequency characterization techniques (of hundreds of GHz) to gain insight into the exchange anisotropy interactions, crystal field, and anti-symmetric interactions [3] present in this system. We analyzed the data using a detailed numerical analysis of spin interactions and our findings regarding the V15 spin Hamiltonian will be discussed. \\[4pt] [1] I. Chiorescu, W. Wernsdorfer, A. M\"uller, H. B\"ogge, B. Barbara, Phys. Rev. Lett. 84, 3454 (2000).\\[0pt] [2] S. Bertaina, S. Gambarelli, T. Mitra, B. Tsukerblat, Nature 453, 203-U5 (2008).\\[0pt] [3] B. Tsukerblat, A. Tarantul and A. Muller, Physics Letters A 353, 48-59 (2006). [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:39PM |
M7.00010: Spin dynamics of molecular nanomagnets unravelled at atomic scale by four-dimensional inelastic neutron scattering Invited Speaker: Michael Baker The application of inelastic neutron scattering (INS) as a microscopic probe of spin dynamics within molecular based magnets (MM) is discussed with focus on results following recent technological developments. It will be shown that recently-developed INS instrumentation enables single crystal studies of MM, yielding the four-dimensional inelastic-neutron scattering function $S(Q_{xyz},E)$ in vast portions of reciprocal space [1]. Such detailed information of neutron momentum transfer enables spin pair correlations within MM to be directly extracted without the need to pass through a model Hamiltonian. INS results for example MM exhibiting interesting physical properties such as magnetic spin frustration [2] and quantum tunnelling will be presented. The potential of four dimensional INS as a new probe of elusive magnetic phenomena present in MM will be explored. For example, the examination of how a quantum fluctuation propagates around a cyclic antiferromagnetic chain is presented and used to test the degree of validity of the N\'{e}el vector tunneling. \\[4pt] [1] M. L. Baker, T. Guidi, S. Carretta, J. Ollivier, H. Mutka, H. U. G\"{u}del, G.A. Timco, E. J. L. McInnes, G. Amoretti, R. E. P. Winpenny and P. Santini., Nature. Phys., 8, 906, (2012).\\[0pt] [2] M. L. Baker, G. A. Timco, S. Piligkos, J. S. Mathieson, H. Mutka, F. Tuna, P. Kozlowski, M. Antkowiak, T. Guidi, T. Gupta, H. Rath, R. J. Woolfson, G. Kamieniarz, R. G. Pritchard, H. Weihe, L. Cronin, G. Rajaraman, D. Collison, E. J. L. McInnes and R. E. P. Winpenny. Proc. Natl. Acad. Sci., 109, 19113, (2012). [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M7.00011: Quantifying the size of linear superpositions in molecular nanomagnets Filippo Troiani, Paolo Zanardi Molecular nanomagnets are relatively complex spin systems that exhibit quantum mechanical behavior at low temperatures. Exploiting quantum-information theoretic measures we quantify the size of linear superpositions that can be generated within the ground multiplet of high-spin nanomagnets [1]. In particular, we consider the prototypical single-molecule magnets (namely Mn$_{12}$ and Fe$_{8})$, characterized by a ferrimagnetic spin ordering in the ground state. We show that the size of these linear superpositions are comparable to those achievable in mesoscopic systems, and be further enhanced by increasing the asymmetry between the sublattices, and by reducing the competition between exchange interactions within the nanomagnets. The same tools are also applied to the study of low-spin molecules, such as Cr$_{7}$Ni and V$_{15}$, characterized by antiferromagnetic interactions between the constituent spins. The size of the linear superpositions that have been generated within their S$=$1/2 ground doublets is contrasted with that of single s$=$1/2 spins. \\[4pt] [1] F. Troiani and P. Zanardi, Phys. Rev. B 88, 094413 (2013); [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M7.00012: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M7.00013: The first radical-based spintronic memristors: Towards resistive RAMs made of organic magnets Karin Goss, Florian Krist, Simon Seyfferle, Udo Hoefel, Alexa Paretzki, Martin Dressel, Lapo Bogani Using molecules as building blocks for electronic devices offers ample possibilities for new device functionalities due to a chemical tunability much higher than that of standard inorganic materials, and at the same time offers a decrease in the size of the electronic component down to the single-molecule level. Purely organic molecules containing no metallic centers such as organic radicals can serve as an electronic component with magnetic properties due to the unpaired electron in the radical state. Here we present memristive logic units based on organic radicals of the nitronyl-nitroxide kind. Integrating these purely molecular units as a spin coated layer into crossbar arrays, electrically induced unipolar resistive switching is observed with a change in resistance of up to 100\%. We introduce a model based on filamentary reorganization of molecules of different oxidation state revealing the importance of the molecular nature for the switching properties. The major role of the oxidation state of these paramagnetic molecules introduces a magnetic field dependence to the device functionality, which goes along with magnetoresistive charactistics observed for the material. These are the first steps towards a spintronic implementation of organic radicals in electronic devices. [Preview Abstract] |
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