Publications by John Gregg

Onset of spin wave time-domain fractals in a dynamic artificial crystal

Journal of Magnetism and Magnetic Materials Elsevier 495 (2019) 165868

A Inglis, J Gregg

<p>We report on the first observation of an exact fractal pattern in the time-domain arising spontaneously from adynamic artificial crystal (DAC). The all-magnon process occurs in a passive, unlithographed magnetic wave-guide. The DAC was created by a standing spin wave of frequency ƒ<sub>DAC</sub> in a region of nonlinear waveguide, resulting in a spatio-temporally periodic potential. The interaction of travelling spin waves of frequency ƒ′ with the DAC resulted in a series of new modes appearing in a comb with intervals Δƒ<sup>(0)</sup> = | ƒ<sub>DAC</sub> − ƒ′|. As the magnetic field <em>H</em> was increased a 1st pre-fractal pattern was observed with frequency interval Δƒ<sup>(1)</sup> = Δƒ<sup>(0)</sup>/2. Finally, the onset of a 2nd pre-fractal was observed with frequency interval Δƒ<sup>(2)</sup> = Δƒ<sup>(1)</sup>/2 = Δƒ<sup>(0)</sup>/4. The magnetic field dependence of the nonlinear signals matched that of the DAC. The spin wave fractal behavior was demonstrated for different values of ƒ′.</p>

Indirect observation of phase conjugate magnons from non-degenerate four-wave mixing

SN Applied Sciences Springer Nature 1 (2019) 480

A Inglis, CJ Tock, JF Gregg

Phase modulation and amplitude modulation interconversion for magnonic circuits

Physical Review Applied American Physical Society 11 (2019) 044065

C Tock, JF Gregg

Magnonic circuit elements generally operate using signals that are either amplitude or phase modulated. We propose a simple all-magnon circuit element capable of converting between these two types of encoding, and support our design with simulations. We highlight the potential of our technique to augment existing devices and propose an alternative schematic for an “equality” gate and exclusive nor gate.

Oscillatory energy exchange between waves coupled by a dynamic artificial crystal.

Phys Rev Lett 108 (2012) 015505-

AD Karenowska, JF Gregg, VS Tiberkevich, AN Slavin, AV Chumak, AA Serga, B Hillebrands

We describe a general mechanism of controllable energy exchange between waves propagating in a dynamic artificial crystal. We show that if a spatial periodicity is temporarily imposed on the transmission properties of a wave-carrying medium while a wave is inside, this wave is coupled to a secondary counterpropagating wave and energy oscillates between the two. The oscillation frequency is determined by the width of the spectral band gap created by the periodicity and the frequency difference between the coupled waves. The effect is demonstrated with spin waves in a dynamic magnonic crystal.

Oscillatory Energy Exchange between Waves Coupled by a Dynamic Artificial Crystal


AD Karenowska, JF Gregg, VS Tiberkevich, AN Slavin, AV Chumak, AA Serga, B Hillebrands

Employing magnonic crystals to dictate the characteristics of auto-oscillatory spin-wave system


AD Karenowska, AV Chumak, AA Serga, JF Gregg, B Hillebrands

Spin information transfer and transport in hybrid spinmechatronic structures


AD Karenowska, JF Gregg, AV Chumak, AA Serga, B Hillebrands

Employing magnonic crystals to dictate the characteristics of auto-oscillatory spin-wave systems

Journal of Physics: Conference Series 303 (2011)

AD Karenowska, AV Chumak, AA Serga, JF Gregg, B Hillebrands

Spin-wave active rings - positive-feedback systems incorporating spin-wave waveguides - provide important insight into fundamental magnetics, enable experimental investigations into nonlinear wave phenomena, and potentially find application in microwave electronics. Such rings break into spontaneous, monomode oscillation at a certain threshold value of feedback gain. In general, the wavenumber of this initially excited, threshold mode is impossible to predict precisely. Here we discuss how, by exploiting resonant spin-wave reflections from a magnonic crystal, an active ring system having a threshold mode with a well-defined and precisely predictable wavenumber may be realized. Our work suggests that study and development of active ring systems incorporating magnonic crystals may deliver useful insight into spin-wave transmission in structured magnetic films as well as devices with technological applicability.

Spin information transfer and transport in hybrid spinmechatronic structures

Journal of Physics: Conference Series 303 (2011)

AD Karenowska, JF Gregg, AV Chumak, AA Serga, B Hillebrands

Spin waves have long been recognized as potential signal carriers in spintronic devices. However, practical development of spin wave based information platforms is its infancy. To date, work in this area has focused on one-dimensional topologies based on purely magnetic thin-film transmission systems, typically exploiting interference phenomena to perform logical operations. In this paper, we describe an alternative approach in which spinmechatronic structures combining spin-wave transmission systems with magnetically loaded micro- and nano-mechanical elements provide spin-information processing functionality.

All-linear time reversal by a dynamic artificial crystal


AV Chumak, VS Tiberkevich, AD Karenowska, AA Serga, JF Gregg, AN Slavin, B Hillebrands

All-linear time reversal by a dynamic artificial crystal

Nature Communications 1 (2010)

AV Chumak, VS Tiberkevich, AD Karenowska, AA Serga, JF Gregg, AN Slavin, B Hillebrands

The time reversal of pulsed signals or propagating wave packets has long been recognized to have profound scientific and technological significance. Until now, all experimentally verified time-reversal mechanisms have been reliant upon nonlinear phenomena such as four-wave mixing. In this paper, we report the experimental realization of all-linear time reversal. The time-reversal mechanism we propose is based on the dynamic control of an artificial crystal structure, and is demonstrated in a spin-wave system using a dynamic magnonic crystal. The crystal is switched from an homogeneous state to one in which its properties vary with spatial period a, while a propagating wave packet is inside. As a result, a linear coupling between wave components with wave vectors k≈π/a and k'=k-2π/a≈-π/a is produced, which leads to spectral inversion, and thus to the formation of a time-reversed wave packet. The reversal mechanism is entirely general and so applicable to artificial crystal systems of any physical nature. © 2010 Macmillan Publishers Limited. All rights reserved.

Optical excitation of a forbidden magnetic resonance mode in a doped lutetium-iron-garnet film via the inverse Faraday effect

Physical Review Letters 105 (2010)

AHM Reid, AV Kimel, A Kirilyuk, JF Gregg, T Rasing

The effective magnetic field induced by a femtosecond pulse of circularly polarized light, via the inverse Faraday effect, is shown to excite a magnetic-dipole forbidden exchange spin resonance in a lutetium iron garnet. An external magnetic field cannot excite this mode, as the iron sublattices have the same gyromagnetic ratio and no net torque can be applied between them. However, since the sublattices have different magneto-optical susceptibilities, the inverse Faraday effect induces different effective fields on different iron sites, allowing excitation.

Magnonic crystal based forced dominant wavenumber selection in a spin-wave active ring

Applied Physics Letters 96 (2010)

AD Karenowska, AV Chumak, AA Serga, JF Gregg, B Hillebrands

Spontaneous excitation of the dominant mode in a spin-wave active ring-a self-exciting positive-feedback system incorporating a spin-wave transmission structure-occurs at a certain threshold value of external gain. In general, the wavenumber of the dominant mode is extremely sensitive to the properties and environment of the spin-wave transmission medium, and is almost impossible to predict. In this letter, we report on a backward volume magnetostatic spin-wave active ring system incorporating a magnonic crystal. When mode enhancement conditions-readily predicted by a theoretical model-are satisfied, the ring geometry permits highly robust and consistent forced dominant wavenumber selection. © 2010 American Institute of Physics.

Investigation of the femtosecond inverse Faraday effect using paramagnetic Dy<inf>3</inf>Al<inf>5</inf>O<inf>12</inf>

Physical Review B - Condensed Matter and Materials Physics 81 (2010)

AHM Reid, AV Kimel, A Kirilyuk, JF Gregg, T Rasing

Pump-probe experiments on the rare-earth paramagnet Dy3Al 5O12 demonstrate that coupling between light and magnetism on the subpicosecond time scale cannot be adequately described by the thermodynamic model of the inverse Faraday effect but instead must be described microscopically by stimulated magneto-Raman scattering. Light-induced paramagnetic resonance, predicted by the thermodynamic theory, is not observed in Dy3Al5O12, however, the formation of a coherent superposition between other magnetic sublevels of the Dy3+ -ion's ground-state multiplet is measured. It is shown that coherence can only be induced between magnetic levels which are connected by the Raman selection rules. © 2010 The American Physical Society.

Electromagnetic field-based position sensor


JF Gregg, A Karenowska

Spintronics: A growing science

Nature Materials 6 (2007) 798-799

JF Gregg

The integration of spintronic elements with silicon technologies in order to produce active spintronic devices with both power gain and spin function that can store and process data. Schmehl and his colleagues have succeeded in making high quality materials with europium oxide, which shows excellent epitaxy on their silicon substrate and their conductivity may be readily and sensitively varied to suit the application. Spintronics uses thin slices of ferromagnetic materials as spin sources and detectors. The epitaxial growth on silicon shows that the interface chemistry problems have been eliminated and goes well for integration with conventional electronics. One most possible field for spintronics is to transferred-electron phenomena to generate a spin-Gunn effects by which spin-dependent negative resistance might be realizable.

A growing science

NATURE MATERIALS 6 (2007) 798-799

JF Gregg

Spin polarized La<inf>0.7</inf>Sr<inf>0.3</inf>MnO<inf>3</inf> thin films on silicon

Journal of Magnetism and Magnetic Materials 312 (2007) 453-457

I Bergenti, V Dediu, E Arisi, M Cavallini, F Biscarini, C Taliani, MP de Jong, CL Dennis, JF Gregg, M Solzi, M Natali

La0.7Sr0.3MnO3 polycrystalline manganite thin films were grown on silicon (Si) substrates covered by SiOx amorphous native oxide. Curie temperatures of about 325 K were achieved for 70-nm-thick films. Strong room temperature XMCD signal was detected indicating high spin polarization at the surface. Cross-sectional TEM images show sharp interface between SiOx and manganite without signature of chemical reaction at the interface. Unusual sharp splitting of the manganite film was observed: on the top of a transition layer characterized by low crystalline order, a magnetically robust layer is formed. © 2007 Elsevier B.V. All rights reserved.

Evidence for electrical spin tunnel injection into silicon

Journal of Applied Physics 100 (2006) 043717 4pp-

JF Gregg, Dennis C L, Ensell G J, S M. Thompson

Flux Compression Sensor


JF Gregg, A Karenowska