111 research outputs found
Magneto-optical imaging of magnetic deflagration in Mn12-Acetate
For the first time, the morphology and dynamics of spin avalanches in
Mn12-Acetate crystals using magneto-optical imaging has been explored. We
observe an inhomogeneous relaxation of the magnetization, the spins reversing
first at one edge of the crystal and a few milliseconds later at the other end.
Our data fit well with the theory of magnetic deflagration, demonstrating that
very slow deflagration rates can be obtained, which makes new types of
experiments possible.Comment: 5 two-column pages, 3 figures, EPL styl
Semiclassical measures and the Schroedinger flow on Riemannian manifolds
In this article we study limits of Wigner distributions (the so-called
semiclassical measures) corresponding to sequences of solutions to the
semiclassical Schroedinger equation at times scales tending to
infinity as the semiclassical parameter tends to zero (when this is equivalent to consider solutions to the non-semiclassical
Schreodinger equation). Some general results are presented, among which a weak
version of Egorov's theorem that holds in this setting. A complete
characterization is given for the Euclidean space and Zoll manifolds (that is,
manifolds with periodic geodesic flow) via averaging formulae relating the
semiclassical measures corresponding to the evolution to those of the initial
states. The case of the flat torus is also addressed; it is shown that
non-classical behavior may occur when energy concentrates on resonant
frequencies. Moreover, we present an example showing that the semiclassical
measures associated to a sequence of states no longer determines those of their
evolutions. Finally, some results concerning the equation with a potential are
presented.Comment: 18 pages; Theorems 1,2 extendend to deal with arbitrary time-scales;
references adde
Magnetic fringe-field control of electronic transport in an organic film
Random, spatially uncorrelated nuclear-hyperfine fields in organic materials dramatically affect electronic transport properties such as electrical conductivity, photoconductivity, and electroluminescence. The influence of these nuclear-hyperfine fields can be overwhelmed by a uniform externally applied magnetic field, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. As a result, even in applied magnetic fields as small as 10 mT, the kinetics of exciton formation, bipolaron formation, and single-carrier hopping are all modified at room temperature, leading to changes in transport properties in excess of 10% in many materials. Here, we demonstrate a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. (The fringe field is the magnetic field emanating from a ferromagnet, associated with magnetic dipole interactions or, equivalently, the divergence of the magnetization within and at the surfaces of the ferromagnet.) The ferromagnet's fringe fields might act as a substitute for either the applied magnetic field or the inhomogeneous hyperfine field. The size of the effect, the magnetic-field dependence, and hysteretic properties rule out a model where the fringe fields from the ferromagnet provide a local magnetic field that changes the electronic transport properties through the hyperfine field, and show that our effects originate from electrical transport through the inhomogeneous fringe fields coming from the ferromagnet. Surprisingly, these inhomogeneous fringe fields vary over length scales roughly 2 orders of magnitude larger than the hopping length in the organic materials, challenging the fundamental models of magnetoresistance in organic layers which require the correlation length of the inhomogeneous field to correspond roughly to the hopping length
Quantum Magnetic Deflagration in Mn12 Acetate
We report controlled ignition of magnetization reversal avalanches by surface
acoustic waves in a single crystal of Mn12 acetate. Our data show that the
speed of the avalanche exhibits maxima on the magnetic field at the tunneling
resonances of Mn12. Combined with the evidence of magnetic deflagration in Mn12
acetate (Suzuki et al., cond-mat/0506569) this suggests a novel physical
phenomenon: deflagration assisted by quantum tunneling.Comment: 4 figure
Direct observation and imaging of a spin-wave soliton with like symmetry
The prediction and realization of magnetic excitations driven by electrical
currents via the spin transfer torque effect, enables novel magnetic
nano-devices where spin-waves can be used to process and store information. The
functional control of such devices relies on understanding the properties of
non-linear spin-wave excitations. It has been demonstrated that spin waves can
show both an itinerant character, but also appear as localized solitons. So
far, it was assumed that localized solitons have essentially cylindrical,
like symmetry. Using a newly developed high-sensitivity time-resolved
magnetic x-ray microscopy, we instead observe the emergence of a novel
localized soliton excitation with a nodal line, i.e. with like symmetry.
Micromagnetic simulations identify the physical mechanism that controls the
transition from to like solitons. Our results suggest a potential new
pathway to design artificial atoms with tunable dynamical states using
nanoscale magnetic devices
Including fringe fields from a nearby ferromagnet in a percolation theory of organic magnetoresistance
Random hyperfine fields are essential to mechanisms of low-field magnetoresistance in organic semiconductors. Recent experiments have shown that another type of random field fringe fields due to a nearby ferromagnet can also dramatically affect the magnetoresistance. A theoretical analysis of the effect of these fringe fields is challenging, as the fringe field magnitudes and their correlation lengths are orders of magnitude larger than that of the hyperfine couplings. We extend a recent theory of organic magnetoresistance to calculate the magnetoresistance with both hyperfine and fringe fields present. This theory describes several key features of the experimental fringe-field magnetoresistance, including the applied fields where the magnetoresistance reaches extrema, the applied field range of large magnetoresistance effects from the fringe fields, and the sign of the effect
Organic magnetoelectroluminescence for room temperature transduction between magnetic and optical information
Magnetic and spin-based technologies for data storage and processing provide unique challenges for information transduction to light because of magnetic metals' optical loss, and the inefficiency and resistivity of semiconductor spin-based emitters at room temperature. Transduction between magnetic and optical information in typical organic semiconductors poses additional challenges, as the spin-orbit interaction is weak and spin injection from magnetic electrodes has been limited to low temperature and low polarization efficiency. Here we demonstrate room temperature information transduction between a magnet and an organic light-emitting diode that does not require electrical current, based on control via the magnet's remanent field of the exciton recombination process in the organic semiconductor. This demonstration is explained quantitatively within a theory of spin-dependent exciton recombination in the organic semiconductor, driven primarily by gradients in the remanent fringe fields of a few nanometre-thick magnetic film
Singlet-to-triplet interconversion using hyperfine as well as ferromagnetic fringe fields
Until recently the important role that spin-physics ('spintronics') plays in organic light-emitting devices and photovoltaic cells was not sufficiently recognized. This attitude has begun to change. We review our recent work that shows that spatially rapidly varying local magnetic fields that may be present in the organic layer dramatically affect electronic transport properties and electroluminescence efficiency. Competition between spin-dynamics due to these spatially varying fields and an applied, spatially homogeneous magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Spatially rapidly varying local magnetic fields are naturally present in many organic materials in the form of nuclear hyperfine fields, but we will also review a second method of controlling the electrical conductivity/electroluminescence, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. Fringe-field magnetoresistance has a magnitude of several per cent and is hysteretic and anisotropic. This new method of control is sensitive to even remanent magnetic states, leading to different conductivity/electroluminescence values in the absence of an applied field. We briefly review a model based on fringe-field-induced polaronpair spin-dynamics that successfully describes several key features of the experimental fringe-field magnetoresistance and magnetoelectroluminescence
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