arXiv subjects

Steffen Wittrock

Publications and source records attributed to Steffen Wittrock.

15 recordsLinked to original sources

arXiv2026

Direct observation of propagating spin waves in a spin-Hall nano-oscillator

Constriction-based spin Hall nano-oscillators (SHNOs) show great promise for application as highly tunable microwave sources with straightforward scalability toward large coupled networks. However, details of the magnetization dynamics within SHNOs have thus far not been addressed experimentally, due to the minute time and length scales involved. In this work, we present direct imaging of the magnetization dynamics within a single CoFeB-based SHNO using time-resolved scanning transmission X-ray microscopy (STXM). Our measurements reveal that the magnon amplitude is the strongest at the two constriction edges, with a pronounced assymetry favoring one edge, and that emitted spin waves exhibit strongly anisotropic propagation. Micromagnetic simulations suggest that grain boundaries and the Dzyaloshinskii-Moriya interaction (DMI) play a key role in both effects. Furthermore, the magnetodynamics changed during the measurement, indicating that the CoFeB/MgO interface may be more susceptible to X-ray induced modifications than previously recognized, challenging its presumed radiation hardness.

cond-mat.mes-hall↗

arXiv2026

Soft-X-ray momentum microscopy of nonlinear magnon interactions below 100-nm wavelength

Magnons are quantised collective excitations of long-range ordered spins. At nanometre wavelengths, exchange interactions increasingly govern their dynamics, giving rise to a largely unexplored regime of couplings between magnons and other quasiparticles. Yet, detecting such short-wavelength spin waves has remained a key experimental challenge. Here, we introduce Magnon Momentum Microscopy (MMM) -- a quasi-elastic, resonant magnetic soft-X-ray scattering technique that directly images magnon populations across two-dimensional momentum space. Owing to its remarkable sensitivity, MMM can capture nonlinear magnon-magnon interactions over large regions of the dispersion plane. Applying MMM to the prototypical magnonic material yttrium iron garnet (YIG), we uncover a rich variety of previously unobserved nonlinear magnon interactions. With its element specificity, bulk sensitivity, as well as intrinsic access to nanometre-scale wavelengths without frequency limitation, soft-X-ray MMM establishes a powerful and versatile platform for exploring short-wavelength and nonlinear magnonics.

cond-mat.mes-hall↗

arXiv2025

A fluctuation-free pathway for a topological magnetic phase transition

Topological magnetic textures are particle-like spin configurations stabilized by competing interactions. Their formation is commonly attributed to fluctuation-driven, first-order nucleation processes requiring activation over a topological energy barrier. Here, we demonstrate an alternative barrier- and fluctuation-free pathway for nucleating topological magnetic textures, triggered in our experiments by an excitation-induced spin reorientation transition. By combining x-ray imaging, scattering and micromagnetic simulations, we show that the system follows a deterministic cascade of symmetry-breaking phase transitions after excitation. First, the system undergoes a second-order phase transition from a homogeneous state to weak stripe domains, then a first-order transition to topologically trivial bubbles, and finally a topological switching event into skyrmionic textures. Through simulations, we generalize our findings and demonstrate that this pathway is active in a vast range of low-anisotropy materials. This previously unrecognized, spontaneous transition pathway suggests strategies for rapid, low-energy generation of topological spin textures and points to a general role of intrinsic modulational instabilities in phase transitions beyond magnetism.

cond-mat.mtrl-sci↗

arXiv2025

Picosecond localization dynamics following ultrafast nanoscale magnetic switching

Ultrashort laser pulses provide the fastest known way to switch magnetic order. Such excitation commonly creates nanometer-scale domains, even after homogeneous illumination when the position of nucleated domains is not externally defined. However, the physics of domain localization during such ultrafast phase transitions remains unresolved. Here, we use shot-resolved pump-probe resonant x-ray scattering together with a material featuring a periodically modulated magnetic anisotropy landscape to track, in real time, the laser-driven nucleation and localization of nanometer-scale spin textures. We find that nucleation and localization are two distinct processes. Nucleation occurs homogeneously via fluctuations at early times, whereas spatially periodic structures emerge only later and, under suitable conditions, localize in less than one nanosecond. Real-space simulations show that this localization is governed by strong lateral variations in spin-texture lifetimes. Our results demonstrate that ultrafast phase-transition dynamics fundamentally differ from conventional transitions, yet still can be controlled through moderate nanometer-scale tailoring of the energy landscape.

cond-mat.mes-hall↗

arXiv2023

Non-hermiticity in spintronics: oscillation death in coupled spintronic nano-oscillators through emerging exceptional points

The emergence of exceptional points (EPs) in the parameter space of a non-hermitian (2D) eigenvalue problem is studied in a general sense in mathematical physics, and has in the last decade successively reached the scope of experiments. In coupled systems, it gives rise to unique physical phenomena, which enable novel approaches for the development of seminal types of highly sensitive sensors. Here, we demonstrate at room temperature the emergence of EPs in coupled spintronic nanoscale oscillators and hence exploit the system's non-hermiticity. We describe the observation of amplitude death of self-oscillations and other complex dynamics, and develop a linearized non-hermitian model of the coupled spintronic system, which properly describes the main experimental features. Interestingly, these spintronic nanoscale oscillators are deployment-ready in different applicational technologies, such as field, current or rotation sensors, radiofrequeny and wireless devices and, more recently, novel neuromorphic hardware solutions. Their unique and versatile properties, notably their large nonlinear behavior, open up unprecedented perspectives in experiments as well as in theory on the physics of exceptional points. Furthermore, the exploitation of EPs in spintronics devises a new paradigm for ultrasensitive nanoscale sensors and the implementation of complex dynamics in the framework of non-conventional computing.

cond-mat.mes-hall↗

arXiv2022

Tailoring Optical Excitation to Control Magnetic Skyrmion Nucleation

In ferromagnetic multilayers, a single laser pulse with a fluence above an optical nucleation threshold can create magnetic skyrmions, which are randomly distributed over the area of the laser spot. However, in order to study the dynamics of skyrmions and for their application in future data technology, a controllable localization of the skyrmion nucleation sites is crucial. Here, it is demonstrated that patterned reflective masks behind a thin magnetic film can be designed to locally tailor the optical excitation amplitudes reached, leading to spatially controlled skyrmion nucleation on the nanometer scale. Using x-ray microscopy, the influence of nanopatterned back-side aluminum masks on the optical excitation is studied in two sample geometries with varying layer sequence of substrate and magnetic Co/Pt multilayer. Surprisingly, the masks' effect on suppressing or enhancing skymion nucleation reverses when changing this sequence. Moreover, optical near-field enhancements additionally affect the spatial arrangement of the nucleated skyrmions. Simulations of the spatial modulation of the laser excitation, and the following heat transfer across the interfaces in the two sample geometries are employed to explain these observations. The results demonstrate a reliable approach to add nanometer-scale spatial control to optically induced magnetization processes on ultrafast timescales.

cond-mat.mtrl-sci↗

arXiv2021

Stabilization of phase noise in spin torque nano oscillators by a phase locked loop

The main limitation in order to exploit spin torque nano-oscillators (STNOs) in various potential applications is their large phase noise. In this work, we demonstrate its efficient reduction by a highly reconfigurable, compact, specifically on-chip designed PLL based on custom integrated circuits. First, we thoroughly study the parameter space of the PLL+STNO system experimentally. Second, we present a theory which describes the locking of a STNO to an external signal in a general sense. In our developed theory, we do not restrict ourselves to the case of a perfect phase locking but also consider phase slips and the corresponding low offset frequency $1/f^2$ noise, so far the main drawback in such systems. Combining experiment and theory allows us to reveal complex parameter dependences of the system's phase noise. The results provide an important step for the optimization of noise properties and thus leverage the exploitation of STNOs in prospective real applications.

physics.app-ph↗

arXiv2021

Flicker and random telegraph noise between gyrotropic and dynamic C-state of a vortex based spin torque nano oscillator

Vortex based spin torque nano oscillators (STVOs) can present more complex dynamics than the spin torque induced gyrotropic (G) motion of the vortex core. The respective dynamic modes and the transition between them can be controlled by experimental parameters such as the applied dc current. An interesting behavior is the stochastic transition from the G- to a dynamic C-state occurring for large current densities. Moreover, the C-state oscillations exhibit a constant active magnetic volume. We present noise measurements in the different dynamic states that allow accessing specific properties of the stochastic transition, such as the characteristic state transition frequency. Furthermore,we confirm, as theoretically predicted, an increase of flicker noise with $I_{dc}^2$ when the oscillation volume remains constant with the current. These results bring insight into the potential optimization of noise properties sought for many potential rf applications with spin torque oscillators. Furthermore, the investigated stochastic characteristics open up new potentialities, for instance in the emerging field of neuromorphic computing schemes.

cond-mat.mes-hall↗

arXiv2020

Spin-torque Dynamics for Noise Reduction in Vortex-based Sensors

Performance of magnetoresistive sensors is today mainly limited by their 1/f low-frequency noise. Here, we study this noise component in vortex-based TMR sensors. We compare the noise level in different magnetization configurations of the device, i.e vortex state or uniform parallel or antiparallel states. We find that the vortex state is at least an order of magnitude noisier than the uniform states. Nevertheless, by activating the spin-transfer induced dynamics of the vortex configuration, we observe a reduction of the 1/f noise, close to the values measured in the AP state, as the vortex core has a lower probability of pinning into defect sites. Additionally, by driving the dynamics of the vortex core by a non-resonant rf field or current we demonstrate that the 1/f noise can be further decreased. The ability to reduce the 1/f low-frequency noise in vortex-based devices by leveraging their spin-transfer dynamics thus enhances their applicability in the magnetic sensors' landscape.

cond-mat.mes-hall↗

arXiv2020

Beyond the gyrotropic motion: dynamic C-state in vortex spin torque oscillators

In the present study, we investigate a dynamical mode beyond the gyrotropic (G) motion of a magnetic vortex core in a confined magnetic disk of a nano-pillar spin torque nano oscillator. It is characterized by the in-plane circular precession associated to a C-shaped magnetization distribution. We show a transition between G and C-state mode which is found to be purely stochastic in a current-controllable range. Supporting our experimental findings with micromagnetic simulations, we believe that the results provide novel opportunities for the dynamic and stochastic control of STOs, which could be interesting to be implemented for example in neuromorphic networks.

cond-mat.mes-hall↗

arXiv2020

Influence of flicker noise and nonlinearity on the frequency spectrum of spin torque nano-oscillators

The correlation of phase fluctuations in any type of oscillator fundamentally defines its spectral shape. However, in nonlinear oscillators, such as spin torque nano oscillators, the frequency spectrum can become particularly complex. This is specifically true when not only considering thermal but also colored $1/f$ flicker noise processes, which are crucial in the context of the oscillator's long term stability. In this study, we address the frequency spectrum of spin torque oscillators in the regime of large-amplitude steady oscillations experimentally and as well theoretically. We particularly take both thermal and flicker noise into account. We perform a series of measurements of the phase noise and the spectrum on spin torque vortex oscillators, notably varying the measurement time duration. Furthermore, we develop the modelling of thermal and flicker noise in Thiele equation based simulations. We also derive the complete phase variance in the framework of the nonlinear auto-oscillator theory and deduce the actual frequency spectrum. We investigate its dependence on the measurement time duration and compare with the experimental results. Long term stability is important in several of the recent applicative developments of spin torque oscillators. This study brings some insights on how to better address this issue.

cond-mat.mes-hall↗

arXiv2019

Spin-current manipulation of photoinduced magnetization dynamics in heavy metal / ferromagnet double layer based nanostructures

Spin currents offer a way to control static and dynamic magnetic properties, and therefore they are crucial for next-generation MRAM devices or spin-torque oscillators. Manipulating the dynamics is especially interesting within the context of photo-magnonics. In typical $3d$ transition metal ferromagnets like CoFeB, the lifetime of light-induced magnetization dynamics is restricted to about 1 ns, which e.g. strongly limits the opportunities to exploit the wave nature in a magnonic crystal filtering device. Here, we investigate the potential of spin-currents to increase the spin wave lifetime in a functional bilayer system, consisting of a heavy metal (8 nm of $β$-Tantalum (Platinum)) and 5 nm CoFeB. Due to the spin Hall effect, the heavy metal layer generates a transverse spin current when a lateral charge current passes through the strip. Using time-resolved all-optical pump-probe spectroscopy, we investigate how this spin current affects the magnetization dynamics in the adjacent CoFeB layer. We observed a linear spin current manipulation of the effective Gilbert damping parameter for the Kittel mode from which we were able to determine the system's spin Hall angles. Furthermore, we measured a strong influence of the spin current on a high-frequency mode. We interpret this mode an an exchange dominated higher order spin-wave resonance. Thus we infer a strong dependence of the exchange constant on the spin current.

cond-mat.mes-hall↗

arXiv2019

Low offset frequency $1/f$ flicker noise in spin torque vortex oscillators

Low frequency noise close to the carrier remains little explored in spin torque nano oscillators. However, it is crucial to investigate as it limits the oscillator's frequency stability. This work addresses the low offset frequency flicker noise of a TMR-based spin-torque vortex oscillator in the regime of large amplitude steady oscillations. We first phenomenologically expand the nonlinear auto-oscillator theory aiming to reveal the properties of this noise. We then present a thorough experimental study of the oscillator's $1/f$ flicker noise and discuss the results based on the theoretical predictions. Hereby, we connect the oscillator's nonlinear dynamics with the concept of flicker noise and furthermore refer to the influence of a standard $1/f$ noise description based on the Hooge formula, taking into account the non-constant magnetic oscillation volume, which contributes to the magnetoresistance.

cond-mat.mes-hall↗

arXiv2015

Spin-transfer torque switching at ultra low current densities

The influence of the tantalum buffer layer on the magnetic anisotropy of perpendicular Co-Fe-B/MgO based magnetic tunnel junctions is studied using magneto-optical Kerr-spectroscopy. Samples without a tantalum buffer are found to exhibit no perpendicular magnetization. The transport of Boron into the tantalum buffer is considered to play an important role on the switching currents of those devices. With the optimized layer stack of a perpendicular tunnel junction, a minimal critical switching current density of only 9.2 kA/cm$^2$ is observed. As of today, this value is the lowest reported value for current-induced magnetization reversal.

cond-mat.mtrl-sci↗

arXiv2013

Spin-transfer torque switching below 20 kA/cm$^2$ in perpendicular magnetic tunnel junctions

We demonstrate the reduction of critical spin-transfer torque (STT) switching currents in Co-Fe-B/MgO based magnetic tunnel junctions (MTJ) with perpendicular magnetization anisotropy (PMA). The junctions yield tunnel magnetoresistance (TMR) ratios of up to 64% at 4 monolayer (ML) tunnel barrier thickness. In this paper, the reduction of the critical switching current density is studied. By optimizing the applied bias field during DC-STT measurements, ultra low critical switching current densities of less than 20 kA/cm$^2$, even down to 9 kA/cm$^2$, are found. With the reduced switching currents, our samples are ideal candidates for further experimental studies such as the theoretical predicted thermally driven spin-transfer torque effect.

cond-mat.mes-hall↗