Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B31: Focus Session: Single Molecule Magnets |
Hide Abstracts |
Sponsoring Units: GMAG DMP Chair: Stephen Hill, National High Magneticm Field Laboratory Room: 207A |
Monday, March 2, 2015 11:15AM - 11:27AM |
B31.00001: The Relationship Between Torsion and Anisotropic Exchange Coupling in a Tb(III)-Radical Complex Michael L. Baker, Takuya Tanaka, Seiko Kawamura, Kenji Nakajima, Takayuki Ishida, Hiroyuki Nojiri The incorporation of paramagnetic ligands within anisotropic rare earth ion clusters has provided significant advance to the design of single molecule magnets with large blocking temperatures [1]. The exchange interaction in such systems is complex, difficult to probe, and little is known about structural relations. Inelastic neutron scattering and sub-THz electron paramagnetic resonance are used complimentary to investigate the large exchange interaction between a rare earth - radical pair in the Tb(hfac)3(2pyNO) complex [2]. Two molecular species exhibiting different Tb-O-N-C torsion angles of the paramagnetic 2pyNO ligand are compared. Antiferromagnetic Ising type $2p-4f$ exchange is determined for a low torsion angle (3.8 degrees) species. A different species with a larger torsion angle (15.8 degrees) is found to have weaker antiferromagnetic exchange and a non-degenerate ground state doublet. The origin of degeneracy lifting is due to an in-plane ferromagnetic component to the exchange matrix originating from $2p-5d$ charge transfer rather than a Dzyaloshinski-Moriya interaction. \\[4pt] [1] J. D. Rinehart, et. al., Nat Chem 3, 538 (2011).\\[0pt] [2] R. Murakami, et. al., Dalton Trans. 42, 13968 (2013). [Preview Abstract] |
Monday, March 2, 2015 11:27AM - 11:39AM |
B31.00002: Magnetic Behavior of a Dy$_8$ Molecular Nanomagnet Qing Zhang, Myriam Sarachik, Michael Baker, Yizhang Chen, Andrew Kent, Theocharis Stamatatos As part of a study of quantum tunneling in a newly synthesized family of dysprosium-based molecular magnets that exhibit a chiral spin structure, we report initial investigations of the magnetic response of a Dy$_8$ cluster with the formula (Et$_4$N)$_4$[Dy$_8$O(nd)$_8$(NO$_3$)$_{10}$(H$_2$O)$_2$]$\cdot$2MeCN [1]. The molecular complex contains triangular arrangements of exchange coupled Dy(III) ions [2]. The compound forms an approximate snub-square Archimedean lattice unit. The measured magnetization of this network of four triangles suggests the presence of multiple spin chiral vortexes. Single crystal susceptibility and magnetization measurements indicate the presence of a hard-axis direction and an easy plane. These principal orientations have been investigated in magnetic fields up to 5 Tesla for temperatures between 1.8 and 100 K using a SQUID-based Quantum Design MPMS magnetometer. Complex easy plane magnetic hysteresis loops emerge at lower temperatures measured using Hall probe magnetometry at sub 1 K temperatures. The analysis of these measurements will be discussed and compared with results of theoretical calculations. [1] D. I. Alexandropoulos, et al., Inorg. Chem. 53, 5420 (2014). [2] J. Luzon, et al., Phys. Rev. Lett. 100, 247201 (2008). [Preview Abstract] |
Monday, March 2, 2015 11:39AM - 11:51AM |
B31.00003: Magnetic Properties of Electrically Contacted Fe$_{4}$ Molecular Magnets Jacob Burgess, Luigi Malavolti, Valeria Lanzilotto, Matteo Mannini, Frederico Totti, Silviya Ninova, Shichao Yan, Deung-Jang Choi, Steffen Rolf-Pissarczyk, Andrea Cornia, Roberta Sessoli, Sebastian Loth Single molecule magnets (SMMs) are often large and fragile molecules. This poses challenges for the construction of SMM based spintronics. Device geometries with two electronic leads contacting a molecule may be explored via scanning tunneling microscopy (STM). The Fe$_{4}$ molecule [1] stands out as a robust, thermally evaporable SMM, making it ideal for such an experiment. Here we present the first STM investigations of individual Fe$_{4}$ molecules thermally evaporated onto a monolayer of Cu$_{2}$N on a Cu (100) crystal. Using inelastic electron tunneling spectroscopy (IETS), spin excitations in single Fe$_{4}$ molecules can be detected at meV energies. Analysis using a Spin Hamiltonian [2] allows extraction of magnetic properties of individual Fe$_{4}$ molecules, and investigation of the influence of the electronic leads. The tip and sample induce small changes in the magnetic properties of Fe$_{4}$ molecules, making Fe$_{4}$ a promising candidate for the development of spintronics devices based on SMMs. \\[4pt] [1] Nature 468, 417 (2010). [2] Nano Letters 12, 518 (2012). [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:27PM |
B31.00004: Single-Molecule Toroics in Ising-type lanthanide molecular clusters Invited Speaker: Liviu Chibotaru The toroidal magnetic moment [1] is an antisymmetric combination of second-order magnetic moments, possessing distinct symmetry from first-order electromagnetic moments due to the sign change under both space and time inversion. It has been observed for the first time in LiCoPO$_{4}$ as a homogeneous distribution of toroidal polarization [2], which was also the first evidence for the fourth fundamental form of ferroic order, the ferrotoroidicity [3]. Recently an almost net toroidal moment has been detected in Dy$_{3}$ triangles, implying the existence of toroidal quantum states in these complexes [4]. Single-molecule toroics (SMTs) are defined, by analogy with single-molecule magnets (SMMs), as bistable molecules with toroidal magnetic state, which seem to be most promising for future applications in quantum computing and information storage and as molecular multiferroic materials with magnetoelectric effect. The key features offering advantages to CMTs as potential units for storage and processing of information are (i) their insensitivity to external homogeneous magnetic fields and a remarkably weak magnetic interaction between themselves and (ii) the possibility to manipulate the toroidal states by electrical means (charge currents and variable electric fields). In this interdisciplinary research area that spans chemistry, physics and material sciences, synthetic chemists have already produced SMT systems suitable for detailed experimental study, while ab initio calculations have proven their reliability in the description of toroidal magnetization [6] In this presentation, I will review the emerging field of SMTs with particular focus on how recent studies tend to address the issue of toroidal arrangement of local magnetic moment on the metal sites. Nine lanthanide-based SMTs will be presented showing, in particular, that the assembly of wheel-shaped complexes with the high symmetry of the molecule unit and combining strong intermetallic dipolar interactions with strong axial anisotropy on the metal sites represents the most promising route toward the design of efficient SMTs. \\[4pt] [1] V.M. Dubovik, VV. Tugushev, \textit{Physics Reports} \textbf{187}, 145 (1990).\\[0pt] [2] B.B. Van Aken, J.-P. Rivera, H. Schmid, M. Fiebig, \textit{Nature} \textbf{449}, 702 (2007).\\[0pt] [3] N.A. Spaldin, M. Fiebig, M. Mostovoy, \textit{J. Phys.: Condens. Matter} \textbf{20}, 434203 (2008).\\[0pt] [4] L.F. Chibotaru, L. Ungur, A. Soncini, \textit{Angew. Chem. Int.} \textbf{47}, 4126 (2008).\\[0pt] [5] L. Ungur, S.-Y. Lin, J. Tang, L.F. Chibotaru, \textit{Chem. Soc. Rev.} \textbf{43}, 6994 (2014). [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B31.00005: Spontaneous Magnetic Deflagration of \boldmath {$\rm {Mn_{12}tBuAc}$} in a Transverse Field Yizhang Chen, A. D. Kent, Qing Zhang, M. P. Sarachik, Michael L. Baker, D. A. Garanin, Najah Mhesn, Christos Lampropoulos Magnetic deflagration has been triggered in molecular magnets with a swept longitudinal magnetic field [1], acoustic waves [2], and by applying a heat pulse [3-4]. In this work we report a study of the conditions for the spontaneous ignition of magnetic deflagration in the axially symmetric single molecule magnet $\rm Mn_{12} tBuAc$. The onset of spontaneous deflagration shows clear resonant features in the $\rm {H_x} \otimes {H_z}$ plane; here $\rm H_z$ is the longitudinal magnetic field, the bias that reduces the height of the magnetic anisotropy barrier, and $\rm H_x$ is the field transverse to the easy axis that mixes spin states on opposite sides of the anisotropy barrier. Consistent with expectations, the conditions $\rm (H_x, H_z)$ for spontaneous ignition vary with temperature. We show that the speed of the deflagration fronts are strongly reduced near quantum tunneling resonances due to magnetic relaxation prior to spontaneous deflagration events.\\[0pt][1] Yoko Suzuki {\it et al.}, PRL {\bf 95}, 147201 (2005); [2] A. Hern\'andez-M\'inguez {\it et al.}, PRL {\bf 95}, 217205 (2005); [3] S. McHugh {\it et al.} PRB {\bf 76}, 172410 (2007); [4] P. Subedi {\it et al.}, PRL {\bf 110}, 207203 (2013). [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B31.00006: Understanding and controlling the magnetic interaction between Ln(III) bis-(phthalocyanine)s ``Double Decker'' molecular nanomagnets and a magnetic substrate Andrea Candini, Simone Marocchi, Valdis Corradini, Filippo Troiani, Valerio Bellini, Roberto Biagi, Valentina De Renzi, Umberto del Pennino, Marco Affronte, Svetlana Klyatskaya, Mario Ruben, David Klar, Heiko Wende Understanding and controlling the interaction between molecules and substrate is of crucial importance for the realization and implementation of moleculecular devices. Here we present the study by means of XAS and XMCD of the magnetic coupling between LnPc$_2$ ``Double Decker'' (Ln = Tb, Dy, Er) molecular nanomagnets sublimated \textit{in situ} on top of a Ni(111) single crystal. We find an antiferromagnetic exchange coupling between the molecules and the Ni substrate. The observed dependence of the coupling strength on the specific Ln ion is explained by the analysis of the Ln spin-polarized density of states as calculated by DFT. This allows us to identify the microscopic origin of the magnetic interaction between the Ln ions and the molecule environment which happens by the mediation of the organic part of the molecule.\footnote{A. Candini et al., \textit{paper submitted}(2014)} This result will be particularly relevant also for the investigation of molecular spintronics devices employing TbPc$_2$ molecules. Finally, we show how this interaction can be further tuned by the insertion of a graphene layer. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B31.00007: Pushing the Limits of Magnetic Anisotropy in a Mononuclear Ni(II) Single-Molecule Magnet: a High-Field EPR Study Lakshmi Bhaskaran, Katie Marriott, Mark Murrie, Stephen Hill Single-Molecule Magnets (SMMs) are potential candidates for nanoscale magnetic information storage, where slow magnetization dynamics (bistability) is realized at low temperatures due to a magnetic anisotropy barrier separating the ``spin-up'' and ``spin-down'' states of the SMMs. Here, we report spectroscopic evidence for a huge easy-axis anisotropy in a trigonal bipyramidal (TBP) [Ni$^{\mathrm{II}}$Cl$_{3}$(Me-dabco)$_{2}$] complex with an orbitally degenerate ground state. Single-crystal EPR studies were carried out in a 35T resistive magnet at the NHMFL. A very strong angle-dependence of the spectrum was observed within a few degrees of the hard plane, suggesting a huge zero-field-splitting (zfs) parameter, \textbar D\textbar \textgreater 300 cm$^{-1}$, associated with first order spin-orbit coupling. This value is considerably larger than previously reported for a Ni$^{\mathrm{II}}$ TBP complex [1], and is thought to be due to the rigidity of the ligand that prevents Jahn-Teller type effects that can reduce D [2]. This is confirmed by the small value of the rhombic parameter, \textbar E\textbar $=$ 0.66 cm$^{-1}$.\\[4pt] [1] R. Ruamps et al., JACS, 135, 3017-3026 (2013).\\[0pt] [2] M. Gruden-Pavlovic et al., Chem. Sci., 5, 1453-1462 (2014) [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B31.00008: Observation of Highly Forbidden Single-Photon Transitions in a Ni$_4$ Single-Molecule Magnet Yiming Chen, Mohammad D. Ashkezari, Rafael Cassaro, Jonathan Friedman We report electron-spin resonance experiments on a crystal of the single-molecule magnet (SMM) [Ni(hmp)(dmb)Cl]$_4$ (hereafter Ni$_4$), which is an $S=4$ system with large uniaxial anisotropy. At 115.54 GHz and low magnetic fields (below the anisotropy field for the SMM), we observe two weak resonances that correspond to highly forbidden transitions between magnetic sublevels, one corresponding to $\Delta m \approx 6$ and another corresponding to $\Delta m \approx -7$. The interpretation of the observed transitions is confirmed by following how the peak positions change with the angle between the sample's easy axis and the applied field. The selection rules forbidding these transitions are lifted by tunneling between $m$ states. The observed forbidden transitions can be viewed as tunneling-assisted direct single-photon transitions between spin states. Equivalently, the forbidden transitions can be interpreted as resonant tunneling between one spin state of the molecule and the dressed state of another spin state. The forbidden transitions have much narrower line widths than the allowed transitions, which suggests that the lines are not inhomogeneously broadened by local fields. Assuming homogeneous broadening, we infer a decoherence time $T_2$ of $\sim$0.5 ns. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B31.00009: Single Molecule Metamagnetism and the Single Energy Scale Model Bellave Shivaram, Pradeep Kumar, Richard Winpenny, Michael Osofsky, V. Celli Measurements of the magnetization isotherms at low temperatures are used to extract the linear, $\chi_{1}$ and third order, $\chi _{3}$, magnetic susceptibilities of two distinct single molecule magnets (SMM), (UO$_{2}$-L)$_{3}$ and Cr$_{9}$F$_{11}$. In (UO$_{2}$-L)$_{3}$ the behavior of $\chi_{3}$ is consistent with a peak appearing at a temperature T$_{3}$ which is $\sim$ 0.5 T$_{1}$ the temperature at which a peak in $\chi_{1}$ is observed. In Cr$_{9}$F$_{11}$ a peak in $\chi _{1}$ is not present neither is a peak in $\chi_{3}$ observed. Nevertheless, a simple phenomenological model with only a single energy scale, is able to account for the two distinct behaviors in the linear and nonlinear magnetic response of these two SMMs. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B31.00010: Role of spin-orbit fluctuations in spin decoherence Mathew Martens, Johan van Tol, Naresh Dalal, Sylvain Bertaina, Irinel Chiorescu We performed a systematic study of the decoherence mechanism in the molecular compound K$_6$[V$^{IV}_{15}$As$^{III}_6$O$_{42}$(D$_2$O)] $\cdot$ 8D$_2$O, in short V$_{15}$\footnote{M. Martens \textit{et. al.}, PRB \textbf{89}, 2014}, utilizing high-field electron spin resonance at 120 GHz, 241 GHz, and 336 GHz. This system has shown important quantum effects such as coherent spin oscillations\footnote{S. Bertaina \textit{et. al.}, Nature (London) \textbf{466}, 2010} as well as interesting out-of-equilibrium spin dynamics due to phonon bottlenecking\footnote{I. Chiorescu \textit{et. al.}, PRL \textbf{84}, 2000}. The spectra of a single V$_{15}$ crystal were measured and linewidths as a function of orientation, temperature, and field were extracted. By analyzing the shape and orientation anisotropy of the linewidths, we study how fluctuations in each term of the spin Hamiltonian contribute to the spin decoherence with much attention given to the spin-orbit coupling that generates $g$-factor anisotropy. Our conclusion is that fluctuations in the spin-orbit coupling can play an important role in the linewidth of a spin resonance. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B31.00011: Low Temperature Scanning Tunneling Spectroscopy of isolated Mn$_{12}$-Ph Single Molecule Magnets K. Reaves, P. Han, K. Iwaya, T. Hitosugi, D. Packwood, H.G. Katzgraber, H. Zhao, K.R. Dunbar, K. Kim, W. Teizer We study Mn$_{12}$O$_{12}$(C$_6$H$_5$COO)$_{16}$(H$_2$O)$_4$ (Mn$_{12}$-Ph) single-molecule magnets on a Cu(111) surface using scanning tunneling microscopy and scanning tunneling spectroscopy at cryogenic temperatures (T $<$ 6K). We report the observation of Mn$_{12}$-Ph in isolation and in thin films, deposited through $in\,situ$ vacuum spray deposition onto clean Cu(111). The tunneling current of isolated Mn$_{12}$-Ph, normalized with respect to the Cu background, shows a strong bias voltage dependence within the molecular interior. The qualitative features of these I vs.V curves differ by spatial location in several intriguing ways (e.g. fixed junction impedance with increasing bias voltages). We explore these normalized I vs. V curves and present a phenomenological explanation for the observed behaviors, corresponding to the physical and electronic structure within the molecule. [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B31.00012: Quantum coherence in Mn-based single molecule magnets C. Abeywardana, F.H. Cho, A. Mowson, G. Christou, S. Takahashi As spin systems in solids, single-molecule magnets (SMMs) form a unique class of materials that have a high-spin, and their spin state and interaction can be easily tuned by changing peripheral organic ligands and solvate molecules. In addition, it has been shown that an individual or a small ensemble of SMMs can be transferred to surface with retention of their magnetic behavior. SMM is therefore a promising system for fundamental quantum science and for applications to dense and efficient quantum memory, computing, and molecular spintronics devices. In spite of diverse interests on quantum properties in SMMs, decoherence properties that ultimately limit such behaviors have not been understood yet. Until now, coherent manipulation of spin states in SMMs has been experimentally demonstrated only in a few SMMs [1-2]. In this presentation, we investigate quantum coherence in Mn-based SMMs using a high-frequency pulsed EPR technique, which has a significant advantage to quench the spin decoherence due to electron spins [3]. \\[4pt] [1] S. Takahashi et al., Nature \textbf{476 }, 76 (2011).\\[0pt] [2] S. Takahashi et al., Phys. Rev. Lett. \textbf{102}, 087603 (2009).\\[0pt] [3] S. Takahashi et al., Phys. Rev. Lett. \textbf{101}, 047601 (2008). [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B31.00013: Interface Effects in Spin-crossover (SCO) Thin Films on Au(111) Sumit Beniwal, Xin Zhang, Patrick Rosa, Jean-Francois Letard, Tatiana Palamarciuc, Bernard Doudin, Peter Dowben, Axel Enders Thin films of the SCO molecules [Fe(H$_{\mathrm{2}}$B(pz)$_{\mathrm{2}})_{\mathrm{2}}$(bipy)] on Au(111) are investigated. The growth mode is determined by low temperature scanning tunneling microscopy, whereas chemical and electronic properties are determined with X-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy (IPES). The role of substrate in determining the electronic structure is determined from thickness and temperature dependent XPS. Thin films exhibit coexistence of Fe(II) and Fe(III) oxidation states, which is different from the Fe(II) oxidation state in bulk. The fraction of molecules in the Fe(II) state increases with film thickness, which suggests that the molecules at the interface are in the Fe(III) state. Cooling the films to 100 K triggers an irreversible transition from Fe(III) to Fe(II). This transition coincides with spin phase transition, where shift of the conduction band edge away from the Fermi level is observed in IPES. These results demonstrate that thin films of this complex have different phase transition behavior as compared to bulk-like samples and underline that substrate interaction is a powerful parameter to control their structural conformation, spin state as well as electronic properties. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700