Tetsuya Sato

Deposition of Diamond-like Carbon Films by Pulsed-Laser Evaporation

Diamond-like carbon thin films were deposited by pulsed-laser evaporation. A carbon target was irradiated by a Xe-Cl laser with a power density of 3×108 W/cm2. Ions were mixed with vaporized atoms. Deposition rates were typically 200 A/ min. Film properties changed with substrate temperature. The films deposited at 50°C were diamond-like, as confirmed by refractive index (2.1 to 2.2), optical transparenty and chemical resistance. Hydrogen-free films were produced. Optical band gap was 1.4 eV and electrical resistivity was 108 Ω-cm. No crystallinity was observed.

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.

Spin Pumping without Three-Magnon Splitting in Polycrystalline Bi1Y2Fe5O12/Pt Bilayer Structure

Spin pumping at various precession frequencies has been investigated in a polycrystalline Bi-substituted yttrium iron garnet (Bi:YIG)/Pt bilayer structure by means of inverse spin Hall effect (ISHE) with comparison to a single crystal La-substituted yttrium iron garnet (La:YIG)/Pt bilayer structure which shows three-magnon splitting. As precession frequency decreases, the magnitude of pumped spin currents in the polycrystalline sample continuously increases to high magnitude comparable to the highest magnitude in the single crystal sample while that in the single crystal sample decreases below a threshold frequency of three-magnon splitting. By comparing ISHE voltage with microwave absorption intensity in the polycrystalline sample, we verify that the large magnitude of the pumped spin currents is maintained at low frequencies through the suppression of three-magnon splitting.

Anomalous anisotropy of re-entrant film

Re-entrant polycrystalline films, 600 A thick, grown on a quartz substrate by using an electron beam evaporation technique, have been investigated. A spin-wave resonance (SWR) technique has been used to study the magnetic properties of the films. The SWR spectra exhibit highly anisotropic behaviour with respect to the film normal. When the sample is rotated around the film normal in a fixed applied field, both the resonance fields and the line-shapes are changed. This unexpected behaviour - for a polycrystalline film - has been attributed to a growth-induced geometric (oblique) anisotropy. The SWR spectra have been successfully analysed by using this oblique anisotropy term in addition to usual magnetocrystalline effective bulk, surface, and unidirectional anisotropy terms in the magnetic free energy. A substantial and temperature-dependent growth-induced geometric anisotropy has been obtained, beside the usual bulk and surface anisotropies. A strong correlation between the surface and oblique anisotropy has been found. This correlation has been attributed to a manifestation of the influence of easy-plane surface anisotropy along the fibre axes of individual grains on the effective magnetic anisotropy energy.

Intraparticle structure in ultra-fine ZnFe2O4 particles

Abstract We have performed small-angle polarized-neutron scattering from ultra-fine ZnFe 2 O 4 particles to confirm the intraparticle magnetic structure, which has been considered the origin of its peculiar magnetic features. On the basis of the Q -dependent differential neutron-scattering intensity, we were unable to detect any magnetic structure consisting of magnetically inactive surface layer with magnetic core. Thus, the magnetic behavior of ultra-fine ZnFe 2 O 4 particles is explained in terms of a deviation in cation distribution from the normal spinel structure.

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.

Ferromagnetism of Pd fine particles

Abstract The field-dependent magnetization consists of the saturated ferromagnetic component and the paramagnetic one which increases linearly with increasing field. The saturation magnetization decreased steeply with increasing temperature below 80 K. The temperature dependence of the high-field susceptibility resembles to that of the Pd bulk. These results support the picture that a Pd particle consists of the ferromagnetic surface and nonmagnetic core.

Size dependent magnetization of PdFe fine particles

Abstract We present the results of magnetic measurements of Pd—0.4 at% Fe fine particle, suggesting that the particle consists of the nonmagnetic surface and the magnetic core. The thickness of the surface layer is estimated to be 33 A based on the core—shell model. Further, it is also conjectured that the core is magnetically enhanced compared to the bulk.

Effects of finite size on spin glass dynamics

In spite of comprehensive studies to clarify a variety of interesting phenomena of spin glasses, their understanding has been insufficiently established. To overcome such a problem, fabrication of a mesoscopic spin glass system, whose dynamics can be observed over the entire range to the equilibrium, is useful. In this review the challenges of research that has been performed up to now in this direction and our recent related studies are introduced. We have established to study the spin glass behaviour in terms of droplet picture using nanofabricated mesoscopic samples to some extent, but some problems that should be clarified have been left. Finally, the direction of some new studies is proposed to solve the problems.

Band Gap Narrowing and Electron Doping by Potassium Encapsulation into Single-Walled Carbon Nanotubes

We have fabricated field-effect transistors (FETs) with pristine and potassium-encapsulated single-walled carbon nanotube (SWNT) films, and the effects of potassium encapsulation are investigated. The transformation from a unipolar characteristic to an ambipolar characteristic by potassium encapsulation is observed from the measurement of the gate voltage dependence (Vgs) of the current (I) for SWNT-film FETs. This result indicates that the potassium encapsulation into SWNTs causes band gap narrowing. In addition, the n-type region of the I–Vgs curve is expanded during annealing of the devices; electron transfer from potassium to SWNTs occurs owing to the removal of the adsorbates. The adsorbate removal is confirmed by photoemission spectroscopy measurement. The FET with an individual potassium-encapsulated SWNT shows an ambipolar characteristic.