Magnonics: Experiment to Prove the Concept

An experimental scheme for studying spin wave propagation across thin film samples is proposed. An experiment upon a periodically layered nanowire is numerically simulated, while the sample might equally well be a continuous film or an array of elements (e.g. nanowires) that either have uniform composition or are periodically layered as in a magnonic crystal. The experiments could be extended to study domain wall induced spin wave phase shifts, and used for creation of the spin wave magnetic logic devices.Comment: Presented as a poster HP-09 at 50th MMM conference, San Jose, CA (Oct 30 - Nov3, 2005

Dispersion of collective magnonic modes in stacks of nanoscale magnetic elements

Copyright © 2011 American Physical SocietyWe report a numerical study of the dispersion of collective magnonic modes in magnonic crystals formed by stacks of magnetostatically coupled magnetic nanoelements. The calculations reveal that the sign of the magnonic dispersion is determined by the spatial character and ellipticity of precession for the eigenmodes of the isolated elements that give rise to the magnonic bands. We identify a critical value of the ellipticity at which the dispersion of the collective magnonic modes changes sign. The critical value is independent of the magnetic parameters and shape of the elements but is a characteristic of their arrangement (superstructure)

Graded-index magnonics

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics or Institute for Low Temperature Physics and Engineering (Kharkov, Ukraine).The published version can be found at http://hdl.handle.net/10871/22077The wave solutions of the Landau–Lifshitz equation (spin waves) are characterized by some of the most complex and peculiar dispersion relations among all waves. For example, the spin-wave (“magnonic”) dispersion can range from the parabolic law (typical for a quantum-mechanical electron) at short wavelengths to the nonanalytical linear type (typical for light and acoustic phonons) at long wavelengths. Moreover, the long-wavelength magnonic dispersion has a gap and is inherently anisotropic, being naturally negative for a range of relative orientations between the effective field and the spin-wave wave-vector. Nonuniformities in the effective field and magnetization configurations enable the guiding and steering of spin waves in a deliberate manner and there-fore represent landscapes of graded refractive index (graded magnonic index). By analogy to the fields of graded-index photonics and transformation optics, the studies of spin waves in graded magnonic landscapes can be united under the umbrella of the graded-index magnonics theme and are reviewed here with focus on the chal-lenges and opportunities ahead of this exciting research direction.Engineering and Physical Sciences Research Council (EPSRC

Negative permeability due to exchange spin-wave resonances in thin magnetic films with surface pinning

Copyright © 2010 The American Physical SocietyWe report a theory of the effective permeability of multilayered metamaterials containing thin ferromagnetic layers with magnetization pinned on either one or both surfaces. Because of the pinning and small film thickness, the lowest frequency magnetic resonances are due to nonuniform exchange spin waves with frequencies far above those expected for uniform ferromagnetic resonance in known magnetic materials. Yet, the coupling of the nonuniform spin-wave modes to the electromagnetic field is shown to be strong enough to lead, for magnetic parameters characteristic for conventional transition metal alloys, to negative values of the effective permeability at frequencies of several hundred gigahertzs. The permittivity of metals is already negative in this frequency range. Hence, this system represents a negative refractive index metamaterial at subterahertz frequencies. The ways by which to maximize the frequency and the strength of the negative magnetic response are analyzed

Use of the Faraday optical transformer for ultrafast magnetization reversal of nanomagnets

Copyright 2007 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.We propose a new strategy for ultrafast magnetization reversal of nanomagnets. Due to the Inverse Faraday Effect, circularly polarized optical pulses induce a pulsed magnetic flux in materials with large magneto-optical susceptibility. Alternatively, intense optical pulses can induce a pulsed magnetic flux by means of ultrafast demagnetization of a metallic thin film or multilayer with a perpendicular magnetic anisotropy. The time varying magnetic flux induces a transient electro-motive force and electric current in a conducting loop on the surface of the illuminated material, and hence a transient magnetic field. The magnetic field pulses due to the transient current appear to be too short for use in the magnetic field or spin-current induced precessional switching of magnetization. However, our calculations suggest that the magnetic field could lead to ultrafast switching of a nanomagnet overlaid on the surface of the conductor and demagnetized by the same optical pulse. In the case of magnetic pulses due to the Inverse Faraday Effect, the switching direction could be controlled by the helicity of the optical pulse

Nanoscale spin wave valve and phase shifter

Copyright © 2012 American Institute of PhysicsWe have used micromagnetic simulations to demonstrate a method for controlling the amplitude and phase of spin waves propagating inside a magnonic waveguide. The method employs a nanomagnet formed on top of a magnonic waveguide. The function of the proposed device is controlled by defining the static magnetization direction of the nanomagnet. The result is a valve or phase shifter for spin waves, acting as the carrier of information for computation or data processing within the emerging spin wave logic architectures of magnonics. The proposed concept offers such technically important benefits as energy efficiency, non-volatility, and miniaturization

Graded-index magnonics

The wave solutions of the Landau–Lifshitz equation (spin waves) are characterized by some of the most complex and peculiar dispersion relations among all waves. For example, the spin-wave (“magnonic”) dispersion can range from the parabolic law (typical for a quantum-mechanical electron) at short wavelengths to the nonanalytical linear type (typical for light and acoustic phonons) at long wavelengths. Moreover, the longwavelength magnonic dispersion has a gap and is inherently anisotropic, being naturally negative for a range of relative orientations between the effective field and the spin-wave wave vector. Nonuniformities in the effective field and magnetization configurations enable the guiding and steering of spin waves in a deliberate manner and therefore represent landscapes of graded refractive index (graded magnonic index). By analogy to the fields of graded-index photonics and transformation optics, the studies of spin waves in graded magnonic landscapes can be united under the umbrella of the graded-index magnonics theme and are reviewed here with focus on the challenges and opportunities ahead of this exciting research direction

Spectroscopic study of optically induced ultrafast electron dynamics in gold

Copyright © 2007 The American Physical SocietyUsing a supercontinuum pulse as a probe, we have measured the transient reflectivity spectra of a thin film of gold for different values of the pump-probe time delay. The wavelength lambda(x) at which the measured transient reflectivity changes sign has been found to depend upon the time delay, leading to bipolar time resolved signals. The time dependence of lambda(x) has been shown to be consistent with calculations that take into account the full dependence of the reflectivity upon the electron occupation number, and to contradict qualitatively a model in which the signal is assumed to be directly proportional to the occupation number. The shift of lambda(x) has been found to persist at time delays that are much longer than the time required for the electrons to thermalize. Therefore the bipolar reflectivity signals do not necessarily contain a contribution from nonthermalized electrons, as has been previously assumed

Ultrafast magnetization dynamics of spintronic nanostructures

Copyright © 2011 The Royal SocietyThe ultrafast (sub-nanosecond) magnetization dynamics of ferromagnetic thin films and elements that find application in spintronic devices is reviewed. The major advances in the understanding of magnetization dynamics in the two decades since the discovery of giant magnetoresistance and the prediction of spin-transfer torque are discussed, along with the plethora of new experimental techniques developed to make measurements on shorter length and time scales. Particular consideration is given to time-resolved measurements of the magneto-optical Kerr effect, and it is shown how a succession of studies performed with this technique has led to an improved understanding of the dynamics of nanoscale magnets. The dynamics can be surprisingly rich and complicated, with the latest studies of individual nanoscale elements showing that the dependence of the resonant mode spectrum upon the physical structure is still not well understood. Finally, the article surveys the prospects for development of high-frequency spintronic devices and highlights areas in which further study of fundamental properties will be required within the coming decade

Precessional dynamics in microarrays of nanomagnets

Copyright © 2005 American Institute of PhysicsTime resolved scanning Kerr microscopy has been used to study the response of square Ni88Fe12/Co80Fe20 bilayer elements to a pulsed magnetic field. Measurements were performed upon a square element of 6000 nm size and upon 64, 120, 220, 425, and 630 nm square elements that formed square arrays of about 4000 nm total size. While the frequency of precession of the magnetization of the 6000 nm element could be described with a macrospin model, the frequencies observed in the arrays of submicron size elements differed from the macrospin prediction. This observation may be understood in terms of the increasing nonuniformity of the demagnetizing field as the element aspect ratio is decreased