arXiv subjects

Jakob Walowski

Publications and source records attributed to Jakob Walowski.

At least 19 recordsLinked to original sources

arXiv2026

Femtosecond all-optical coherent control of spin polarization in altermagnets

Altermagnets constitute an emerging materials platform for spintronic technologies by combining compensated magnetic order with ferromagnet-like spin-split electronic bands. Here, we investigate the proposed d-wave altermagnetic material RuO2 using circularly polarized ultrashort laser pulses. Time-resolved magneto-optical Kerr effect measurements, which are intrinsically sensitive to surface and interface states, reveal the ultrafast spin response of RuO2. In contrast to the demagnetization dynamics characteristic of conventional ferromagnets, we observe a distinct coherent contribution to the complex Kerr rotation that appears during the light-matter interaction and lasts for about 200 fs. Similar signatures have been associated with spin-momentum locking and directional band splitting in spin-split surface states of topological insulators as well as spin-orbit-coupled semiconductors. They are governed by a finite Raman coherence time. We interpret this coherent response as evidence for transient spin-polarized surface states in RuO2, consistent with the emergence of altermagnetic surface states that are directly relevant to spin-polarized transport at surfaces and interfaces.

cond-mat.other↗

arXiv2026

Non-Volatile Vortex MTJs with Opto-Electrical and Spin-Diode Nonlinearities as Multifunctional Neuromorphic Platforms

The human brain achieves exceptional energy efficiency by co-locating memory and processing, yet reproducing this principle in hardware remains challenging because many neuromorphic devices require standby power, offer limited programmability, or separate state storage from nonlinear computation. Here we demonstrate a multifunctional spintronic platform based on storage-layer-enabled vortex magnetic tunnel junctions (MTJs) that unifies non-volatile weight storage, optoelectrically driven nonlinear computation, and multilevel readout within a single nanopillar. A thermally programmable FM/AFM storage layer retains analog synaptic weights with zero standby power and enables non-volatile tuning of the vortex gyrotropic resonance over ${\sim}15$~MHz. Under optoelectrical operation, combined laser heating and dc bias drive the junction into the bias-enhanced tunnel magneto-Seebeck (bTMS) regime, where the thermoelectric response exhibits a pronounced cubic nonlinearity providing a compact, hardware-native transfer function for weighted analog computation. The electrical and thermoelectric channels switch at matched coercive fields but with distinct amplitudes, yielding an effective four-level readout space. Crossbar-array simulations parameterized by measured device response maps evaluate two neuromorphic modes -- a bTMS mode (optical input, dc-bias weights) and a spin-diode mode (RF-frequency input, RF-power weights) -- achieving image-classification accuracies of $95.4\%$ and $94.9\%$, comparable to a digital single-layer network with sigmoid activations. Smaller 600~nm devices consistently outperform larger ones, identifying nonlinear-response engineering as a key device-level lever. Because bTMS and spin-diode rectification coexist in the same junction, a combined regime could enable nonlinear multi-input interactions, including quadratic cross-terms, within a single nanoscale element.

physics.app-ph↗

arXiv2025

Femtosecond photocurrents by the Dresselhaus bulk spin-galvanic effect in an inversion-asymmetric ferromagnet

We study ultrafast photocurrents in thin films of a model ferromagnetic metal with broken bulk inversion symmetry, the half-metallic Heusler compound NiMnSb, following excitation with an optical pump pulse with photon energy 1.55 eV. Remarkably, in terms of the direction of the sample magnetization M, all photocurrents are found to be a superposition of a component with Rashba- and Dresselhaus-type symmetry. We explain the Dresselhaus bulk photocurrent as follows: Pump-induced electron heating induces an excess of spin {\mu}_s||M, which transfers spin angular momentum into states with Dresselhaus-type spin-momentum locking. The resulting charge current relaxes on a time scale of 10 fs by momentum relaxation and, thus, follows {\mu}_s quasi-instantaneously. The relaxation of {\mu}_s is governed by the cooling of the electrons and not by the significantly slower spin-lattice relaxation of half-metals. Our findings add the Dresselhaus spin-galvanic effect (SGE) to the set of ultrafast spin-charge-conversion phenomena. They indicate a route to more efficient spintronic terahertz emitters and detectors based on the volume scaling of the bulk SGE.

cond-mat.mtrl-sci↗

arXiv2023

Comparative Analysis of THz Signal Emission from SiO$_2$/CoFeB/Metal Heterostructures: Wideband and High-Frequency THz Signal Advantage of PtBi-based Emitter

Spintronic THz emitters have attracted much attention due to their desirable properties, such as affordability, ultra-wideband capability, high efficiency, and tunable polarization. In this study, we investigate the characteristics of THz signals, including their frequency, bandwidth, and amplitude, emitted from a series of heterostructures with ferromagnetic (FM) and nonmagnetic (NM) materials. The FM layer consists of a wedge-shaped CoFeB layer with a thickness of 0 to 5 nm, while the NM materials include various metals such as Pt, Au, W, Ru, Pt$_{\%92}$Bi$_{\%8}$, and Ag$_{\%90}$Bi$_{\%10}$ alloys. Our experiments show that the emitter with Pt-NM layer has the highest amplitude of the emitted THz signal. However, the PtBi-based emitter exhibits a higher central THz peak and wider bandwidth, making it a promising candidate for broadband THz emitters. These results pave the way for further exploration of the specific compositions of Pt$_{1-x}$Bi$_{x}$ for THz emitter design, especially with the goal of generating higher frequency and wider bandwidth THz signals. These advances hold significant potential for applications in various fields such as high-resolution imaging, spectroscopy, communications, medical diagnostics, and more.

physics.optics↗

arXiv2023

Enhancing Spin Transfer Torque in Magnetic Tunnel Junction Devices: Exploring the Influence of Capping Layer Materials and Thickness on Device Characteristics

We have developed and optimized two categories of spin transfer torque magnetic tunnel junctions (STT-MTJs) that exhibit a high tunnel magnetoresistance (TMR) ratio, low critical current, high outputpower in the micro watt range, and auto-oscillation behavior. These characteristics demonstrate the potential of STT-MTJs for low-power, high-speed, and reliable spintronic applications, including magnetic memory, logic, and signal processing. The only distinguishing factor between the two categories, denoted as A-MTJs and B-MTJs, is the composition of their free layers, 2 CoFeB/0.21 Ta/6 CoFeSiB for A-MTJs and 2 CoFeB/0.21 Ta/7 NiFe for B-MTJs. Our study reveals that B-MTJs exhibit lower critical currents for auto-oscillation than A-MTJs. We found that both stacks have comparable saturation magnetization and anisotropy field, suggesting that the difference in auto-oscillation behavior is due to the higher damping of A-MTJs compared to B-MTJs. To verify this hypothesis, we employed the all-optical time-resolved magneto-optical Kerr effect (TRMOKE) technique, which confirmed that STT-MTJs with lower damping exhibited auto-oscillation at lower critical current values. Additionally, our study aimed to optimize the STT-MTJ performance by investigating the impact of the capping layer on the device's response to electronic and optical stimuli.

physics.optics↗

arXiv2022

Identification and characterization of various plastics using THz-spectroscopy

THz spectroscopy has reached imaging capability with a spatial resolution of a few micrometres. This property enables measurements and imaging of biological samples like cells. THz photons have very low photon energies in the millielectronvolt range. These energies interact mainly with vibrations in the molecules. Therefore, the investigated samples are neither ionized nor their DNA damaged, making this the perfect method for application on living cell tissue. In the future, material databases with absorption spectra will empower this method to distinguish even artificial particles like microplastics inside biological tissue. Current research aims at the impact of plastic nanoparticles on cell tissue considering various aspects. One of them is the investigation of the potential harm, the interaction of these particles with human body cells can cause, as these highly abundant particles are found anywhere in the environment. THz absorption spectroscopy offers the opportunity to investigate the polymers and identify them using their specific absorption lines as a fingerprint. In this publication, we study different polymers and analyse their particular fingerprints in the THz frequency spectrum from 0.1 up to 4 THz using a commercially available THz spectrometer. With this work we are contributing to the development, using THz waves for imaging, by identifying the fingerprints of the four most used plastics by humankind. At the end of this development, we envision a method to investigate the influence of nanometre-sized plastic particles on the human body and other biological organisms.

cond-mat.soft↗

arXiv2022

Coupling broadband terahertz dipoles to microscale resonators

Spintronic emitters are a unique class of terahertz (THz) sources due to their quasi-two-dimensional geometry and thereby their capability to couple to resonator near fields. Global excitation of the emitters often obstructs the intricate details of the coupling mechanisms between local THz dipoles and the individual modes of resonator structures. Here, we demonstrate the spatial mapping of the coupling strength between a local terahertz source on a spintronic emitter and far-field light mediated by a structured metallic environment. For a bow-tie geometry, experimental results are reproduced by a numerical model providing insights into the microscopic coupling mechanisms. The broad applicability of the technique is showcased by extracting the THz mode structure in split-ring resonator metasurfaces and linear arrays. With these developments, planar THz sources with tailored spectral and angular emission profiles are accessible.

physics.optics↗

arXiv2022

Magnetic domain scanning imaging using phase-sensitive THz-pulse detection

In our study, we determine the alignment of magnetic domains in a CoFeB layer using THz radiation. We generate THz-pulses by fs-laser-pulses in magnetized CoFeB/Pt heterostructures, based on spin currents. An LT-GaAs Auston switch detects the radiation phase-sensitively and allows to determine the magnetization alignment. Our scanning technique with motorized stages with step sizes in the sub-micrometer range, allows to image two dimensional magnetic structures. Theoretically the resolution is restricted to half of the wavelength if focusing optics in the far-field limit are used. By applying near-field imaging, the spatial resolution is enhanced to the single digit micrometer range. For this purpose, spintronic emitters in diverse geometric shapes, e.g. circles, triangles, squares, and sizes are prepared to observe the formation of magnetization patterns. The alignment of the emitted THz radiation can be influenced by applying unidirectional external magnetic fields. We demonstrate how magnetic domains with opposite alignment and different shapes divided by domain walls are created by demagnetizing the patterns using minor loops and imaged using phase sensitive THz radiation detection. For analysis, the data is compared to Kerr microscope images. The possibility to combine this method with THz range spectroscopic information of magnetic texture or antiferromagnets in direct vicinity to the spintronic emitter, makes this detection method interesting for much wider applications probing THz excitation in spin systems with high resolution beyond the Abbe diffraction limit, limited solely by the laser excitation area.

physics.optics↗

arXiv2021

Spintronic emitters for super-resolution in THz-spectral imaging

THz-spectroscopy is an attractive imaging tool for scientific research, especially in life science, offering non-destructive interaction with matter due to its low photon energies. However, wavelengths above $100μm$ principally limit its spatial resolution in the far-field by diffraction to this regime, making it not sufficient to image biological cells in the micrometer scale. Therefore, super-resolution imaging techniques are required to overcome this restriction. Near-field-imaging using spintronic emitters offers the most feasible approach because of its simplicity and potential for wide-ranging applications. In our study, we investigate THz-radiation generated by fs-laser-pulses in CoFeB/Pt heterostructures, based on spin currents, detected by commercial LT-GaAs Auston switches. The spatial resolution is evaluated applying a 2D scanning technique with motorized stages allowing scanning steps in the sub-micrometer range. By applying near-field imaging we can increase the spatial resolution to the dimensions of the laser spot size in the micrometer scale. For this purpose, the spintronic emitter is directly evaporated on a gold test pattern separated by a 300 nm spacer layer. Moving these structures with respect to the femtosecond laser spot which generates the THz radiation allows for resolution determination using the knife-edge method. We observe a full-width half-maximum THz beam diameter of $4.9(4)μ$m at 1 THz. The possibility to deposit spintronic emitter heterostructures on simple glass substrates makes them an interesting candidate for near-field imaging for a large number of applications.

physics.optics↗

arXiv2021

Observation of multi-skyrmion objects created by size and density control in Ta/CoFeB/MgO films

Magnetic skyrmions are chiral spin textures with a nontrivial topology that offer a potential for future magnetic memory and storage devices. The controlled formation and adjustment of size and density of magnetic skyrmions in Ta/CoFeB/MgO trilayers is demonstrated. It is the ideal candidate for the use as a bottom electrode integration into CoFeB/MgO/CoFeB magnetic tunnel junctions. Varying the CoFeB thickness close to the out-of-plane to in-plane magnetic phase transition, we find that subtle energy contributions enable the skyrmion formation in a narrow thickness window, corresponding to only around 10 pm variation in CoFeB thickness. Using magneto-optical imaging with quantitative image processing, variations in skyrmion diameter and distribution below the Abbe limit can be analyzed. We demonstrate a high degree of diameter and density control. Zero-field stable skyrmions can be set with proper magnetic field initialization. This demonstrated tunability and degree of comprehension of skyrmion formation, paves the way for future skyrmion based magnetic memory. Moreover, we demonstrate a controlled merging of individual skyrmions to complex topological objects. We compare our results with the baby-Skyrme model, developed to describe the soliton nature, for any topological charge n, and demonstrate the ability to form multi-skyrmion objects. These objects will be interesting for fundamental mathematical studies of the topological behavior of solitons in the future.

cond-mat.mes-hall↗

arXiv2021

The impact of metallic contacts on spin-polarized photocurrents in topological insulator $\text{Bi}_2\text{Se}_3$ nanowires

Recently, a new quantum phase, the topological insulator, has been vividly investigated in a variety of materials. Its unique bandstructure allows for optical generation and control of spin-polarized currents based on the circular photogalvanic effect. In this paper, we generate and distinguish the different photocurrent contributions via the the polarization of the driving light wave. We discuss the helicity-dependent spin-polarized current and the polarization independent thermoelectric current as spatially resolved maps, focusing on the influence of the topological insulator/metallic contact interface. We observe for both current contributions a significant enhancement of the current values at the topological insulator/metallic contact interface and moreover a dipole-like distribution of the spin-polarized current close to the contacts. We discuss the general behavior of the thermovoltage as a three-material Seebeck effect and explain the enhanced values by the acceleration of the photoelectrons generated in the space charge region of the topological insulator/metallic contact interface. Furthermore, we interpret the temperature gradient together with the spin Nernst effect as a possible origin for the enhancement and dipole-like distribution of the spin-polarized current.

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↗

arXiv2018

Petahertz Spintronics

The enigmatic coupling between electronic and magnetic phenomena was one of the riddles propelling the development of modern electromagnetism. Today, the fully controlled electric field evolution of ultrashort laser pulses permits the direct and ultrafast control of electronic properties of matter and is the cornerstone of light-wave electronics. In sharp contrast, because there is no first order interaction between light and spins, the magnetic properties of matter can only be affected indirectly on the much slower tens-of-femtosecond timescale in a sequence of optical excitation followed by the rearrangement of the spin structure. Here we record an orders of magnitude faster magnetic switching with sub-femtosecond response time by initiating optical excitations with near-single-cycle laser pulses in a ferromagnetic layer stack. The unfolding dynamics are tracked in real-time by a novel attosecond time-resolved magnetic circular dichroism (atto-MCD) detection scheme revealing optically induced spin and orbital momentum transfer (OISTR) in synchrony with light field driven charge relocation. In tandem with ab-initio quantum dynamical modelling, we show how this mechanism provides simultaneous control over electronic and magnetic properties that are at the heart of spintronic functionality. This first incarnation of attomagnetism observes light field coherent control of spin-dynamics in the initial non-dissipative temporal regime and paves the way towards coherent spintronic applications with Petahertz clock rates.

physics.app-ph↗

arXiv2018

Tunnel magneto-Seebeck effect

The interplay of charge, spin and heat transport is investigated in the fascinating research field of spin caloritronics, the marriage of spintronics and thermoelectrics. Here, many new spin-dependent thermal transport phenomena in magnetic nanostructures have been explored in the recent years. One of them is the tunnel magneto-Seebeck (TMS) effect in magnetic tunnel junctions (MTJs) that has large potential for future nanoelectronic devices, such as nanostructured sensors for three-dimentional thermal gradients, or scanning tunneling microscopes driven by temperature differences. The TMS describes the dependence of the MTJ's thermopower on its magnetic cofiguration when a thermal gradient is applied. In this review, we highlight the successful way from first observation of the TMS in 2011 to current ongoing developments in this research area. We emphasize on different heating techniques, material designs, applications, and additional physical aspects such as the role of the thermal conductivity of the barrier material. We further demonstrate the efficient interplay between ab initio calculations and experiments within this field, as this has led, e.g., to the detection of large TMS ratios in MTJs with half-metallic Heusler electrodes.

cond-mat.mes-hall↗

arXiv2018

Thermal conductivity of thin insulating films determined by tunnel magneto-Seebeck effect measurements and finite-element modeling

In general, it is difficult to access the thermal conductivity of thin insulating films experimentally just by electrical means. Here, we present a new approach utilizing the tunnel magneto-Seebeck effect (TMS) in combination with finite-element modeling (FEM). We detect the laser-induced TMS and the absolute thermovoltage of laser-heated magnetic tunnel junctions with 2.6 nm thin barriers of MgAl$_2$O$_4$ (MAO) and MgO, respectively. A second measurement of the absolute thermovoltage after a dielectric breakdown of the barrier grants insight into the remaining thermovoltage of the stack. Thus, the pure TMS without any parasitic Nernst contributions from the leads can be identified. In combination with FEM via COMSOL, we are able to extract values for the thermal conductivity of MAO ($0.7$ W/(K$\cdot$m)) and MgO ($5.8$ W/(K$\cdot$m)), which are in very good agreement with theoretical predictions. Our method provides a new promising way to extract the experimentally challenging parameter of the thermal conductivity of thin insulating films.

cond-mat.mtrl-sci↗

arXiv2018

Anomalous Nernst effect on the nanometer scale: Exploring three-dimensional temperature gradients in magnetic tunnel junctions

Localized laser heating creates temperature gradients in all directions and thus leads to three-dimensional electron flux in metallic materials. Temperature gradients in combination with material magnetization generate thermomagnetic voltages. The interplay between these direction-dependent temperature gradients and the magnetization along with their control enable to manipulate the generated voltages, e.g. in magnetic nanodevices. We identify the anomalous Nernst effect (ANE) generated on a nanometer length scale by micrometer sized temperature gradients in magnetic tunnel junctions (MTJs). In a systematic study, we extract the ANE by analyzing the influence of in-plane temperature gradients on the tunnel magneto-Seebeck effect (TMS) in three dimensional devices. To investigate these effects, we utilize in-plane magnetized MTJs based on CoFeB electrodes with an MgO tunnel barrier. Due to our measurement configuration, there is no necessity to disentangle the ANE from the spin Seebeck effect in inverse spin-Hall measurements. The temperature gradients are created by a tightly focused laser spot. The spatial extent of the measured effects is defined by the MTJ size, while the spatial resolution is given by the laser spot size and the step size of its lateral translation. This method is highly sensitive to low voltages and yields an ANE coefficient of $K_N\approx 1.6\cdot 10^{-8}\,\mathrm{V/TK}$ for CoFeB. In general, TMS investigations in MTJs are motivated by the usage of otherwise wasted heat in magnetic memory devices for read/write operations. Here, the additionally generated ANE effect allows to expand the MTJs' functionality from simple memory storage to nonvolatile logic devices and opens new application fields such as direction dependent temperature sensing with the potential for further downscaling.

cond-mat.mes-hall↗

arXiv2017

Enhancement of thermovoltage and tunnel magneto-Seebeck effect in CoFeB based magnetic tunnel junctions by variation of the MgAl$_2$O$_4$ and MgO barrier thickness

We investigate the influence of the barrier thickness of Co$_{40}$Fe$_{40}$B$_{20}$ based magnetic tunnel junctions on the laser-induced tunnel magneto-Seebeck effect. Varying the barrier thickness from 1nm to 3nm, we find a distinct maximum in the tunnel magneto-Seebeck effect for 2.6nm barrier thickness. This maximum is independently measured for two barrier materials, namely MgAl$_2$O$_4$ and MgO. Additionally, samples with an MgAl$_2$O$_4$ barrier exhibit a high thermovoltage of more than 350$μ$V in comparison to 90$μ$V for the MTJs with MgO barrier when heated with the maximum laser power of 150mW. Our results allow for the fabrication of improved stacks when dealing with temperature differences across magnetic tunnel junctions for future applications in spin caloritronics, the emerging research field that combines spintronics and themoelectrics.

cond-mat.mtrl-sci↗

arXiv2017

Pumping laser excited spins through MgO barriers

We present a study of the tunnel magneto-Seebeck (TMS) effect in MgO based magnetic tunnel junctions (MTJs). The electrodes consist of CoFeB with in-plane magnetic anisotropy. The temperature gradients which generate a voltage across the MTJs layer stack are created using laser heating. Using this method, the temperature can be controlled on the micrometer length scale: here, we investigate, how both, the TMS voltage and the TMS effect, depend on the size, position and intensity of the applied laser spot. For this study, a large variety of different temperature distributions was created across the junction. We recorded two-dimensional maps of voltages generated by heating in dependence of the laser spot position and the corresponding calculated TMS values. The voltages change in value and sign, from large positive values when heating the MTJ directly in the centre to small values when heating the junction on the edges and even small negative values when heating the sample away from the junction. Those zero crossings lead to very high calculated TMS ratios. Our systematic analysis shows, that the distribution of the temperature gradient is essential, to achieve high voltage signals and reasonable resulting TMS ratios. Furthermore, artefacts on the edges produce misleading results, but also open up further possibilities of more complex heating scenarios for spincaloritronics in spintronic devices.

cond-mat.mes-hall↗