Daiji Hasegawa

Synthesis and magnetic properties of monodisperse magnetite nanocubes

The monodisperse Fe3O4 nanocubes with controllable sizes from 6.5to30.0nm have been synthesized in one pot. The shape-induced texture of the self-assembled nanocube superlattices has been deposited onto substrates by a convective assembly technique. The cubelike shapes affect the crystalline orientation of individual particles within the self-assembled superlattices compared with the nanoparticles with an isotropic spherical shape. The nanocubes with a size less than 25nm show a typical superparamagnetic behavior at room temperature. Comparison of the hysteresis loop at 5K of cubes with different sizes reveals a size-dependent behavior of saturation magnetization and coercivity.

Achieving a noninteracting magnetic nanoparticle system through direct control of interparticle spacing

Monodisperse magnetite (Fe3O4) nanoparticles (NPs) were synthesized and coated using a SiO2 shell with controlled thickness ranging from 3.0 to 20.0 nm. The temperature-dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetizations of the 7.5 nm Fe3O4 NPs with systematically increasing interparticle spacing were studied using the continuous and intermittent cooling protocol. The experimental evidence from dc magnetization and simulated ZFC/FC curves reveal that the increasing interparticle spacing modulated the collective magnetic behavior by effectively lowering the interparticle dipolar coupling, and for 7.5 nm Fe3O4 NPs a noninteracting particle system formed with interparticle spacing above 31.5 nm.

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.

Quantitative evaluation of magnetocrystalline anisotropy of columnar grains and thickness of initial layer in CoCr-based perpendicular media

We propose a quantitative analysis using perpendicular torquemetry to evaluate the thickness of the initial layer which consists of nanocrystalline grains, whose c axes are three-dimensionally randomly oriented, and the intrinsic magnetocrystalline anisotropy of the columnar grains with its c plane parallel to the film plane for CoCr-based perpendicular thin-film media. By applying this analysis to CoNiCrTa, CoCrPtTa, and CoCrPtB media, we found that the initial layer can be completely removed in the CoCrPtB medium epitaxially grown on an hcp-structured Co60Cr40 intermediate layer.

Structural and magnetic properties of monolayer film of CoPt nanoparticles synthesized by polyol process

Co43Pt57 nanoparticle dispersion was synthesized by using the polyol process, and monolayer film of the particles with [3-(2-aminoethylamino)propyl]trimethoxysilane coupling layer on a silicon substrate was fabricated and characterized. After the film was annealed at 873K, the Co43Pt57 nanoparticles transformed from their as-deposited fcc phase to L10 structure. However, the resulting c∕a ratio was only 0.993, indicating incomplete transformation. Grain size of the annealed film was 4.9nm against 4.1nm for the as-deposited state. The annealed film showed the presence of high anisotropic phase with the remanence coercivity of 4.2kOe and anisotropy field over 25kOe at RT.

Direct synthesis of Pt based L10 structured nanoparticles (invited)

Equiatomic FePt and CoPt nanoparticles with the ordered L10 structure are attractive as ultrahigh density magnetic recording media. In a recent work, chemically synthesized fcc-FePt nanoparticles with narrow size distribution and their self-assembled array with close-packed microstructure has been achieved successfully. However, the particles coalesced during the subsequent annealing step necessary to obtain L10 FePt nanoparticles. In the present study, we have successfully demonstrated the direct synthesis of L10 FePt nanoparticles at low temperature of 553K using the “modified polyol method” without subsequent annealing, whose diameter is 5–10nm and intrinsic magnetocrystalline anisotropy field (Hk) is 31kOe. This indicates that precisely controlling the reaction kinetics, especially low reduction rate through optimizing the polyol/Pt mole ratio and type of polyol are very important for directly synthesizing the L10 FePt nanoparticles. Furthermore, we investigated the size, morphology and composition de...

Semi-quantitative evaluation of stacking faults in pseudo-hcp thin films by laboratory-scale in-plane x-ray diffraction

Measurement of the stacking faults (SFs) formed in a pseudo-hexagonal closed packed (pseudo-hcp) film is proposed using a laboratory-scale x-ray diffractometer. The pseudo-hcp structure includes (1?1?1)-oriented face-centred-cubic (fcc), c-plane oriented hcp and their mediate structures with the SFs. Diffractions from the (11.0) and (10.0) planes, DH and DL, were observed in the in-plane x-ray diffraction (XRD) profiles of the pseudo-hcp materials. The structure factor revealed that DH and DL originate from the number of total atomic layers and the imbalance of the number of A, B and C atomic layers, respectively. Therefore, the intensity ratio of DH to DL, corrected by Lorentz and atomic scattering factors (corrected IL/IH), is defined as the degree of SFs for pseudo-hcp materials from the stacking probability, independent of thickness, compositional atoms and lattice constants. Theoretical values of the corrected IL/IH are 0.25 for perfect hcp stacking and 0 for perfect fcc stacking and statistical calculations revealed that this index is very sensitive to the approach of perfect hcp stacking order rather than that of perfect fcc stacking order. By applying this evaluation to experimentally sputtered thin films, it was clarified that: (1) in Pt based alloy films, addition of Cr, Mo and W is found to stabilize ?A?B?A? stacking, (2) the corrected IL/IH for a c-plane oriented Co film is only 0.04, which corresponds to an hcp stacking probability of 0.9 for a pure Co film and (3) in both cases of Co?Ir and Co?Pt alloy films, the atomic-layered structure approaches perfect hcp stacking for additive alloying element contents of 22?25?at%, which is considered to be one of the factors contributing to the large absolute value of uniaxial magnetocrystalline anisotropy.

Facile large-scale synthesis of monodisperse Fe nanoparticles by modest-temperature decomposition of iron carbonyl

A facile synthesis method for monodisperse Fe nanoparticles has been developed by the injection of iron carbonyl into kerosene at a modest temperature, typically 180°C. By controlling the reaction time, the molar ratio of surfactants to Fe(CO)5, and the Fe(CO)5 concentration in kerosene, 3–12nm monodisperse Fe nanoparticles were prepared. The size distribution is very narrow and any size-selective treatment is not required even when several grams of Fe nanoparticles are synthesized in one pot. Magnetic measurements reveal that the saturation magnetization (Ms) of the 9.3nm Fe nanoparticles is always over 150emu∕g at room temperature.

Magnetization Process of h.c.p.-CoIr Nanoparticles With Negative Uniaxial Magnetocrystalline Anisotropy

To increase the superparamagnetic blocking frequency, we propose magnetic nanoparticles with negative uniaxial magnetocrystalline anisotropy, K_u ^grain. Using this material, both high ferromagnetic resonance frequency, f_r, and low magnetic potential height, DeltaE, can be satisfied simultaneously. Thus, f_b can be greatly improved. From the results of DC magnetization process of highly oriented h.c.p. Co₈₃Ir₁₇ nanogranular with negative K_u ^grain, it is verified that the nanoparticles with negative K_u ^grain have superparamagnetic properties only in the c-plane

Facile Synthesis, Phase Transfer, and Magnetic Properties of Monodisperse Magnetite Nanocubes

The monodisperse Fe₃O₄ nanocubes with controllable sizes from 6.5 to 30.0 nm have been synthesized in organic phase. By the formation of surfactant bilayers with charged hydrophilic ions of alpha -cyclodextrin, Fe₃O₄ nanocubes were successfully ldquopulledrdquo into water from hexane for a possible biomedical application. The Fe₃O₄ nanocubes with the surfactant bilayers show a good stability in aqueous solution compared with those in aqueous solution which were transferred from organic phase by ligand exchange using PVP with strong binding group. The hysteresis loop at 300 K of Fe₃O₄ nanocubes with different sizes reveals that a size-dependent behavior of saturation magnetization and the nanocubes with a size less than 25 nm show a typical superparamagnetic behavior at room temperature. The comparison of magnetization of the hydrophilic Fe₃O₄ nanocubes in CD and PVP aqueous solution reveals that the surfactant bilayers can provide a good stability of magnetization.