Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T48: Invited Session: Single Molecule Magnets |
Hide Abstracts |
Sponsoring Units: GMAG Chair: Enrique del Barco, University of Central Florida Room: Mile High Ballroom 1A-1B |
Thursday, March 6, 2014 11:15AM - 11:51AM |
T48.00001: Onset of a Propagating Self-Sustained Spin Reversal Front in a Magnetic System Invited Speaker: Andrew D. Kent The energy released in a magnetic material by reversing spins as they relax toward equilibrium can lead to a dynamical magnetic instability in which all the spins in a sample rapidly reverse in a run-away process known as magnetic deflagration. A well-defined front separating reversed and un-reversed spins develops that propagates at a constant speed. This process is akin to a chemical reaction in which a flammable substance ignites and the resulting exothermic reaction leads via thermal conduction to increases in the temperature of an adjacent unburned substance that ignites it. In a magnetic system the reaction is the reversal of spins that releases Zeeman energy and the magnetic anisotropy barrier is the reaction's activation energy. An interesting aspect of magnetic systems is that these key energies--the activation energy and the energy released--can be independently controlled by applied magnetic fields enabling systematic studies of these magnetic instabilities. We have studied the instability that leads to the ignition of magnetic deflagration in a thermally driven Mn$_{12}$-Ac molecular magnet single crystal. Each Mn$_{12}$-ac molecule is a uniaxial nanomagnet with spin 10 and energy barrier of 60 K. We use a longitudinal field (a field parallel to the easy axis) to set the energy released and a transverse field to control the activation energy. A heat pulse is applied to one end of the crystal to initiate the process. We study the crossover between slow magnetic relaxation and rapid, self-sustained magnetic deflagration as a function of these fields at low temperature (0.5 K). An array of Hall sensors adjacent to a single crystal is used to detect and measure the speed of the spin-reversal front. I will describe a simple model we developed based on a reaction-diffusion process that describes our experimental findings. I will also discuss prospects for observing spin-fronts driven by magnetic dipole interactions between molecules that can be sonic, i.e. travel near the speed of sound ($\sim 1000$ m/s). \\[4pt] P. Subedi, S. Velez, F. Macia, S. Li, M. P. Sarachik, J. Tejada, S. Mukherjee, G. Christou and A. D. Kent, Physical Review Letters {\bf 110}, 207203 (2013) [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:27PM |
T48.00002: Magnetic Deflagration and Turbulent Fronts of Quantum Detonation in Molecular Magnets Invited Speaker: Dmitry Garanin Spin tunneling in molecular magnets such as Mn-12, boosted by a strong transverse field, should result in quantum effects in magnetic burning or deflagration. As the dipolar field can block or unblock tunneling resonances, a new possibility of propagating fronts of spin flips opens up that coexists with the standard magnetic deflagration. Here this process is being considered within a full three-dimensional model for an elongated magnet including heat conduction, spin tunneling, and dipolar field created by the changing sample's magnetization. It is shown that within the so-called dipolar window around tunneling resonances, where spin tunneling is possible, the deflagration front is non-flat and similar to a cone with the central part of the front leading. With increasing bias toward the right end of the dipolar window, dipolar instability makes the front turbulent. The latter destroys the exact resonance condition for spins in the front core that leads to fast propagating fronts within the simplified 1d theory. Nevertheless, the dependence of the front speed on the bias is similar to that of the 1d model and the speed reaches sonic values. The latter is a signature of detonation, although here the physical nature of the process is different. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 1:03PM |
T48.00003: Geometric-Phase Interference in a Mn$_{12}$ Single-Molecule Magnet with Truly Fourfold Symmetry Invited Speaker: Jonathan Friedman A single-molecule magnet (SMM) is a large-spin system with an anisotropy barrier separating preferred ``up'' and ``down'' orientations. The spin can tunnel between these directions when an external longitudinal magnetic field brings levels in opposite wells into resonance. When there exist more than one energetically equivalent paths for tunneling, those paths can interfere, a geometric-phase effect that modulates the rate at which spins flip direction. The interference can be controlled by a magnetic field applied perpendicular to the spin's easy magnetization axis. In a ground-breaking experiment, Wernsdorfer and Sessoli~[1] found oscillations in the probability of spin tunneling as a function of the field applied along the hard axis of the Fe$_8$ SMM. This observation confirmed a theoretical prediction by Garg~[2]. Similar geometric-phase interference has been observed in other SMMs that have effective two-fold symmetry, where tunneling involves the interference between two equal-amplitude paths. Such interference effects have not previously been seen in systems with four-fold rotational symmetry. In recent work~[3], my group has seen evidence of the observation of a geometric-phase interference effect in the Mn$_{12}$-$^t$BuAc SMM, a variant of the bellwether Mn$_{12}$-Ac SMM that has true four-fold rotational symmetry (being free of the solvent disorder that breaks the four-fold symmetry in the latter). The spin relaxation rate as a function of the applied transverse magnetic field shows a modulated behavior, with retarded relaxation near where one expects destructive interference between tunneling paths associated with excited states. Tuning the direction of the transverse field away from the hard axis washes out the observed interference effect by favoring one tunneling path over others. Detailed master-equation calculations are used to fit the observed behavior and yield anisotropy parameters consistent with values determined by other groups. Unlike previous observations of geometric-phase interference, which involved ground-state tunneling, the interference effect we observe in Mn$_{12}$-$^t$BuAc takes place in the thermally assisted tunneling regime where tunneling occurs near the top of the barrier. The interference effect enables us to clearly identify which levels participate in the thermally assisted process. Some preliminary results on geometric-phase interference in a version of Mn$_{12}$-Ac that is crystalized without solvent disorder will also be presented.\\[4pt] [1] W. Wernsdorfer and R. Sessoli, Science {\bf 284}, 133 (1999).\\[0pt] [2] A. Garg, Europhys. Lett. {\bf 22}, 205 (1993).\\[0pt] [3] S. T. Adams \emph{et al.}, Phys. Rev. Lett., {\bf 110}, 087205 (2013). [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:39PM |
T48.00004: Three-Leaf Quantum Interference Clovers in a Single-Molecule Magnet Invited Speaker: Enrique del Barco The study of single-molecule magnets bridges the world of the simplest quantum spin systems (S = 1/2) and the macroscopic ensembles that merge with the classical experience. By examining the magnetic behavior of these molecules at low temperature, where the obfuscating effects of thermal fluctuations are practically eliminated, a wealth of detail is revealed about the spin dynamics and the corresponding role played by internal molecular degrees of freedom, with ramifications for the structural symmetry and the specifics of the individual constituent ions. This is the case of the molecular magnet reported in this talk, where the trigonal symmetry imposed by the spatial arrangement of three constituent manganese ions and the corresponding orientations of their single-ion anisotropy tensors results in a fascinating three-fold angular modulation of the quantum tunneling of the magnetization (QTM) rates, as well as in trigonal quantum interference patterns that mimic the form of a three-leaf clover. Interestingly, although expected in all the QTM resonances for a trigonal molecular symmetry, the three-fold modulation only appears at resonances for which a longitudinal magnetic field is applied (i.e. resonances numbers $|$k$|$ $>$ 0). At k = 0, where no longitudinal field is present, the QTM probability displays a six-fold transverse field modulation. This comes as a direct consequence of a three-fold corrugation of the hard anisotropy plane, a predicted but previously unobserved feature which acts as an effective internal longitudinal field that varies the precise conditions required to maintaining a resonance when a transverse field is applied. The sophisticated behavior of the QTM in this molecule allows an unequivocal association of the trigonal distortion of the local spin-orbit interactions with the spatial disposition of the constituent ions. Finally, and of particular significance for the molecular magnetism community, the clear elucidation of the behavior of different resonances with the magnitude of an applied transverse magnetic field unveils the applicability of the spin selection rules within the nature of QTM, including tunneling in odd-numbered resonances. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 2:15PM |
T48.00005: Direct Observation of Magnetic Anisotropy in an Individual Fe_4 Single-Molecule Magnet Invited Speaker: H. S. J. van der Zant |
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