Meinhard Schilling

Magnetorelaxometry of magnetic nanoparticles in magnetically unshielded environment utilizing a differential fluxgate arrangement

A magnetorelaxometry system based on a differential fluxgate arrangement is presented. Compared to the single fluxgate setup, the use of two fluxgate magnetometers increases the relaxation signal from the sample by a factor of 2, the signal-to-noise ratio by a factor 2, and allows one to perform magnetorelaxation measurements without any magnetic shielding. For a sample with superparamagnetic Fe3O4 nanoparticles and a volume of 150μl, 100 nmol Fe could be detected, limited by the intrinsic noise of the fluxgate sensors.

Characterization of superparamagnetic nanoparticles by analyzing the magnetization and relaxation dynamics using fluxgate magnetometers

We have investigated the magnetization and relaxation dynamics of diluted, freeze-dried superparamagnetic magnetite (Fe3O4) nanoparticle samples with organic shells using a differential fluxgate magnetorelaxometry system. The experimental data were analyzed within the framework of the moment superposition model (MSM), providing information on size and size distribution of particle cores as well as on magnetic properties such as saturation magnetization and anisotropy constant. The MSM was refined by introducing an expression for the Neel time constant depending on magnetic field, anisotropy energy, saturation magnetization, and orientation of the magnetic moment of an individual magnetic nanoparticle (MNP) with respect to the external magnetic field. It is shown that especially the dependence of the magnetization and relaxation curves on magnetizing field and magnetization time provides valuable information for an unambiguous and comprehensive determination of MNP core parameters. All experimental finding...

Fluxgate based detection of magnetic nanoparticle dynamics in a rotating magnetic field

We have developed a measurement setup allowing the investigation of the dynamics of magnetic nanoparticle suspensions in a rotating magnetic field. To determine the vector of the sample magnetization, sensitive fluxgate magnetometers are utilized detecting the sample’s stray field. The phase lag between sample magnetization and rotating magnetic field vector is determined via the cross correlation spectrum. The phase lag spectra measured for various rotating field amplitudes on aqueous magnetite nanoparticle suspensions show good agreement with theory if the multidispersity of core and hydrodynamic size is taken into account.

Optimization of Magnetic Nanoparticles for Magnetic Particle Imaging

Systematic studies of the magnetic properties of magnetic nanoparticles from FeraSpin R and the FeraSpin Series with respect to their suitability as tracers for magnetic particle imaging are presented. Magnetic particle spectroscopy measurements indicate that FeraSpin R exhibits a harmonic spectrum very similar to that of Resovist, whereas FeraSpin L, XL, and XXL show a 2.5-fold increase of harmonic amplitudes compared to FeraSpin R. To understand the differences between the various samples of the FeraSpin Series, representing size-selected, narrowly distributed particles of identical composition extracted from FeraSpin R, measurements of the ac susceptibility (ACS), magnetorelaxometry (MRX), and static M-H curves were performed on suspended and immobilized particle samples. ACS and MRX measurements on immobilized samples indicate a wide distribution of anisotropy energies despite the narrow distribution of hydrodynamic sizes. Static magnetization measurements show that all samples exhibit a bimodal distribution of magnetic moments: The fraction of larger moments corresponds to the contribution from the overall particle core, whereas the smaller is attributed to the contribution from the crystallites comprising the core.

Magnetic fluid dynamics in a rotating magnetic field

The dynamics of a magnetic fluid in a rotating magnetic field in the presence of thermal noise were studied by performing numerical simulations based on the Fokker-Planck equation. We first clarified the dynamic properties by numerical simulation such as the frequency dependence of the fluid magnetization, the field-dependent relaxation time, and the M-H curve in a rotating magnetic field. Using the simulation results, we modified an existing analytical model and obtained an empirical expression to quantitatively describe the particle dynamics. The simulation results were compared with experimental results in a rotating magnetic field. The frequency dependence of the magnetization of the magnetic fluid was measured over the linear and nonlinear regions. In order to make a quantitative comparison, the hydrodynamic and effective core size distributions were independently estimated from measurements of the ac susceptibility and the M-H curve. The phase lag and amplitude in a rotating magnetic field obtained from our simulation agreed well with the experimental results.

Determination of core and hydrodynamic size distributions of CoFe2O4 nanoparticle suspensions using ac susceptibility measurements

The complex susceptibility was measured on CoFe2O4 nanoparticle suspensions in the frequency range between 1 kHz and 1 MHz for different values of a superimposed static magnetic field. The maximum in the imaginary part χ″ of the ac susceptibility shifts to higher frequencies with increasing static magnetic field. The shift is theoretically modeled utilizing the magnetic field dependence of the Brownian relaxation time constant and assuming a distribution of hydrodynamic particle sizes. The mean hydrodynamic size as determined from the maximum of χ″ in zero field and the mean core size as obtained from the shift of the χ″ peak with static field agree very well with the data from transmission electron microscopy and dynamic light scattering measurements, respectively. The results indicate that both core and hydrodynamic size distributions can be determined from measurements on nanoparticle suspensions proposed that magnetic dipole-dipole interactions are negligible.

Effect of HF concentration on physical and electronic properties of electrochemically formed nanoporous silicon

The most common fabrication technique of porous silicon (PS) is electrochemical etching of a crystalline silicon wafer in a hydrofluoric (HF) acid-based solution. The electrochemical process allows for precise control of the properties of PS such as thickness of the porous layer, porosity, and average pore diameter. The effect ofHF concentration in the used electrolyte on physical and electronic properties of PS was studied by visual color observation, measuring nitrogen sorption isotherm, field emission type scanning electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. It was found that with decrease in HF concentration, the pore diameter increased. The PS sample with large pore diameter, that is, smaller nanocrystalline size of Si between the pores, was found to lead to a pronounced photoluminescence peak. The systematic rise of photoluminescence peak with increase of pore diameter and porosity of PS was attributed to quantum confinement. The changes in nanocrystalline porous silicon were also clearly observed by an asymmetric broadening and shift of the optical silicon phonons in Raman spectra. The change in electronic properties of PS with pore diameter suggests possibilities of use of PS material as a template for fundamental physics as well as an opticalmaterial for technological applications.

Characterization of magnetic nanoparticle systems with respect to their magnetic particle imaging performance

Abstract The optimization of magnetic nanoparticles (MNPs) as markers for magnetic particle imaging (MPI) requires an understanding of the relationship between the harmonics spectrum and the structural and magnetic properties of the MNPs. Although magnetic particle spectroscopy (MPS) – carried out at the same excitation frequency as the given MPI system – represents a straightforward technique to study MNPs for their suitability for MPI, a complete understanding of the mechanisms and differences between different tracer materials requires additional measurements of the static and dynamic magnetic behavior covering additional field and time ranges. Furthermore, theoretical models are needed, which correctly account for the static and dynamic magnetic properties of the markers. In this paper, we give an overview of currently used theoretical models for the explanation of amplitude and phase of the harmonics spectra as well as of the various static and dynamic magnetic techniques, which are applied for the comprehensive characterization of MNPs for MPI. We demonstrate on two multicore MNP model systems, Resovist® and FeraSpin™ Series, how a detailed picture of the MPI performance can be obtained by combining various static and dynamic magnetic measurements.

Properties of magnetic nanoparticles in the Brownian relaxation range for liquid phase immunoassays

Properties of magnetic nanoparticles in the Brownian relaxation region were studied. Using the magnetic nanoparticles that exhibit remanence, we measured the magnetic properties, such as static magnetization, magnetic relaxation, and alternating current susceptibility, in a solution. Comprehensive comparisons were made between the experimental results and the theoretical ones predicted from the Brownian relaxation. From the comparison, the distributions of the particle parameters, i.e., the magnetic moment and the relaxation time, were estimated. It was shown that all the magnetic properties can be well explained when we take into account the parameter distributions in the sample.

Extraction of SSVEP signals of a capacitive EEG helmet for Human Machine Interface

The use of capacitive electrodes for measuring EEG eliminates the preparation procedure known from classical noninvasive EEG measurements. The insulated interface to the brain signals in combination with steady-state visual evoked potentials (SSVEP) enables a zero prep human machine interface triggered by brain signals. This paper presents a 28-channel EEG helmet system based on our capacitive electrodes measuring and analyzing SSVEPs even through scalp hair. Correlation analysis is employed to extract the stimulation frequency of the EEG signal. The system is characterized corresponding to the available detection time with different subjects. As demonstration of the use of capacitive electrodes for SSVEP measurements, preliminary online Brain-Computer Interface (BCI) results of the system are presented. Detection times lie about a factor of 3 higher than in galvanic EEG SSVEP measurements, but are low enough to establish a proper communication channel for Human Machine Interface (HMI).