Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A31: Focus Session: Molecular Nanomagnets |
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Sponsoring Units: GMAG DMP Chair: Michael Pechan, Miami University Room: 335 |
Monday, March 16, 2009 8:00AM - 8:36AM |
A31.00001: Molecular Spintronics using Molecular Nanomagnets Invited Speaker: A revolution in electronics is in view, with the contemporary evolution of two novel disciplines, spintronics and molecular electronics. A fundamental link between these two fields can be established using molecular magnetic materials and, in particular, single-molecule magnets [1], which combine the classic macroscale properties of a magnet with the quantum properties of a nanoscale entity. The resulting field, molecular spintronics aims at manipulating spins and charges in electronic devices containing one or more molecules. In this context, we want to fabricate, characterize and study molecular devices (molecular spin-transistor, molecular spin-valve and spin filter, molecular double-dot devices, carbon nanotube nano-SQUIDs, etc.) in order to read and manipulate the spin states of the molecule and to perform basic quantum operations. The talk will discuss this--still largely unexplored--field and present our the first important results [2,3].\\[4pt] [1] L. Bogani \& W. Wernsdorfer, Nature Mat. 7, 179 (2008).\\[0pt] [2] J.-P. Cleuziou, W. Wernsdorfer, V. Bouchiat, T. Ondar\c{c}uhu, M. Monthioux, Nature Nanotech. 1, 53-59 (2006).\\[0pt] [3] N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer, F. Balestro, Nature 453, 633 (2008). [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A31.00002: Entrapment of magnetic micro-crystals for on-chip ESR studies Nickolas Groll, Sylvain Bertaina, Mekhala Pati, Naresh S. Dalal, Irinel Chiorescu On-chip Electronic Spin Resonance (ESR) of magnetic molecules requires the ability to precisely position nanosized samples in antinodes for a maximum magnetic coupling. A method is developed to entrap micro-crystals containing spins in a well defined location on the substrate surface. Through the use of photolithography, this method has achieved positioning of single to tens of crystals with micron scale resolution. The method has allowed Q-band EPR measurements of a 175 micron diameter single crystal of BDPA at 34 GHz. Polycrystalline diluted Cr$^{5+}$ spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ systems [1] have been entrapped in 500 micron squares for which the lower limit of the EPR measurement sensitivity was approached. This method gives way to on-chip ESR measurements at dilution refrigerator temperatures by allowing the samples to be positioned inside an on-chip superconducting cavity. [1] N. Sarita et al, Phys. Rev. Lett. 99, 137601 (2007). [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A31.00003: Experimental determination of the dipolar field in Mn$_{12}$-acetate Sean McHugh, R. Jaafar, M.P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, G. Christou Crystals of the molecular magnet Mn$_{12}$-acetate are known to contain a small fraction of defect (minor species) molecules with a small anisotropy barrier against spin reversal. The lower barrier leads to faster magnetic relaxation and lower coercive field. We exploit the low coercive fields of the minor species, and the location of the minor species tunneling resonances, to make a direct determination of the dipole field in Mn$_{12}$-ac. We find that the dipolar field of a fully magnetized crystal is $51.5 \pm 8.5$ mT, consistent with theoretical expectations. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A31.00004: Magnetization barrier reduction in Mn$_{12}$ single-molecule magnets Gage Redler, Changhyun Koo, Saiti Datta, Christos Lampropoulos, Theocharis C. Stamatatos, George Christou, Stephen Hill High-frequency electron paramagnetic resonance (HFEPR) and AC susceptibility data will be presented for a new high-symmetry Mn$_{12}${\-}Ac complex, [Mn$_{12}$O$_{12}$(OAc)$_{16}$(MeOH)$_{4}$]$_{~}\cdot $~MeOH, in which the acetic acid solvent is replaced by a single methanol. The results are compared with those of several other Mn$_{12}$ single-molecule magnets (SMMs), including Mn$_{12}${\-}Ac$_{~}\cdot _{~}$2CH$_{3}$COOH. AC susceptibility studies indicate that Mn$_{12}${\-}Ac$_{~}\cdot _{~}$MeOH has a relatively large effective barrier, $U_{eff}$~$\sim $~74 K, in comparison to Mn$_{12}${\-}Ac$_{~}\cdot _{~}$2CH$_{3}$COOH. Meanwhile, EPR studies suggest more-or-less identical zero-field-splitting parameters for the two complexes. Based on these findings, we discuss the factors that can lead to reductions in $U_{eff}$ in various Mn$_{12}$ SMMs. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A31.00005: The role of quantum tunneling in magnetic avalanches in Mn$_{12}$-acetate Xiang Ma, Bo Wen, S. McHugh, M.P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, G. Christou Steps occur in the hysteresis loop of the molecular magnet, Mn$_{12}$-ac due to quantum tunneling at ``resonant" magnetic fields where the energies of levels on opposite sides of the anisotropy barrier corresponding to different spin projections cross. The effect of quantum tunneling is also evident when magnetic relaxation occurs abruptly as a magnetic avalanche where spin reversal occurs along a narrow front that travels at subsonic speed. In particular, studies have shown that the ignition temperature displays minima and the velocity of the avalanche front shows maxima at the resonant fields. We report measurements of the avalanche speeds triggered in an external magnetic field applied at an angle with respect the c-axis of the crystal, where the transverse component provides a symmetry-breaking field that increases the tunneling rate and magnetic relaxation. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A31.00006: Tuning magnetization avalanches in Mn$_{12}$-acetate Bo Wen, S. McHugh, Xiang Ma, M. P. Sarachik, Y. Myasoedov, H. Shtrikman, E. Zeldov, R. Bagai, G. Christou We report the results of a systematic study of magnetic avalanches (abrupt magnetization reversals) in the molecular magnet Mn$_{12}$-acetate using a micron-sized Hall sensor array. Measurements were taken for: (a) fixed magnetic field (constant barrier against spin reversal); and (b) fixed energy release obtained by adjusting the barrier and $\Delta M$. A detailed comparison with the theory of magnetic deflagration of Garanin and Chudnovsky [1] will be presented and discussed. [1] D. A. Garanin and E. M. Chudnovsky, Phys. Rev. B 76, 054410 (2007) [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A31.00007: Non-adiabatic spin transition in the presence of phonon bottleneck effect Lei Chen, Irinel Chiorescu We present a study on deviations of the magnetization cycle of a two-level spin system from a reversible function into an opened hysteresis cycle due phonon bottleneck effect combined with Landau-Zener transitions. In the case of large zero-field level repulsion the magnetization curves can be described by a simple phonon-bottleneck model, in agreement with recent experiments on molecular magnets (V15 and Ru2 [1]). In the case of small tunneling gaps, as for large spin systems (Mn12 or Fe8), the spin will tunnel with a probability given by the Landau-Zener mechanism. The phonon-bottleneck model is here generalized into a model able to blend the non-adiabatic dynamics of spins with the presence of a non-equilibrium phonon bath [2]. Bloch equations are written in the eigenbasis of the effective spin Hamiltonian, assumed to be a two-level system at low temperatures, with a relaxation term driven by the phonon-bottleneck mechanism. \\[0pt] [1] L. Chen et al, Applied Phys. Lett. 89, 252502 (2006) \\[0pt] [2] L. Chen, I. Chiorescu, cond-mat/0810.2502 [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A31.00008: Geometric-Phase Effect in the Thermally Assisted Resonant Tunneling of Mn$_{12}$-tBuAc J.R. Friedman, E. H. da Silva Neto, C. Lampropoulos, G. Christou, N. Avraham, Y. Myaesoedov, H. Shtrikman, E. Zeldov Mn$_{12}$-tBuAc, like the better-known single-molecule magnet Mn$_{12}$-Ac, relaxes between up and down spin states by thermally assisted resonant tunneling when a longitudinal magnetic field (H$_{L})$ brings energy levels into resonance. In Mn$_{12}$-Ac, tunneling is induced by a second-order transverse anisotropy produced by local solvent disorder. Such disorder makes the observation of any possible geometric-phase interference effect impractical. Mn$_{12}$-tBuAc, in contrast, has negligible solvent disorder and an intrinsic fourth-order transverse anisotropy. We present experimental data on the transverse-field (H$_{T})$ dependence of the magnetic relaxation rate for Mn$_{12}$-tBuAc. When on resonance (H$_{L}$=0), the rate increases as a function of H$_{T }$ in a series of steps and plateaus due to abrupt changes in the dominant tunneling pair of levels. Surprisingly, a similar effect occurs when off resonance (i.e. large H$_{L})$. Detailed numerical simulations show that the experimental results, both on and off of resonance, can be well described if the fourth-order anisotropy is included in the spin Hamiltonian. The results can be understood as arising from a geometric-phase effect that occurs when H$_{T}$ is applied along the hard axis. Support: NSF grant {\#}DMR-0449516. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A31.00009: GGA+U study of exchange interactions in a Mn5 single-molecule magnet Emalee Popoff, Salvador Barraza-Lopez, Kyungwha Park, Hui-Lien Tsai Electronic structure of a single-molecule magnet (SMM) Mn5 is investigated using GGA+U formalism. There are two types of Mn ions in the SMM Mn5: Mn3+ (S=2) and Mn2+ (S=5/2). In a prototype single-molecule magnet Mn12, superexchange interactions between Mn ions through oxygen anions are known to be antiferromagnetic. Our calculation on Mn5, however, showed that the Mn ions are all ferromagnetically coupled to each through various ligands. This results in the ground state spin of S=11, which is in good agreement with experiment. We discuss the nature of the ferromagnetic coupling between the Mn ions by analyzing calculated projected density of states. We also present calculated exchange coupling constants considering various broken symmetry states. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A31.00010: ABSTRACT HAS BEEN MOVED TO SESSION S1 |
Monday, March 16, 2009 10:24AM - 10:36AM |
A31.00011: Magnetism and magnetic anisotropies of small organic molecules Jaime Ferrer, Diego Carrascal, Lucas Fernandez Seivane The ability to enhance and tailor the magnetism of small atomic cluster and molecules will determine whether nanospintronics can be used as a storage technology. We present here our ab initio studies on the magnetism of small organic molecules containing transition metal atoms. We focus specially on 5d atoms like gold, platinum and iridium. These have a large spin-orbit interaction, which generates large magnetic anisotropies in small atomic clusters[1]. [1] L. Fernandez Seivane and J. Ferrer, Phys. Rev. Lett. 99, 183401 (2007). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A31.00012: Anisotropic exchange in tetranuclear Co$^{II }$complexes Saiti Datta, Junjie Liu, Jon Lawrence, Christopher C. Beedle, David N. Hendrickson, Stephen Hill High-frequency electron paramagnetic resonance (HFEPR) studies of the tetranuclear Co$^{II}$ complex [Co(hmp)(dmb)Cl]$_{4}$ (\textbf{1) }reveal the presence of significant zero-field-splitting (ZFS) within the ground state spin multiplet. Meanwhile, low-temperature hysteresis measurements of \textbf{1} provide evidence for slow magnetization relaxation, suggesting that it could be a single-molecule magnet (SMM). However, HFEPR studies of a Zn analog of \textbf{1}, doped with a small quantity of Co$^{II}$, show the ground state of the Co$^{II}$ ions to be an effective spin \textit{S$\prime $}~=~1/2 Kramers doublet with a highly anisotropic $g$-tensor.$^{ }$To understand the origin of the ZFS within the ground state spin multiplet of \textbf{1}, as well as the slow magnetization relaxation, we consider the effect of anisotropic and antisymmetric exchange interactions between the ions within the tetranuclear complex. Our model provides an explanation for the ZFS in the ground state observed via HFEPR, and can also account for qualitative features observed through magnetic measurements. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A31.00013: Magnetization studies of a new single molecule magnet [Net4]3[Mn3Zn2(salox)3O(N3)6Br2]. John Henderson, Enrique del Barco, Changhyun Koo, Stephen Hill, Patrick Feng, David Hendrickson, Motohiro Nakano We present magnetization studies of a novel S=6 single molecule magnet (SMM) [Net4]3[Mn3Zn2(salox)3O(N3)6Br2] (Mn3). The results reveal extremely clean changes in the magnetization associated to the high crystalline quality of the compound. The sample allows a detailed study of the role of molecular symmetry on the nature of the magnetic quantum tunneling relaxation of these molecules. Experiments at temperatures down to 30 mK carried out in a 3D vector superconducting magnet will be presented. The possible role of phonons as originators of the tunneling relaxation of the magnetization in this SMM will be used to explain interesting experimental observations. [Preview Abstract] |
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