Abstract. Nanocrystalline mainly superparamagnetic ferrite particles ≈ 10 nm are used for the pre... more Abstract. Nanocrystalline mainly superparamagnetic ferrite particles ≈ 10 nm are used for the preparation of magnetic fluids. Barium hexaferrite BaFe12−2xTixCoxO19 powders with mean particle sizes < 30 nm show the transition to single domain Stoner-Wohlfarth behaviour. ...
ABSTRACT The objective of the present study was to investigate the feasibility of a minimal-invas... more ABSTRACT The objective of the present study was to investigate the feasibility of a minimal-invasive method for the elimination of tumors in the breast by local heating. Heating is generated by an intratumoral iron oxide (magnetite) loading of the tumor and exposure of the breast to an alternating magnetic field. The methodology consisted in the implantation of human adenocarcinomas (MX-1 cells) into 10 immunodeficient mice. After application of coated iron oxide particles (average total particle diameter: 10 nm) the intratumoral particle deposition was controlled by radiography. Iron contents in selected organs were determined using atomic absorption spectrometry at 50 min p.i.. During exposure of mice (4 min) to an AC magnetic field (frequency: 400 kHz, amplitude: 6.5 kA/m) intratumoral and rectal temperatures were measured. 6.7±2.3%; 2.8± 0.9% and 2.4 ± 0.1% of injected iron dose per g dried tissue was found in the spleen, liver and blood, respectively at 50 min p.i.. No injected iron was found in the lung. Temperatures of 87 ± 7°C and 71 ± 8°C (distal and proximal tumor periphery, respectively) were measured at the end of magnetic field exposure. From the data we deduce that the method enables the generation of localized heat spots that damage tumor cells within several minutes and that the prototype particles are adequately retained at the tumor site. The introduction of the technique in future interventional radiological procedures is promising. Tables 1, Figs 4, Refs 14.
Nanometer-scale particles are interesting because of their unique magnetic properties. Barium fer... more Nanometer-scale particles are interesting because of their unique magnetic properties. Barium ferrite with particle sizes &amp;amp;amp;lap; 10 nm behave superparamagnetically and show at bigger sizes the transition to single domain behaviour. Beside the particle size, the anisotropy energy K_1\cdot V, and thus the Neel relaxation time, depends also on the amount of doping. The glass crystallisation method was used for
ABSTRACT A YBCO single crystal having only one twin domain with very uniform twin spacing is inve... more ABSTRACT A YBCO single crystal having only one twin domain with very uniform twin spacing is investigated by torque magnetometry with field rotation parallel to a {110} plane which is either perpendicular or parallel to twin planes. Experimental data are analyzed with respect to irreversible and reversible torque contributions. From the latter one an anisotropy ratio γ = 9 is determined. Several rotational hysteresis loss contributions are identified, the most remarkable of which are very anisotropic losses due to intrinsic pinning and twin pinning. By using different measuring geometries it is shown that twin boundaries do not assist in the intrinsic pinning process as it was suggested in the literature. Isotropic losses are ascribed to impurity atoms. Viscous losses arising during vortex motion are small in comparison to all other losses. The results are discussed in terms of recent pinning models from literature.Ein YBCO-Einkristall mit einem einzigen Zwillingsbereich mit einheitlichem Zwillingsabstand wird mittels Drehmomentenmessung bei Feldrotation parallel zu einer {110}-Ebene, die entweder senkrecht oder parallel zu den Zwillingsebenen ist, untersucht. Die experimentellen Ergebnisse werden hinsichtlich der irreversiblen und reversiblen Drehmomentbeiträge analysiert. Aus letzteren wird das Anisotropiever-hältnis γ = 9 bestimmt. Mehrerer Beiträge zur Rotationsverlusthysterese werden identifiziert, die bedeutendsten davon sind stark anisotrope Verluste, die durch Eigen- und Zwilungsverankerung entstehen. Durch Verwendung unterschiedlicher Meßgeometrien wird gezeigt, daß Zwillingsgrenzen den gittereignen Verankerungsprozeß nicht unterstützen, wie in der Literatur angenommen wurde. Isotrope Verluste werden Fremdatomen zugeschrieben. Viskose-Verluste aufgrund von Vortex-bewegungen sind im Vergleich zu anderen Verlusten klein. Die Ergebnisse werden im Rahmen der kürzlich in der Literatur präsentierten Pinning-Modelle diskutiert.
ABSTRACT YBa2 Cu3 O7-δ single crystals with different microstructure of the twin pattern were stu... more ABSTRACT YBa2 Cu3 O7-δ single crystals with different microstructure of the twin pattern were studied by means of torque magnetometry. Current densities and pinning force volume densities were calculated from torque hysteresis data. Pinning force volume density Fp in dependence on the component Bz of the flux density parallel to the c-axis direction revealed different regimes of collective pinning: For very low Bz , in the single vortex pinning regime, current density is independent on Bz . With increasing Bz a transition towards the regime of collective pinning of small flux bundles occurs. Here two different cases can be distinguished. For pinning at randomly distributed point pinning centers a strong influence of thermal oscillations of flux lines within the pinning barriers can be observed. In contrast, for single crystals having a ``mosaic&#39;&#39; twin structure, the influence of thermal oscillations can be neglected due to a fraction of flux lines which are strongly correlated pinned by twin planes and hinder the thermal oscillations of the other weakly pinned flux lines in the flux bundles. In both cases an increase of the current density with increasing Bz (i.e., ``fishtail&#39;&#39; effect) can be observed, which is characterized, at low Bz , by a power-law behavior Fp ∼Bzp&gt; with p=7/4 for collective random point pinning of small flux bundles and p=2 for pinning of small flux bundles with additional correlated pinning contributions from the mosaic twin structures, respectively. For YBa2 Cu3 O7-δ single crystals, with only medium current density but with a mosaic twin structure, a ``lock-in&#39;&#39; transition from pure collective random point pinning of large flux bundles, which is strongly influenced by thermal oscillations, towards collective pinning of small bundles with contributions of correlated pinning can be observed.
ABSTRACT A novel coil system as a replacement for Helmholtz coil systems will be presented. It co... more ABSTRACT A novel coil system as a replacement for Helmholtz coil systems will be presented. It consists of three pairs of coils with one pair having opposite polarity. In comparison to Helmholtzcoils the novel coil system has the following advantages: (a) The magnetic field amplitude on the coil axis (z-axis) outside the coil system declines with H ∼ z-5 significantly faster than in a Helmholtz coil (H ∼ z-3). (b) It provides an homogeneity up to the sixth order of the magnetic field along the coil axis. Hence a larger homogeneous volume in comparison to a Helmholtz coil system with equipollent dimensions could be achieved. The novel coil system can be applied for the stray field insensitive measurement of magnetic moments and for the generation of magnetic fields especially inside of magnetic shielding chambers whose walls should not be magnetized in any circumstance. An excellent agreement between calculated field profiles and field measurements of prototypes has been found. The homogeneity of the field profile of a prototype could be further improved by connecting a resistor in parallel to three of the six coils. This way precision measurements by nuclear magnetic resonance could be accomplished.
The ldquofishtailrdquo-effect in strong pinning YBCO single crystals has been investigated by mea... more The ldquofishtailrdquo-effect in strong pinning YBCO single crystals has been investigated by means of torque magnetometry. We show that this effect can be explained in the framework of the collective pinning theory of Feigel'man and Vinokur
Abstract. Nanocrystalline mainly superparamagnetic ferrite particles ≈ 10 nm are used for the pre... more Abstract. Nanocrystalline mainly superparamagnetic ferrite particles ≈ 10 nm are used for the preparation of magnetic fluids. Barium hexaferrite BaFe12−2xTixCoxO19 powders with mean particle sizes < 30 nm show the transition to single domain Stoner-Wohlfarth behaviour. ...
Relaxation measurements using torque magnetometry were performed for textured Bi2Sr2Ca2Cu3Ox tape... more Relaxation measurements using torque magnetometry were performed for textured Bi2Sr2Ca2Cu3Ox tapes at 77 K. Experiments were carried out for different orientations of the external field with respect to the texture axis (defined as the c axis), which coincides with the tape normal. The experimental torque data were separated into reversible and irreversible anisotropic contributions. It is shown that the irreversible
This paper discusses the design and use of gradient annealing devices. Generally, it is intended ... more This paper discusses the design and use of gradient annealing devices. Generally, it is intended to use such devices for the rapid optimization of thin film materials by simultaneous thermal processing at different temperatures. Furthermore, these devices are efficient for short- time annealing experiments. They are used in order to quickly vary the annealing parameters (temperature, time) from sample to sample. Here, nanoscale Fe/Pt multilayer precursor thin films for the fabrication of hard magnetic Fe-Pt thin films are investigated as a test system. First results prove the usefulness of the gradient annealing devices for high-throughput experiments.
RöFo - Fortschritte auf dem Gebiet der Röntgenstrahlen und der bildgebenden Verfahren, 2002
ABSTRACT To find an optimal imaging modality for the assessment of magnetite agglomerations used ... more ABSTRACT To find an optimal imaging modality for the assessment of magnetite agglomerations used as the heating sources during magnetic thermoablation of tumors. 1 to 107 mg of coated (starch) magnetite particles were directly administered to an in vitro tumor model (swine lymph nodes) and investigated immediately (radiography) or after being embedded within a 4 % agar-phantom (sonography). T1-weighted MR images (TR = 400 ms, TE = 14 ms) were acquired from lymph nodes containing 0.5 to 25 mg magnetite. All investigated magnetite masses were qualitatively detectable by radiography. Sonographically, only mass agglomerations containing 107 mg magnetite were appropriately discernible. MRT images revealed distinct susceptibility artifacts. Based on the investigated imaging modalities, radiography is the method of choice for assessment of magnetite agglomerations using relevant dosages for magnetic thermoablation of tumors.
RöFo - Fortschritte auf dem Gebiet der Röntgenstrahlen und der bildgebenden Verfahren, 2005
The therapeutic strategy for breast cancer is changing, especially for early tumor stages with go... more The therapeutic strategy for breast cancer is changing, especially for early tumor stages with good prognosis. One potential minimally invasive therapy modality consists in the accumulation of a well-tolerated magnetic material (iron oxides, particularly magnetite) in the target tissue. By applying an alternating magnetic field, energy is selectively absorbed and induces harmful heating of the tumor. The present review deals with the essential conditions and parameters as studied in vitro and in vivo in animal experiments. Extrapolations to the clinical situation are discussed, in particular, the heating potential of the magnetic material, the selection of the magnetic field parameters, the occurrence of eddy currents, the generation of localized heating spots and the expected temperature rises and their effects on the tumor area.
Abstract. Nanocrystalline mainly superparamagnetic ferrite particles ≈ 10 nm are used for the pre... more Abstract. Nanocrystalline mainly superparamagnetic ferrite particles ≈ 10 nm are used for the preparation of magnetic fluids. Barium hexaferrite BaFe12−2xTixCoxO19 powders with mean particle sizes < 30 nm show the transition to single domain Stoner-Wohlfarth behaviour. ...
ABSTRACT The objective of the present study was to investigate the feasibility of a minimal-invas... more ABSTRACT The objective of the present study was to investigate the feasibility of a minimal-invasive method for the elimination of tumors in the breast by local heating. Heating is generated by an intratumoral iron oxide (magnetite) loading of the tumor and exposure of the breast to an alternating magnetic field. The methodology consisted in the implantation of human adenocarcinomas (MX-1 cells) into 10 immunodeficient mice. After application of coated iron oxide particles (average total particle diameter: 10 nm) the intratumoral particle deposition was controlled by radiography. Iron contents in selected organs were determined using atomic absorption spectrometry at 50 min p.i.. During exposure of mice (4 min) to an AC magnetic field (frequency: 400 kHz, amplitude: 6.5 kA/m) intratumoral and rectal temperatures were measured. 6.7±2.3%; 2.8± 0.9% and 2.4 ± 0.1% of injected iron dose per g dried tissue was found in the spleen, liver and blood, respectively at 50 min p.i.. No injected iron was found in the lung. Temperatures of 87 ± 7°C and 71 ± 8°C (distal and proximal tumor periphery, respectively) were measured at the end of magnetic field exposure. From the data we deduce that the method enables the generation of localized heat spots that damage tumor cells within several minutes and that the prototype particles are adequately retained at the tumor site. The introduction of the technique in future interventional radiological procedures is promising. Tables 1, Figs 4, Refs 14.
Nanometer-scale particles are interesting because of their unique magnetic properties. Barium fer... more Nanometer-scale particles are interesting because of their unique magnetic properties. Barium ferrite with particle sizes &amp;amp;amp;lap; 10 nm behave superparamagnetically and show at bigger sizes the transition to single domain behaviour. Beside the particle size, the anisotropy energy K_1\cdot V, and thus the Neel relaxation time, depends also on the amount of doping. The glass crystallisation method was used for
ABSTRACT A YBCO single crystal having only one twin domain with very uniform twin spacing is inve... more ABSTRACT A YBCO single crystal having only one twin domain with very uniform twin spacing is investigated by torque magnetometry with field rotation parallel to a {110} plane which is either perpendicular or parallel to twin planes. Experimental data are analyzed with respect to irreversible and reversible torque contributions. From the latter one an anisotropy ratio γ = 9 is determined. Several rotational hysteresis loss contributions are identified, the most remarkable of which are very anisotropic losses due to intrinsic pinning and twin pinning. By using different measuring geometries it is shown that twin boundaries do not assist in the intrinsic pinning process as it was suggested in the literature. Isotropic losses are ascribed to impurity atoms. Viscous losses arising during vortex motion are small in comparison to all other losses. The results are discussed in terms of recent pinning models from literature.Ein YBCO-Einkristall mit einem einzigen Zwillingsbereich mit einheitlichem Zwillingsabstand wird mittels Drehmomentenmessung bei Feldrotation parallel zu einer {110}-Ebene, die entweder senkrecht oder parallel zu den Zwillingsebenen ist, untersucht. Die experimentellen Ergebnisse werden hinsichtlich der irreversiblen und reversiblen Drehmomentbeiträge analysiert. Aus letzteren wird das Anisotropiever-hältnis γ = 9 bestimmt. Mehrerer Beiträge zur Rotationsverlusthysterese werden identifiziert, die bedeutendsten davon sind stark anisotrope Verluste, die durch Eigen- und Zwilungsverankerung entstehen. Durch Verwendung unterschiedlicher Meßgeometrien wird gezeigt, daß Zwillingsgrenzen den gittereignen Verankerungsprozeß nicht unterstützen, wie in der Literatur angenommen wurde. Isotrope Verluste werden Fremdatomen zugeschrieben. Viskose-Verluste aufgrund von Vortex-bewegungen sind im Vergleich zu anderen Verlusten klein. Die Ergebnisse werden im Rahmen der kürzlich in der Literatur präsentierten Pinning-Modelle diskutiert.
ABSTRACT YBa2 Cu3 O7-δ single crystals with different microstructure of the twin pattern were stu... more ABSTRACT YBa2 Cu3 O7-δ single crystals with different microstructure of the twin pattern were studied by means of torque magnetometry. Current densities and pinning force volume densities were calculated from torque hysteresis data. Pinning force volume density Fp in dependence on the component Bz of the flux density parallel to the c-axis direction revealed different regimes of collective pinning: For very low Bz , in the single vortex pinning regime, current density is independent on Bz . With increasing Bz a transition towards the regime of collective pinning of small flux bundles occurs. Here two different cases can be distinguished. For pinning at randomly distributed point pinning centers a strong influence of thermal oscillations of flux lines within the pinning barriers can be observed. In contrast, for single crystals having a ``mosaic&#39;&#39; twin structure, the influence of thermal oscillations can be neglected due to a fraction of flux lines which are strongly correlated pinned by twin planes and hinder the thermal oscillations of the other weakly pinned flux lines in the flux bundles. In both cases an increase of the current density with increasing Bz (i.e., ``fishtail&#39;&#39; effect) can be observed, which is characterized, at low Bz , by a power-law behavior Fp ∼Bzp&gt; with p=7/4 for collective random point pinning of small flux bundles and p=2 for pinning of small flux bundles with additional correlated pinning contributions from the mosaic twin structures, respectively. For YBa2 Cu3 O7-δ single crystals, with only medium current density but with a mosaic twin structure, a ``lock-in&#39;&#39; transition from pure collective random point pinning of large flux bundles, which is strongly influenced by thermal oscillations, towards collective pinning of small bundles with contributions of correlated pinning can be observed.
ABSTRACT A novel coil system as a replacement for Helmholtz coil systems will be presented. It co... more ABSTRACT A novel coil system as a replacement for Helmholtz coil systems will be presented. It consists of three pairs of coils with one pair having opposite polarity. In comparison to Helmholtzcoils the novel coil system has the following advantages: (a) The magnetic field amplitude on the coil axis (z-axis) outside the coil system declines with H ∼ z-5 significantly faster than in a Helmholtz coil (H ∼ z-3). (b) It provides an homogeneity up to the sixth order of the magnetic field along the coil axis. Hence a larger homogeneous volume in comparison to a Helmholtz coil system with equipollent dimensions could be achieved. The novel coil system can be applied for the stray field insensitive measurement of magnetic moments and for the generation of magnetic fields especially inside of magnetic shielding chambers whose walls should not be magnetized in any circumstance. An excellent agreement between calculated field profiles and field measurements of prototypes has been found. The homogeneity of the field profile of a prototype could be further improved by connecting a resistor in parallel to three of the six coils. This way precision measurements by nuclear magnetic resonance could be accomplished.
The ldquofishtailrdquo-effect in strong pinning YBCO single crystals has been investigated by mea... more The ldquofishtailrdquo-effect in strong pinning YBCO single crystals has been investigated by means of torque magnetometry. We show that this effect can be explained in the framework of the collective pinning theory of Feigel'man and Vinokur
Abstract. Nanocrystalline mainly superparamagnetic ferrite particles ≈ 10 nm are used for the pre... more Abstract. Nanocrystalline mainly superparamagnetic ferrite particles ≈ 10 nm are used for the preparation of magnetic fluids. Barium hexaferrite BaFe12−2xTixCoxO19 powders with mean particle sizes < 30 nm show the transition to single domain Stoner-Wohlfarth behaviour. ...
Relaxation measurements using torque magnetometry were performed for textured Bi2Sr2Ca2Cu3Ox tape... more Relaxation measurements using torque magnetometry were performed for textured Bi2Sr2Ca2Cu3Ox tapes at 77 K. Experiments were carried out for different orientations of the external field with respect to the texture axis (defined as the c axis), which coincides with the tape normal. The experimental torque data were separated into reversible and irreversible anisotropic contributions. It is shown that the irreversible
This paper discusses the design and use of gradient annealing devices. Generally, it is intended ... more This paper discusses the design and use of gradient annealing devices. Generally, it is intended to use such devices for the rapid optimization of thin film materials by simultaneous thermal processing at different temperatures. Furthermore, these devices are efficient for short- time annealing experiments. They are used in order to quickly vary the annealing parameters (temperature, time) from sample to sample. Here, nanoscale Fe/Pt multilayer precursor thin films for the fabrication of hard magnetic Fe-Pt thin films are investigated as a test system. First results prove the usefulness of the gradient annealing devices for high-throughput experiments.
RöFo - Fortschritte auf dem Gebiet der Röntgenstrahlen und der bildgebenden Verfahren, 2002
ABSTRACT To find an optimal imaging modality for the assessment of magnetite agglomerations used ... more ABSTRACT To find an optimal imaging modality for the assessment of magnetite agglomerations used as the heating sources during magnetic thermoablation of tumors. 1 to 107 mg of coated (starch) magnetite particles were directly administered to an in vitro tumor model (swine lymph nodes) and investigated immediately (radiography) or after being embedded within a 4 % agar-phantom (sonography). T1-weighted MR images (TR = 400 ms, TE = 14 ms) were acquired from lymph nodes containing 0.5 to 25 mg magnetite. All investigated magnetite masses were qualitatively detectable by radiography. Sonographically, only mass agglomerations containing 107 mg magnetite were appropriately discernible. MRT images revealed distinct susceptibility artifacts. Based on the investigated imaging modalities, radiography is the method of choice for assessment of magnetite agglomerations using relevant dosages for magnetic thermoablation of tumors.
RöFo - Fortschritte auf dem Gebiet der Röntgenstrahlen und der bildgebenden Verfahren, 2005
The therapeutic strategy for breast cancer is changing, especially for early tumor stages with go... more The therapeutic strategy for breast cancer is changing, especially for early tumor stages with good prognosis. One potential minimally invasive therapy modality consists in the accumulation of a well-tolerated magnetic material (iron oxides, particularly magnetite) in the target tissue. By applying an alternating magnetic field, energy is selectively absorbed and induces harmful heating of the tumor. The present review deals with the essential conditions and parameters as studied in vitro and in vivo in animal experiments. Extrapolations to the clinical situation are discussed, in particular, the heating potential of the magnetic material, the selection of the magnetic field parameters, the occurrence of eddy currents, the generation of localized heating spots and the expected temperature rises and their effects on the tumor area.
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Papers by Robert Hiergeist