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  • 1.
    Ferguson, Richard Matthew
    et al.
    University of Washington, USA.
    Khandhar, Amit P.
    University of Washington, USA.
    Johansson, Christian
    RISE., Swedish ICT, Acreo.
    Blomgren, Jakob
    RISE., Swedish ICT, Acreo.
    Johansson, Christer
    Krishnan, K. M.
    University of Washington, USA.
    Size-Dependent Relaxation Properties of Monodisperse Magnetite Nanoparticles Measured Over Seven Decades of Frequency by AC Susceptometry2013Ingår i: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 49, s. 3441-3444Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Magnetic relaxation is exploited in innovative biomedical applications of magnetic particles such as magnetic particle imaging (MPI), magnetic fluid hyperthermia, and bio-sensing. Relaxation behavior should be optimized to achieve high performance imaging, efficient heating, and good SNR in bio-sensing. Using two AC susceptometers with overlapping frequency ranges, we have measured the relaxation behavior of a series of monodisperse magnetic particles and demonstrated that this approach is an effective way to probe particle relaxation characteristics from a few Hz to 10 MHz, the frequencies relevant for MPI, hyperthermia, and sensing.

  • 2.
    Fornara, Andrea
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Kemi Material och Ytor.
    Magnetic, structural, and particle size analysis of single- and multi-core magnetic nanoparticles2014Ingår i: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, nr 11, s. 6971714-Artikel i tidskrift (Refereegranskat)
  • 3.
    Ludwig, F.
    et al.
    Technische Universität Braunschweig, Germany.
    Kazakova, O.
    National Physical Laboratory, UK.
    Barquin, L. F.
    University of Cantabria, Spain.
    Fornara, A.
    RISE., SP – Sveriges Tekniska Forskningsinstitut.
    Trahms, L.
    Physikalisch-Technische Bundesanstalt, Germany.
    Steinhoff, U.
    Physikalisch-Technische Bundesanstalt, Germany.
    Svedlindh, P.
    Uppsala University, Sweden.
    Wetterskog, E
    Uppsala University, Sweden.
    Pankhurst, Q. A.
    University College of London, UK.
    Southern, P.
    University College of London, UK.
    Morales, P.
    University of Cantabria, Spain.
    Hansen, M. F.
    DTU Technical University of Denmark, Denmark.
    Frandsen, C.
    DTU Technical University of Denmark, Denmark.
    Olsson, E.
    Chalmers University of Technology, Sweden.
    Gustafsson, S.
    Chalmers University of Technology, Sweden.
    Gehrke, N.
    NanoPET Pharma GmbH, Germany.
    Ludtke-Buzug, K.
    University of Lübeck, Germany.
    Gruttner, C.
    Micromod Partikeltechnologie GmbH, Germany.
    Jonasson, Christian
    RISE - Research Institutes of Sweden, ICT, Acreo.
    Johansson, Christer
    RISE., Swedish ICT, Acreo.
    Magnetic, Structural, Particle Size Analysis of Single- Multi-Core Magnetic Nanoparticles2014Ingår i: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, nr 11, s. 5300204-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have measured and analyzed three different commercial magnetic nanoparticle systems, both multi-core and single-core in nature, with the particle (core) size ranging from 20 to 100 nm. Complementary analysis methods and same characterization techniques were carried out in different labs and the results are compared with each other. The presented results primarily focus on determining the particle size - both the hydrodynamic size and the individual magnetic core size - as well as magnetic and structural properties. The used analysis methods include transmission electron microscopy, static and dynamic magnetization measurements, and Mössbauer spectroscopy. We show that particle (hydrodynamic and core) size parameters can be determined from different analysis techniques and the individual analysis results agree reasonably well. However, in order to compare size parameters precisely determined from different methods and models, it is crucial to establish standardized analysis methods and models to extract reliable parameters from the data. 

  • 4.
    Ludwig, Frank
    et al.
    TU Braunschweig, Germany.
    Balceris, Christoph
    TU Braunschweig, Germany.
    Johansson, Christer
    RISE - Research Institutes of Sweden, ICT, Acreo.
    The Anisotropy of the AC Susceptibility of Immobilized Magnetic Nanoparticles-the Influence of Intra-Potential-Well Contribution on the AC Susceptibility Spectrum2017Ingår i: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 53, nr 11, artikel-id 7894242Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 5.
    Ludwig, Frank
    et al.
    TU Braunschweig, Germany.
    Balceris, Christoph
    TU Braunschweig, Germany.
    Jonasson, Christian
    RISE - Research Institutes of Sweden, ICT, Acreo.
    Johansson, Christer
    RISE - Research Institutes of Sweden, ICT, Acreo.
    Analysis of AC Susceptibility Spectra for the Characterization of Magnetic Nanoparticles2017Ingår i: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 53, nr 11, artikel-id 7898860Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

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