Time-resolved Kerr Microscopy Research Articles

Observation of pinning and pinning evasion dynamics of a magnetic vortex core

An understanding of the dynamics of magnetic textures as they interact with defects in the host material is critical for proposed applications that depend on controlled translation of these magnetic textures. We report on the experimental observation of magnetic vortex core (VC) trajectories as they are affected by a potential energy landscape arising from defects in thin, permalloy disks. We first map the VC pinning landscape using equilibrium Kerr microscopy, and then use multiplexed time-resolved Kerr microscopy to track in-plane dynamics of a magnetic VC in response to a magnetic field pulse with ns time resolution, and several-nm spatial resolution. The initial position of the VC is set by applying a static magnetic field to shift the dynamics relative to the pinning landscape. We observe the expected spiral trajectory as the VC relaxes to equilibrium over several tens of nanoseconds. However, in the presence of strong pinning, we also observe pinning evasion as the VC trajectory is deflected due to the pinning potential, and enhanced damping as the vortex becomes pinned. We compare to a simple model based on the Thiele equation of motion to understand these results.

Time-resolved detection of spin–orbit torque switching of magnetization and exchange bias

Current-induced magnetization switching driven by spin–orbit torques on sub-nanosecond timescales could be used to create fast and low-power spintronic devices. The time-resolved detection and analysis of switching trajectories in ferromagnet/antiferromagnet exchange-biased structures are the key to designing spin–orbit torque devices with high speed, but insight remains limited. Here we report the time-resolved detection of spin–orbit torque switching of the magnetization and exchange bias in platinum/cobalt/iridium–manganese heterostructures. Using time-resolved magneto-optical Kerr microscopy, combined with micromagnetic simulations, we show that the ferromagnets, as well as interfacial antiferromagnetic spins and exchange bias, can be partially switched by sub-nanosecond current pulses, which allows the switching probabilities to be flexibly controlled at multiple levels. We also show that the spin–orbit-torque-induced switching of the exchange bias, which intimately depends on the current density, can stabilize multilevelled magnetization switching within sub-nanosecond current pulses.

Gate-Tunable Helical Currents in Commensurate Topological Insulator/Graphene Heterostructures.

van der Waals heterostructures made from graphene and three-dimensional topological insulators promise very high electron mobilities, a nontrivial spin texture, and a gate-tunability of electronic properties. Such a combination of advantageous electronic characteristics can only be achieved through proximity effects in heterostructures, as graphene lacks a large enough spin-orbit interaction. In turn, the heterostructures are promising candidates for all-electrical control of proximity-induced spin phenomena. Here, we explore epitaxially grown interfaces between graphene and the lattice-matched topological insulator Bi2Te2Se. For this heterostructure, spin-orbit coupling proximity has been predicted to impart an anisotropic and electronically tunable spin texture. Polarization-resolved second-harmonic generation, Raman spectroscopy, and time-resolved magneto-optic Kerr microscopy are combined to demonstrate that the atomic interfaces align in a commensurate symmetry with characteristic interlayer vibrations. By polarization-resolved photocurrent measurements, we find a circular photogalvanic effect which is drastically enhanced at the Dirac point of the proximitized graphene. We attribute the peculiar gate-tunability to the proximity-induced interfacial spin structure, which could be exploited for, e.g., spin filters.

Observation of Defect-Assisted Magnetic Vortex Core Reversal at Ultralow Critical Velocity

The stability of structures with nontrivial topology is a subject of great interest, both from fundamental and technological perspectives. The topology of a magnetic vortex, for example, results in high stability of the vortex core polarity, which is attractive for applications. This high stability, however, greatly impedes the required deterministic polarity switching, which is typically only achieved with large magnetic fields or strong dynamic driving. Here, we show that the interaction between the vortex core and manufactured nanoscale defects in the magnetic material lowers the required driving strength for core polarity reversal by more than an order of magnitude. We excite vortex dynamics in thin permalloy disks, and map the two-dimensional (2D) vortex core trajectory using 3D time-resolved Kerr microscopy. In pristine samples, we observe normal gyrotropic motion of the vortex core. After laser-induced generation of defects, however, we observe repeated vortex core reversal at much-reduced driving strength. Micromagnetic simulations reveal how local reduction of exchange coupling and saturation magnetization can create vortex core reversal sites for deterministic vortex core switching.

Dynamic detection of current-induced spin-orbit magnetic fields

Current induced spin-orbit torques (SOTs) in ferromagnet/non-magnetic metal heterostructures open vast possibilities to design spintronic devices to store, process and transmit information in a simple architecture. It is a central task to search for efficient SOT-devices, and to quantify the magnitude as well as the symmetry of current-induced spin-orbit magnetic fields (SOFs). Here, we report a novel approach to determine the SOFs based on magnetization dynamics by means of time-resolved magneto-optic Kerr microscopy. A microwave current in a narrow Fe/GaAs (001) stripe generates an Oersted field as well as SOFs due to the reduced symmetry at the Fe/GaAs interface, and excites standing spin wave (SSW) modes because of the lateral confinement. Due to their different symmetries, the SOFs and the Oersted field generate distinctly different mode patterns. Thus it is possible to determine the magnitude of the SOFs from an analysis of the shape of the SSW patterns. Specifically, this method, which is conceptually different from previous approaches based on lineshape analysis, is phase independent and self-calibrated. It can be used to measure the current induced SOFs in other material systems, e.g., ferromagnetic metal/non-magnetic metal heterostructures.

Anisotropic expansion of drifting spin helices in GaAs quantum wells

The drift of electron spin helices in an external in-plane electric field in GaAs quantum wells is studied by means of time-resolved magneto-optical Kerr microscopy. The evolution of the spin distribution measured for different excitation powers reveals that, for short delay times and higher excitation powers, the spin helix drift slows down while its envelope becomes anisotropic. The effect is understood as a local decrease of the electron gas mobility due to electron collisions with nonequilibrium holes within the excitation spot and is reproduced well in the kinetic theory framework. For larger delay times, when the electrons constituting the spin helix and nonequilibrium holes are separated by an electric field, the spin helix drift accelerates and the mobility reaches its unperturbed value again.

Current-induced picosecond magnetization dynamics in a Ta/CoFeB/MgO hall bar

Time-resolved Kerr microscopy (TRSKM) has been used to explore the small amplitude picosecond magnetization dynamics induced by spin–orbit torques in a Ta(4 nm)/Co40Fe40B20(1 nm)/MgO(1.6 nm)/Ta(1 nm) Hall bar structure. The time dependent polar magneto optical Kerr effect was recorded following injection of a current pulse of 70 ps duration. Macrospin simulations provide a reasonable description of the precession and a transient background response as the field strength and current polarity are varied, while confirming that the in-plane spin–orbit torque is dominant within this system. Increasing the current density within the simulations leads to coherent magnetization reversal. Inclusion of a modest in-plane bias field is found to reduce both the switching current and the time required for switching. The orientation of the in-plane field relative to the direction of the current determines whether the magnetization can be switched backwards and forwards by current pulses of the same or opposite polarity.

Transport of a persistent spin helix drifting transverse to the spin texture

Time-resolved magneto-optic Kerr microscopy measures the effect of in-plane electric fields on the dynamics of a photoexcited spin distribution in a modulation-doped GaAs quantum well. The structure features nearly equal Dresselhaus and Rashba coefficients, such that there is negligible impact of spin-orbit coupling for electrons moving along the $[1\overline{1}0]$ or $[\overline{1}10]$ directions. Meanwhile, spin texture emerges for electrons moving in the $[110]$ or $[\overline{1}\overline{1}0]$ directions. The overall spin pattern resembles a persistent spin helix. An in-plane electric field, applied transverse to the spin texture (along the $[1\overline{1}0]$ or $[\overline{1}10]$ directions), introduces a drift of the spin packet and additional Larmor precessions, i.e., a marked decrease of the spatial periodicity of the spin pattern. The in-plane electric field also increases the temporal frequency of the evolving spin distribution, which is directly linked to the cubic Dresselhaus spin-orbit coupling term. Moreover, the in-plane field increases the diffusion coefficient by more than an order of magnitude. We attribute this effect to carrier heating and the separation of the photogenerated electron-hole dipole.

Ultrafast magnetization dynamics in a nanoscale three-dimensional cobalt tetrapod structure.

Three-dimensional magnetic nanostructures are now attracting intense interest due to their potential as ultrahigh density future magnetic storage devices. Here, we report on the study of ultrafast magnetization dynamics of a complex three-dimensional magnetic nanostructure. Arrays of magnetic tetrapod structures were fabricated using a combination of two-photon lithography (TPL) and electrodeposition. All-optical time-resolved magneto-optical Kerr microscopy was exploited to probe the spin-wave modes from the junction of a single tetrapod structure. Micromagnetic simulations reveal that the nature of these modes originates from the intricate three-dimensional tetrapod structure. Our findings enhance the basic knowledge about the dynamic control of spin waves in complex three-dimensional magnetic elements which are imperative for the construction of modern spintronic devices.

Magnetization dynamics and damping behavior of Co/Ni multilayers with a graded Ta capping layer

Magnetization dynamics of a Co/Ni multilayer film capped by a wedged Ta (0.3–1.3 nm) layer were investigated using time-resolved magneto-optic Kerr microscopy. The ferromagnetic resonance was observed for a wide range of magnetic fields in a variety of geometries. Using a macrospin approximation and the Kittel formula to evaluate the relaxation frequency, a significant increase of the effective anisotropy field was observed as the Ta layer thickness is increased. Furthermore, the damping behavior of the magnetization precession also shows a strong dependence on the Ta layer thickness. Using a model to account for variations of the local anisotropy fields, we reproduce the field evolution of the observed damping using a single Gilbert damping parameter for the Co/Ni system. These findings have direct implications for the rational design of novel spintronic devices.

Tunable Magnetization Dynamics in Interfacially Modified Ni81Fe19/Pt Bilayer Thin Film Microstructures.

Interface modification for control of ultrafast magnetic properties using low-dose focused ion beam irradiation is demonstrated for bilayers of two technologically important materials: Ni81Fe19 and Pt. Magnetization dynamics were studied using an all-optical time-resolved magneto-optical Kerr microscopy method. Magnetization relaxation, precession, damping and the spatial coherence of magnetization dynamics were studied. Magnetization precession was fitted with a single-mode damped sinusoid to extract the Gilbert damping parameter. A systematic study of the damping parameter and frequency as a function of irradiation dose varying from 0 to 3.3 pC/μm2 shows a complex dependence upon ion beam dose. This is interpreted in terms of both intrinsic effects and extrinsic two-magnon scattering effects resulting from the expansion of the interfacial region and the creation of a compositionally graded alloy. The results suggest a new direction for the control of precessional magnetization dynamics, and open the opportunity to optimize high-speed magnetic devices.

Direct observation of closure domain wall mediated spin waves

The generation and guiding of spin waves from and by magnetic domain walls are demonstrated. The spin waves radiate from pinned and oscillating magnetic closure domain walls and propagate linearly along a narrow path formed by the surrounding 180° asymmetric Bloch domain walls. The propagating spin wave modes are directly visualized by time-resolved magneto-optical Kerr microscopy with picosecond temporal resolution. A linear relationship between excitation frequency, wavelength, and number of spin waves per domain exists. Independent of the field excitation frequency, a constant phase velocity of spin waves propagation is obtained. Spin waves characteristics can be tuned by varying the magnetic domain dynamics, allowing for variable spin wave characteristics with magnetic field characteristics and histories.

Ferromagnetic resonance of patterned chromium dioxide thin films grown by selective area chemical vapour deposition

A selective area chemical vapour deposition technique has been used to fabricate continuous and patterned epitaxial CrO2 thin films on (100)-oriented TiO2 substrates. Precessional magnetization dynamics were stimulated both electrically and optically, and probed by means of time-resolved Kerr microscopy and vector network analyser ferromagnetic resonance techniques. The dependence of the precession frequency and the effective damping parameter upon the static applied magnetic field were investigated. All films exhibited a large in-plane uniaxial anisotropy. The effective damping parameter was found to exhibit strong field dependence in the vicinity of the hard axis saturation field. However, continuous and patterned films were found to possess generally similar dynamic properties, confirming the suitability of the deposition technique for fabrication of future spintronic devices.

Time-domain detection of current controlled magnetization damping in Pt/Ni81Fe19 bilayer and determination of Pt spin Hall angle

The effect of spin torque from the spin Hall effect in Pt/Ni81Fe19 rectangular bilayer film was investigated using time-resolved magneto-optical Kerr microscopy. Current flow through the stack resulted in a linear variation of effective damping up to ±7%, attributed to spin current injection from the Pt into the Ni81Fe19. The spin Hall angle of Pt was estimated as 0.11 ± 0.03. The modulation of the damping depended on the angle between the current and the bias magnetic field. These results demonstrate the importance of optical detection of precessional magnetization dynamics for studying spin transfer torque due to spin Hall effect.

Spatial constraints on the source of uniaxial anisotropy in (Ga,Mn)As films

We use time-resolved Kerr microscopy to perform ferromagnetic resonance experiments on nanostructured elements to show that strain is not the origin of the strong uniaxial in-plane anisotropy observed in (Ga,Mn)As films. From local angular dependent ferromagnetic resonance measurements we are able to extract the anisotropy constants. We observe no change of the strong uniaxial anisotropy in disks as small as 200 nm.

Time-resolved Kerr microscopy of coupled transverse domain walls in a pair of curved nanowires

Time-resolved scanning Kerr microscopy has been used to directly observe magnetostatically coupled transverse domain walls (TDWs) in a pair of closely spaced, curved nanowires (NWs). Kerr images of the precessional response of the magnetic domain to either side of the TDW revealed the TDW as a minimum in the Kerr signal in the region of closest NW separation. When the TDWs were ejected from the NW pair, the minimum in the Kerr signal was no longer observed. By imaging this transition, the static de-coupling field was estimated to be in the range from 38 to 48 Oe in good agreement with a simple micromagnetic model. This work provides a novel technique by which DC and microwave assisted decoupling fields of TDWs may be explored in NW pairs of different width, separation, and curvature.

Picosecond real-space imaging of electron spin diffusion in GaAs

We use time-resolved two-color Kerr microscopy to study low-temperature electron spin diffusion in bulk GaAs with high spatial and temporal resolution. This method enables direct real-space imaging of the advancing spread of an optically excited electron spin packet by spin diffusion. We observe a high initial expansion rate of the spin packet, which is strongly reduced with increasing time. Comparison with continuous-wave Kerr spectroscopy demonstrates that this decrease is caused by the influence of the transient cooling of hot photocarriers on the electron spin diffusion coefficient.

Magnetization reversal asymmetry in [Co/Pt]/CoFe/IrMn multilayers with enhanced perpendicular exchange bias

The magnetization reversal of perpendicular exchange biased [Co/Pt]/Co60Fe40/IrMn and [Co/Pt]/Co/IrMn multilayers was investigated by time-resolved surface magneto-optical Kerr effect and Kerr microscopy. Compared with the nearly symmetric reversal of the [Co/Pt]/Co/IrMn with a smaller exchange bias field, significantly asymmetric domain evolution in the [Co/Pt]/Co60Fe40/IrMn with a larger exchange bias field was directly observed by Kerr microscopy. The asymmetric magnetization reversal is discussed in terms of the average thermally activated energy barriers as well as the dispersions of the barriers. The substitution of the interfacial Co60Fe40 layer for Co layer results in an enhancement of the interfacial exchange coupling εint and absolute dispersion of interfacial exchange coupling, σεint, and consequently increases the exchange bias field and reversal asymmetry.

Reduced stochasticity in domain wall motion with increasing pinning density in thin Fe films

We report in this paper the decrease of the stochasticity of magnetization half-reversal time with increasing domain wall (DW) pinning in Fe films investigated by means of time-resolved magneto-optic Kerr microscopy. The domain images in the films reveal that the density of DW pinning sites increases with increasing Fe thickness. However, we found that the stochasticity of the magnetization half-reversal time significantly decreases with increasing DW pinning. The major reason for the reduced stochasticity is shown to be due to a thermally activated DW creep mechanism that becomes dominant during magnetization reversal due to increased DW pinning.

Large amplitude magnetization dynamics and the suppression of edge modes in a single nanomagnet

Large amplitude magnetization dynamics of a single square nanomagnet have been studied by time-resolved Kerr microscopy. Experimental spectra revealed that only a single mode was excited for all bias field values. Micromagnetic simulations demonstrate that at larger pulsed field amplitudes the center mode dominates the dynamic response while the edge mode is almost completely suppressed. Controlled suppression of edge modes in a single nanomagnet has potential applications in the operation of nanoscale spin transfer torque oscillators and bistable switching devices for which the amplitude of the magnetization trajectory is often large and a more uniform dynamic response is desirable.