Alexy Davison Karenowska

All-linear time reversal by a dynamic artificial crystal

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−2ππ/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.

Temporal evolution of inverse spin Hall effect voltage in a magnetic insulator-nonmagnetic metal structure

It is demonstrated that the temporal evolution of a spin-wave induced inverse spin Hall effect voltage in a magnetic insulator–nonmagnetic metal structure is distinctly different from that of the directly excited (microwave pulse driven) spin-wave mode from which it originates. The differences in temporal behavior provide compelling evidence that incoherent secondary spin-wave modes, having a range of different characteristic lifetimes, make an important contribution to spin pumping at the insulator-metal interface.

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

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.

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

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.

The Dynamic Magnonic Crystal: New Horizons in Artificial Crystal Based Signal Processing

In this chapter, we describe the development and properties of the first experimental dynamic magnonic crystal devices and highlight certain aspects of the intriguing new physics that they have to offer us. We discuss the significance of the dynamic magnonic crystal both in the context of the furtherance and technological application of magnonics, and in the understanding of general wave dynamics in metamaterial systems.

Measurement of a magnonic crystal at millikelvin temperatures

Hybrid systems combining magnons and superconducting quantum circuits have attracted increasing interest in recent years. Magnonic crystals (MCs) are one of the building blocks of room-temperature magnonics and can be used to create devices with an engineered band structure. These devices, exhibiting tunable frequency selectivity and the ability to store travelling excitations in the microwave regime, may form the basis of a set of new tools to be used in the context of quantum information processing. In order to ascertain the feasibility of such plans, MCs must be demonstrated to work at the low temperatures required for microwave-frequency quantum experiments. We report the first measurements of the transmission of microwave signals through an MC at 20 mK and observe a magnonic bandgap in both continuous-wave and pulsed excitation experiments. The spin-wave damping at low temperatures in our yttrium iron garnet MC is higher than expected, indicating that further work is necessary before the full potential of quantum experiments using magnonic crystals can be realised.

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

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.

ICMM 2012 Foreword From the Publication Committee

The content of this special issue is testimony to the excellent scientific and technical quality of the 2012 International ICMM Meeting.

Spin information transfer and transport in hybrid spinmechatronic structures

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.