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Register a new userSingle-particle versus collective effects in assemblies of nanomagnets: screening
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We discuss experimentally realizable situations in which surface effects may screen out the dipolar interactions in an assembly of nanomagnets, which then behaves as a noninteracting system. We consider three examples of physical observables, equilibrium magnetization, ac susceptibility and ferromagnetic resonance spectrum, to illustrate this screening effect. For this purpose, we summarize the formalism that accounts for both the intrinsic features of the nanomagnets and their collective effects within an assembly the condition for screening.
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I. S. Tupitsyn Pacific Institute for Theoretical Physics
, n University of British Columbia
2004
The spin coherence phenomena and the possibility of their observation in nanomagnetic insulators attract more and more attention in the last several years. Recently it has been shown that in these systems in large transverse magnetic field there can be a fairly narrow coherence window for phonon and nuclear spin-mediated decoherence. What kind of spin dynamics can then be expected in this window in a crystal of magnetic nanomolecules coupled to phonons, to nuclear spin bath and it to each other via dipole-dipole interactions? Studying multispin correlations, we determine the region of parameters where coherent clusters of collective spin excitations can appear. Although two particular systems, namely crystals of $Fe_8$-triazacyclonane and $Mn_{12}$-acetate molecules, are used in this work to illustrate the results, here we are not trying to predict an existence of collective coherent dynamics in some particular system. Instead, we discuss the way how any crystalline system of dipole-dipole coupled nanomolecules can be analyzed to decide whether this system is suitable for attempts to observe coherent dynamics. The presented analysis can be useful in the search for magnetic systems showing the spin coherence phenomena.
The effect of noise on the process of high-speed remagnetization of vortex state of a pentagonal array of five circular magnetic nanoparticles is studied by means of computer simulation of Landau-Lifshits model. The mean switching time and its standard deviation of the reversal between the counterclockwise and clockwise vorticities have been computed. It has been demonstrated that with the reversal by the pulse with sinusoidal shape, the optimal pulse duration exists, which minimizes both the mean switching time (MST) and the standard deviation (SD). Besides, both MST and SD significantly depend on the angle between the reversal magnetic field and pentagon edge, and the optimal angle roughly equals 10 degrees. Also, it is demonstrated that the optimization of the angle, duration and the amplitude of the driving field leads to significant decrease of both MST and SD. In particular, for the considered parameters, the MST can be decreased from 60 ns to 2-3 ns. Such a chain of magnetic nanoparticles can effectively be used as an element of magnetoresistive memory, and at the temperature 300K the stable operation of the element is observed up to rather small size of nanoparticles with the raduis of 20 nm.
We demonstrate, through experiment and theory, enhanced high-frequency current oscillations due to magnetically-induced conduction resonances in superlattices. Strong increase in the ac power originates from complex single-electron dynamics, characterized by abrupt resonant transitions between unbound and localized trajectories, which trigger and shape propagating charge domains. Our data demonstrate that external fields can tune the collective behavior of quantum particles by imprinting configurable patterns in the single-particle classical phase space.
The excitation spectrum and the collective modes of graphene antidot lattices (GALs) are studied in the context of a $pi$-band tight-binding model. The dynamical polarizability and dielectric function are calculated within the random phase approximation. The effect of different kinds of disorder, such as geometric and chemical disorder, are included in our calculations. We highlight the main differences of GALs with respect to single-layer graphene (SLG). Our results show that, in addition to the well-understood bulk plasmon in doped samples, inter-band plasmons appear in GALs. We further show that the static screening properties of undoped and doped GALs are quantitatively different from SLG.
Many living systems use assemblies of soft and slender structures whose deflections allow them to mechanically probe their immediate environment. In this work, we study the collective response of artificial soft hair assemblies to a shear flow by imaging their deflections. At all hair densities, the deflection is found to be proportional to the local shear stress with a proportionality factor that decreases with density. The measured collective stiffening of hairs is modeled both with a microscopic elastohydrodynamic model that takes into account long range hydrodynamic hair-hair interactions and a phenomenological model that treats the hair assemblies as an effective porous medium. While the microscopic model is in reasonable agreement with the experiments at low hair density, the phenomenological model is found to be predictive across the entire density range.
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