Time-resolved Scanning Kerr Microscopy Research Articles

Optically detected spin–orbit torque ferromagnetic resonance in an in-plane magnetized ellipse

Time-resolved scanning Kerr microscopy has been used to perform optically detected, phase-resolved spin–orbit torque ferromagnetic resonance (SOT-FMR) measurements on a microscale CoFeB ellipse at the center of a Pt Hall cross subject to RF and DC current. Time-resolved polar Kerr images revealed localized dynamics with large amplitude at the center and weaker amplitude at the edges. Therefore, field swept SOT-FMR spectra were acquired from the so-called center mode to probe the SOTs active at the center of the ellipse, thus minimizing non-uniform edge contributions. When the magnetic field was applied at 30° from the hard axis of the ellipse and a DC current was applied, a marked asymmetry was observed in the amplitude and linewidth of the FMR peaks as the applied field was reversed. Both absorptive and dispersive parts of the spectra were in good agreement with a macrospin calculation. The damping parameter (α) and the Slonczewski torque parameter were determined to be 0.025 and (6.75 ± 0.75) × 10−7 Oe A−1 cm2, respectively. The hard axis SOT-FMR linewidth was found to be almost independent of the DC current value, suggesting that the SOT has a minimal influence in the hard axis configuration and that thermal effects were insignificant. This study paves the way for spatially resolved measurements of SOT probed using localized modes of microscale devices that go beyond the spatially averaged capability of electrical measurement techniques.

Time-domain imaging of curling modes in a confined magnetic vortex and a micromagnetic study exploring the role of spiral spin waves emitted by the core

The curling spin wave modes of a ferromagnetic vortex confined to a microscale disc have been directly imaged in response to a microwave field excitation using time-resolved scanning Kerr microscopy. Micromagnetic simulations have been used to explore the interaction of gyrotropic vortex core dynamics with the curling modes observed in the region of circulating in-plane magnetization. Hybridization of the fundamental gyrotropic mode with the degenerate, lowest-frequency, azimuthal modes has previously been reported to lead to their splitting and counter propagating motion, as we observe in our spectra and measured images. The curling nature of the modes can be ascribed to asymmetry in the static and dynamic magnetization across the disc thickness, but here we also present evidence that spiral spin waves emitted by the core can influence the spatial character of higher frequency curling modes for which hybridization is only permitted with gyrotropic modes of the same sense of azimuthal motion. While it is challenging to identify if such modes are truly hybridized from the mode dispersion in a confined disc, our simulations reveal that spiral spin waves from the core may act as mediators of the interaction between the core dynamics and azimuthal modes. At higher frequency, modes with radial character only do not exhibit marked curling, but instead show evidence of interaction with spin waves generated at the edge of the disc. The measured spatio-temporal character of the observed curling modes is accurately reproduced by our simulations, which reveal the emission of propagating short-wavelength spiral spin waves from both core and edge regions of the disc. Our simulations suggest that the propagating modes are not inconsequential, but may play a role in the dynamic overlap required for hybridization of modes of the core and in-plane magnetised regions.

Integration and characterization of micron-sized YIG structures with very low Gilbert damping on arbitrary substrates

We present a process that allows the transfer of monocrystalline yttrium-iron-garnet microstructures onto virtually any kind of substrate. The process is based on a recently developed method that allows the fabrication of freestanding monocrystalline YIG bridges on gadolinium-gallium-garnet. Here, the bridges' spans are detached from the substrate by a dry etching process and immersed in a watery solution. Using drop-casting, the immersed YIG platelets can be transferred onto the substrate of choice, where the structures finally can be reattached and, thus, be integrated into complex devices or experimental geometries. Using time-resolved scanning Kerr microscopy and inductively measured ferromagnetic resonance, we can demonstrate that the structures retain their excellent magnetic quality. At room temperature, we find a ferromagnetic resonance linewidth of μ0ΔHHWHM≈195 μT and we were even able to inductively measure magnon spectra on a single micrometer-sized yttrium-iron-garnet platelet at a temperature of 5 K. The process is flexible in terms of substrate material and shape of the structure. In the future, this approach will allow for types of spin dynamics experiments until now unthinkable.

Monocrystalline Freestanding Three-Dimensional Yttrium-Iron-Garnet Magnon Nanoresonators

Nano resonators in which mechanical vibrations and spin waves can be coupled are an intriguing concept that can be used in quantum information processing to transfer information between different states of excitation. Until now, the fabrication of free standing magnetic nanostructures which host long lived spin wave excitatons and may be suitable as mechanical resonators seemed elusive. We demonstrate the fabrication of free standing monocrystalline yttrium iron garnet (YIG) 3D nanoresonators with nearly ideal magnetic properties. The freestanding 3D structures are obtained using a complex lithography process including room temperature deposition and lift-off of amorphous YIG and subsequent crystallization by annealing. The crystallization nucleates from the substrate and propagates across the structure even around bends over distances of several micrometers to form e.g. monocrystalline resonators as shown by transmission electron microscopy. Spin wave excitations in individual nanostructures are imaged by time resolved scanning Kerr microscopy. The narrow linewidth of the magnetic excitations indicates a Gilbert damping constant of only $\alpha = 2.6 \times 10^{-4}$ rivalling the best values obtained for epitaxial YIG thin film material. The new fabrication process represents a leap forward in magnonics and magnon mechanics as it provides 3D YIG structures of unprecedented quality. At the same time it demonstrates a completely new route towards the fabrication of free standing crystalline nano structures which may be applicable also to other material systems.

Time-resolved imaging of magnetization dynamics in double nanocontact spin torque vortex oscillator devices

Double nanocontact (NC) spin transfer vortex oscillator devices, in which NCs of 100-nm diameter have center-to-center separation ranging from 200 to 1100 nm, have been studied by means of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM). The NCs were positioned close to the edge of the top electrical contact so that the magnetization dynamics of the adjacent region could be probed optically. The electrical measurements showed different ranges of frequency operation for devices with different NC separations. For 900-nm NC separation, TRSKM showed magnetic contrast consistent with the formation of a magnetic vortex at each NC, while for 200-nm NC separation a lack of magnetic contrast near the NC region suggests that the magnetization dynamics occur closer to the NC and underneath the top contact. TRSKM also reveals the presence of additional localized dynamical features far from the NCs, which are not seen by electrical measurements; has not been reported previously for double NCs with different separations; and provides insight into how the dynamic state of the phase-locked oscillators is established and stabilized.

Spatial mapping of torques within a spin Hall nano-oscillator

Time-resolved scanning Kerr microscopy (TRSKM) was used to study precessional magnetization dynamics induced by a radio frequency (RF) current within a ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$/Py(5 nm)/Pt(6 nm)/Au(150 nm) spin Hall nano-oscillator structure. The Au layer was formed into two needle-shaped electrical contacts that concentrated the current in the center of a Py/Pt mesa of 4 $\ensuremath{\mu}\mathrm{m}$ diameter. Due to the spin Hall effect, current within the Pt layer drives a spin current into the Py layer, exerting a spin transfer torque (STT). By injecting RF current and exploiting the phase sensitivity of TRSKM and the symmetry of the device structure, the STT and Oersted field torques have been separated and spatially mapped. The STT and torque due to the in-plane Oersted field are observed to exhibit minima at the device center that is ascribed to spreading of RF current that is not observed for DC current. Torques associated with the RF current may destabilize the position of the self-localized bullet mode excited by the DC current and inhibit injection locking. The present study demonstrates the need to characterize both DC and RF current distributions carefully.

Time resolved imaging of the non-linear bullet mode within an injection-locked nano-contact spin Hall nano-oscillator

Time-resolved scanning Kerr microscopy (TRSKM) has been used to image precessional magnetization dynamics excited by a DC current within a nano-contact (NC) spin Hall nano-oscillator (SHNO). Injection of a radio frequency (RF) current was used to phase lock the SHNO to TRSKM. The out of plane magnetization was detected by means of the polar magneto optical Kerr effect (MOKE). However, longitudinal MOKE images were dominated by an artifact arising from the edges of the Au NCs. Time resolved imaging revealed the simultaneous excitation of a non-linear “bullet” mode at the centre of the device, once the DC current exceeded a threshold value, and ferromagnetic resonance (FMR) induced by the RF current. However, the FMR response observed for sub-critical DC current values exhibits an amplitude minimum at the centre, which is attributed to spreading of the RF spin current due to the reactance of the device structure. This FMR response can be subtracted to yield images of the bullet mode. As the DC current is increased above threshold, the bullet mode appears to increase in size, suggesting increased translational motion. The reduced spatial overlap of the bullet and FMR modes, and this putative translational motion, may impede the injection locking and contribute to the reduced locking range observed within NC-SHNO devices. This illustrates a more general need to control the geometry of an injection-locked oscillator so that the autonomous dynamics of the oscillator exhibit strong spatial overlap with those resulting from the injected signal.

Observation of a Goos-Hänchen-like Phase Shift for Magnetostatic Spin Waves.

In optics, a light beam experiences a spatial shift in the beam plane upon total internal reflection. This shift is usually referred to as the Goos-Hänchen shift. When dealing with plane waves, it manifests itself as a phase shift between an incoming and reflected wave that depends on the wave vector component along the interface. In the experiments presented here, plane spin waves are excited in a 60-nm-thick Permalloy film and propagate towards the edge of the film. By means of time-resolved scanning Kerr microscopy, we are able to directly detect a phase shift between the incoming and reflected wave. With the help of a numerical model, we show that this phase shift naturally occurs for spin waves in the dipolar regime.

Spin-wave wavelength down-conversion at thickness steps

We report a systematic experimental study on the refraction and reflection of magnetostatic spin-waves at a thickness step between two Permalloy films of different thickness. The transmitted spin-waves for the transition from a thick film to a thin film have a higher wave vector compared to the incoming waves. Consequently, such systems may find use as passive wavelength transformers in magnonic networks. We investigate the spin-wave transmission behavior by studying the influence of the external magnetic field, incident angle, and thickness ratio of the films using time-resolved scanning Kerr microscopy and micro-focused Brillouin light scattering.

A platform for time-resolved scanning Kerr microscopy in the near-field.

Time-resolved scanning Kerr microscopy (TRSKM) is a powerful technique for the investigation of picosecond magnetization dynamics at sub-micron length scales by means of the magneto-optical Kerr effect (MOKE). The spatial resolution of conventional (focused) Kerr microscopy using a microscope objective lens is determined by the optical diffraction limit so that the nanoscale character of the magnetization dynamics is lost. Here we present a platform to overcome this limitation by means of a near-field TRSKM that incorporates an atomic force microscope (AFM) with optical access to a metallic AFM probe with a nanoscale aperture at its tip. We demonstrate the near-field capability of the instrument through the comparison of time-resolved polar Kerr images of magnetization dynamics within a microscale NiFe rectangle acquired using both near-field and focused TRSKM techniques at a wavelength of 800 nm. The flux-closure domain state of the in-plane equilibrium magnetization provided the maximum possible dynamic polar Kerr contrast across the central domain wall and enabled an assessment of the magneto-optical spatial resolution of each technique. Line profiles extracted from the Kerr images demonstrate that the near-field spatial resolution was enhanced with respect to that of the focused Kerr images. Furthermore, the near-field polar Kerr signal (∼1 mdeg) was more than half that of the focused Kerr signal, despite the potential loss of probe light due to internal reflections within the AFM tip. We have confirmed the near-field operation by exploring the influence of the tip-sample separation and have determined the spatial resolution to be ∼550 nm for an aperture with a sub-wavelength diameter of 400 nm. The spatial resolution of the near-field TRSKM was in good agreement with finite element modeling of the aperture. Large amplitude electric field along regions of the modeled aperture that lie perpendicular to the incident polarization indicate that the aperture can support plasmonic excitations. The comparable near-field and focused polar Kerr signals suggest that such plasmonic excitations may lead to an enhanced near-field MOKE. This work demonstrates that near-field TRSKM can be performed without significant diminution of the polar Kerr signal in relatively large, sub-wavelength diameter apertures, while development of a near-field AFM probe utilizing plasmonic antennas specifically designed for measurements deeper into the nanoscale is discussed.

Broadband conversion of microwaves into propagating spin waves in patterned magnetic structures

We have used time-resolved scanning Kerr microscopy and micromagnetic simulations to demonstrate that, when driven by the spatially uniform microwave field, the edges of patterned magnetic samples represent both efficient and highly tunable sources of propagating spin waves. The excitation is due to the local enhancement of the resonance frequency induced by the non-uniform dynamic demagnetizing field generated by precessing magnetization aligned with the edges. Our findings represent a crucial step forward in the design of nanoscale spin-wave sources for magnonic architectures and are also highly relevant to the understanding and interpretation of magnetization dynamics driven by spatially uniform magnetic fields in patterned magnetic samples.

Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting**This article belongs to the Special Issue: Emerging Leaders, which features invited work from the best early-career researchers working within the scope of J. Phys. D. This project is part of the Journal of Physics series' 50th anniversary celebrations in 2017. Paul Keatley was selected by the Editorial Board

Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-to-device variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by the injected RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.

Magnetic properties of spin waves in thin yttrium iron garnet films

We report spin wave propagation experiments in thin yttrium iron garnet (YIG) films. Using time-resolved scanning Kerr microscopy we extract the mode structure of the spin waves. For quasi-single-mode excitation, the spin wave decay can be fitted with a damped harmonic oscillator function providing us with information about the attenuation length. We measure values of about 2.7 and 3.6 \ensuremath{\mu}m for the spin wave decay length of 38- and 49-nm-thick YIG samples, respectively. Micromagnetic simulations are performed to compare experimental and simulated modes. The data are in very good agreement with these simulations.

Design and construction of a spin-wave lens.

In this work, we present the focusing of a Damon-Eshbach wave in a Ni80Fe20 film by a shaped, discrete transition of the film thickness. We devised an algorithm to determine the required shape of a spin-wave lens. Due to the anisotropy three geometries qualify as plano-convex lenses. One lens geometry has been realized experimentally and the emitted spin-wave pattern is investigated by time-resolved scanning Kerr microscopy.

Superharmonic injection locking of nanocontact spin-torque vortex oscillators

Super-harmonic injection locking of single nano-contact (NC) spin-torque vortex oscillators (STVOs) subject to a small microwave current has been explored. Frequency locking was observed up to the fourth harmonic of the STVO fundamental frequency $f_{0}$ in microwave magneto-electronic measurements. The large frequency tunability of the STVO with respect to $f_{0}$ allowed the device to be locked to multiple sub-harmonics of the microwave frequency $f_{RF}$, or to the same sub-harmonic over a wide range of $f_{RF}$ by tuning the DC current. In general, analysis of the locking range, linewidth, and amplitude showed that the locking efficiency decreased as the harmonic number increased, as expected for harmonic synchronization of a non-linear oscillator. Time-resolved scanning Kerr microscopy (TRSKM) revealed significant differences in the spatial character of the magnetization dynamics of states locked to the fundamental and harmonic frequencies, suggesting significant differences in the core trajectories within the same device. Super-harmonic injection locking of a NC-STVO may open up possibilities for devices such as nanoscale frequency dividers, while differences in the core trajectory may allow mutual synchronisation to be achieved in multi-oscillator networks by tuning the spatial character of the dynamics within shared magnetic layers.

Direct observation of magnetization dynamics generated by nanocontact spin-torque vortex oscillators

Time-resolved scanning Kerr microscopy has been used to directly image the magnetization dynamics of nano-contact (NC) spin-torque vortex oscillators (STVOs) when phase-locked to an injected microwave (RF) current. The Kerr images reveal free layer magnetization dynamics that extend outside the NC footprint, where they cannot be detected electrically, but which are crucial to phase-lock STVOs that share common magnetic layers. For a single NC, dynamics were observed not only when the STVO frequency was fully locked to that of the RF current, but also for a partially locked state characterized by periodic changes in the core trajectory at the RF frequency. For a pair of NCs, images reveal the spatial character of dynamics that electrical measurements show to have enhanced amplitude and reduced linewidth. Insight gained from these images may improve understanding of the conditions required for mutual phase-locking of multiple STVOs, and hence enhanced microwave power emission.

Time-resolved scanning Kerr microscopy of flux beam formation in hard disk write heads

To meet growing data storage needs, the density of data stored on hard disk drives must increase. In pursuit of this aim, the magnetodynamics of the hard disk write head must be characterized and understood, particularly the process of “flux beaming.” In this study, seven different configurations of perpendicular magnetic recording (PMR) write heads were imaged using time-resolved scanning Kerr microscopy, revealing their detailed dynamic magnetic state during the write process. It was found that the precise position and number of driving coils can significantly alter the formation of flux beams during the write process. These results are applicable to the design and understanding of current PMR and next-generation heat-assisted magnetic recording devices, as well as being relevant to other magnetic devices.

Interface modes at step edges of media with anisotropic dispersion

Time-resolved scanning Kerr microscopy data are presented that illustrate the refraction and total reflection of spin waves in a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ film at a film-thickness step. In our experiments spin waves are excited with microwaves on coplanar wave guides. By changing the orientation of an externally applied magnetic field, we can manipulate the group velocity of the wave and hence control the angle of incidence on the step. An interface mode bound at the step is observed, when the magnetic field vector is perpendicular to the step. We point out the general conditions under which such interface modes can be expected in media with strongly anisotropic dispersion.

Direct observation of isolated Damon-Eshbach and backward volume spin-wave packets in ferromagnetic microstripes.

The analysis of isolated spin-wave packets is crucial for the understanding of magnetic transport phenomena and is particularly interesting for applications in spintronic and magnonic devices, where isolated spin-wave packets implement an information processing scheme with negligible residual heat loss. We have captured microscale magnetization dynamics of single spin-wave packets in metallic ferromagnets in space and time. Using an optically driven high-current picosecond pulse source in combination with time-resolved scanning Kerr microscopy probed by femtosecond laser pulses, we demonstrate phase-sensitive real-space observation of spin-wave packets in confined permalloy (Ni80Fe20) microstripes. Impulsive excitation permits extraction of the dynamical parameters, i.e. phase- and group velocities, frequencies and wave vectors. In addition to well-established Damon-Eshbach modes our study reveals waves with counterpropagating group- and phase-velocities. Such unusual spin-wave motion is expected for backward volume modes where the phase fronts approach the excitation volume rather than emerging out of it due to the negative slope of the dispersion relation. These modes are difficult to excite and observe directly but feature analogies to negative refractive index materials, thus enabling model studies of wave propagation inside metamaterials.

Interfacial Dzyaloshinskii-Moriya interaction studied by time-resolved scanning Kerr microscopy

We investigate the influence of the interfacial Dzyaloshinskii-Moriya interaction (DMI) on the propagation of Damon-Eshbach spin waves in micrometer-sized Pt(2 nm)/Co(0.4 nm)/Py(5 nm)/MgO(5 nm) stripes. We use time-resolved scanning Kerr microscopy to image the spin waves excited by a microwave antenna and to directly access their dispersion. The presence of an interfacial DMI manifests itself in an asymmetry in the dispersion for counterpropagating spin waves which reverses sign upon reversal of the direction of the externally applied magnetic field. From this asymmetry we deduce the strength of the interfacial DMI. Micromagnetic simulations confirm that the observed difference in the wave numbers and the signature of the asymmetry are characteristic for the occurrence of an interfacial DMI at the Pt/Co interface and cannot be explained by the uniaxial perpendicular magnetic anisotropy field originating from the same interface.