Henning Ulrichs

The building blocks of magnonics

Abstract Novel material properties can be realized by designing waves’ dispersion relations in artificial crystals. The crystal’s structural length scales may range from nano- (light) up to centimeters (sound waves). Because of their emergent properties these materials are called metamaterials. Different to photonics, where the dielectric constant dominantly determines the index of refraction, in a ferromagnet the spin-wave index of refraction can be dramatically changed already by the magnetization direction. This allows a different flexibility in realizing dynamic wave guides or spin-wave switches. The present review will give an introduction into the novel functionalities of spin-wave devices, concepts for spin-wave based computing and magnonic crystals. The parameters of the magnetic metamaterials are adjusted to the spin-wave k -vector such that the magnonic band structure is designed. However, already the elementary building block of an antidot lattice, the singular hole, owns a strongly varying internal potential determined by its magnetic dipole field and a localization of spin-wave modes. Photo-magnonics reveal a way to investigate the control over the interplay between localization and delocalization of the spin-wave modes using femtosecond lasers, which is a major focus of this review. We will discuss the crucial parameters to realize free Bloch states and how, by contrast, a controlled localization might allow us to gradually turn on and manipulate spin-wave interactions in spin-wave based devices in the future.

Magnetic nano-oscillator driven by pure spin current

With the advent of pure-spin-current sources, spin-based electronic (spintronic) devices no longer require electrical charge transfer, opening new possibilities for both conducting and insulating spintronic systems. Pure spin currents have been used to suppress noise caused by thermal fluctuations in magnetic nanodevices, amplify propagating magnetization waves, and to reduce the dynamic damping in magnetic films. However, generation of coherent auto-oscillations by pure spin currents has not been achieved so far. Here we demonstrate the generation of single-mode coherent auto-oscillations in a device that combines local injection of a pure spin current with enhanced spin-wave radiation losses. Counterintuitively, radiation losses enable excitation of auto-oscillation, suppressing the nonlinear processes that prevent auto-oscillation by redistributing the energy between different modes. Our devices exhibit auto-oscillations at moderate current densities, at a microwave frequency tunable over a wide range. These findings suggest a new route for the implementation of nanoscale microwave sources for next-generation integrated electronics.

Nanomagnonic devices based on the spin-transfer torque

Magnonics is based on signal transmission and processing by spin waves (or their quanta, called magnons) propagating in a magnetic medium. In the same way as nanoplasmonics makes use of metallic nanostructures to confine and guide optical-frequency plasmon-polaritons, nanomagnonics uses nanoscale magnetic waveguides to control the propagation of spin waves. Recent advances in the physics of nanomagnetism, such as the discovery of spin-transfer torque, have created possibilities for nanomagnonics. In particular, it was recently demonstrated that nanocontact spin-torque devices can radiate spin waves, serving as local nanoscale sources of signals for magnonic applications. However, the integration of spin-torque sources with nanoscale magnetic waveguides, which is necessary for the implementation of integrated spin-torque magnonic circuits, has not been achieved to date. Here, we suggest and experimentally demonstrate a new approach to this integration, utilizing dipolar field-induced magnonic nanowaveguides. The waveguides exhibit good spectral matching with spin-torque nano-oscillators and enable efficient directional transmission of spin waves. Our results provide a practical route for the implementation of integrated magnonic circuits utilizing spin transfer.

Synchronization of spin Hall nano-oscillators to external microwave signals

Recently, a novel type of spin-torque nano-oscillators driven by pure spin current generated via the spin Hall effect was demonstrated. Here we report the study of the effects of external microwave signals on these oscillators. Our results show that they can be efficiently synchronized by applying a microwave signal at approximately twice the frequency of the auto-oscillation, which opens additional possibilities for the development of novel spintronic devices. We find that the synchronization exhibits a threshold determined by magnetic fluctuations pumped above their thermal level by the spin current, and is significantly influenced by the nonlinear self-localized nature of the auto-oscillatory mode.

Magnonic spin-wave modes in CoFeB antidot lattices

In this paper time-resolved magneto-optical Kerr effect experiments on structured CoFeB films are presented. The geometries considered are two dimensional square lattices of micrometer-sized antidots, fabricated by a focused ion beam. The spin-wave spectra of these magnonic crystals show a precessional mode, which can be related to a Bloch state at the zone boundary. Additionally, another magnetic mode of different nature appears, whose frequency displays no dependence on the externally applied magnetic field. These findings are interpreted as delocalized and localized modes, respectively.

Spin-torque nano-emitters for magnonic applications

In the emerging field of magnonics, spin waves are utilized as a medium for high-speed transmission and processing of electrical signals on the nanometer scale. The implementation of high-performance magnonic devices requires efficient nano-scaled sources of spin waves. Here, we demonstrate experimentally that this goal can be accomplished by utilizing a modified design of spin-torque devices, in which the internal dipolar field is compensated in the active device area by a built-in micromagnet. We show experimentally that the proposed device geometry enables efficient emission of spin waves with a large propagation length for practical implementation of integrated magnonic circuits.

Optimization of Pt-based spin-Hall-effect spintronic devices

We study experimentally the routes to improve the characteristics of the spin-Hall-effect devices based on permalloy/Pt bilayers by optimization of the Pt layer thickness and by the addition of an antiferromagnetic spin-sinking layer. We experimentally determine the spin-diffusion length in Pt and show that Pt thickness can be reduced down to 2 nm without degradation of the device characteristics caused by the spin accumulation effects, which provides possibilities for significant reduction of the required driving currents. We also show that the addition of a spin-sinking layer results in a non-monotonic dependence of device efficiency on the Pt thickness.

Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses

Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.

Micromagnetic study of auto-oscillation modes in spin-Hall nano-oscillators

We present a numerical study of magnetization dynamics in a recently introduced spin torque nano-oscillator, whose operational principle relies on the spin-Hall effect—spin-Hall nano-oscillators. Our numerical results show good agreement with the experimentally observed behaviors and provide detailed information about the features of the primary auto-oscillation mode observed in the experiments. They also clarify the physical nature of the secondary auto-oscillation mode, which was experimentally observed under certain conditions only.

Single antidot as a passive way to create caustic spin-wave beams in yttrium iron garnet films

Single antidot as a passive point excitation source has been implemented to create caustic spin-wave beams in yttrium iron garnet film. Diffraction of surface magnetostatic spin waves from such antidot is investigated experimentally and theoretically. Our experimental results from Brillouin light scattering spectroscopy indicate that the diffraction beams, which manifest as reflection, extinction, and semicaustic lines, are tunable by the frequency and direction of the incident wave. Numerical calculations are in good agreement with the experimental findings and explain the directions of caustic beams caused by the diffraction.