arXiv subjects

Andreas Ney

Publications and source records attributed to Andreas Ney.

17 recordsLinked to original sources

arXiv2026

Noncollinear spin structure in Dy-doped classical ferrimagnet

Noncollinear spin structures have attracted tremendous attention because they offer a versatile platform for spin control and manipulation, essential in spintronics. Realizing noncollinearity in ferrimagnetic insulators is of particular interest as they can be potentially utilized in low-damping spintronics with tunable magnetic order. Within the spinel-ferrite family, Zn and Al-substituted nickel ferrite (NiZAF) has emerged as an excellent choice for low-damping spintronics. However, realizing noncollinearity in such systems remains challenging. Here, we present evidence of noncollinear spin structure in the NiZAF thin films induced by the rare earth Dy-doping, utilizing the soft x-ray spectroscopy methods such as magnetic circular dichroism and x-ray resonant magnetic reflectivity (XRMR). In particular, XRMR reveals a spiral-type spin structure, which is attributed to the Dzyaloshinskii-Moriya interaction, arising due to broken inversion symmetry by the Dy-induced local strain field as confirmed by our theoretical calculations. The realization of noncollinearity in the spinel-ferrite opens pathway to explore the possibility of chiral magnetic domains and topological spin textures exhibiting promise for oxide-based spintronics

cond-mat.mtrl-sci↗

arXiv2025

Long-range propagating paramagnon-polaritons in organic free radicals

Materials are commonly distinguished by their magnetic response into diamagnetic, paramagnetic, and magnetically ordered (ferro-, ferri-, and antiferromagnetic) phases. Diamagnets and paramagnets lack spontaneous long-range order, whereas ordered magnets develop such order below their Curie or N\'eel temperature and support single spin-wave excitations (magnons). Magnons have found applications in radio-frequency technologies and computation, magneto-optics, and foundational quantum experiments. Above the Curie/N\'eel temperature, long-range order is lost and the material transitions to a paramagnetic phase, with localised spin alignment in small patches, producing paramagnons with only short-range propagation. Here we show that long-range coherence is preserved in the organic free radical 2,2,6,6-tetramethylpiperidin-1-oxyl above the N\'eel temperature using all-electrical propagating spin-wave spectroscopy in external magnetic fields. We observe coherently excited low-energy paramagnon-polaritons up to $\mathbf{23\,\mathrm{GHz}}$ , propagating over $\mathbf{8\,\mathrm{mm}}$ at supersonic group velocities exceeding $\mathbf{100\,\mathrm{km\,s^{-1}}}$. Using free radicals as magnon carriers integrates organic materials with spintronics and opens the way to organic electronics, dense information storage, and quantum technologies.

cond-mat.mes-hall↗

arXiv2024

Probing Berry curvature in magnetic topological insulators through resonant infrared magnetic circular dichroism

Probing the quantum geometry and topology in condensed matter systems has relied heavily on static electronic transport experiments in magnetic fields. Yet, contact-free optical measurements have rarely been explored. Magnetic dichroism (MCD), the nonreciprocal absorption of circular polarized light, was theoretically linked to the quantized anomalous Hall effect in magnetic insulators and can identify the bands and momenta responsible for the underlying Berry Curvature (BC). Detecting BC through MCD faces two challenges: First, the relevant inter-band transitions usually generate MCD in the infrared (IR) range, requiring large samples with high quality. Second, while most magnetic materials are metallic, the relation between MCD and BC in metals remains unclear. Here, we report the observation of MCD in the IR range along with the anomalous Hall effect in thin film MnBi2Te4. Both phenomena emerge with a field-driven phase transition from an antiferromagnet to a canted ferromagnet. By theoretically relating the MCD to the anomalous Hall effect via BC in a metal, we show that this transition accompanies an abrupt onset of BC, signaling a topological phase transition from a topological insulator to a doped Chern insulator. Our density functional theory calculation suggests the MCD signal mainly originates from an optical transition at the Brillouin zone edge, hinting at a potential new source of BC away from the commonly considered {\Gamma} point. Our findings demonstrate a novel experimental approach for detecting BC and identifying the responsible bands and momenta, generally applicable to magnetic materials.

cond-mat.mes-hall↗

arXiv2023

Quantifying the spin-wave asymmetry in single and double rectangular Ni$_{80}$Fe$_{20}$ microstrips by TR-STXM, FMR and micromagnetic simulations

The asymmetry of spin-wave patterns in confined rectangular Ni$_{80}$Fe$_{20}$ microstrips, both in single and double-strip geometries, is quantified. The results of TR-STXM and micromagnetic simulations are compared. For the TR-STXM measurements and the corresponding simulations the excitation was a uniform microwave field with a fixed frequency of 9.43 GHz, while the external static magnetic field was swept. In the easy axis orientation of the analyzed microstrip, the results show a higher asymmetry for the double microstrip design, indicating an influence of the additional microstrip placed in close proximity to the analyzed one.

cond-mat.mes-hall↗

arXiv2023

Room-temperature spin glass behavior in zinc ferrite epitaxial thin films

Zinc ferrite (ZnFe$_{\text{2}}$O$_{\text{4}}$) epitaxial thin films were grown by reactive magnetron sputtering on MgAl$_{\text{2}}$O$_{\text{4}}$ and Al$_{\text{2}}$O$_{\text{3}}$ substrates varying a range of preparation parameters. The resulting structural and magnetic properties were investigated using a range of experimental techniques confirming epitaxial growth of ZnFe$_{\text{2}}$O$_{\text{4}}$ with the nominal stoichiometric composition and long range magnetic order at and above room temperature. The main preparation parameter influencing the temperature $T_{\text{f}}$ of the bifurcation between $M(T)$ curves under field cooled and zero-field cooled conditions was found to be the growth rate of the films, while growth temperature or the Ar:O$_2$ ratio did not systematically influence $T_{\text{f}}$. Furthermore $T_{\text{f}}$ was found to be systematically higher for MgAl$_{\text{2}}$O$_{\text{4}}$ as substrate and $T_{\text{f}}$ extends to above room temperature. While in some samples $T_{\text{f}}$ seems to be more likely correlated with superparamagentism, the highest $T_{\text{f}}$ occurs in ZnFe$_{\text{2}}$O$_{\text{4}}$ epitaxial films where experimental signatures of magnetic glassiness can be found. Element-selective X-ray magnetic circular dichroism measurements aim at associating the magnetic glassiness with the occurrence of a different valence state and lattice site incorporation of Fe pointing to a complex interplay of various competing magnetic interactions in ZnFe$_{\text{2}}$O$_{\text{4}}$.

cond-mat.other↗

arXiv2022

Collective topological spin dynamics in a correlated spin glass

The interplay between spin-orbit interaction (SOI) and magnetic order is currently one of the most active research fields in condensed matter physics and leading the search for materials with novel and tunable magnetic and spin properties. Here we report on a variety of unexpected and unique observations in thin multiferroic \Ge$_{1-x}$Mn$_x$Te films. The ferrimagnetic order in this ferroelectric semiconductor is found to reverse with current pulses six orders of magnitude lower as for typical spin-orbit torque systems. Upon a switching event, the magnetic order spreads coherently and collectively over macroscopic distances through a correlated spin-glass state. Lastly, we present a novel methodology to controllably harness this stochastic magnetization dynamics, allowing us to detect spatiotemporal nucleation of topological spin textures we term ``skyrmiverres''.

cond-mat.mtrl-sci↗

arXiv2022

Effect of impurity scattering on percolation of bosonic islands and reentrant superconductivity in Fe implanted NbN thin films

A reentrant temperature dependence of the thermoresistivity $ρ_{\mathrm{xx}}(T)$ between an onset local superconducting ordering temperature $T_\mathrm{loc}^\mathrm{onset}$ and a global superconducting transition at $T=T_\mathrm{glo}^\mathrm{offset}$ has been reported in disordered conventional 3-dimensional (3D) superconductors. The disorder of these superconductors is a result of either an extrinsic granularity due to grain boundaries, or of an intrinsic granularity ascribable to the electronic disorder originating from impurity dopants. Here, the effects of Fe doping on the electronic properties of sputtered NbN layers with a nominal thickness of 100 nm are studied by means of low-$T$/high-$μ_{0}H$ magnetotransport measurements. The doping of NbN is achieved $via$ implantation of 35 keV Fe ions. In the as-grown NbN films, a local onset of superconductivity at $T_\mathrm{loc}^\mathrm{onset}=15.72\,\mathrm{K}$ is found, while the global superconducting ordering is achieved at $T_\mathrm{glo}^\mathrm{offset}=15.05\,\mathrm{K}$, with a normal state resistivity $ρ_{\mathrm{xx}}=22\,{μΩ}\cdot{\mathrm{cm}}$. Moreover, upon Fe doping of NbN, $ρ_{\mathrm{xx}}=40\,{μΩ}\cdot{\mathrm{cm}}$ is estimated, while $T_\mathrm{loc}^\mathrm{onset}$ and $T_\mathrm{glo}^\mathrm{offset}$ are measured to be 15.1 K and 13.5K, respectively. In Fe:NbN, the intrinsic granularity leads to the emergence of a bosonic insulator state and the normal-metal-to-superconductor transition is accompanied by six different electronic phases characterized by a $N$-shaped $T$ dependence of $ρ_{\mathrm{xx}}(T)$. The bosonic insulator state in a $s$-wave conventional superconductor doped with dilute paramagnetic impurities is predicted to represent a workbench for emergent phenomena, such as gapless superconductivity, triplet Cooper pairings and topological odd frequency superconductivity.

cond-mat.supr-con↗

arXiv2022

Nonstanding spin waves in a single rectangular permalloy microstrip under uniform magnetic excitation

Ferromagnetic resonance modes in a single rectangular Ni$_{80}$Fe$_{20}$ microstrip were directly imaged using time-resolved scanning transmission x-ray microscopy combined with a phase-locked ferromagnetic resonance excitation scheme and the findings were corroborated by micromagnetic simulations. Although under uniform excitation in a single confined microstructure typically standing spin waves are expected, all imaged spin waves showed a nonstanding character both, at and off resonance, the latter being additionally detected with microantenna-based ferromagnetic resonance. The effect of the edge quality on the spin waves was observed in micromagnetic simulations.

cond-mat.mes-hall↗

arXiv2022

Element-specific visualization of dynamic magnetic coupling in a Co/Py bilayer microstructure

We present the element-specific and time resolved visualization of uniform ferromagnetic resonance excitations of a Permalloy (Py) disk - Cobalt (Co) stripe bilayer microstructure. The transverse high frequency component of the resonantly excited magnetization is sampled in the ps regime by a combination of ferromagnetic resonance (FMR) and scanning transmission X-ray microscopy (STXM- FMR) recording snapshots of the local magnetization precession of Py and Co with nanometer spatial resolution. The approach allows us to individually image the resonant dynamic response of each element, and we find that angular momentum is transferred from the Py disk to the Co stripe and vice versa at their respective resonances. The integral (cavity) FMR spectrum of our sample shows an unexpected additional third resonance. This resonance is observed in the STXM-FMR experiments as well and our microscopic findings suggest that it is governed by magnetic exchange between Py and Co, showing for the Co stripe a difference in relative phase of the magnetization due to stray field influence.

cond-mat.mes-hall↗

arXiv2021

Control of site occupancy by variation of the Zn and Al content in NiZnAl ferrite epitaxial films with low magnetic damping

The structural and magnetic properties of Zn/Al doped nickel ferrite thin films can be adjusted by changing the Zn and Al content. The films are epitaxially grown by reactive magnetron sputtering using a triple cluster system to sputter simultaneously from three different targets. Upon the variation of the Zn content the films remain fully strained with similar structural properties, while the magnetic properties are strongly affected. The saturation magnetization and coercivity as well as resonance position and linewidth from ferromagnetic resonance (FMR) measurements are altered depending on the Zn content in the material. The reason for these changes can be elucidated by investigation of the x-ray magnetic circular dichroism spectra to gain site and valence specific information with elemental specificity. Additionally, from a detailed investigation by broadband FMR a minimum in g-factor and linewidth could be found as a function of film thickness. Furthermore, the results from a variation of the Al content using the same set of measurement techniques is given. Other than for Zn, the variation of Al affects the strain and even more pronounced changes to the magnetic properties are apparent.

cond-mat.mtrl-sci↗

arXiv2020

Influence of structure and cation distribution on magnetic anisotropy and damping in Zn/Al doped nickel ferrites

An in-depth analysis of Zn/Al doped nickel ferrites grown by reactive magnetron sputtering is relevant due to their promising characteristics for applications in spintronics. The material is insulating and ferromagnetic at room temperature with an additional low magnetic damping. By studying the complex interplay between strain and cation distribution their impact on the magnetic properties, i.e. anisotropy, damping and g-factor is unravelled. In particular, a strong influence of the lattice site occupation of Ni$^{2+}_{\text{Td}}$ and cation coordination of Fe$^{2+}_{\text{Oh}}$ on the intrinsic damping is found. Furthermore, the critical role of the incorporation of Zn$^{2+}$ and Al$^{3+}$ is evidenced by comparison with a sample of altered composition. Especially, the dopant Zn$^{2+}$ is evidenced as a tuning factor for Ni$^{2+}_{\text{Td}}$ and therefore unquenched orbital moments directly controlling the g-factor. A strain-independent reduction of the magnetic anisotropy and damping by adapting the cation distribution is demonstrated.

cond-mat.mtrl-sci↗

arXiv2019

Imaging non-standing spin-waves in confined micron-sized ferromagnetic structures under uniform excitation

A non-standing character of directly imaged spin-waves in confined micron-sized ultrathin permalloy (Ni\textsubscript{80}Fe\textsubscript{20}) structures is reported along with evidence of the possibility to alter the observed state by modifications to the sample geometry. Using micromagnetic simulations the presence of the spin-wave modes excited in the permalloy stripes along with the quasi-uniform modes were calculated. The predicted spin-waves were imaged in direct space using time resolved scanning transmission X-ray microscopy, combined with a ferromagnetic resonance excitation scheme (STXM-FMR). STXM-FMR measurements revealed a non-standing character of the spin-waves. Also it was shown by micromagnetic simulations and confirmed with STXM-FMR results that the observed character of the spin-waves can be influenced by the local magnetic fields in different sample geometries.

cond-mat.mes-hall↗

arXiv2019

Spin Pumping from Permalloy into Uncompensated Antiferromagnetic Co doped Zinc Oxide

Heterostructures of Co-doped ZnO and Permalloy were investigated for their static and dynamic magnetic interaction. The highly Co-doped ZnO is paramagentic at room temperature and becomes an uncompensated antiferromagnet at low temperatures, showing a narrowly opened hysteresis and a vertical exchange bias shift even in the absence of any ferromagnetic layer. At low temperatures in combination with Permalloy an exchange bias is found causing a horizontal as well as vertical shift of the hysteresis of the heterostructure together with an increase in coercive field. Furthermore, an increase in the Gilbert damping parameter at room temperature was found by multifrequency FMR evidencing spin pumping. Temperature dependent FMR shows a maximum in magnetic damping close to the magnetic phase transition. These measurements also evidence the exchange bias interaction of Permalloy and long-range ordered Co-O-Co structures in ZnO, that are barely detectable by SQUID due to the shorter probing times in FMR.

cond-mat.mtrl-sci↗

arXiv2019

The Magnetic Grüneisen Parameter for Model Systems

The magneto-caloric effect (MCE), which is the refrigeration based on the variation of the magnetic entropy, is of great interest in both technological applications and fundamental research. The MCE is quantified by the magnetic Grüneisen parameter $Γ_{\textmd{mag}}$. We report on an analysis of $Γ_{\textmd{mag}}$ for the classical Brillouin-like paramagnet, for a modified Brillouin function taking into account a zero-field splitting originated from the spin-orbit (SO) interaction and for the one-dimensional Ising (1DI) model under longitudinal field. For both Brillouin-like model with SO interaction and the longitudinal 1DI model, for $ T \rightarrow$ 0 and vanishing field a sign change of the MCE is observed, suggestive of a quantum phase transition. SO interaction leads to a narrowing of the critical fluctuations upon approaching the critical point. Our findings emphasize the relevance of $Γ_{\textmd{mag}}$ for exploring critical points. Also, we show that the Brillouin model with and without SO interaction can be recovered from the 1DI model in the regime of high-temperatures and vanishing coupling constant $J$.

cond-mat.str-el↗

arXiv2019

Evaluation protocol for revealing magnonic contrast in STXM-FMR measurements

We present a statistically motivated method to extract magnonic contrast from STXM-FMR measurement with microwave frequencies of the order of \unit[10]{GHz}. With this method it is possible to generate phase and amplitude profiles with a spatial resolution of about \unit[30]{nm} given by the STXM resolution, furthermore this method allows fo a rigoros transformation to reciprocal \vec{k}-space, revealing \vec{k}-dependent magnon properties.

cond-mat.str-el↗

arXiv2015

Microwave soft x-ray microscopy for nanoscale magnetization dynamics in the 5-10 GHz frequency range

We present a scanning transmission x-ray microscopy setup combined with a novel microwave synchronization scheme in order to study high frequency magnetization dynamics at synchrotron light sources. The sensitivity necessary to detect small changes of the magnetization on short time scales and nanometer spatial dimensions is achieved by combination of the developed excitation mechanism with a single photon counting electronics that is locked to the synchrotron operation frequency. The required mechanical stability is achieved by a compact design of the microscope. Our instrument is capable of creating direct images of dynamical phenomena in the 5-10 GHz range, with 35 nm resolution. When used together with circularly polarized x-rays, the above capabilities can be combined to study magnetic phenomena at microwave frequencies, such as ferromagnetic resonance (FMR) and spin waves. We demonstrate the capabilities of our technique by presenting phase resolved images of a 6 GHz nanoscale spin wave generated by a spin torque oscillator, as well as the uniform ferromagnetic precession with ~0.1 deg amplitude at 9 GHz in a micrometer-sized cobalt strip.

cond-mat.mes-hall↗

arXiv2011

Irradiation Enhanced paramagnetism on graphene nanoflakes

We have studied the magnetization of vertically aligned graphene nanoflakes irradiated with nitrogen ions of 100 KeV energy and doses in the range 10^11- 10^17 ions/cm2. The non-irradiated graphene nanoflakes show a paramagnetic contribution, which is increased progressively by ion irradiation at low doses up to 10^15 /cm^2. However, further increase on implantation dose reduces the magnetic moment which coincides with the onset of amorphization as verified by both Raman and X-ray photoelectron spectroscopic data. Overall, our results demonstrate the absence of ferromagnetism on either implanted or unimplanted samples from room temperature down to a temperature of 5K

cond-mat.mes-hall↗