arXiv subjects

Cathrine Frandsen

Publications and source records attributed to Cathrine Frandsen.

13 recordsLinked to original sources

arXiv2026

Unravelling Challenges in Heating Power Measurements for Magnetic Hyperthermia -- the RADIOMAG Round Robin Study Revisited

Non-adiabatic AC calorimetry is the most widely used technique for estimating the heating power of magnetic nanoparticles in magnetic hyperthermia. However, it is prone to systematic errors which lead to a standard deviation in the intrinsic loss power (ILP) of approximately 30-40%, as revealed by the RADIOMAG EU COST Action TD1402 round-robin study involving 21 European laboratories. In this study, we re-examine the RADIOMAG dataset to both uncover previously unreported instrumentation issues, and to explore more deeply some of the reported instrumentation issues. We identify four common sources of error: i) Insufficient temperature resolution, ii) AC-field sensitive thermometers, iii) Non-physical temperature oscillations, and iv) Apparent non-linear heat loss. Based on these findings, we propose criteria for sufficient measurement quality and apply them to re-estimate the ILP values. These results have a standard deviation of 18-30%., demonstrating that addressing instrumentation and analysis issues can improve measurement reliability and decrease the inter-laboratory deviation by up to 38%. When re-estimating ILP, we used the initial slope method, arguing that the corrected slope method, which was previously used to investigate the RADIOMAG data, could introduce misleading interpretations of systematic ILP deviations due to sub-optimal measurement conditions and the unnoticed influence of non-linear heat losses. However, we emphasise that the corrected slope method is preferred, given a linear heat loss. Based on our analysis, we introduce a diagnostic protocol by using slope curves - a simple yet effective plot type - for identifying and solving common instrumentation challenges proactively before the data acquisition phase.

physics.app-ph↗

arXiv2026

The impact of sample insulation on estimating the heating power of magnetic nanoparticles by AC calorimetry

Correct estimation of the heating power of magnetic nanoparticles is important for magnetic hyperthermia treatment. This work investigates the impact of sample insulation in AC calorimetry. We show that temperature increase in the insulation can lead to systematic errors when estimating the heating power by the corrected slope method. The errors arise if the temperature of the sample environment is kept fixed at its initial temperature in the data analysis. To correct for this, we propose the use of a local temperature difference between the sample and the sample environment.

physics.app-ph↗

arXiv2026

Enhanced nanocomposite susceptibility by field-alignment of superparamagnetic particles

Nanocomposites comprised of insulated magnetic single-domain particles are promising candidates for high-frequency, eddy current free, soft magnetic materials, but tend to suffer from low magnetic susceptibility ($<20$). Particle alignment has been proposed to increase nanocomposite susceptibility and reduce magnetic losses but experimental verification has been lacking. Here, magnetic nanocomposites containing 3-57 vol\% field-aligned 11$\pm$3 nm maghemite particles in a poly-vinyl matrix were investigated for potential use as high-frequency inductor core materials. The particles were aligned by a homogenous static alignment field during nanocomposite drying, fixating the particle orientation. Particle aggregation was disproved by small-angle scattering. The dependence of the alignment field strength and particle concentration on the nanocomposite's susceptibility and hysteresis losses were investigated from DC up to 922 kHz by vibrating sample magnetometry, AC-susceptibility and high-frequency hysteresis measurements. Nanocomposite susceptibility increased super-linearly with particle fraction due to weak particle interactions. Alignment of the particles increased the nanocomposite susceptibility from 21 to 50 for samples with a particle content of 57 vol\%. Hence, the synergy between particle alignment and interaction allows for a higher than expected susceptibility of nanocomposites. The results show that magnetically aligning particles in a nanocomposite reduces magnetic losses when using well-dispersed single-domain superparamagnetic nanoparticles. Measured nanocomposite susceptibility could be modelled by a combination of directional dependent Debye-models including mean-field interaction effects and partial particle alignment. Measured susceptibility of 50 is among the highest obtained for nanocomposites, making it a relevant candidate for applications in power electronics.

physics.app-ph↗

arXiv2026

The effect of demagnetization on the susceptibility of single-domain particles and assemblies

According to the classical laws of magnetism, the shape of magnetically soft objects limits the effective susceptibility. For example, spherical soft magnets cannot display an effective susceptibility larger than 3. Although this is true for macroscopic multi-domain magnetic materials, we explain why magnetic nanoparticles in a single-domain state do not suffer from this limitation. For single-domain particles, the differences between demagnetization factors along principal axes are relevant and can influence susceptibility but do not limit the susceptibility to an upper value as in the case for multi-domain particles. We experimentally validated this result on spherical nanoparticles with varying diameter (9 to 150 nm) and varying volume fraction (0.1 to 47 vol%). In agreement with our predictions, we measure single-domain particle susceptibilities largely above 3, in fact up to more than 250. Moreover, contrary to an existing model for assemblies of particles, we find that the susceptibility of materials composed of non-interacting single-domain particles in a non-magnetic matrix scales linearly with the volume fraction of particles. This implies that high susceptibilities (>100) are achievable for nanoparticle-based composites and is relevant for the design of magnetically soft materials that are operational at MHz-GHz frequencies with negligible power losses.

cond-mat.mes-hall↗

arXiv2026

Solvothermal vapor annealing and environmental control setup with adjustable magnetic field module for GISAXS studies

A compact, modular environmental control and solvothermal vapor annealing chamber designed for maintaining a controlled atmosphere with regard to solvent humidity and temperature is presented. The setup allows ex situ and in situ grazing incidence small-angle X-ray scattering (GISAXS) investigations of thin film self-assembly and reorganization. Its modular slotting system enables stable reconfiguration, including the integration of an adjustable magnetic field module. The temperature is maintained via a water-based heating and cooling loop supplemented by resistive elements, and the solvent vapor environment is regulated using a commercial controlled mixing and evaporation unit. The performance of the setup is validated through measurements of fill and quench times together with magnetic field mapping with Gauss meter measurements and finite element simulations. Further, the versatility of the setup is demonstrated with four research examples using the chamber for solvothermal vapor annealing of block copolymer thin films together with lab-based ex situ and in situ GISAXS measurements. The portable new design offers robust environmental control and flexibility for advanced thin film investigations both in the lab and at large scale facilities. The design can be adapted for grazing incidence small-angle neutron scattering, GISANS.

cond-mat.soft↗

arXiv2025

AC Magnetometry Loop Tracer Compatible with Magnetic Calorimetry for Power Loss Analysis

Magnetic nanoparticles (MNPs) have garnered significant attention for various applications in the high-kHz-to-MHz range, although their magnetic characterization at these operational conditions has been limited. However, a number of recent studies have showcased high-frequency and high-field amplitude AC magnetometry loop tracers capable of retrieving the magnetic AC hysteresis curve and the associated magnetic properties. In this paper, we present an easily constructable loop tracer that is retrofitted into an existing AC calorimetry setup. This enables the loop tracer to function simultaneously with the AC calorimetry setup and also to be run as a high-frequency AC susceptometer. The loop tracer is shown to work in the frequency range 160-922 kHz with maximum applied fields from 18 to 46 mT depending on the frequency. An iron oxide nanoflower sample is used to test the loop tracer, showcasing high reproducibility in measured magnetic parameters as well as quantitative agreement between the different measurement methods in the setup.

physics.ins-det↗

arXiv2025

Printable Nanocomposites with Superparamagnetic Maghemite ($γ$-Fe$_2$O$_3$) Particles for Microinductor-core Applications

We here present printable and castable magnetic nanocomposites containing superparamagnetic 11$\pm$3 nm $γ$-Fe$_2$O$_3$ particles in an insulating poly-vinyl alcohol polymer matrix. The nanocomposites feature well-dispersed particles with volume fractions between 10 and 45 \%, as confirmed by small-angle neutron scattering. The magnetic volume susceptibility is as high as 17, together with negligible hysteresis at low frequency, and constant AC-response up to the high-kHz range. Measured hysteresis curves at 100-900 kHz with up to 110 mT induced $B$-fields in the nanocomposite show that power losses depend on $B$-field squared, and frequency to the power of 1-1.3. The only loss mechanism in the nanocomposite is hysteresis losses at $>$100 kHz frequencies, where the largest particles in the 11$\pm$3 nm distribution transition from the superparamagnetic to blocked regime. To mitigate the resulting hysteresis losses (up 10$^2$-10$^5$ kW/m$^3$) a more narrow particle size distribution could be used for future materials. The presented material is eddy current-free and easily integrated into micro-fabrication protocols, as we demonstrate by fabrication of 3-turn print circuit board based inductors with cast/manual printed nanocomposite inductor cores, on which induction has been measured up to 100 MHz.

cond-mat.mes-hall↗

arXiv2025

Design of superparamagnetic nanoparticle-materials for high-frequency inductor cores

The progress in the semiconductor industry has resulted in great demand for high-frequency magnetic materials applicable in microfabricated inductor cores. Nanocomposite materials, containing magnetic nanoparticles in a non-conducting matrix, may provide a solution for materials with high susceptibility or permeability and low power loss in the MHz regime, where traditional ferrites fail in performance. Here, we present a design guide for usage of magnetic nanoparticles in such materials. We use statistical mechanics methods to derive the magnetic susceptibility of nanoparticles in case of uniaxial or cubic anisotropy, as function of particle size and applied field direction, and investigate shape and interaction effects on the susceptibility. Using the derived susceptibilities, with inductor-core applications in mind, we show that close-to-spherical particles of materials with high saturation magnetization and low magnetic anisotropy, such as FeNi$_3$, are optimal. Additionally, the particle size shall be optimized to be as large as possible while maintaining superparamagnetic behaviour at the relevant frequency. Based on this, we predict that high particle susceptibilities of $>$700 (/$>$1500) are possible for randomly oriented (/uniaxially aligned) 20$\pm$1 nm diameter FeNi$_3$ particles, together with high-frequency stability, shown by low out-of-phase component at 2 MHz. This implies that materials containing nanoparticles have the potential to be tuned to outperform state-of-the-art ferrite inductor-core materials at MHz-frequencies.

cond-mat.mes-hall↗

arXiv2024

Dipolar Attraction of Superparamagnetic Nanoparticles

For superparamagnetic nanoparticles (SMNPs), it is often claimed that the rapid thermal fluctuations of their magnetic moments negates the magnetic dipolar attraction, hence preventing aggregation in liquid suspension. However we find that this is a misconception. Using Langevin dynamics, we simulate SMNP pairs and the dimer clusters they form which is the simplest case of aggregation. To quantify the tendency to aggregate, we introduce the dimer debonding time and calculate the average magnetic force of attraction which results from correlations in the fluctuating moments. Neither quantity has any dependence on the magnetocrystalline anisotropy, which determines the rate of superparamagnetic reversals, and comparing with computed Néel relaxation times we show that this holds for both blocked and superparamagnetic particles. These results imply that the phenomenon of superparamagnetism does not affect aggregation. Because the key dimensionless parameter for the Néel relaxation of a lone SMNP and the one for magnetic attraction have the same size and temperature scaling, there is a strong correlation between superparamagnetism and colloidal stability, as observed experimentally, but no causal relation.

cond-mat.soft↗

arXiv2023

Magnetic levitation by rotation

A permanent magnet can be levitated simply by placing it in the vicinity of another permanent magnet that rotates in the order of 200 Hz. This surprising effect can be easily reproduced in the lab with off-the-shelf components. Here we investigate this novel type of magnetic levitation experimentally and clarify the underlying physics. Using a 19 mm diameter spherical NdFeB magnet as rotor magnet, we capture the detailed motion of levitating, spherical NdFeB magnets, denoted floater magnets. We find that as levitation occurs, the floater magnet frequency-locks with the rotor magnet, and, noticeably, that the magnetization of the floater is oriented close to the axis of rotation and towards the like pole of the rotor magnet. This is in contrast to what might be expected by the laws of magnetostatics as the floater is observed to align its magnetization essentially perpendicular to the magnetic field of the rotor. Moreover, we find that the size of the floater has a clear influence on the levitation: the smaller the floater, the higher the rotor speed necessary to achieve levitation, and the further away the levitation point shifts. We verify that magnetostatic interactions between the rotating magnets are responsible for creating the equilibrium position of the floater. Hence, this type of magnetic levitation does not rely on gravity as a balancing force to achieve an equilibrium position. Based on theoretical arguments and a numerical model, we show that a constant, vertical field and eddy-current enhanced damping is sufficient to produce levitation from rest. This enables a gyroscopically stabilised counter-intuitive steady-state moment orientation, and the resulting magnetostatically stable, mid-air equilibrium point. The numerical model display the same trends with respect to rotation speed and the floater magnet size as seen in the experiments.

physics.app-ph↗

arXiv2023

Conservation laws for interacting magnetic nanoparticles at finite temperature

We establish a general Langevin Dynamics model of interacting, single-domain magnetic nanoparticles in liquid suspension at finite temperature. The model couples the LLG equation for the moment dynamics with the mechanical rotation and translation of the particles. Within this model, we derive expressions for the instantaneous transfer of energy, linear and angular momentum between the particles and with the environment. We demonstrate by numerical tests that all conserved quantities are fully accounted for, thus validating the model and the transfer expressions. The energy transfer expressions derived here are also useful analysis tools to decompose the instantaneous, non-equilibrium power loss at each MNP into different loss channels. To demonstrate the model capabilities, we analyse simulations of MNP collisions and high-frequency hysteresis in terms of power and energy contributions.

cond-mat.mes-hall↗

arXiv2022

Nanocriticality in the magnetic phase transition of CoO nanoparticles

The universal theory of critical phase transitions describes the critical behavior at second-order phase transitions in infinitely large systems. With the increased contemporary interest in nanoscale materials, we investigated CoO nanoparticles by means of neutron scattering and found how the theory of critical phenomena breaks down in the nanoscale regime. Using CoO as a model system, we have identified a size-dependent nanocritical temperature region close to the antiferromagnetic phase transition where the magnetic correlation length of the nanoparticles converges to a constant value, which is significantly smaller than that of the saturated state found at low temperatures. This is in clear contrast to the divergence around $T_{\rm N}$ observed for bulk systems. Our findings of nanocriticality in the magnetic phase transition is of great importance for the understanding of phase transitions at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2018

Dipolar-coupled moment correlations in clusters of magnetic nanoparticles

Here, we investigate the nature of the moment coupling between 10-nm DMSA-coated magnetic nanoparticles, in both colloidal dispersion and in powder form. The individual iron oxide cores were composed of > 95% maghemite and agglomerated to clusters. At room temperature the ensemble behaved as a superparamagnet according to Mössbauer and magnetization measurements, however, with clear signs of dipolar interactions at low temperatures. Analysis of temperature-dependent AC susceptibility data in the superparamagnetic regime indicates a tendency for dipolar coupled anticorrelations of the core moments within the clusters. To resolve the directional correlations between the particle moments we performed polarized small-angle neutron scattering and determined the magnetic spin-flip cross-section of the powder in low magnetic field at 300 K. We extract the underlying pair distance distribution function of the magnetization vector field by an indirect Fourier transform of the cross-section, and which suggests positive as well as negative correlations between nearest neighbor moments, with anticorrelations clearly dominating for next-nearest moments. These tendencies are confirmed by Monte Carlo simulations of such core-clusters.

cond-mat.mes-hall↗