Hiroaki Sukegawa

Direct Synthesis of MOF‐Derived Nanoporous Carbon with Magnetic Co Nanoparticles toward Efficient Water Treatment

Nanoporous carbon particles with magnetic Co nanoparticles (Co/NPC particles) are synthesized by one-step carbonization of zeolitic imidazolate framework-67 (ZIF-67) crystals. After the carbonization, the original ZIF-67 shapes are preserved well. Fine magnetic Co nanoparticles are well dispersed in the nanoporous carbon matrix, with the result that the Co/NPC particles show a strong magnetic response. The obtained nanoporous carbons show a high surface area and well-developed graphitized wall, thereby realizing fast molecular diffusion of methylene blue (MB) molecules with excellent adsorption performance. The Co/NPC possesses an impressive saturation capacity for MB dye compared with the commercial activated carbon. Also, the dispersed magnetic Co nanoparticles facilitate easy magnetic separation.

Synthesis of Prussian Blue Nanoparticles with a Hollow Interior by Controlled Chemical Etching

Hollow particles, an important class of materials with large internal cavities and thin shells, present a wide range of potential applications, such as energy storage, chemical catalysis, photonics, and biomedical carriers. The properties of hollow particles are strongly affected by the compositions and exquisite nanostructures of the shell regions. Many efforts have been made to control these parameters. Recently, hollow particles with nanoporous shells have attracted great interest because the large pore volumes and high surface areas provided by the nanoporous shells show large storage capacity and allow guest species to pass easily into the internal cavity. For instance, metal oxide hollow particles present a superior lithium storage capacity and good cycle performance, 8] which could improve gas sensitivity and catalytic performance 10] in other applications. Inspired by the superiority of nanoporous shells, the creation of microporous crystal shells with outstanding properties is also expected. Porous coordination polymers (PCPs) (or metal–organic frameworks, MOFs) and zeolites are representative microporous crystals. Having adjustable porosity and properties, microporous crystals show great potential in applications such as separation, catalysis, adsorption, and gas storage. 13] To date, however, there have been no reports on the successful synthesis of uniformly nanosized hollow particles with microporous crystalline shells. 15] Very few reports on microporous zeolite and PCP hollow particles have been published, and several problems have been encountered. A major problem is that the synthesized hollow particles are very large and have a broad size distribution. Considering their practical use in various catalysis reaction processes, uniformly sized hollow particles are ideal because they can densely fill reactors or columns. Another problem is that the microporous crystallinity in the shells decreases seriously during the formation process (that is, amorphous regions are formed in the shells), causing a loss of thermal stability, acidic sites, and surface area. Therefore, the development of a general method of synthesizing uniformly nanosized hollow particles with highly crystalline microporous shells is in demand. Herein, we present a facile route to the fabrication of uniform-sized Prussian blue (PB) hollow particles by utilizing a controlled self-etching reaction in the presence of PVP. PB crystals consisting of metal ions coordinated by CN bridges are very typical coordination polymers with a high surface area, showing excellent properties in many applications such as catalysis, sensors, molecular magnets, gas storage, and bioimaging. The critical point in our procedure is the selection of PB mesocrystals as a starting material. PB mesocrystals are formed by the aggregation of PB nanocrystals in an oriented way, thereby showing single-crystal-like behavior. Through small pores (or defects) in the aggregated PB nanocrystals, the etching solution can be diffused into the core of the mesocrystals. We succeeded in the formation of an interior hollow cavity with the retention of the original PB crystallinity. Although a few attempts to prepare PB hollow structures have been reported, the obtained shells were amorphous or poorly crystalline, with large organic impurities. Such hollow particles cannot show high surface areas and good magnetic responses. Two types of PB mesocrystals with different particle sizes were used as starting materials (details regarding the synthetic conditions are given in the Experimental Section). SEM images indicated that the particle-size distributions of the original PB mesocrystals were very narrow and their average diameters were around 110 nm (Figure 1a) and 190 nm (Supporting Information, Figure S1a). From highly magnified SEM images (insets of Figure 1 a; Supporting Information, Figure S1a), very rough surfaces were observed at the edges and corners of the cubes, suggesting that the cube shapes were created by the aggregation of small PB nano[*] Dr. M. Hu, Dr. Y. Nemoto, Prof. Dr. Y. Yamauchi World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) E-mail: yamauchi.yusuke@nims.go.jp

Hard x-ray photoelectron spectroscopy of buried Heusler compounds

This work reports on high energy photoelectron spectroscopy from the valence band of buried Heusler thin films (Co2MnSi and Co2FeAl0.5Si0.5) excited by photons of about 6?keV energy. The measurements were performed on thin films covered by MgO and SiOx with different thicknesses from 1 to 20?nm of the insulating layer and additional AlOx or Ru protective layers. It is shown that the insulating layer does not affect the high energy spectra of the Heusler compound close to the Fermi energy. The high resolution measurements of the valence band close to the Fermi energy indicate a very large electron mean free path of the electrons through the insulating layer. The spectra of the buried thin films agree well with previous measurements from bulk samples. The valence band spectra of the two different Heusler compounds exhibit clear differences in the low lying s bands as well as close to the Fermi energy.

Bulk and interfacial scatterings in current-perpendicular-to-plane giant magnetoresistance with Co2Fe(Al0.5Si0.5) Heusler alloy layers and Ag spacer

We report the transport properties of a current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) device with Co2Fe(Al0.5Si0.5) (CFAS) Heusler alloy ferromagnetic layers and a Ag spacer layer. The CPP-GMR devices showed relatively high ΔRA values and MR ratios up to 17 m Ω μm2 and 80% at 14 K, and 8 m Ω μm2 and 34% at 290 K. The spin diffusion length ∼3 nm and the bulk spin asymmetry ∼0.77 for the CFAS alloy at 14 K were estimated by the Valet–Fert model, indicating a large contribution of the interfacial scattering.

Giant tunneling magnetoresistance up to 330% at room temperature in sputter deposited Co2FeAl/MgO/CoFe magnetic tunnel junctions

Magnetoresistance ratio up to 330% at room temperature (700% at 10 K) has been obtained in a spin-valve-type magnetic tunnel junction (MTJ) consisting of a full-Heusler alloy Co2FeAl electrode and a MgO tunnel barrier fabricated on a single crystal MgO (001) substrate by sputtering method. The output voltage of the MTJ at one-half of the zero-bias value was found to be as high as 425 mV, which is the largest reported to date in MTJs using Heusler alloy electrodes. The present finding suggests that Co2FeAl may be one of the most promising candidates for future spintronics devices applications.

Current-perpendicular-to-plane giant magnetoresistance in spin-valve structures using epitaxial Co2FeAl0.5Si0.5/Ag/Co2FeAl0.5Si0.5 trilayers

A current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve using epitaxial layers of Co2FeAl0.5Si0.5 (CFAS) Heusler alloy as ferromagnetic electrodes is reported. A multilayer stack of Cr/Ag/CFAS/Ag/CFAS/Co75Fe25/Ir22Mn78/Ru was deposited on a MgO (001) single crystal substrate. Epitaxial growth of the Cr, Ag, and CFAS layers in the (001) orientation up to the top CFAS layer was confirmed. Large MR ratios of 6.9% at room temperature and 14% at 6 K were observed for the CPP-GMR device. High spin polarization of epitaxial CFAS is the possible reason for high MR ratios.

Perpendicular magnetization of Co2FeAl full-Heusler alloy films induced by MgO interface

The perpendicular magnetization of Co2FeAl (CFA) full-Heusler alloy films was achieved in the structures of CFA/MgO and MgO/CFA with the perpendicular magnetic anisotropy energy density (KU) of 2–3×106 erg/cm3, which can be used as the perpendicular ferromagnetic electrodes of MgO-based magnetic tunnel junctions (MTJs) with high thermal stability at sub-50-nm dimension. The CFA thickness dependence of KU was investigated at different annealing temperatures, indicating that the perpendicular anisotropy of CFA is contributed by the interfacial anisotropy between CFA and MgO. This letter will open up a way for obtaining perpendicular magnetization of Co-based full-Heusler alloys, which is promising for further reduction in the critical current of current induced magnetization switching in MgO-based MTJ nanopillars with perpendicular full-Heusler alloy electrodes.

Evaluation of Spin Hall Angle and Spin Diffusion Length by Using Spin Current-Induced Ferromagnetic Resonance

We experimentally demonstrate the thickness dependence of the spin current-induced ferromagnetic resonance in Ni80Fe20/nonmagnetic (Pt,Pd) bilayer thin films. The spectrum shape depends on the Ni80Fe20 layer thickness, due to extrinsic excitation in addition to the spin Hall effect. Detailed analysis of the thickness dependence of the spectrum, both for Ni80Fe20 and nonmagnetic layers, provides the spin Hall angle (Pt: 0.022±0.004, Pd: 0.008±0.002) and spin diffusion length (Pt: 1.2±0.06 nm, Pd: 2.0±0.09 nm).

Tunnel magnetoresistance with improved bias voltage dependence in lattice-matched Fe/spinel MgAl2O4/Fe(001) junctions

We fabricated fully epitaxial Fe/MgAl2O4/Fe(001) magnetic tunnel junctions using plasma oxidation of an Mg/Al bilayer. The MgAl2O4 showed a (001)-oriented spinel-type structure, and there were few misfit dislocations at the interfaces between the MgAl2O4 and the two Fe layers due to a small lattice mismatch (∼1%). Tunnel magnetoresistance (TMR) ratios up to 117% (165%) were obtained at room temperature (15 K). The bias voltage for one-half of the zero-bias TMR ratio (Vhalf) was relatively large, ranging from 1.0 to 1.3 V at room temperature, which is attributed to the small misfit dislocation density.

A 4‐Fold‐Symmetry Hexagonal Ruthenium for Magnetic Heterostructures Exhibiting Enhanced Perpendicular Magnetic Anisotropy and Tunnel Magnetoresistance

A 4-fold-symmetry hexagonal Ru emerging in epitaxial MgO/Ru/Co2 FeAl/MgO heterostructures is reported, in which an approximately Ru(022¯3) growth attributes to the lattice matching between MgO, Ru, and Co2 FeAl. Perpendicular magnetic anisotropy of the Co2 FeAl/MgO interface is substantially enhanced. The magnetic tunnel junctions (MTJs) incorporating this structure give rise to the largest tunnel magnetoresistance for perpendicular MTJs using low damping Heusler alloys.