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Shinpei Yamamoto,
Gallage Ruwan,
Yoshinori Tamada,
Kaori Kohara,
Yoshihiro Kusano,
Tatsuya Sasano,
Kohji Ohno,
Yoshinobu Tsujii,
Hiroshi Kageyama,
Teruo Ono,
Mikio Takano
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ABSTRACT: Nanoparticles of α-Fe, an excellent soft magnet, have been successfully made corrosion-resistant and dispersible in polar and nonpolar solvents by coating these with inner and outer layers of amorphous silica and organics like poly(ethylene glycol), respectively. The double coating was facilitated by using stable and easy-to-handle oxide particles as the core to be subsequently metallized at temperatures low enough to keep the organic layer intact. Use of CaH2 as a reductant lowered the working temperature down to 200−300 °C, where thermal particle adhesion did not take place, formation of impurities like iron silicates was suppressed, and the overall morphological features of the starting particles were preserved. The feasibility of organo-functionalization of the surface will open a way for this nanomagnet toward bioscientific and medical applications.Keywords: α-Fe nanoparticle; CaH2; low-temperature reduction
02/2011;
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ABSTRACT: Recent discoveries of large magnetoresistance in non-magnetic semiconductors have gained much attention because the size of the effect is comparable to, or even larger than, that of magnetoresistance in magnetic systems. Conventional magnetoresistance in doped semiconductors is straightforwardly explained as the effect of the Lorentz force on the carrier motion, but the reported unusually large effects imply that the underlying mechanisms have not yet been fully explored. Here we report that a simple device, based on a lightly doped silicon substrate between two metallic contacts, shows a large positive magnetoresistance of more than 1,000 per cent at room temperature (300 K) and 10,000 per cent at 25 K, for magnetic fields between 0 and 3 T. A high electric field is applied to the device, so that conduction is space-charge limited. For substrates with a charge carrier density below approximately 10(13) cm(-3), the magnetoresistance exhibits a linear dependence on the magnetic field between 3 and 9 T. We propose that the observed large magnetoresistance can be explained by quasi-neutrality breaking of the space-charge effect, where insufficient charge is present to compensate the electrons injected into the device. This introduces an electric field inhomogeneity, analogous to the situation in other semiconductors in which a large, non-saturating magnetoresistance was observed. In this regime, the motions of electrons become correlated, and thus become dependent on magnetic field. Although large positive magnetoresistance at room temperature has been achieved in metal-semiconductor hybrid devices, we have now realized it in a simpler structure and in a way different from other known magnetoresistive effects. It could be used to develop new magnetic devices from silicon, which may further advance silicon technology.
Nature 03/2009; 457(7233):1112-5. · 36.28 Impact Factor
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ABSTRACT: We investigated structural and magnetic properties of the easy-axis aligned L10-FePt nanoparticles by the combined use of x-ray diffraction (XRD), magnetization, and 57Fe Mössbauer measurements. The L10-FePt nanoparticles were fixed in a polystyrene matrix by performing free radical polymerization of styrene under an aligning external magnetic field. Mössbauer spectrum of the L10-FePt nanoparticles/polystyrene composite showed tremendous decrease in the second and fifth absorption lines under the condition that the incident γ ray was parallel to the aligning field. This result indicates that the easy axes of the L10-FePt nanoparticles in the composite have a strong preferred orientation with a finite distribution. We estimated the distribution of easy-axis orientation by using the Mössbauer hyperfine parameters, which is in good agreement with that determined by the XRD rocking curve.
Phys. Rev. B. 12/2008; 78(21).
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ABSTRACT: We report observation of a very large magnetoresistance (MR) effect in the space‐charge regime in lightly doped silicon with the Schottky contacts. It is found that the device resistance increases more than 30,000% when the magnetic field is changed from 0 to 1 T at 25 K. We propose that charge correlation in magnetic field in the space‐charge regime in the Schottky barrier enhances the MR effect.
PHYSICS OF SEMICONDUCTORS: 29th International Conference on the Physics of Semiconductors; 07/2008
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ABSTRACT: We studied the dynamics of photoluminescence (PL) and energy transfer in close-packed monolayer films of CdSe and Au nanoparticles (NPs) assembled using the Langmuir-Blodgett technique. The PL intensity and dynamics depended on the ratio of CdSe to Au NPs in the mixed films. The PL quenching of CdSe NPs occurs through rapid energy transfer from excitons in CdSe NPs to plasmons in Au NPs. The PL decay curves of the mixed NPs monolayers are determined by three decay rates: the direct energy transfer between the nearest-neighbor CdSe and Au NPs (CdSe-->Au), the stepwise energy transfer from CdSe to CdSe to Au NPs (CdSe-->CdSe-->Au), and the radiative recombination in CdSe NPs.
Physical Review Letters 05/2008; 100(20):207404. · 7.37 Impact Factor
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07/2007;
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ABSTRACT: It was found that the well-ordered L1<sub>0</sub> structure is formed in the FePt nanoparticles synthesized by the improved “ Si O <sub>2</sub> -nanoreactor” method, whereas the previously employed annealing condition has suffered from the presence of the unconverted fcc-FePt nanoparticles which are superparamagnetic at room temperature. The L1<sub>0</sub>- Fe Pt nanoparticles prepared by this method showed a smooth hysteresis loop with no kink, and the room temperature coercivity reaches an extremely large value of 28 kOe , even though the particle size is 6.7 nm in diameter. The Mössbauer hyperfine parameters of the nanoparticles are very close to those of the bulk L1<sub>0</sub>- Fe Pt alloy, indicating that they possess magnetic moments comparable to the bulk even at the particle surface.
Applied Physics Letters 05/2007; · 3.84 Impact Factor
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ABSTRACT: A method to prepare monodisperse and highly coercive L10-FePt nanoparticles which are dispersible in nonpolar organic solvents such as toluene, chloroform, and hexane was developed. By vigorously stirring the SiO2-coated L10-FePt nanoparticles synthesized by the SiO2-nanoreactor method (Appl. Phys. Lett. 2005, 87, 032503) in a mixture of an aqueous NaOH solution, chloroform, and hexadecyltrimethlyammonium bromide, the SiO2 coating was dissolved off and bare FePt nanoparticles could be extracted to the chloroform phase without degrading their magnetic properties. These particles showed a tendency to form a close packed lattice on slow drying of the chloroform-based solution. The present success may promote the practical application to ultra-high density magnetic recording and also may open the door to providing these particles with new physical and/or chemical functions.
08/2006;
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ABSTRACT: We investigated magnetic properties of the L10-FePt nanoparticles synthesized by the SiO2-nanoreactor method by means of Moessbauer spectroscopy from the microscopic point of view. Almost all of the nanoparticles were revealed to have nearly the same Moessbauer hyperfine parameters as those of the bulk L10-FePt alloy, indicating that they have well-defined L10 structure equivalent to the bulk state in spite of their small size of 6.5 nm.
08/2006;
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ABSTRACT: We report a successful synthesis of monodisperse L10-FePt nanocrystals which are not only superior in magnetism but also easy to handle through being dispersible in solvents. Thanks to a thick SiO2-coating, the thermal treatment needed to form the L10 structure can be done even at 900 oC without inducing coalescence and coarsening. The protecting shell was thereafter removed in a specific way that enabled us to recover the L10-FePt nanocrystals in a water-dispersion. The SiO2-coated nanocrystals show a high coercivity of 18.5 kOe at room temperature in spite of their core size of only 6.5 nm in diameter, and the hysteresis loop of the water-dispersed nanocrystals, which were frozen under an external field, was almost rectangular indicating that their magnetic and structural orientation could be attained. Comment: 14 pages, 5 figures
04/2005;
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ABSTRACT: The phonon states of iron atoms in the easy-axis aligned L1_{0}-FePt nanoparticles were investigated using 57Fe nuclear-resonant inelastic scattering. It was revealed that the phonon density of states (PDOS) in the L1_{0}-FePt has a pronounced anisotropy. To study the results from a theoretical point of view, we calculated the PDOS of the L1_{0}-FePt in the bulk state by means of the first-principles method. The results were in good agreement with the experimental results.
