arXiv subjects

Sibylle Sievers

Publications and source records attributed to Sibylle Sievers.

15 recordsLinked to original sources

arXiv2026

On the influence of reference sample properties on magnetic force microscopy calibrations

Magnetic force microscopy (MFM) allows the characterization of magnetic stray field distributions with high sensitivity and spatial resolution. Based on a suitable calibration procedure, MFM can also yield quantitative magnetic field values. This process typically involves measuring a reference sample to determine the distribution of the tip's stray field or stray field gradient at the sample surface. This distribution is called the tip transfer function (TTF) and is derived through regularized deconvolution in Fourier space. The properties of the reference sample and the noise characteristics of the detection system significantly influence the derived TTF, thereby limiting its validity range. In a recent study, the tip stray field distribution, and hence the TTF, of an MFM tip was independently measured in real space using a nitrogen vacancy center as a quantum sensor, revealing considerable discrepancies with the reference-sample-based TTF. Here, we analyze the influence of the feature distribution of the reference sample and the MFM measurement parameters on the resulting TTF. We explain the observed differences between quantum-calibrated stray field distributions and the classical approach by attributing them to a loss of information due to missing or suppressed spectral components. Furthermore, we emphasize the importance of the spectral coverage of the TTF. Our findings indicate that for high-quality reconstruction of the stray field of a sample under test (SUT), it is more critical to ensure a strong overlap of frequency components between the reference sample and the SUT than to achieve an accurate real-space reconstruction of the tip stray field distribution.

cond-mat.mes-hall↗

arXiv2024

Magnetic Force Microscopy: High Quality-Factor Two-Pass Mode

Magnetic force microscopy (MFM) is a well-established technique in scanning probe microscopy that allows for the imaging of magnetic samples with a spatial resolution of tens of nm and stray fields down to the mT range. The spatial resolution and field sensitivity can be significantly improved by measuring in vacuum conditions. This improvement originates from the higher quality-factor (Q-factor) of the cantilever's oscillation in vacuum compared to ambient conditions. However, while high Q-factors are desirable as they directly enhance the magnetic measurement signal, they pose a challenge when performing standard MFM two-pass (lift) mode measurements. At high Q-factors, amplitude-based topography measurements become impossible, and the MFM phase response behaves non-linearly. Here, we present a modified two-pass mode implementation in a vacuum atomic force microscope (AFM) that addresses these issues. By controlling the Q-factor in the first pass and using a phase-locked loop (PLL) technique in the second pass, high Q-factor measurements in vacuum are enabled. Measuring the cantilever's frequency shift instead of the phase shift eliminates the issue of emerging nonlinearities. The improvements in MFM signal-to-noise ratio are demonstrated using a nano-patterned magnetic sample. The elimination of non-linear response is highlighted through measurements performed on a well-characterized multilayer reference sample. Finally, we discuss a technique that avoids topography-induced artifacts by following the average sample slope. The newly developed, sensitive, and distortion-free high quality-factor two-pass mode has the potential to be widely implemented in commercial setups, facilitating high-resolution MFM measurements and advancing studies of modern magnetic materials.

cond-mat.mes-hall↗

arXiv2023

Optical creation and annihilation of skyrmion patches in a bulk chiral magnet

A key challenge for the realization of future skyrmion devices comprises the controlled creation, annihilation and detection of these topologically non-trivial magnetic spin textures. In this study, we report an all-optical approach for writing, deleting, and reading skyrmions in the cubic chiral magnet Fe$_{0.25}$Co$_{0.75}$Si based on thermal quenching. Using focused femtosecond laser pulses, patches of a skyrmion state are created and annihilated locally, demonstrating unprecedented control of thermally metastable skyrmions in a bulk compound. The skyrmion state is read-out by analyzing the microwave spin excitations in time-resolved magneto-optical Kerr effect measurements. Extracting the magnetic field and laser fluence dependence, we find well-separated magnetic field regimes and different laser fluence thresholds for the laser-induced creation and annihilation of metastable skyrmions. The all-optical skyrmion control, as established in this study for a model system, represents a promising and energy-efficient approach for the realization of skyrmions as magnetic bits in future storage devices, reminiscent of magneto-optical storage devices in the past.

cond-mat.mes-hall↗

arXiv2022

Optically excited spin dynamics of thermally metastable skyrmions in Fe$_{0.75}$Co$_{0.25}$Si

We investigate the microwave spin excitations of the cubic chiral magnet Fe$_{0.75}$Co$_{0.25}$Si as driven by the thermal modulation of magnetic interactions via laser heating and probed by time-resolved measurements of the magneto-optical Kerr effect. Focusing on the topologically nontrivial skyrmion lattice state, the dynamic properties in thermodynamic equilibrium are compared with those of a metastable state prepared by means of rapid field cooling. In both cases, we find precessional and exponential contributions to the dynamic response, characteristic of a breathing mode and energy dissipation, respectively. When taking into account the universal scaling as a function of temperature, the precession frequencies in the equilibrium and metastable skyrmion state are in excellent quantitative agreement. This finding highlights that skyrmion states far from thermal equilibrium promise great flexibility, for instance with respect to temperature and field scales, both for possible microwave applications and the study of fundamental properties.

cond-mat.mes-hall↗

arXiv2022

A Ti/Pt/Co multilayer stack for transfer function based magnetic force microscopy calibrations

Magnetic force microscopy is a widespread technique for imaging magnetic structures with a resolution of some 10 nanometers. MFM can be calibrated to obtain quantitative spatially resolved magnetization data in units of A/m by determining the calibrated point spread function of the instrument, its instrument calibration function (ICF), from a measurement of a well-known reference sample. Beyond quantifying the MFM data, a deconvolution of the MFM image data with the ICF also corrects the smearing caused by the finite width of the MFM tip stray field distribution. However, the quality of the calibration depends critically on the calculability of the magnetization distribution of the reference sample. Here, we discuss a Ti/Pt/Co multilayer stack that shows a stripe domain pattern as a suitable reference material. A precise control of the fabrication process, combined with a characterization of the sample micromagnetic parameters, allows reliable calculation of the sample's magnetic stray field, proven by a very good agreement between micromagnetic simulations and qMFM measurements. A calibrated qMFM measurement using the Ti/Pt/Co stack as a reference sample is shown and validated, and the application area for quantitative MFM measurements calibrated with the Ti/Pt/Co stack is discussed

cond-mat.mes-hall↗

arXiv2021

Quantum calibrated magnetic force microscopy

We report the quantum calibration of a magnetic force microscope (MFM) by measuring the two-dimensional magnetic stray field distribution of the MFM tip using a single nitrogen vacancy (NV) center in diamond. From the measured stray field distribution and the mechanical properties of the cantilever a calibration function is derived allowing to convert MFM images to quantum calibrated stray field maps. This novel approach overcomes limitations of prior MFM calibration schemes and allows quantum calibrated nanoscale stray field measurements in a field range inaccessible to scanning NV magnetometry. Quantum calibrated measurements of a stray field reference sample allow its use as a transfer standard, opening the road towards fast and easily accessible quantum traceable calibrations of virtually any MFM.

cond-mat.mes-hall↗

arXiv2020

Traceably Calibrated Scanning Hall Probe Microscopy at Room Temperature

Fabrication, characterization and comparison of gold and graphene micro- and nano-size Hall sensors for room temperature scanning magnetic field microscopy applications is presented. The Hall sensors with active areas from 5 $μ$m down to 50 nm were fabricated by electron-beam lithography. The calibration of the Hall sensors in an external magnetic field revealed a sensitivity of 3.2 mV/(AT) $\pm$ 0.3 % for gold and 1615 V/(AT) $\pm$ 0.5 % for graphene at room temperature. The gold sensors were fabricated on silicon nitride cantilever chips suitable for integration into commercial scanning probe microscopes, allowing scanning Hall microscopy (SHM) under ambient conditions and controlled sensor-sample distance. The height dependent stray field distribution of a magnetic scale was characterized using a 5 $μ$m gold Hall sensor. The uncertainty of the entire Hall sensor based scanning and data acquisition process was analyzed allowing traceably calibrated SHM measurements. The measurement results show good agreement with numerical simulations within the uncertainty budget.

physics.ins-det↗

arXiv2020

Thermoelectric signature of individual skyrmions

We experimentally study the thermoelectrical signature of individual skyrmions in chiral Pt/Co/Ru multilayers. Using a combination of controlled nucleation, single skyrmion annihilation, and magnetic field dependent measurements the thermoelectric signature of individual skyrmions is characterized. The observed signature is explained by the anomalous Nernst effect of the skyrmions spin structure. Possible topological contributions to the observed thermoelectrical signature are discussed. Such thermoelectrical characterization allows for non-invasive detection and counting of skyrmions and enables fundamental studies of topological thermoelectric effects on the nano scale

cond-mat.mes-hall↗

arXiv2020

Uncertainty Analysis of Stray Field Measurements by Quantitative Magnetic Force Microscopy

Magnetic force microscopy (MFM) measurements generally provide phase images which represent the signature of domain structures on the surface of nanomaterials. To quantitatively determine magnetic stray fields based on an MFM image requires calibrated properties of the magnetic tip. In this work, an approach is presented for calibrating a magnetic tip using a Co/Pt multilayered film as a reference sample which shows stable well-known magnetic properties and well-defined perpendicular band domains. The approach is based on a regularized deconvolution process in Fourier domain with a Wiener filter and the L-curve method for determining a suitable regularization parameter to get a physically reasonable result. The calibrated tip is applied for a traceable quantitative determination of the stray fields of a test sample which has a patial frequency spectrum covered by that of the reference sample. According to the "Guide to the expression of uncertainty in measurement", uncertainties of the processing algorithm are estimated considering the fact that the regularization influences significantly the quantitative analysis. We discuss relevant uncertainty components and their propagations between real domain and Fourier domain for both, the tip calibration procedure and the stray field calculation, and propose an uncertainty evaluation procedure for quantitative magnetic force microscopy.

physics.data-an↗

arXiv2017

Investigation of a possible electronic phase separation in the magnetic semiconductors Ga$_{1-x}$Mn$_{x}$As and Ga$_{1-x}$Mn$_{x}$P by means of fluctuation spectroscopy

We present systematic temperature-dependent resistance noise measurements on a series of ferromagnetic Ga$_{1-x}$Mn$_{x}$As epitaxial thin films covering a large parameter space in terms of the Mn content $x$ and other variations regarding sample fabrication. We infer that the electronic noise is dominated by switching processes related to impurities in the entire temperature range. While metallic compounds with $x>2$ % do not exhibit any significant change in the low-frequency resistance noise around the Curie temperature $T_\mathrm{C}$, we find indications for an electronic phase separation in films with $x<2$ % in the vicinity of $T_\mathrm{C}$, manifesting itself in a maximum in the noise power spectral density. These results are compared with noise measurements on an insulating Ga$_{1-x}$Mn$_{x}$P reference sample, for which the evidence for an electronic phase separation is even stronger and a possible percolation of bound magnetic polarons is discussed. Another aspect addressed in this work is the effect of ion-irradiation induced disorder on the electronic properties of Ga$_{1-x}$Mn$_{x}$As films and, in particular, whether any electronic inhomogeneities can be observed in this case. Finally, we put our findings into the context of the ongoing debate on the electronic structure and the development of spontaneous magnetization in these materials.

cond-mat.mtrl-sci↗

arXiv2014

Domain wall magneto-Seebeck effect

The interplay between charge, spin, and heat currents in magnetic nano systems subjected to a temperature gradient has lead to a variety of novel effects and promising applications studied in the fast-growing field of spincaloritronics. Here we explore the magnetothermoelectrical properties of an individual magnetic domain wall in a permalloy nanowire. In thermal gradients of the order of few Kelvin per micrometer along the long wire axis, we find a clear magneto-Seebeck signature due to the presence of a single domain wall. The observed domain wall magneto-Seebeck effect can be explained by the magnetization-dependent Seebeck coefficient of permalloy in combination with the local spin configuration of the domain wall.

cond-mat.mes-hall↗

arXiv2014

Towards wafer scale inductive characterization of spin transfer torque critical current density of magnetic tunnel junction stacks

We explore the prospects of wafer scale inductive probing of the critical current density $j^{c0}$ for spin transfer torque switching of a CoFeB/MgO/CoFeB magnetic tunnel junction with varying MgO thickness. From inductive measurements magnetostatic parameters and the effective damping are derived and $j^{c0}$ is calculated based on spin transfer torque equations. The inductive values compare well to the values derived from current induced switching measurements on individual nanopillars. Using a wafer scale inductive probe head could in the future enable wafer probe station based metrology of $j^{c0}$.

cond-mat.mtrl-sci↗

arXiv2013

Towards wafer scale inductive determination of magnetostatic and dynamic parameters of magnetic thin films and multilayers

We investigate an inductive probe head suitable for non-invasive characterization of the magnetostatic and dynamic parameters of magnetic thin films and multilayers on the wafer scale. The probe is based on a planar waveguide with rearward high frequency connectors that can be brought in close contact to the wafer surface. Inductive characterization of the magnetic material is carried out by vector network analyzer ferromagnetic resonance. Analysis of the field dispersion of the resonance allows the determination of key material parameters such as the saturation magnetization MS or the effective damping parameter Meff. Three waveguide designs are tested. The broadband frequency response is characterized and the suitability for inductive determination of MS and Meff is compared. Integration of such probes in a wafer prober could in the future allow wafer scale in-line testing of magnetostatic and dynamic key material parameters of magnetic thin films and multilayers.

physics.ins-det↗

arXiv2013

Broadband ferromagnetic resonance characterization of GaMnAs thin films

The precessional magnetization dynamics of GaMnAs thin films are characterized by broadband network analyzer ferromagnetic resonance (FMR) in a coplanar geometry at cryogenic temperatures. The FMR frequencies are characterized as function of in-plane field angle and field amplitude. Using an extended Kittel model of the FMR dispersion the magnetic film parameters such as saturation magnetization and anisotropies are derived. The modification of the FMR behavior and of the magnetic parameters of the thin film upon annealing is analyzed.

cond-mat.mes-hall↗

arXiv2010

Quantitative measurement of the magnetic moment of an individual magnetic nanoparticle by magnetic force microscopy

We demonstrate the quantitative measurement of the magnetization of individual magnetic nanoparticles (MNP) using a magnetic force microscope (MFM). The quantitative measurement is realized by calibration of the MFM signal using an MNP reference sample with traceably determined magnetization. A resolution of the magnetic moment of the order of 10^(-18) Am^2 under ambient conditions is demonstrated which is presently limited by the tip's magnetic moment and the noise level of the instrument. The calibration scheme can be applied to practically any MFM and tip thus allowing a wide range of future applications e.g. in nanomagnetism and biotechnology.

cond-mat.mes-hall↗