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Papers by Christer Johansson

Abstract. A correct estimate of the size distribution (ie, median diameter D and geometric standa... more Abstract. A correct estimate of the size distribution (ie, median diameter D and geometric standard deviation σ) of the magnetic nanocrystals (MNCs) embedded in magnetic multicore particles is a necessity in most applications relying on the magnetic response of these ...

Journal of Physics: Condensed Matter, 1995

We studied the magnetic properties of frozen magnetic liquids containing amorphous Fe1-xCx partic... more We studied the magnetic properties of frozen magnetic liquids containing amorphous Fe1-xCx particles. By varying the particle size and concentration, we could separate single particle effects from interactions. In samples containing particles with median diameters 5.0 nm and 3.8 nm and with spontaneous saturation magnetizations 7.1 kA m-1 and 9.2 kA m-1, effects of particle interactions are observed. For 3.2 nm no interactions were observed. In samples with negligible interactions the superparamagnetic blocking temperature increases with the particle size. The effective anisotropy constant increases with decreasing particle size, implying that the anisotropy is enhanced by surface effects.

Magnetic Resonance in Medicine, 1996

The T,* contrast efficacy of paramagnetic contrast agents is dependent on their magnetic properti... more The T,* contrast efficacy of paramagnetic contrast agents is dependent on their magnetic properties. Vibrating sample magnetometry (VSM) and the Live Chan NMR method have been used to evaluate the influence of ligand structure on the bulk magnetic susceptibility (BMS) of low-molecular weight (LMW) lanthanide chelates. VSM was also used for the BMS assessment of LMW lanthanide chelates covalently attached to cross-linked starch particles. The ligand structure had no influence on the BMS of the gadolinium (Gd) and dysprosium (Dy) chelates. The mean BMS value of the Dy-chelates was 1.8 fold higher than that of the Gd-chelates. The holmium (Ho) DTPA-BMA chelate had a similar BMS to that of Dy-DTPA-BMA while the lowest BMS was found for europium (Eu(ll1)) DTPA-BMA. The covalent attachment of Gd-DTPA and Dy-DTPA to a cross-linked starch particle had no impact on their intrinsic magnetic properties. The BMS data were in good accordance with those obtained for non-particulate bound LMW Dy-and Gd-chelates. The magnetic susceptibility of the Gd-DTPA labeled particles was described by the Curie law, indicative of no magnetic interactions between Gd-DTPA molecules. The magnetic susceptibility of the Dy-DTPA labeled particles followed the Curie-Weiss law with a Curie-Weiss temperature of about -2 K, indicating magnetic interactions. The magnetic susceptibility of Dy-DTPA will, however, not be affected by such magnetic interactions at physiological temperatures.

Fuel Processing Technology, 2015

ABSTRACT A method for 3-dimensional (3D) magnetic tracer particle tracking in a fluid dynamically... more ABSTRACT A method for 3-dimensional (3D) magnetic tracer particle tracking in a fluid dynamically downscaled fluidized bed is presented. The method applies anisotropic magnetoresistive (AMR) sensors to track a magnetic tracer particle in the form of an NdFeB-based permanent magnet. The fluid dynamically downscaled bed has a cross-section of 0.17 × 0.17 m2 and is operated at ambient conditions with bronze powder as bulk solids and the tracer particle corresponding to a fuel particle. After up-scaling the bed corresponds to a 0.85 × 0.85 m2 bed of fuel ash or silica sand operated with air at 900 °C using anthracite coal as a fuel. Thus, the method provides continuous tracking of the tracer particle trajectory which, combined with the fluid dynamic scaling yields quantitative information applicable to the study of fuel mixing in large-scale fluidized-bed processes operating under hot conditions. Application of the method represents a significant step forward compared to other experimental studies which are limited to qualitative interpretations; performed in 2D units and in cold 3D units which are not fluid dynamically scaled.

Journal of Magnetism and Magnetic Materials, 1991

The magnetic susceptibility of magnetic liquids has been measured for frequencies up to 70 MHz. T... more The magnetic susceptibility of magnetic liquids has been measured for frequencies up to 70 MHz. The imaginary part of the susceptibility exhibits a maximum around 10 MHz. This feature is interpreted as due to single-particle Ntel relaxation, where particle interactions cause a slight concentration dependence in the position of the maximum.

IEEE Transactions on Magnetics, 1994

ABSTRACT

IEEE Transactions on Magnetics, 2000

Magnetic relaxation is exploited in innovative biomedical applications of magnetic particles such... more 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.

Journal of Magnetism and Magnetic Materials, 1996

Hyperfine Interactions, 1994

Ferrofluids with ultrafine magnetic particles of amorphous Fe1-xCx were studied by Mössbauer spec... more Ferrofluids with ultrafine magnetic particles of amorphous Fe1-xCx were studied by Mössbauer spectroscopy, transmission electron microscopy and magnetic measurements. Magnetic measurements showed that the initial susceptibility follows a Curie-Weiss law, indicating magnetic interaction between the particles. The influence of the magnetic interaction on the estimation of the particle size, made from the field dependence of the induced hyperfine splitting, is discussed.

J Magn Magn Mater, 2005

X-ray diffraction, SEM analysis, Mössbauer spectroscopy and magnetic measurements were used to ch... more X-ray diffraction, SEM analysis, Mössbauer spectroscopy and magnetic measurements were used to characterize three different magnetic beads (Dynabeads ®). Maghemite ( γ-Fe 2O 3) is the predominant crystalline phase. The nanoparticles were evenly spread in the beads, and the crystal sizes were in the range of 8 nm. The nanoparticles showed superparamagnetic behaviour. The particle's intrinsic magnetization of about 340 kA/m is typical for nanoparticles of maghemite.

Journal of Magnetism and Magnetic Materials, May 31, 2005

... Characterisation of Dynabeads ® by magnetization measurements and Mössbauer spectroscopy. Gei... more ... Characterisation of Dynabeads ® by magnetization measurements and Mössbauer spectroscopy. Geir Fonnum a , Corresponding Author Contact Information , E-mail The Corresponding Author , Christer Johansson b , Astrid Molteberg a , Steen Mørup c and Elin Aksnes a. ...

Sensors and Actuators A: Physical, 2013

ABSTRACT This study reports on the poling and characteristics of a melt-spun piezoelectric bicomp... more ABSTRACT This study reports on the poling and characteristics of a melt-spun piezoelectric bicomponent fiber with poly(vinylidene fluoride) (PVDF) as its sheath component and a conductive composite with carbon black (CB) and high density polyethylene (HDPE) as its core component. The influence of poling conditions on the piezoelectric properties of the fibers has been investigated. The poling parameters temperature, time and poling voltage have been varied and the piezoelectric effect of both contact- and corona-poled yarns have been evaluated. The results show that a high piezoelectric effect is achieved when the poling voltage is high as possible and the poling temperature is between 60 °C and 120 °C. It was also shown that permanent polarization is achieved in a time as short as 2 s in corona-poled fibers. A yarn exposed to a sinusoidal axial tension of 0.07% strain (the corresponding force amplitude was 0.05 N) shows an intrinsic voltage output of 4 V. The mean power from a 25 mm length of yarn is estimated to be 15 nW. To demonstrate the fibers sensor properties, they are woven into a textile fabric from which a force sensor is manufactured and used to detect the heartbeat of a human.

Materials Science and Engineering: R: Reports, 2007

This paper provides a detailed overview of developments in transducer materials technology relati... more This paper provides a detailed overview of developments in transducer materials technology relating to their current and future applications in micro-scale devices. Recent advances in piezoelectric, magnetostrictive and shape-memory alloy systems are discussed and emerging transducer materials such as magnetic nanoparticles, expandable micro-spheres and conductive polymers are introduced. Materials properties, transducer mechanisms and end applications are described and the potential for integration of the materials with ancillary systems components is viewed as an essential consideration. The review concludes with a short discussion of structural polymers that are extending the range of micro-fabrication techniques available to designers and production engineers beyond the limitations of silicon fabrication technology.