arXiv subjects

A. Michels

Publications and source records attributed to A. Michels.

14 recordsLinked to original sources

arXiv2026

Reduced vortex descriptors linking polycrystallinity in magnetic nanoparticles with polarized magnetic small-angle neutron scattering

Analytical vortex models reduce polarized magnetic small-angle neutron scattering (SANS) from nanoparticle ensembles to a small set of texture descriptors. In this work, we apply this reduction to micromagnetic simulations of polycrystalline iron oxide nanoflowers at a fixed particle size and examine how a controlled parametrization of multigrain disorder is reflected in the remanent descriptors. The particles are represented by explicit Voronoi microstructures, and intraparticle disorder is varied through the intergrain exchange coupling and anisotropy-axis coherence. Fitting each remanent magnetization state to a hyperbolic vortex model reveals a predominantly two-channel organization: the intergrain exchange coupling is associated mainly with the radial vortex profile, whereas the anisotropy-axis coherence is associated mainly with the orientational moment of the vortex-axis distribution. The normalized spin-flip SANS cross sections are accurately represented by independent fits of the analytical linear-vortex SANS expression obtained from the first-order expansion of the hyperbolic profile. The fitted orientational descriptor agrees closely with its independent real-space estimate, whereas the corresponding radial descriptors exhibit a strong global nonlinear relation. This separation identifies which information from the micromagnetic vortex textures is robustly retained by the reduced analytical representation.

cond-mat.mes-hall↗

arXiv2026

Effect of spin disorder on the specific loss power of a nanomagnet

Spin non-collinearities in magnetic nanostructures arise from a variety of sources, including structural defects, finite-size effects, boundary or surface effects, Dzyaloshinskii-Moriya exchange coupling, and magnetic vortex formation. While strong forms of spin disorder generally require a numerical treatment, relatively weak non-collinearities induced by surface anisotropy are amenable to the analytical framework of the effective one-spin problem (EOSP). In this work, we exploit this framework to present a qualitative, semi-analytical study of the effect of spin disorder on the specific loss power (SLP) of a single nanomagnet within linear-response theory. Surface-induced spin misalignment mainly manifests as an additional quartic (cubic-symmetry) contribution to the anisotropy energy, parametrized by the ratio $\zeta \equiv K_4/K_2$. We derive a semi-analytical expression for the SLP as a function of $\zeta$ by combining the $\zeta$-dependent equilibrium susceptibility and the relaxation rate obtained within Langer's approach. Our results show that, for systems in the slow-relaxation regime, the SLP is enhanced by spin misalignment, predominantly through the increase of the relaxation rate caused by the lowering of the effective energy barrier. Retaining the full Debye factor reveals that for moderate reduced barriers $\sigma$, where the system is close to the superparamagnetic regime, the SLP can actually \emph{decrease} with increasing spin disorder. The enhancement is asymmetric with respect to the sign of $\zeta$ and depends on the nanomagnet shape (sphere versus cube) through the geometric prefactors in the EOSP mapping.

physics.app-ph↗

arXiv2025

Spin-disorder-induced angular anisotropy in polarized magnetic neutron scattering

We experimentally report a hitherto unseen angular anisotropy in the polarized small-angle neutron scattering (SANS) cross section of a magnetically strongly inhomogeneous material. Based on an analytical prediction using micromagnetic theory, the difference between the spin-up and spin-down SANS cross sections is expected to show a spin-disorder-induced anisotropy. The effect is particularly pronounced in inhomogeneous magnetic materials, such as nanoporous ferromagnets, magnetic nanocomposites, or steels, which exhibit large nanoscale jumps in the saturation magnetization at internal pore-matrix or particle-matrix interfaces. Analysis of the experimental neutron data constitutes a method for determining the exchange-stiffness constant. Our results are generic to the nuclear-magnetic interference terms contained in the polarized magnetic neutron scattering cross section and might also be of relevance to other neutron techniques.

cond-mat.mes-hall↗

arXiv2024

Low-frequency signature of magnetization nutation in nanomagnets

In this work, we show that surface anisotropy in nanomagnets induces a nutational motion of their magnetization at various frequencies, the lowest of which can be described by the macrospin model whose dynamics is governed by an effective energy potential. We derive analytical expressions for the precession and nutation frequencies and amplitudes as functions of the size of the nanomagnet and its atomistic parameters, such as the exchange coupling and the onsite anisotropy. Our analytical model predicts a reduction of the precession frequency with increased surface anisotropy. We also simulate the dynamics of the corresponding atomistic many-spin system and compare the results with the effective model. We thereby show that the first nutation mode induced by the finite size and surface anisotropy occurs at a frequency that is four times larger than the precession frequency, thus lending itself to a relatively easy detection by standard experiments of magnetic resonance.

cond-mat.mes-hall↗

arXiv2023

Spatial magnetization profile in spherical nanomagnets with surface anisotropy: Green's function approach

We consider a single spherical nanomagnet and investigate the spatial magnetization profile $\mathbf{m}\left(\mathbf{r}\right)$ in the continuum approach, using the Green's function formalism. The energy of the (many-spin) nanomagnet comprises an isotropic exchange interaction, a uniaxial anisotropy in the core and Néel's surface anisotropy, and an external magnetic field. We derive a semi-analytical expression for the magnetization vector field $\mathbf{m}\left(\mathbf{r}\right)$ for an arbitrary position $\mathbf{r}$ within and on the boundary of the nanomagnet, as a solution of a homogeneous Helmholtz equation with inhomogeneous Neumann boundary conditions. ... For a more plausible comparison with experiments, e.g. using the technique of small-angle magnetic neutron scattering, we have averaged over the direction solid angle and derived the spatial profile in terms of the distance $r$. We believe that the predictions of the present study could help to characterize and understand the effects of size and surface anisotropy on the magnetization configurations in nanomagnet assemblies such as arrays of well-spaced platelets.

cond-mat.mes-hall↗

arXiv2022

Generic role of the Dzyaloshinskii-Moriya interaction in nanocrystalline ferromagnets

Motivated by recent experimental polarized neutron results, we present a numerical micromagnetic study of the interfacial (intergrain) Dzyaloshinskii-Moriya interaction (DMI) in nanocrystalline terbium. We demonstrate that the DMI-induced spin misalignment between adjacent nanograins is the reason for the formation of the asymmetric positive-negative pattern seen in polarized neutron scattering experiments. Analysis of the remagnetization process suggests the generic impact of the DMI on the macroscopic magnetic parameters of polycrystalline defect-rich materials.

cond-mat.mes-hall↗

arXiv2022

Small-angle neutron scattering by spatially inhomogeneous ferromagnets with a nonzero average uniaxial anisotropy

Micromagnetic small-angle neutron scattering theory is well established for analyzing spin-misalignment scattering data of bulk ferromagnets. Here, this theory is extended to allow for a global uniaxial magnetic anisotropy (texture) of the material, in addition to the already included random zero-average local anisotropy. Macroscopic cross-sections and spin-misalignment response functions are computed analytically for several practically relevant mutual anisotropy and external magnetic field orientations in both parallel and perpendicular scattering geometries for field magnitudes both above and below the rotational saturation. Some of these expressions are tested on published experimental data of magnetic-field-annealed Vitroperm and plastically-deformed Ni, allowing to determine the corresponding global uniaxial anisotropy quality factors.

cond-mat.mes-hall↗

arXiv2022

Uniaxial polarization analysis of bulk ferromagnets: Theory and first experimental Results

Based on Brown's static equations of micromagnetics, we compute the uniaxial polarization of the scattered neutron beam of a bulk magnetic material. The theoretical expressions are compared to experimental data on a soft magnetic nanocrystalline alloy. The micromagnetic SANS theory provides a general framework for polarized real-space neutron methods, and it opens up a new avenue for magnetic neutron data analysis on magnetic microstructures.

cond-mat.mtrl-sci↗

arXiv2020

Anisometric mesoscale nuclear and magnetic texture in sintered Nd-Fe-B magnets

By means of temperature and wavelength-dependent small-angle neutron scattering (SANS) experiments on sintered isotropic and textured Nd-Fe-B magnets we provide evidence for the existence of an anisometric structure in the microstructure of the textured magnets. This conclusion is reached by observing a characteristic cross-shaped angular anisotropy in the total unpolarized SANS cross section at temperatures well above the Curie temperature. Comparison of the experimental SANS data to a microstructural model based on the superquadrics form factor allows us to estimate the shape and lower bounds for the size of the structure. Subtraction of the scattering cross section in the paramagnetic regime from data taken at room temperature provides the magnetic SANS cross section. Surprisingly, the anisotropy of the magnetic scattering is very similar to the nuclear SANS signal, suggesting that the nuclear structure is decorated by the magnetic moments via spin-orbit coupling. Based on the computation of the two-dimensional correlation function we estimate lower bounds for the longitudinal and transversal magnetic correlation lengths.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic Guinier law

Small-angle scattering of x-rays and neutrons is a routine method for the determination of nanoparticle sizes. The so-called Guinier law represents the low-q approximation for the small-angle scattering curve from an assembly of particles. The Guinier law has originally been derived for nonmagnetic particle-matrix-type systems, and it is successfully employed for the estimation of particle sizes in various scientific domains (e.g., soft matter physics, biology, colloidal chemistry, materials science). An important prerequisite for it to apply is the presence of a discontinuous interface separating particles and matrix. Here, we introduce the Guinier law for the case of magnetic small-angle neutron scattering (SANS) and experimentally demonstrate its applicability for the example of nanocrystalline cobalt. It is well-known that the magnetic microstructure of nanocrystalline ferromagnets is highly nonuniform on the nanometer length scale and characterized by a spectrum of continuously varying long-wavelength magnetization fluctuations, i.e., these systems do not manifest sharp interfaces in their magnetization profile. The magnetic Guinier radius depends on the applied magnetic field, on the magnetic interactions (exchange, magnetostatics), and on the magnetic anisotropy-field radius, which characterizes the size over which the magnetic anisotropy field is coherently aligned into the same direction. In contrast to the nonmagnetic conventional Guinier law, the magnetic version can be applied to fully dense random-anisotropy-type ferromagnets.

cond-mat.mes-hall↗

arXiv2013

Critical behavior of nanocrystalline gadolinium: Evidence for a new universality class

We report on how nanocrystal size affects the critical behavior of the rare-earth metal Gd near the ferromagnetic-to-paramagnetic phase transition. The asymptotic critical behavior of the coarse-grained polycrystalline sample (with an average crystallite size of $L \cong \unit[100]{\mu m}$) is that of a (pure) \textsl{uniaxial dipolar} ferromagnet, as is the case with single-crystal Gd, albeit the width of the asymptotic critical region (ACR) is reduced. As the grain size approaches $\sim \unit[30]{nm}$, the ACR is so narrow that it could not be accessed in the present experiments. Inaccessibly narrow ACR for $L \sim \unit[30]{nm}$ and the continuous increase in the width of ACR as $L$ decreases from $\unit[16]{nm}$ to $\unit[9.5]{nm}$ basically reflects a crossover to the \textsl{random uniaxial dipolar} fixed point caused by the quenched random-exchange disorder prevalent at the internal interfaces (grain boundaries).

cond-mat.mes-hall↗

arXiv2011

Magnetic neutron scattering on nanocomposites: decrypting cross-section images using micromagnetic simulations

We have used numerical micromagnetics for the calculation of the magnetic (small-angle) neutron scattering cross section of nanocomposites. The novel aspect of our approach consists in the possibility to study the applied-field dependence of the individual contributions to the total magnetic scattering. Such a micromagnetic tool ideally complements neutron experiments in which one generally measures only a weighted sum of the Fourier components of the magnetization. The procedure furnishes unique and fundamental information regarding the magnetic microstructure and corresponding magnetic scattering from nanomagnets. In particular, our simulation results explain the recent observation of dipolar correlations in two-phase nanocomposites and provide an answer to the question of the explicit dependence of the magnetization Fourier coefficients on the scattering vector.

cond-mat.mes-hall↗

arXiv1996

Domain walls and perturbation theory in high temperature gauge theory: SU(2) in 2+1 dimensions

We study the detailed properties of Z_2 domain walls in the deconfined high temperature phase of the d=2+1 SU(2) gauge theory. These walls are studied both by computer simulations of the lattice theory and by one-loop perturbative calculations. The latter are carried out both in the continuum and on the lattice. We find that leading order perturbation theory reproduces the detailed properties of these domain walls remarkably accurately even at temperatures where the effective dimensionless expansion parameter, g^2/T, is close to unity. The quantities studied include the surface tension, the action density profiles, roughening and the electric screening mass. It is only for the last quantity that we find an exception to the precocious success of perturbation theory. All this shows that, despite the presence of infrared divergences at higher orders, high-T perturbation theory can be an accurate calculational tool.

hep-lat↗

arXiv1994

Gauge Theory in d=2+1 at High Temperature: Z_N interface

We calculate on the lattice the interface tension in the SU(2) pure gauge theory in d=2+1 at high temperature. The result is compared to the perturbative prediction. The agreement confirms applicability of the perturbation theory in this case.

hep-lat↗