arXiv subjects

Robert Frömter

Publications and source records attributed to Robert Frömter.

15 recordsLinked to original sources

arXiv2026

Methods for traceable scanning magnetometry using single nitrogen vacancy centers in diamond: determining orientation, distance and localization

Individual, scannable nitrogen vacancy (NV) centers in single crystal diamond nanostructures enable nanoscale, quantitative imaging of magnetic stray fields. Nevertheless, important parameters like distance between the NV center and the sample and the orientation of the NV high symmetry axis are often not known precisely and enter data evaluation as free fitting parameters. We here use scanning NV imaging on micro-patterned, perpendicularly magnetized stripes and discs. From these measurements, we directly infer NV - sample distance d_NV and the NV's azimuthal orientation without the need for an external vector magnet control. We determine d_NV = 31.5 nm, while we infer the azimuthal orientation with a precision of 3°. We additionally employ commercially available silicon needles to image the apex topography of our diamond nanostructures to detect surface contamination. Simultaneously, monitoring NV fluorescence as a function of the needle's position allows us to estimate the lateral placement of the NV inside the diamond nanostructure.

cond-mat.mes-hall↗

arXiv2025

Controlling Skyrmion Lattice Orientation with Local Magnetic Field Gradients

Precise control over the formation and arrangement of magnetic skyrmion lattices is essential for understanding their emergent behavior and advancing their integration into spintronic and magnonic devices. We report on a simple and minimally invasive technique to nucleate and manipulate skyrmion lattices in soft magnetic CoFeB using single-pass magnetic force microscopy (MFM). By tuning the scan-line spacing to match the intrinsic stripe domain periodicity, the stray field gradient from the MFM tip induces reversible transitions from stripe domains to isolated skyrmions and locally ordered lattices. The resulting skyrmion positions are extracted to compute the local orientational order parameter $ψ_6$, enabling quantitative evaluation of lattice ordering. A systematic improvement in $\langle |ψ_6| \rangle$ is observed with repeated scanning, indicating a transition from a disordered state to ordered hexagonal lattices. Furthermore, we demonstrate that the lattice orientation can be deterministically rotated by changing the scanning direction, as confirmed by both real-space analysis and fast Fourier transformations. This method enables the controlled creation, reordering, and deletion of metastable skyrmion textures on demand. Our approach establishes a practical and accessible platform for studying two-dimensional phase behavior in topological spin systems, offering direct and reconfigurable control over lattice symmetry, order, and orientation.

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arXiv2025

Correlating on-the-fly Electrical and Optical Skyrmion Readout

Magnetic skyrmions, topologically stabilized spin textures, are promising candidates for future memory devices and non-conventional computing applications due to their enhanced stability, non-linear interactions, and low-power manipulation capabilities. Despite their significant potential, the reliable electrical readout of individual skyrmions remains a fundamental challenge. While magnetic tunnel junctions and anomalous Hall effect (AHE)-based techniques have demonstrated skyrmion detection capabilities, they currently fail to reliably detect single moving skyrmions as required for applications. Our approach leverages thermally activated skyrmions, where a low constant drive current simultaneously generates both skyrmion motion and the Hall voltage necessary for detection. We demonstrate the reliability of this method through real-time correlation between measured Hall voltage signals and direct Kerr microscopy imaging. Two consecutive Hall crosses allow for determining the skyrmion velocity, in accordance to Kerr microscopy videos. We present an analytical formula for the skyrmion AHE readout signal, demonstrating scalability from micrometer to nanometer skyrmions. These advances establish a robust platform for skyrmion-based sensors, counters and unconventional computing systems that depend on precise individual skyrmion control and detection.

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arXiv2025

Observation of a non-reciprocal skyrmion Hall effect of hybrid chiral skyrmion tubes in synthetic antiferromagnetic multilayers

A hybrid chiral skyrmion tube is a well-known example of a 3D topological spin texture, exhibiting an intriguing chirality transition along the thickness direction. This transition progresses from left-handed to right-handed Néel-type chirality, passing through a Bloch-type intermediate state. Such an exotic spin configuration potentially exhibits distinctly different dynamics from that of the common skyrmion tube that exhibits a homogeneous chirality; yet these dynamics have not been ascertained so far. Here we reveal the distinct features of current-induced dynamics that result from the hybrid chiral skyrmion tube structure in synthetic antiferromagnetic (SyAFM) multilayers. Strikingly, the SyAFM hybrid chiral skyrmion tubes exhibit a non-reciprocal skyrmion Hall effect in the flow regime. The non-reciprocity can even be tuned by the degree of magnetic compensation in the SyAFM systems. Our theoretical modeling qualitatively corroborates that the non-reciprocity stems from the dynamic oscillation of skyrmion helicity during its current-induced motion. The findings highlight the critical role of the internal degrees of freedom of these complex skyrmion tubes for their current-induced dynamics.

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arXiv2025

Antiferromagnetic Skyrmion Scattering Revealed by Direct Time-Resolved Imaging of Collective Dynamics

Scattering analysis offers a fundamental route to revealing particle interactions with direct implications for device technologies relying on ensembles of particles such as magnetic skyrmions. Here, we directly visualize, in real time, the nanosecond current-driven dynamics of an antiferromagnetic (AFM) skyrmion lattice using element-specific pump-probe X-ray microscopy. By tuning spin-orbit torque relative to local pinning potentials, we reveal two regimes: incoherent flow, where mobile skyrmions scatter from pinned ones, inducing recoil dynamics with 3-20 ns relaxation, and coherent flow, where the lattice translates uniformly. Quantification of the reproducible post-pulse relaxation trajectories via an inverse analyis method based on the Thiele equation yields the nanoscale AFM skyrmion-skyrmion scattering potential, which decays exponentially with a range of 30 nm, in full agreement with micromagnetic simulations. At higher current densities, the lattice exhibits coherent motion free from detectable Hall and inertial effects or dynamical deformation, enabling robust GHz operation. These findings establish a quantitative framework for AFM skyrmion interactions and demonstrate deterministic control of their collective dynamics over billions of cycles even in the incoherent flow regime, thereby paving the way for multi-skyrmion spintronic devices.

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arXiv2024

Laser-induced helicity and texture-dependent switching of nanoscale stochastic domains in a ferromagnetic film

Controlling magnetic textures at ever smaller length and time scales is of key fundamental and technological interest. Achieving nanoscale control often relies on finding an external stimulus that is able to act on that small length scales, which is highly challenging. A promising alternative is to achieve nanoscale control using the inhomogeneity of the magnetic texture itself. Using a multilayered ferromagnetic Pt/Co/Pt thin-film structure as a model system, we employ a magnetic force microscope to investigate the change in magnetic nanotextures induced by circularly polarized picosecond laser pulses. Starting from a saturated magnetic state, we find stochastic nucleation of complex nanotextured domain networks. In particular, the growth of these domains depends not only on their macroscopic magnetization but also on the complexity of the domain texture. This helicity and texture-dependent effect contrasts with the typical homogeneous growth of magnetic domains initiated by an effective magnetic field of a circularly polarized laser pulse. We corroborate our findings with a stochastic model for the nucleation of magnetic domains, in which the nucleation and annihilation probability not only depends on the helicity of light but also on the relative magnetization orientation of neighboring domains. Our results establish a new approach to investigate ultrafast nanoscale magnetism and photo-excitation across first-order phase transitions.

cond-mat.mtrl-sci↗

arXiv2024

The role of magnetic dipolar interactions in skyrmion lattices

Magnetic skyrmions are promising candidates for information and storage technologies. In the last years, magnetic multilayer systems have been tuned to enable room-temperature skyrmions, stable even in the absence of external magnetic field. There are several models describing the properties of an isolated skyrmion in a homogeneous background for single repetition multilayer stack, however, the description on how the equilibrium skyrmion size in lattices scales with increasing the number of repetitions of the stack remains unaddressed. This question is essential for fundamental and practical perspectives, as the behaviour of an ensemble of skyrmions differs from the isolated case. Based on a multilayer stack hosting a skyrmion lattice, we have carried out a series of imaging experiments scaling up the dipolar interaction by repeating $n$ times the multilayer unit, from $n =1$ up to $n=30$. We have developed an analytical description for the skyrmion radius in the whole multilayer regime, $i.e.$, from thin to thick film limits. Furthermore, we provide insight on how nucleation by an externally applied field can give rise to a lattice with more skyrmions (thus, overfilled) than the predicted by the calculations.

cond-mat.mtrl-sci↗

arXiv2023

Homochiral antiferromagnetic merons, antimerons and bimerons realized in synthetic antiferromagnets

The ever-growing demand for device miniaturization and energy efficiency in data storage and computing technology has prompted a shift towards antiferromagnetic (AFM) topological spin textures as information carriers, owing to their negligible stray fields, leading to possible high device density and potentially ultrafast dynamics. We realize, in this work, such chiral in-plane (IP) topological antiferromagnetic spin textures, namely merons, antimerons, and bimerons in synthetic antiferromagnets by concurrently engineering the effective perpendicular magnetic anisotropy, the interlayer exchange coupling, and the magnetic compensation ratio. We demonstrate by three-dimensional vector imaging of the Néel order parameter, the topology of those spin textures and reveal globally a well-defined chirality, which is a crucial requirement for controlled current-induced dynamics. Our analysis reveals that the interplay between interlayer exchange and interlayer magnetic dipolar interactions plays a key role in significantly reducing the critical strength of the Dzyaloshinskii-Moriya interaction required to stabilize topological spin textures, such as AFM merons, making synthetic antiferromagnets a promising platform for next-generation spintronics applications.

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arXiv2022

Controlling 3D spin textures by manipulating sign and amplitude of interlayer DMI with electrical current

The recently discovered interlayer Dzyaloshinskii-Moriya interaction (IL-DMI) in multilayers with perpendicular magnetic anisotropy favors the canting of spins in the in-plane direction and could thus enable new exciting spin textures such as Hopfions in continuous multilayer films. A key requirement is to control the IL-DMI and so in this study, the influence of an electric current on the IL-DMI is investigated by out-of-plane hysteresis loops of the anomalous Hall effect under applied in-plane magnetic fields. The direction of the in-plane field is varied to obtain a full azimuthal dependence, which allows us to quantify the effect on the IL-DMI. We observe a shift in the azimuthal dependence of the IL-DMI with increasing current, which can be understood from the additional in-plane symmetry breaking introduced by the current flow. Using an empirical model of two superimposed cosine functions we demonstrate the presence of a current-induced term that linearly increases the IL-DMI in the direction of current flow. With this, a new easily accessible possibility to manipulate 3D spin textures by current is realized. As most spintronic devices employ spin-transfer or spin-orbit torques to manipulate spin textures, the foundation to implement current-induced IL-DMI into thin-film devices is broadly available.

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arXiv2021

Polarization amplification by spin-doping in nanomagnetic/graphene hybrid systems

The generation of non-equilibrium electron spin polarization, spin transport, and spin detection are fundamental in many quantum devices. We demonstrate that a lattice of magnetic nanodots enhances the electron spin polarization in monolayer graphene via carrier exchange. We probed the spin polarization through a resistively-detected variant of electron spin resonance (ESR) and observed resonance amplification mediated by the presence of the nanodots. Each nanodot locally injects a surplus of spin-polarized carriers into the graphene, and the ensemble of all "spin hot spots" generates a non-equilibrium electron spin polarization in the graphene layer at macroscopic lengths. This occurs whenever the interdot distance is comparable or smaller than the spin diffusion length.

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arXiv2019

Magnetic chirality controlled by the interlayer exchange interaction

Chiral magnetism, wherein there is a preferred sense of rotation of the magnetization, has become a key aspect for future spintronic applications. It determines the chiral nature of magnetic textures, such as skyrmions, domain walls or spin spirals, and a specific magnetic chirality is often required for spintronic applications. Current research focuses on identifying and controlling the interactions that define the magnetic chirality. The influence of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) and, recently, the dipolar interactions have previously been reported. Here, we experimentally demonstrate that an indirect interlayer exchange interaction can be used as an additional tool to effectively manipulate the magnetic chirality. We image the chirality of magnetic domain walls in a coupled bilayer system using scanning electron microscopy with polarization analysis (SEMPA). Upon increasing the interlayer exchange coupling, we induce a transition of the magnetic chirality from clockwise rotating Néel walls to degenerate Bloch-Néel domain walls and we confirm our findings with micromagnetic simulations. In multi-layered systems relevant for skyrmion research a uniform magnetic chirality across the magnetic layers is often desired. Additional simulations show that this can be achieved for reduced iDMI values when exploiting the interlayer exchange interaction. This work opens up new ways to control and tailor the magnetic chirality by the interlayer exchange interaction.

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arXiv2019

Measuring the Dzyaloshinskii-Moriya interaction of the epitaxial Co/Ir(111) interface

The in-plane orientation of the magnetization in the center of domain walls is measured in Co/Ir(111) as a function of Co thickness via scanning electron microscopy with polarization analysis. Uncapped, thermally evaporated cobalt on an Ir(111) single-crystal surface is imaged in situ in ultra-high vacuum. The initial pseudomorphic growth with an atomically flat interface of cobalt on iridium ensures comparability to theoretical calculations and provides a study of an interface that is as ideal as possible. Below a cobalt thickness of 8.8 monolayers, the magnetic domain walls are purely Néel oriented and show a clockwise sense of rotation. For larger thicknesses the plane of rotation changes and the domain walls show a significant Bloch-like contribution, allowing to calculate the strength of the Dzyaloshinskii-Moriya interaction (DMI) from energy minimization. From the angle between the plane of rotation and the domain-wall normal an interfacial DMI parameter $D_s = -(1.07 \pm 0.05)$ pJ/m is determined, which corresponds to a DMI energy per bond between two Co atoms at the interface of $d_{tot} = -(1.04 \pm 0.05)$ meV.

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arXiv2019

Tuning magnetic chirality by dipolar interactions

Chiral magnetism has gained enormous interest in recent years because of the anticipated wealth of applications in nanoelectronics. The demonstrated stabilization of chiral magnetic domain walls and skyrmions has been attributed to the actively investigated Dzyaloshinskii-Moriya interaction. Recently, however, predictions were made that suggest dipolar interactions can also stabilize chiral domain walls and skyrmions, but direct experimental evidence has been lacking. Here we show that dipolar interactions can indeed stabilize chiral domain walls by directly imaging the magnetic domain walls using scanning electron microscopy with polarization analysis. We further show that the competition between the Dzyaloshinskii-Moriya and dipolar interactions can reverse the domain-wall chirality. Finally, we suggest that this competition can be tailored by a Ruderman-Kittel-Kasuya-Yosida interaction. Our work therefore reveals that dipolar interactions play a key role in the stabilization of chiral spin textures. This insight will open up new routes towards balancing interactions for the stabilization of chiral magnetism.

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arXiv2017

Scanning electron microscopy with polarization analysis for multilayered chiral spin textures

We show that scanning electron microscopy with polarization analysis (SEMPA) that is sensitive to both in-plane magnetization components can be used to image the out-of-plane magnetized multi-domain state in multilayered chiral spin textures. By depositing a thin layer of Fe on top of the multilayer we image the underlying out-of-plane domain state through the mapping of its stray fields in the Fe. We also demonstrate that SEMPA can be used to image the domain wall chirality in these systems after milling away the capping layer and imaging the topmost magnetic layer directly.

cond-mat.mes-hall↗

arXiv2015

Influence of elastically pinned magnetic domain walls on magnetization reversal in multiferroic heterostructures

In elastically coupled multiferroic heterostructures that exhibit full domain correlations between ferroelectric and ferromagnetic sub-systems, magnetic domain walls are firmly pinned on top of ferroelectric domain boundaries. In this work we investigate the influence of pinned magnetic domain walls on the magnetization reversal process in a Co40Fe40B20 wedge film that is coupled to a ferroelectric BaTiO3 substrate via interface strain transfer. We show that the magnetic field direction can be used to select between two distinct magnetization reversal mechanisms, namely (1) double switching events involving alternate stripe domains at a time or (2) synchronized switching of all domains. Furthermore, scaling of the switching fields with domain width and film thickness is also found to depend on field orientation. These results are explained by considering the dissimilar energies of the two types of pinned magnetic domain walls that are formed in the system.

cond-mat.mtrl-sci↗