Yasushi Takemura

Effects of particle dipole interaction on the ac magnetically induced heating characteristics of ferrite nanoparticles for hyperthermia

Magnetic particle dipole interaction was revealed as a crucial physical parameter to be considered in optimizing the ac magnetically induced heating characteristics of magnetic nanoparticles. The ac heating temperature of soft MFe2O4 (M=Mg,Ni) nanoparticles was remarkably increased from 17.6 to 94.7 °C (MgFe2O4) and from 13.1 to 103.1 °C (NiFe2O4) by increasing the particle dipole interaction energy at fixed ac magnetic field of 140 Oe and frequency of 110 kHz. The increase in “magnetic hysteresis loss” that resulted from the particle dipole interaction was the main physical reason for the significant improvement of ac heating characteristics.

Applications of NiFe2O4 nanoparticles for a hyperthermia agent in biomedicine

Self-heating temperature rising characteristics, cytotoxicity, and magnetic properties of NiFe2O4 nanoparticles have been investigated to confirm the effectiveness as an in vivo hyperthermia agent in biomedicine. NiFe2O4 nanoparticles showed both superparamagnetic and ferrimagnetic behaviors depending on particle sizes. The quantitative cytotoxicity test verified that both uncoated and chitosan-coated NiFe2O4 nanoparticles had noncytotoxicity. The solid state 35nm size NiFe2O4 nanoparticles first exhibited a maximum self-heating temperature of 44.2°C at H0f=5.1×108Am−1s−1. The physical nature of the self-heating was primarily thought to be due to the magnetic hysteresis loss, Neel rotations, and Brownian rotations of 35nm size NiFe2O4 nanoparticles.

Characterization of FeSe thin films prepared on GaAs substrate by selenization technique

FeSe thin films were prepared on GaAs(100) substrate by the selenization of Fe films using molecular-beam epitaxy. FeSe compound thin films were obtained at a substrate temperature above 380 °C. From the depth profiles of Fe and Se in the selenized film measured by Auger electron spectroscopy, it was confirmed that an FeSe layer with a constant ratio of Fe/Se was formed. The measured composition ratio of Fe/Se in the film was 1/3. It was different from the composition in Fe3Se4 or Fe7Se8, which is a stable bulk FeSe compound. From the measured M–H curve, it was found that the obtained FeSe film consisted of two phases with different magnetic properties.

AC Magnetic-Field-Induced Heating and Physical Properties of Ferrite Nanoparticles for a Hyperthermia Agent in Medicine

AC magnetic-field-induced heating, cytotoxicity, and bio-related physical properties of two kinds of spinel ferrite nanoparticles, soft (NiFe₂O₄) and hard (CoFe₂O₄), with different mean particle sizes were investigated in this paper to confirm the effectiveness for an in vivo magnetic nanoparticle hyperthermia agent in biomedicine. AC magnetically induced heating temperature of the nanoparticles measured both in a solid and an agar state at different applied magnetic fields and frequencies clarified that the maximum heating temperature of NiFe₂O₄ nanoparticles is much higher than that of CoFe₂O₄ nanoparticles. In addition, it was demonstrated that solid-state NiFe₂O₄ nanoparticles with 24.8 and 35 nm mean particle size exhibited a promisingly high heating temperature (21.5degC-45degC) for a hyperthermia agent in the physiologically tolerable range of the ac magnetic field with less than 50 kHz of applied frequency. According to the magnetic and physical analysis results, the superior ac magnetically induced heating performance of NiFe₂O₄ nanoparticles was primarily due to their higher magnetic susceptibility (permeability) that directly induces a larger magnetic minor hysteresis loop area at the low magnetic field. Cytotoxicity test results, quantitatively estimated by methylthiazol tetrazolium bromide test method, verified that uncoated NiFe₂O₄, chitosan-coated NiFe₂O₄, and CoFe₂O₄ showed a noncytotoxicity, which is clinically suitable for a hyperthermia agent application.

Physical limits of pure superparamagnetic Fe3O4 nanoparticles for a local hyperthermia agent in nanomedicine

Magnetic and AC magnetically induced heating characteristics of Fe3O4 nanoparticles (IONs) with different mean diameters, d, systematically controlled from 4.2 to 22.5 nm were investigated to explore the physical relationship between magnetic phase and specific loss power (SLP) for hyperthermia agent applications. It was experimentally confirmed that the IONs had three magnetic phases and correspondingly different SLP characteristics depending on the particle sizes. Furthermore, it was demonstrated that pure superparamagnetic phase IONs (d < 9.8 nm) showed insufficient SLPs critically limiting for hyperthermia applications due to smaller AC hysteresis loss power (Neel relaxation loss power) originated from lower out-of-phase magnetic susceptibility.

REACTIVE MOLECULAR-BEAM EPITAXY OF GAN LAYERS DIRECTLY ON 6H-SIC(0001)

We investigate the quality of GaN layers directly grown on 6H–SiC(0001) substrates by reactive molecular-beam epitaxy. Despite a pure three-dimensional nucleation, step-flow growth is achieved by in situ adjusting conditions such that the (2×2) reconstruction observed during growth is maximized in intensity. The resulting surface morphology exhibits large terraces separated by mono- and multiatomic steps, and is clearly superior to that obtained by plasma-assisted growth. Furthermore, the structural and optical properties of these layers are comparable to those of layers grown by plasma-assisted molecular-beam epitaxy.

Dependence of Frequency and Magnetic Field on Self-Heating Characteristics of NiFe

Self-heating temperature-rising characteristics of nano-size controlled NiFe2O4 particles were analyzed as a function of applied frequency and magnetic field in order to investigate the physical principle of self-heating and to confirm the possibility for a real in vivo hyperthermia application. According to the magnetic properties of 35-nm size NiFe2O4 nanoparticles, it was confirmed that the physical mechanism of self-heating is mainly attributed to the hysteresis loss. In addition, it was found that the self-heating temperature was linearly increased by increasing frequency and was proportionally square to the applied magnetic field. The self-heating temperature was rapidly increased in an initial stage and then it reached to the maximum. The maximum self-heating temperature was controlled from 2.8degC to 72.6degC by changing the applied frequency and magnetic field. The corresponding product of the frequency and the strength of magnetic field H0f was between 1.9times108 Am-1s-1 and 13.4times10 8 Am-1s-1. These values are in the biological safety and tolerable range for hyperthermia considering deleterious physiological response of human body during hyperthermia treatment

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Magnetic nanoparticles can potentially be used in drug delivery systems and for hyperthermia therapy. The applicability of Fe₃O₄, CoFe₂O₄, MgFe₂O₄, and NiFe₂O₄ nanoparticles for the same was studied by evaluating their magnetization, thermal efficiency, and biocompatibility. Fe₃O₄ and CoFe₂O₄ nanoparticles exhibited large magnetization. Fe₃O₄ and NiFe₂O₄ nanoparticles exhibited large induction heating. MgFe₂O₄ nanoparticles exhibited low magnetization compared to the other nanoparticles. NiFe₂O₄ nanoparticles were found to be cytotoxic, whereas the other nanoparticles were not cytotoxic. This study indicates that Fe₃O₄ nanoparticles could be the most suitable ones for hyperthermia therapy.

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A novel lithography technique for fabricating ferromagnetic metal/insulator/ferromagnetic metal (FM/I/FM)-based nanoscale devices was investigated using a scanning probe microscope (SPM) local oxidation process. This technique was applied to the surface modification of Ni thin films. Planar-type Ni/Ni oxide/Ni-based diodes and ferromagnetic single-electron transistor (FMSET) structures were fabricated by SPM local oxidation. Ni-based diodes clearly showed nonlinear current-voltage (I-V) characteristics, and capacitively coupled FMSET structures with a Ni-based double ferromagnetic tunnel junction exhibited Coulomb blockade effects.

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The magnetic characterization of polyethyleneimine (PEI)-coated magnetite (Fe3O4) nanoparticles having diameters of 20-30 nm that were dispersed in a solution was performed, and their self-heating property was investigated. The hydrodynamic diameter of PEI-coated Fe3O4 nanoparticles was approximately 155 nm on an average. To investigate the self-heating property, the increase in the temperature of a sample heated by an applied ac magnetic field was measured, and the specific loss power (SLP) was calculated. The effect of magnetic relaxation loss was estimated by determining the dependence of the SLP on the frequency of an applied magnetic field. For the magnetic characterization of the nanoparticles, dc and ac magnetization curves of the sample were measured and compared with each other to elucidate the heating mechanism. Uncoated Fe3O4 nanoparticles having diameters of 20-30 nm exhibited ferromagnetic characteristics in dry condition. However, the PEI-coated Fe3O4 nanoparticles dispersed in a solution did not exhibit hysteresis of the dc magnetization curve because the particles could easily rotate with the changing magnetic field. The magnetization curve measured under an ac magnetic field had a large area compared to the dc magnetization curve. The results indicate that Brownian relaxation is dominant during magnetization reversal.