Masahiko Nishijima

Evidence for fractional crystallization of wadsleyite and ringwoodite from olivine melts in chondrules entrained in shock-melt veins.

Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pressure polymorphs of olivine, ringwoodite and wadsleyite, in diverse textures and settings in fragments entrained in shock-melt veins. Among these settings are complete olivine porphyritic chondrules. We encountered few squeezed and flattened olivine porphyritic chondrules entrained in shock-melt veins of this meteorite with novel textures and composition. The former chemically unzoned (Fa24–26) olivine porphyritic crystals are heavily flattened and display a concentric intergrowth with Mg-rich wadsleyite of a very narrow compositional range (Fa6–Fa10) in the core. Wadsleyite core is surrounded by a Mg-poor and chemically stark zoned ringwoodite (Fa28–Fa38) belt. The wadsleyite–ringwoodite interface denotes a compositional gap of up to 32 mol % fayalite. A transmission electron microscopy study of focused ion beam slices in both regions indicates that the wadsleyite core and ringwoodite belt consist of granoblastic-like intergrowth of polygonal crystallites of both ringwoodite and wadsleyite, with wadsleyite crystallites dominating in the core and ringwoodite crystallites dominating in the belt. Texture and compositions of both high-pressure polymorphs are strongly suggestive of formation by a fractional crystallization of the olivine melt of a narrow composition (Fa24–26), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa24–26). Our findings could erase the possibility of the resulting unrealistic time scales of the high-pressure regime reported recently from other shocked L-6 chondrites.

Fe-Mg partitioning between perovskite and ferropericlase in the lower mantle

Abstract Fe-Mg partitioning between perovskite and ferropericlase in the MgO-FeO-SiO2 system has been studied up to about 100 GPa at around 2000 K using a laser-heated diamond anvil cell (LHDAC). The compositions of both phases were determined by using analytical transmission electron microscopy (ATEM) on the recovered samples. Present results reveal that the Fe-Mg apparent partition coefficient between perovskite and ferropericlase [KDPv/Fp = (XFePv XMgFp)/(XMgPv XFeFp)] decreases with increasing pressure for a constant FeO of the system, and it decreases with increasing FeO content of ferropericlase. The gradual decrease of KDPv/Fp with increasing pressure is consistent with the spin transition in ferropericlase occurring in the broad pressure range from 50 to 100 GPa at around 2000 K.

Magnetic properties of 120-mm wide ribbons of high Bs and low core-loss NANOMET® alloy

A 120-mm wide amorphous ribbon of a Fe-Co-Si-B-P-Cu NANOMET® alloy has been successfully produced by a single roll melt spinning technique. The optimally annealed samples exhibited low coercivity (Hc) of 5–7 A/m and high saturation magnetic flux density (Bs) of 1.83 T. The plots of Hc and Bs vs. annealing temperature (Ta) revealed basin-like and plateau-like characteristics, respectively, indicating the good annealing controllability for nanocrystallization and for obtaining soft-magnetic properties with high Bs. The excellent magnetic softness was attributed to the nanocrystalline structure composed of homogeneously dispersed α-Fe grains (with a size of 15–20 nm in diameter) emerged from the amorphous structure after optimum annealing. The nanocrystalline ribbons also exhibited low core-losses (W at 50 Hz) of 0.37 and 0.64 W/kg under maximum flux density of 1.5 T and 1.7 T, respectively. The magnetic properties were comparable with those of laboratory-scale small-width ribbons and confirmed to be indepen...

Memory characteristics of metal-oxide-semiconductor capacitor with high density cobalt nanodots floating gate and HfO2 blocking dielectric

In this letter, cobalt nanodots (Co-NDs) had been formed via a self-assembled nanodot deposition. High resolution transmission electron microscopy and x-ray photoelectron spectroscopy analyses clearly show that the high metallic Co-ND is crystallized with small size of ∼2 nm and high density of (4–5)×1012/cm2. The metal-oxide-semiconductor device with high density Co-NDs floating gate and high-k HfO2 blocking dielectric exhibits a wide range memory window (0–12 V) due to the charge trapping into and distrapping from Co-NDs. After 10 years retention, a large memory window of ∼1.3 V with a low charge loss of ∼47% was extrapolated. The relative longer data retention demonstrates the advantage of Co-NDs for nonvolatile memory application.

Effect of Ion Implantation on DLC Preparation Using PBIID Process

This paper discusses the effects of ion implantation on a diamond-like carbon (DLC) preparation using a hybrid process of plasma-based ion implantation and deposition (PBIID) using superimposed RF and negative high-voltage pulses. Adhesion strength of a DLC film on A-5052 and SUS304 was enhanced by carbon ion implantation to substrate materials. Cross section of interface between the DLC film and substrate was observed by scanning transmission electron microscopy (STEM) and analyzed by energy dispersive X-ray spectroscopy (EDS). It was found that the amorphouslike mixing layer of graded carbon component and substrate materials was produced in the ion-implanted region of substrate and the oxide layer on the substrate surface was destroyed. Besides the reduction of residual stress in the DLC film, the formation of amorphouslike mixing layer and the destruction of oxide layer led to the enhancement in adhesion strength of the DLC film. Residual stress, sp3 fraction, hardness, density, and hydrogen content of the DLC films deposited from acetylene and toluene plasma have the variation with negative pulsed voltage for ion implantation

Formation of high density tungsten nanodots embedded in silicon nitride for nonvolatile memory application

In this letter, the formation of high density tungsten nanodots (W-NDs) embedded in silicon nitride via a self-assembled nanodot deposition is demonstrated. In this method, tungsten and silicon nitride are cosputtered in high vacuum rf sputtering equipment. The W-NDs with small diameters (1–1.5 nm) and high density (∼1.3×1013/cm2) were achieved easily by controlling W composition; this is the ratio of total area of W chips to that of silicon nitride target. The metal-oxide-semiconductor memory device was fabricated with high density W-NDs floating gate and high-k HfO2 blocking dielectric. A wide range memory window (0–29 V) was obtained after bidirectional gate voltages sweeping with range of ±1–±23 V. It is feasible to design the memory window with propriety power consumption for nonvolatile memory application.

Memory characteristics of self-assembled tungsten nanodots dispersed in silicon nitride

In this letter, tungsten nanodots (W-NDs) in silicon nitride formed by a self-assembled nanodot deposition method have been investigated as a floating gate of nonvolatile memory (NVM). Observations from transmission electron microscopy and x-ray diffraction pattern clearly confirm the formation of crystallized W-NDs with a diameter of ∼5 nm. The metal-oxide-semiconductor device with W-NDs in silicon nitride exhibits a larger memory window (∼4.1 V at ±12 V sweep), indicating charge trapping and distrapping between the W-ND and a silicon substrate. The program/erase behaviors and data retention characteristics were evaluated. After 10 years retention, a large memory window of ∼3.4 V with a low charge loss of ∼15% was extrapolated. These results demonstrate advantages of W-NDs in silicon nitride for the NVM application.

Phase transition from fcc to bcc structure of the Cu-clusters during nanocrystallization of Fe85.2Si1B9P4Cu0.8 soft magnetic alloy

A role of Cu on the nanocrystallization of an Fe85.2Si1B9P4Cu0.8 alloy was investigated by X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM). The Cu K-edge XAFS results show that local structure around Cu is disordered for the as-quenched sample whereas it changes to fcc-like structure at 613 K. The fcc Cu-clusters are, however, thermodynamically unstable and begin to transform into bcc structure at 638 K. An explicit bcc structure is observed for the sample annealed at 693 K for 600 s in which TEM observation shows that precipitated bcc-Fe crystallites with ∼12 nm are homogeneously distributed. The bcc structure of the Cu-clusters transforms into the fcc-type again at 973 K, which can be explained by the TEM observations; Cu segregates at grain boundaries between bcc-Fe crystallites and Fe3(B,P) compounds. Combining the XAFS results with the TEM observations, the structure transition of the Cu-clusters from fcc to bcc is highly correlated with the preliminary precipitation...

Role of P in Nanocrystallization of

The role of P in the nanocrystallization of a newly developed Fe-based Fe₈₅Si₂B₈P₄Cu₁ soft magnet (NANOMET) is investigated by differential scanning calorimetry (DSC), high-energy X-ray diffraction (XRD), and transmission electron microscope. DSC results show that addition of P into Fe₈₅Si₂B₈P₄Cu₁ retards growth of α-Fe precipitates and also hinders the onset of crystallization of the residual amorphous phase. High-energy XRD results indicate that a lattice parameter of the precipitated α-Fe is 2.86652 Å, which is very close to that of pure bcc Fe. A small amount of P and/or Si, however, are resolved in nanocrystalline Fe, which causes Curie temperature (TC) of the precipitated α-Fe to decrease by ΔT_C = 31 K lower than the value of pure bcc Fe. With increase in the volume fraction of the precipitated α-Fe, the concentration of residual amorphous becomes close to that of the most stable amorphous alloy, i.e., Fe₇₆Si₉B₁₀P₅ when the volume fraction approaches to 40%. P in NANOMET contributes to hinder the formation of Fe-B(Si,P) compounds by stabilizing residual amorphous phase.

{\rm Fe}_{85}{\rm Si}_{2}{\rm B}_{8}{\rm P}_{4}{\rm Cu}_{1}

Microstructure of a nanocrystalline soft magnetic Fe85Si2B8P4Cu1 alloy (NANOMET®) was investigated by the state of the art spherical aberration-corrected TEM/STEM. Observation by TEM shows that the microstructure of NANOMET® heat treated at 738 K for 600 s which exhibits the optimum soft magnetic properties has homogeneously distributed bcc-Fe nanocrystallites with the average grain size of 30 nm embedded in an amorphous matrix. Elemental mappings indicate that P is excluded from bcc-Fe grains and enriched outside the grains, which causes to retard the grain growth of bcc-Fe crystallites. The aberration-corrected STEM-EDS analysis with the ultrafine electron probe successfully proved that Cu atoms form nanometre scale clusters inside and/or outside the bcc-Fe nanocrystallites.