| 1. |
- Bender, P., et al.
(författare)
-
Relating Magnetic Properties and High Hyperthermia Performance of Iron Oxide Nanoflowers
- 2018
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:5, s. 3068-3077
-
Tidskriftsartikel (refereegranskat)abstract
- We investigated, in depth, the interrelations among structure, magnetic properties, relaxation dynamics and magnetic hyperthermia performance of magnetic nanoflowers. The nanoflowers are about 39 nm in size, and consist of densely packed iron oxide cores. They display a remanent magnetization, which we explain by the exchange coupling between the cores, but we observe indications for internal spin disorder. By polarized small-angle neutron scattering, we unambiguously confirm that, on average, the nanoflowers are preferentially magnetized along one direction. The extracted discrete relaxation time distribution of the colloidally dispersed particles indicates the presence of three distinct relaxation contributions. We can explain the two slower processes by Brownian and classical Néel relaxation, respectively. The additionally observed very fast relaxation contributions are attributed by us to the relaxation of disordered spins within the nanoflowers. Finally, we show that the intrinsic loss power (ILP, magnetic hyperthermia performance) of the nanoflowers measured in colloidal dispersion at high frequency is comparatively large and independent of the viscosity of the surrounding medium. This concurs with our assumption that the observed relaxation in the high frequency range is primarily a result of internal spin relaxation, and possibly connected to the disordered spins within the individual nanoflowers.
|
|
| 2. |
- Fock, J., et al.
(författare)
-
Field-dependent dynamic responses from dilute magnetic nanoparticle dispersions
- 2018
-
Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 10:4, s. 2052-2066
-
Tidskriftsartikel (refereegranskat)abstract
- The response of magnetic nanoparticles (MNPs) to an oscillating magnetic field outside the linear response region is important for several applications including magnetic hyperthermia, magnetic resonance imaging and biodetection. The size and magnetic moment are two critical parameters for the performance of a colloidal MNP dispersion. We present and demonstrate the use of optomagnetic (OM) and AC susceptibility (ACS) measurements vs. frequency and magnetic field strength to obtain the size and magnetic moment distributions including the correlation between the distributions. The correlation between the size and the magnetic moment contains information on the morphology and intrinsic structure of the particle. In OM measurements, the variation of the second harmonic light transmission through a dispersion of MNPs is measured in response to an oscillating magnetic field. We solve the Fokker-Planck equations for MNPs with a permanent magnetic moment, and develop analytical approximations to the ACS and the OM signals that also account for the change in the curve shapes with increasing field strength. Further, we describe the influence of induced magnetic moments on the signals, by solving the Fokker-Planck equation for particles, which apart from the permanent magnetic moment may also have an induced magnetic moment and shape anisotropy. Using the results from the Fokker-Planck calculations we fit ACS and OM measurements on two multi-core particle systems. The obtained fit parameters also describe the correlations between the magnetic moment and size of the particles. From such an analysis on a commercially available polydisperse multicore particle system with an average particle size of 80 nm, we find that the MNP magnetic moment is proportional to the square root of the hydrodynamic size.
|
|
| 3. |
- Ludwig, Frank, et al.
(författare)
-
Analysis of AC Susceptibility Spectra for the Characterization of Magnetic Nanoparticles
- 2017
-
Ingår i: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 53:11
-
Tidskriftsartikel (refereegranskat)abstract
- Measurements of the ac susceptibility (ACS) as a function of frequency have been widely applied for the determination of structure parameters of magnetic nanoparticles (MNP). The analysis of spectra of real and imaginary parts measured on suspensions of MNP is generally based on the Debye model, extended by distributions of size parameters. Here, we compare different modifications of the Debye model with experimental data recorded on suspensions of single-core and multi-core iron-oxide nanoparticles. The applied models also depend on whether the nanoparticle's magnetic moments are thermally blocked and whether both Brownian and Néel relaxation have to be taken into account. The obtained core and hydrodynamic size parameters are compared with those from transmission electron microscopy and dynamic light scattering. Whereas structure parameters can be reliably determined for single-core nanoparticles, the interpretation of ACS spectra measured on multi-core nanoparticles is more complicated, especially regarding the contribution of particles relaxing via the Néel mechanism. Depending on the packing density and thus the interaction between cores in a particle, the effective core parameters derived from the spectrum must be interpreted with care.
|
|
| 4. |
- Ludwig, Frank, et al.
(författare)
-
Size analysis of single-core magnetic nanoparticles
- 2017
-
Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 427, s. 19-24
-
Tidskriftsartikel (refereegranskat)abstract
- Single-core iron-oxide nanoparticles with nominal core diameters of 14 nm and 19 nm were analyzed with a variety of non-magnetic and magnetic analysis techniques, including transmission electron microscopy (TEM), dynamic light scattering (DLS), static magnetization vs. magnetic field (M-H) measurements, ac susceptibility (ACS) and magnetorelaxometry (MRX). From the experimental data, distributions of core and hydrodynamic sizes are derived. Except for TEM where a number-weighted distribution is directly obtained, models have to be applied in order to determine size distributions from the measurand. It was found that the mean core diameters determined from TEM, M-H, ACS and MRX measurements agree well although they are based on different models (Langevin function, Brownian and Néel relaxation times). Especially for the sample with large cores, particle interaction effects come into play, causing agglomerates which were detected in DLS, ACS and MRX measurements. We observed that the number and size of agglomerates can be minimized by sufficiently strong diluting the suspension.
|
|
| 5. |
- Ludwig, Frank, et al.
(författare)
-
The Anisotropy of the AC Susceptibility of Immobilized Magnetic Nanoparticles-the Influence of Intra-Potential-Well Contribution on the AC Susceptibility Spectrum
- 2017
-
Ingår i: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 53:11
-
Tidskriftsartikel (refereegranskat)abstract
- We have experimentally verified the ac susceptibility model by Shliomis and Stepanov which accounts for the anisotropy caused by the different directions of the easy axes of the magnetic nanoparticle (MNP) cores with respect to the applied ac magnetic field. To experimentally access the anisotropy, single-core MNPs were immobilized in the absence of a magnetic field, thus causing random orientations of easy axes, and in a static magnetic field of 170 mT, thus orienting the nanoparticles' easy axes either parallel or perpendicular to the ac field. In agreement with theory, the real part of the sample with easy axes aligned perpendicularly to the ac field is constant while the imaginary part is zero. In contrast, the real part of the sample with parallel-oriented easy axes decays with increasing frequency. The susceptibility spectra of the sample with random orientation of easy axes are in between the other two cases according to χω= χIIω+ 2χω/3. This anisotropic behavior also explains why the real part of suspensions of thermally blocked MNPs does not drop down to zero at high frequencies but to a finite value. This value allows one to independently estimate the effective anisotropy constant of MNPs.
|
|
