arXiv subjects

Jeffrey McCord

Publications and source records attributed to Jeffrey McCord.

16 recordsLinked to original sources

arXiv2026

Geometry-calibrated equilibrium sensing for inverse design of nonlocal topological photonic lattices

Topological photonic lattices are commonly designed using short-range Hamiltonians, yet realistic nanophotonic structures are governed by geometry- and wavelength-dependent long-range electromagnetic interactions. Here we introduce a geometry-calibrated quantum equilibrium-propagation framework for inference and inverse design in finite nonlocal plasmonic Su-Schrieffer-Heeger lattices. A two-qubit equilibrium sensor is trained in effective-coupling space to distinguish boundary-localized from trivial finite-lattice responses. Because labels inherited from the nearest-neighbor SSH model become unreliable in the presence of nonlocal hopping, each sample is independently relabeled using nonlocal winding numbers and a finite-gap criterion. On this physics-verified evaluation set, the sensor achieves 99.8% sensitivity and 98.1% specificity. A physics-gated robustness score then ranks verified configurations by boundary response, response contrast, gap stability and compatibility with the selected nonlocality regime. Full-wave extinction spectra of isolated and paired gold nanoparticles establish a geometry-to-coupling calibration linking particle size and separation to wavelength-resolved pair couplings. Projecting this calibration onto the verified coupling landscape identifies a finite plasmonic geometry supporting spectrally distinct corner- and edge-dominated responses at 546 and 642 nm, with sector-to-bulk intensity contrasts of $2.4 \times 10^{4}$ and $1.5 \times 10^{3}$, respectively. The framework links realistic electromagnetic geometry to nonlocal topological design, moving beyond nearest-neighbor design rules with physics-verified, geometry-resolved inference.

physics.optics↗

arXiv2026

Magneto-optical magnetoelectric voltage sensor

Applications in high-voltage and electromagnetically harsh environments require reliable galvanically isolated voltage sensing, which can be achieved using optical readout. While established optical voltage sensors rely on electro-optic effects or piezoelectric strain with direct optical detection, strain-mediated magnetoelectric coupling combined with magneto-optical readout offers an alternative voltage sensing principle that remains largely unexplored. Here, such a sensor based on a bismuth-substituted yttrium iron garnet magneto-optical indicator film mechanically coupled to a piezoelectric actuator is presented. Voltage induced stress results in changes of the out-of-plane magnetization via magnetoelastic coupling which is detected through magneto-optical Faraday rotation. A critical state of the domain structure is set via an applied bias field, in which voltage-induced nucleation and domain-wall motion dominates the response. In this high sensitivity regime, both AC and DC voltage readout modes are demonstrated, based on either voltage-driven magnetization reversal or voltage-induced modifications of the magnetization loop shape. Equivalent voltage noise densities in the millivolt per root hertz range are achieved. The results establish strain-mediated magneto-optical voltage sensing as a distinct approach to optically isolated voltage measurement.

cond-mat.mtrl-sci↗

arXiv2026

Decoding magnetic texture

In magnetically ordered materials, magnetic field and temperature variations modify the magnetic texture through their coupling to the local energy landscape, imprinting distinct fingerprints in the resulting magnetic domain patterns. Retrieving these conditions from the pattern remains challenging, as stochastic nucleation and hysteresis produce a nonlinear, multivariate, and ambiguous relationship between magnetic domain morphology and external stimuli. To decode these fingerprints, we designed a controlled magneto-optical inference experiment that reconstructs magnetic field, temperature, and magnetic history from a single fine-scale, high-contrast, pixel-resolved optical polarization map of feature-rich magnetic domain textures in a bismuth-substituted yttrium iron garnet film. Deep convolutional neural networks are complemented by feature-based neural-network inference using hand-crafted, physically interpretable descriptors of measured magneto-optical image data, linking the decoded information to material-dependent features and exploring their contributions. Together, these results establish magnetic texture as a high-fidelity record of external conditions enabling accurate single image multiparametric sensing and paving the way for data-driven explorations of complex magnetic states. Uncovering the physically interpretable features that encode this record sheds new light on the physics of magnetic domain formation.

cond-mat.mtrl-sci↗

arXiv2025

Impedance Modeling of Magnetometers: A Path Toward Low-Noise Readout Circuits

Optimizing sensor readout schemes and integrated circuit designs for both open-loop and closed-loop implementations requires precise modeling and simulation strategies. This study introduces a novel two-port impedance model to estimate the behavior of a converse Magnetoelectric (cME) sensor. This model provides a possible framework for calculating transfer functions and simulating magnetometer behavior in both continuous- and discrete-time simulation environments, and it is also possibly transferable to other magnetometer types. Common S-parameters were measured experimentally using an impedance analyzer and converted to Z-parameters to create a transfer function for system-level simulations. The model was validated through an analysis of output-related noise using MATLAB and LTSpice simulations to optimize the noise of the analog circuit parts of the system. The simulation results were compared with experimental measurements using a Zurich Instruments lock-in amplifier and the custom-designed low-noise printed circuit board (PCB) under model considerations. The proposed methodology derives noise considerations and the transfer function of a magnetometer. These are essential for readout schemes for mixed-signal circuit design. This allows low-noise electronics to be designed and extended to other sensor interface electronics, broadening their applicability in high-performance magnetic sensing.

eess.SP↗

arXiv2025

Operation Regimes and Design Principles of Delta-E Effect Sensors

Delta-E effect-based magnetoelectric sensors have emerged as promising technology for detecting weak magnetic fields at low frequencies. However, the performance of such sensors remains difficult to predict, as signal and noise characteristics are dictated by interdependent parameters such as magnetic layer geometry, magnetic microstructure, and loss. In this work, we present a systematic experimental study of sub-mm-sized delta-E effect sensors, comprising 24 device configurations that vary in magnetic layer thickness and lateral dimensions. The sensors are statistically analyzed to identify the influence of magnetic layer geometry on performance through a combination of measurements and simulations. Our findings reveal three distinct operation regimes - dominated by electronic noise, magnetic noise, and nonlinearities - whose boundaries shift systematically with magnetic layer thickness. This regime behavior governs the trade-offs between sensitivity and noise, ultimately determining the sensor's limit of detection. Based on these results, the dependency of the regime boundaries on key device parameters is discussed in detail, providing fundamental insights for tailoring sensor performance. As such, this study establishes a necessary foundation for targeted performance optimization and the scalable design of advanced delta-E effect sensor systems.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic Field-Controlled Mixed Modulation in Magnetoelectric Sensors

Magnetoelectric (ME) magnetic field sensors commonly rely on one of the two modulation principles: the nonlinear dependence of magnetostrictive strain on the applied field or the stress-induced change in magnetization susceptibility. While both effects coexist in any ME device, different readout schemes can be chosen to utilize one or the other effect for magnetic field sensing. This work demonstrates that both principles can be simultaneously implemented in a single electrically modulated ME sensor with inductive readout (a converse ME sensor). This mixed modulation approach significantly enhances low-frequency sensitivity while not affecting the sensitivity at higher frequencies. This leads to a nontrivial dependency of the sensor sensitivity on the frequency of the magnetic field to be measured and can effectively decrease the sensor bandwidth by up to an order of magnitude. We show that the contribution of the modulation from the nonlinearity of the magnetostrictive strain to the sensor sensitivity can be changed by applying a magnetic bias field, offering an additional dimension to the design of ME sensors, especially for potential applications in the unshielded environment.

cond-mat.mtrl-sci↗

arXiv2025

Modeling of High-Sensitivity SAW Magnetic Field Sensors with Au-SiO2 Phononic Crystals

The development of magnetic field sensors with high sensitivity is crucial for accurate detection of magnetic fields. In this context, we present a theoretical model of a highly sensitive surface acoustic wave (SAW) magnetic field sensor that utilizes phononic crystal (PnC) structures composed of Au pillars embedded within a SiO2 guiding layer. We study rectangular and triangular PnCs and assess their potential for application in thin-film magnetic field sensors. In our design, the PnC is integrated into the SiO2 guiding layer, preserving the continuous magnetostrictive layer and maximizing its interaction with the SAW. The sensor achieves nearly two orders of magnitude higher sensitivity compared to a continuous delay line of similar dimensions and an eight-fold improvement over the previous sensor design with PnCs composed of FeCoSiB pillars. The enhanced sensitivity is attributed to resonance effects within the PnC leading to an increased interaction between the SAW and the continuous FeCoSiB layer covering the PnC. Our results highlight the significant potential of incorporating PnCs into the guiding layer of SAWs for future high performance magnetic field sensors.

physics.app-ph↗

arXiv2024

Simultaneous mapping of magnetic and atomic structure for direct visualization of nanoscale magnetoelastic coupling

Achieving a correlative measurement of both magnetic and atomic structures at the nanoscale is imperative to understand the fundamental magnetism of matters and for fostering the development of new magnetic nanomaterials. Conventional microscopy methods fall short in providing the two information simultaneously. Here, we develop a new approach to simultaneously map the magnetic field and atomic structure at the nanoscale using Lorentz 4-dimensional scanning transmission electron microscopy (Ltz-4D-STEM). This method enables precise measurement of the characteristic atomic and magnetic structures across an extensive field of view, a critical aspect for investigating real-world ferromagnetic materials. It offers a comprehensive visualization and statistical evaluation of the different structural information at a pixel-by-pixel correlation. The new method allows to directly visualize the magnetoelastic coupling and the resulting complex magnetization arrangement as well as the competition between magnetoelastic and magnetostatic energy. This approach opens new avenues for in-depth studying the structure-property correlation of nanoscale magnetic materials.

cond-mat.mtrl-sci↗

arXiv2023

Miniaturized Double-Wing Delta-E Effect Sensors

Magnetoelastic composites are integral elements of sensors and actuators utilizing magnetostriction for their functionality. Their sensitivity typically scales with the saturation magnetostriction and inversely with magnetic anisotropy. However, this makes the devices prone to minuscule residual anisotropic stress from the fabrication process, impairing their performance and reproducibility, hence limiting their suitability for arrays. This study presents a shadow mask deposition technology combined with a free-free magnetoelectric microresonator design intended to minimize residual stress and inhomogeneity in the magnetoelastic layer. Resonators are experimentally and theoretically analyzed regarding local stress anisotropy, magnetic anisotropy, and the ΔE effect in several resonance modes. Further, the sensitivity is analyzed in the example of ΔE-effect sensors. The results demonstrate a device-to-device variation of the resonance frequency < 0.2 % with sensitivities comparable with macroscopic ΔE-effect sensors. The reproducibility is drastically improved over previous magnetoelastic device arrays. This development marks a step forward in the reproducibility and homogeneity of magnetoelastic resonators and contributes to the feasibility of large-scale, integrated sensor arrays.

cond-mat.mtrl-sci↗

arXiv2023

Generation of imprinted strain gradients for spintronics

In this work, we propose and evaluate an inexpensive and CMOS-compatible method to locally apply strain on a Si/SiOx substrate. Due to high growth temperatures and different thermal expansion coefficients, a SiN passivation layer exerts a compressive stress when deposited on a commercial silicon wafer. Removing selected areas of the passivation layer alters the strain on the micrometer range, leading to changes in the local magnetic anisotropy of a magnetic material through magnetoelastic interactions. Using Kerr microscopy, we experimentally demonstrate how the magnetoelastic energy landscape, created by a pair of openings, in a magnetic nanowire enables the creation of pinning sites for in-plane vortex walls that propagate in a magnetic racetrack. We report substantial pinning fields up to 15 mT for device-relevant ferromagnetic materials with positive magnetostriction. We support our experimental results with finite element simulations for the induced strain, micromagnetic simulations and 1D model calculations using the realistic strain profile to identify the depinning mechanism. All the observations above are due to the magnetoelastic energy contribution in the system, which creates local energy minima for the domain wall at the desired location. By controlling domain walls with strain, we realize the prototype of a true power-on magnetic sensor that can measure discrete magnetic fields or Oersted currents. This utilizes a technology that does not require piezoelectric substrates or high-resolution lithography, thus enabling wafer-level production.

physics.app-ph↗

arXiv2023

Optimization of Permalloy properties for magnetic field sensors using He$^+$ irradiation

Permalloy, despite being a widely utilized soft magnetic material, still calls for optimization in terms of magnetic softness and magnetostriction for its use in magnetoresistive sensor applications. Conventional annealing methods are often insufficient to locally achieve the desired properties for a narrow parameter range. In this study, we report a significant improvement of the magnetic softness and magnetostriction in a 30 nm Permalloy film after He$^+$ irradiation. Compared to the as-deposited state, the irradiation treatment reduces the induced anisotropy by a factor ten and the hard axis coercivity by a factor five. In addition, the effective magnetostriction of the film is significantly reduced by a factor ten - below $1\times10^{-7}$ - after irradiation. All the above mentioned effects can be attributed to the isotropic crystallite growth of the Ni-Fe alloy and to the intermixing at the magnetic layer interfaces under light ion irradiation. We support our findings with X-ray diffraction analysis of the textured Ni$_{81}$Fe$_{19}$ alloy. Importantly, the sizable magnetoresistance is preserved after the irradiation. Our results show that compared to traditional annealing methods, the use of He$^+$ irradiation leads to significant improvements in the magnetic softness and reduces strain cross sensitivity in Permalloy films required for 3D positioning and compass applications. These improvements, in combination with the local nature of the irradiation process make our finding valuable for the optimization of monolithic integrated sensors, where classic annealing methods cannot be applied due to complex interplay within the components in the device.

cond-mat.mtrl-sci↗

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

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↗

arXiv2020

Phase Sensitivity and Phase Noise of Cantilever-Type Magnetoelastic Sensors Based on the $Δ$E Effect

Magnetoelastic sensors for the detection of low-frequency and low-amplitude magnetic fields are in the focus of research since more than 30 years. In order to minimize the limit of detection (LOD) of such sensor systems, it is of high importance to understand and to be able to quantify the relevant noise sources. In this contribution, cantilever-type electromechanic and magnetoelastic resonators, respectively, are comprehensively investigated and mathematically described not only with regard to their phase sensitivity but especially to the extent of the sensor-intrinsic phase noise. Both measurements and calculations reveal that the fundamental LOD is limited by additive phase noise due to thermal-mechanical noise of the resonator, i.e. by thermally induced random vibrations of the cantilever, and by thermal-electrical noise of the piezoelectric material. However, due to losses in the magnetic material parametric flicker phase noise arises, limiting the overall performance. In particular it is shown that the LOD is virtually independent of the magnetic sensitivity but is solely determined by the magnetic losses. Instead of the sensitivity, the magnetic losses, represented by the material's effective complex permeability, should be considered as the most important parameter for the further improvement of such sensors in the future. This implication is not only valid for magnetoelastic cantilevers but also applies to any type of magnetoelastic resonator.

physics.ins-det↗

arXiv2018

Magnetic domain walls as broadband spin wave and elastic magnetisation wave emitters

We report on the direct observation of spin wave and elastic wave emission from magnetic domain walls in ferromagnetic thin films. Driven by alternating homogeneous magnetic fields the magnetic domain walls act as coherent magnetisation wave sources. Directional and low damped elastic waves below and above the ferromagnetic resonance are excited. The wave vector of the magnetoelastically induced acoustic shear waves is linearly tuned by varying the excitation frequency. Domain wall emitted magnetostatic surface spin waves occur at higher frequencies, which characteristics are confirmed by micromagnetic simulations. The distinct modes of magnetisation wave excitation from micromagnetic objects are a general physical phenomenon relevant for dynamic magnetisation processes in structured magnetic films. Magnetic domain walls can act as reconfigurable antennas for spin wave and elastic wave generation with control of the wave orientation.

cond-mat.mtrl-sci↗

arXiv2011

The Formation and Coarsening of the Concertina Pattern

The concertina is a magnetization pattern in elongated thin-film elements of a soft material. It is a ubiquitous domain pattern that occurs in the process of magnetization reversal in direction of the long axis of the small element. Van den Berg argued that this pattern grows out of the flux closure domains as the external field is reduced. Based on experimental observations and theory, we argue that in sufficiently elongated thin-film elements, the concertina pattern rather bifurcates from an oscillatory buckling mode. Using a reduced model derived by asymptotic analysis and investigated by numerical simulation, we quantitatively predict the average period of the concertina pattern and qualitatively predict its hysteresis. In particular, we argue that the experimentally observed coarsening of the concertina pattern is due to secondary bifurcations related to an Eckhaus instability. We also link the concertina pattern to the magnetization ripple and discuss the effect of a weak (crystalline or induced) anisotropy.

cond-mat.mtrl-sci↗