Hiroaki Kura

Size control and characterization of wustite (core)/spinel (shell) nanocubes obtained by decomposition of iron oleate complex

Monodisperse wustite (core)/spinel (shell) nanocubes with controllable size from 9 to 22 nm were synthesized by the decomposition of iron oleate complex at high temperature. The composition of the nanocubes was confirmed by X-ray diffraction and magnetic analysis, meanwhile the distributions of wustite and spinel phases within the nanocubes were directly observed by high resolution transmission electron microscopy using the dark-field image technique. The core/shell structure is quite unique, in which spinel phase is distributed not only preferentially on the surface, but also in the interior, while almost all of the wustite phase is located in the core of the nanocubes. The formation of wustite is inherent in the decomposition of the iron oleate complex, as indirectly inferred through the detection of a huge quantity of carbon monoxide generated from the reactor.

Size dependence of martensite transformation temperature in ferromagnetic shape memory alloy FePd

Martensite transformation temperature of ferromagnetic shape memory alloy FePd was studied in the shape of nanoparticle and the polycrystalline samples with grain size in micrometers based on the x-ray diffraction and magnetic measurement as a function of sample size. Both the forward transformation start temperature Ms and reverse transformation finish temperature Af of polycrystalline sample monotonically decreased with decreasing grain size and were not observed in the nanoparticles. The size dependence of transformation temperature is explained based on the change in transformation mode, i.e., the decrease in sample size induces the change from the multivariant mode to single variant mode in which the strain energy is large. In the small sample, the strain energy becomes large, and thus the large driving force is required for the transformation. As a result, the large amount of undercooling occurs, which leads to the lowering of Ms. The measurement of heat capacity indicates that the strain energy is ...

Facile synthesis of Fe3O4 nanoparticles by reduction phase transformation from γ-Fe2O3 nanoparticles in organic solvent

A phase transformation induced by the reduction of as-synthesized gamma-maghemite (gamma-Fe(2)O(3)) nanoparticles was performed in solution by exploiting the reservoir of reduction gas (CO) generated from the incomplete combustion reaction of organic substances in the reactor. Results from X-ray diffraction, color indicator, and magnetic analysis using a SQUID strongly support this phase transformation. Based on this route, monodisperse magnetite (Fe(3)O(4)) nanoparticles were simply produced in the range from 260 to 300 degrees C. Almost all aspects of the original gamma-Fe(2)O(3) nanoparticles, such as shape, size, and monodispersity, were maintained in the produced Fe(3)O(4) nanoparticles.

Synthesis and growth mechanism of long ultrafine gold nanowires with uniform diameter

Homogeneous Au nanowires with 1.5 nm diameters and lengths of over 100 μm were synthesized in an oleylamine matrix via the simple reduction of aurichloride in a limited reaction temperature range around 85 °C. Oleylamine has multifunctional roles as solvent, surfactant, and reductant, and the surfactant induce anisotropic growth by adsorbing on the specific Au crystalline surface. As a result, Au nanowires were grown along the ⟨111⟩ direction of fcc-Au having many hcp atomic stacks. In this synthesis method, various shapes of Au nanostructures were produced simultaneously and this was strongly dependent on the reaction temperature. Au nanowires were provided by reconstruction from nanoparticles or their agglomeration. The growth mechanism of the Au nanowire in this synthesis was found to be quite unique and different from that for a conventional one-dimensional nanostructure which is obtained by anisotropical growth with supplying atoms from external resources.

Extreme enhancement of blocking temperature by strong magnetic dipoles interaction of α-Fe nanoparticle-based high-density agglomerate

High-volume fraction ?-Fe nanoparticle (NP) agglomerates were prepared using chemically synthesized NPs. In the agglomerate, NPs are separated by surfactant and NP superlattice with a hexagonal close-packed structure is locally realized. Volume fractions of NPs at 20% and 42% were obtained in agglomerates consisting of 2.9?nm and 8.2?nm diameter NPs, respectively. The high saturation magnetization of ?-Fe NPs and high volume fraction of NPs in the agglomerate provide strong magnetic dipole?dipole interaction. The interaction energy of the agglomerate became much larger than the anisotropic energy of individual NPs. As a result, the blocking temperature of the 8.2?nm NP agglomerate was significantly enhanced from 52.2?K to around 500?K.

Transformation to L10 structure in FePd nanoparticles synthesized by modified polyol process

Abstract FePd nanoparticles were prepared by the modified polyol process, i.e. simultaneous reduction of palladium acetylacetonate and thermal decomposition of iron pentacarbonyl in a solvent. The well-dispersed FePd particles, with the average particle size of ~8.4 nm, were obtained using diphenyl ether as a solvent. The structure of FePd nanoparticles transformed to L10 structure from face centered cubic (fcc) structure after annealing at 600 °C for 1 h in vacuum. The coercivity of the ordered FePd nanoparticles increased to ~2 kOe at 5 K from ~0.6 kOe in the fcc as-made FePd nanoparticle. On the other hand, FePd nanoparticles, synthesized based on the conventional modified polyol process using dioctyl ether as a solvent, showed no structural transformation after annealing up to 700 °C. This is explained in terms of the interruption of thermal diffusion of Fe and Pd atoms during the thermal treatment due to the mixture of interstitial carbon impurity in the synthesis process using dioctyl ether as a solvent.

Spin-glasslike behavior of magnetic ordered state originating from strong interparticle magnetostatic interaction in α-Fe nanoparticle agglomerate

In strongly interacting nanoparticle agglomerate through dipolar coupling, prepared using chemically synthesized α-Fe nanoparticle, the temperature Tpeak at which the zero-field-cooled magnetization exhibits its peak reaches 385 K. This indicates that the magnetic ordering appears above room temperature. Aging phenomena and memory effect observed below Tpeak are intrinsically same as that of spin glasses. The appearance of spin-glasslike behavior near room temperature should be attributed to the three-dimensional interparticle coupling that is purely magnetostatic.

Synthesis of L10-(FeyPt100−y)100−xCux nanoparticles with high coercivity by annealing at 400°C

(FeyPt100−y)100−xCux nanoparticles with various compositions were synthesized by the simultaneous polyol reduction of Pt(acac)2 and Cu(acac)2 and the thermal decomposition of Fe(CO)5. The as-made particles had a disordered fcc structure and an average diameter of ∼4.8nm. The result of x-ray diffraction indicated a transformation from fcc structure to fct structure in some samples after annealing at 400°C for 1h, and the c∕a ratio of lattice constant decreased with increasing Cu content. The doping of Cu clearly plays a significant role on the reduction of the ordering temperature in the ternary (FeyPt100−y)100−xCux alloy. In addition, the ordering of (FeyPt100−y)100−xCux nanoparticles is significantly dependent on the composition of Fe and Pt. As a result, (Fe47Pt53)74Cu26 nanoparticles have the coercive fields of ∼10 and ∼2kOe at 10K and room temperature, respectively.

Manipulation of Faraday rotation in Bi-substituted yttrium-iron garnet film using electromagnetic interaction between Au nanoparticles in two-dimensional array

The Faraday rotation in Bi-substituted yttrium-iron garnet thin films, in which an artificially fabricated Au nanoparticle array is embedded, is studied as a function of lattice spacing compared with the extinction spectra. With decreasing lattice spacing in the Au array, the wavelengths corresponding to the enhanced Faraday rotation and the extinction peak showed blueshifts in the same manner. This indicates that Faraday rotation can be manipulated by means of the wavelength shift of localized surface plasmon resonance that originates from the change in electromagnetic interaction between Au nanoparticles.

Room temperature ferromagnetism in diluted magnetic semiconductor Zn1−xCrxTe nanoparticles synthesized by chemical method

Nanoparticles of diluted magnetic semiconductor Zn1−xCrxTe with various Cr concentrations were synthesized by a chemical method, and their magnetic properties were investigated. The synthesized nanoparticles show ferromagnetism, and the saturation magnetization increased with increasing Cr concentration in the range from x=0.002tox=0.02. The Zn0.98Cr0.02Te nanoparticles showed ferromagnetism above room temperature, where the Curie temperature TC was estimated to be about 520K. This is much higher than the reported value of TC in the Zn1−xCrxTe thin films.