河智 史朗

We developed a resistance measurement using radio frequency reflection to investigate the electrical transport characteristics under destructive pulsed magnetic fields above 100 T. A sample stage consisting of a homemade flexible printed circuit reduced the noise caused by the induced voltage from the pulsed magnetic fields, improving the accuracy of the measurements of the reflected waves. From the obtained reflectance data, the absolute value of the magnetoresistance was successfully determined by analyzing the phase with admittance charts. These developments enable more accurate and comprehensive measurements of electrical resistance in pulsed magnetic fields.

L1(0)-ordered ferromagnetic nanowires with large coercivity are essential for realizing next-generation spintronic devices. Ferromagnetic nanowires have been commonly fabricated by first L1(0)-ordering of initially disordered ferromagnetic films by annealing and then etching them into nanowire structures using lithography. If the L1(0)-ordered nanowires can be fabricated using only lithography and subsequent annealing, the etching process can be omitted, which leads to an improvement in the fabrication process for spintronic devices. However, when nanowires are subjected to annealing, they easily transform into droplets, which is well-known as Plateau-Rayleigh instability. Here, we propose a concept of "nanostructure-induced L1(0)-ordering" of twinned single-crystals in CoPt ferromagnetic nanowires with a 30 nm scale ultrafine linewidth on Si/SiO2 substrates. The driving forces for nanostructure-induced L1(0)-ordering during annealing are atomic surface diffusion and extremely large internal stress at ultrasmall 10 nm scale curvature radii of the nanowires. (Co/Pt)(6) multilayer nanowires are fabricated by a lift-off process combining electron-beam lithography and electron-beam evaporation, followed by annealing. Cross-sectional scanning transmission electron microscope images and nano-beam electron diffraction patterns clearly indicate nanostructure-induced L1(0)-ordering of twinned single-crystals in the CoPt ferromagnetic nanowires, which exhibit a large coercivity of 10 kOe for perpendicular, longitudinal, and transversal directions of the nanowires. Two-dimensional grazing incidence X-ray diffraction shows superlattice peaks with Debye-Scherrer ring shapes, which also supports the nanostructure-induced L1(0)-ordering. The fabrication method for nanostructure-induced L1(0)-ordered CoPt ferromagnetic nanowires with twinned single-crystals on Si/SiO2 substrates would be significant for future silicon-technology-compatible spintronic applications.

L1(0)-ordered CoPt with a large coercivity (H (c)) of 13 kOe was demonstrated on Si/SiO2 substrates by hydrogen annealing. Equiatomic 11.2 nm thick (Co/Pt)(4) multilayer thin films were fabricated by electron-beam evaporation and were annealed at 500 degrees C-900 degrees C for 10-90 min under an Ar/H-2 mixed gas atmosphere. The annealing temperature and time dependences of the crystal structures, magnetic properties, and surface morphologies of the films were systematically analyzed based on the experimental results obtained from grazing incidence X-ray diffraction (GI-XRD), vibrating sample magnetometer, and scanning electron microscope, respectively. Hydrogen annealing effectively promoted the out-of-plane c-axis orientation of L1(0)-ordered CoPt compared to the vacuum annealing according to the GI-XRD patterns. A maximum H (c) of 13.3 kOe was obtained in L1(0)-ordered CoPt with angular-outlined isolated grains by hydrogen annealing at 800 degrees C for 60 min, where the c-axis of L1(0)-ordered CoPt was randomly distributed.

We report the origins of the coloration of Bi2O3-SiO2, Bi2O3-B2O3, and Bi2O3-P2O5 glasses. The glasses were quantitatively analyzed to reveal the Bi valence state and network connectivity of silicate, borate, and phosphate. The electronic structure was correlated with Bi valence states, giving two types of signals: those related to optical bandgaps with Bi3+ s-p intra-transition states at 3.6-4.7 eV and visible absorption bands at 2.5-2.7 eV originating from the charge transfer between Bi3+ and Bi5+ and/or plasmonic Bi-0 clusters in the colored glasses. These Bi oxide glasses with tvisible transparent and high refractive index are attractive to replace toxic Pb-containing optical glasses.

Material properties depend largely on the dimensionality of the crystal structures and the associated electronic structures. If the crystal-structure dimensionality can be switched reversibly in the same material, then a drastic property change may be controllable. Here, we propose a design route for a direct three-dimensional (3D) to 2D structural phase transition, demonstrating an example in (Pb1-xSnx)Se alloy system, where Pb2+ and Sn2+ have similar ns(2) pseudo-closed shell configurations, but the former stabilizes the 3D rock-salt-type structure while the latter a 2D layered structure. However, this system has no direct phase boundary between these crystal structures under thermal equilibrium. We succeeded in inducing the direct 3D-2D structural phase transition in (Pb1-xSnx)Se alloy epitaxial films by using a nonequilibrium growth technique. Reversible giant electronic property change was attained at x similar to 0.5 originating in the abrupt band structure switch from gapless Dirac-like state to semi-conducting state.

We report on the development of a capacitance measuring system that allows measurements of capacitance in pulsed magnetic fields up to 61 T. By using this system, magnetic-field responses of various physical quantities, such as magnetostriction, magnetic-field-induced change in complex dielectric constant, and magneto-caloric effect, can be investigated in pulsed-magnetic-field conditions. Here, we examine the validity of our system for investigations of these magnetic-field-induced phenomena in pulse magnets. For the magnetostriction measurement, magnetostriction of a specimen can be measured through a change in the capacitance between two aligned electrodes glued on the specimen and a dilatometer. We demonstrate a precise detection of valley polarization in semimetallic bismuth through a magnetostriction signal with a resolution better than 10(-6) of the relative length change. For the magnetic-field-induced change in complex dielectric constant, we successfully observed clear dielectric anomalies accompanied by magnetic/magnetoelectric phase transitions in multiferroic Pb(TiO)Cu4(PO4)4. For the measurement of magneto-caloric effect, a magnetic-field-induced change in sample temperature was verified for Gd3Ga5O12 with a capacitance thermometer made of a non-magnetic ferroelectric compound KTa1-xNbxO3 (x = 0.02) whose capacitance is nearly field-independent. These results show that our capacitance measuring system is a promising tool to study various magnetic-field-induced phenomena, which have been difficult to detect in pulsed magnetic fields.

L1(2)-ordered CoPt3 and L1(0)-ordered CoPt are formed in electron-beam-deposited (Co/Pt)(4) multilayer thin films with and without a Ti underlayer, respectively, on Si/SiO2 substrates by rapid thermal annealing. The crystal structures, magnetic properties, and surface morphologies of the films are investigated by grazing incidence X-ray diffraction (GI-XRD), vibrating sample magnetometer (VSM), and scanning electron microscope (SEM), respectively. In the film without a Ti underlayer, L1(0)-ordered CoPt with an isolated round grain structure is confirmed, showing an in-plane coercivity of 2.7 kOe. In contrast, in the film with a Ti underlayer, L1(2)-ordered CoPt3 is confirmed together with Co-rich A1-disordered CoPt, showing an in-plane coercivity of 500 Oe, which exhibits an angular-outlined continuous film structure. The three sets of experimental results from GI-XRD, VSM, and SEM coincide well with each other. (C) 2020 The Japan Society of Applied Physics

Preparation of ordered CoPt on Si substrates is significant for expanding future applications of spintronic devices. In this study, ordered CoPt alloys including the L1(0) phase with a maximum coercivity of 2.1 kOe are formed in electron-beam-deposited 11.4 nm thick Pt/Co bilayer thin films on Si/SiO2 substrates via interdiffusion during rapid thermal annealing (RTA). The effects of RTA temperature on the magnetic properties, crystal structures, cross-sectional elemental profiles, and surface morphologies of the films are analyzed by vibrating sample magnetometer (VSM), grazing incidence x-ray diffraction (GI-XRD), energy-dispersive x-ray spectroscopy (EDX), and scanning electron microscope (SEM), respectively. For the as-deposited film, polycrystalline Pt was confirmed by uniform Debye-Scherrer rings of Pt. At 200 degrees C, interdiffusion between Co and Pt atoms in the film started to be observed by EDX elemental maps, and at 300 degrees C, alloying of Co and Pt atoms was confirmed by diffraction peaks corresponding to A1-disordered CoPt. At 400 degrees C, the in-plane coercivity of the film began to increase. At 700 degrees C, ordered CoPt alloys were confirmed by superlattice diffraction peaks. At 800 degrees C, a graded film containing L1(0)-ordered CoPt was found to be formed and a maximum coercivity of 2.1 kOe was observed by VSM, where the easy axis of magnetization was oriented along the in-plane direction. At 900 degrees C, deformation of the ordered CoPt alloys was observed by GI-XRD, and the grain size of the film reached a maximum.

Formamidinium [FA,HC(NH2)(2)(+)] lead iodide and its cation mixture have attracted interest as potentials in applications for efficient solar cells superior to well-known methylammonium lead iodide. We investigated the crystal structure and thermodynamic properties of high-quality single crystals of FA(1-x)Cs(x)PbI(3) for x = 0 and 0.1 through X-ray diffraction and heat capacity measurements. Both alpha-FA(0.9)Cs(0.1)PbI(3) as well as alpha-FAPbI(3) crystallize in a cubic Pm3m structure with orientationally disordered FA molecules confined in the nondistorted Pb-I framework. In FAPbI(3), we observed a second-order transition at 280 K and two first-order transitions at 141.2 and 130.2 K in between beta- and gamma-phases instead of the previously known single beta-gamma transition. After doping with 10% Cs, the multiple first-order transitions disappeared, leading to phase transitions emerging at 300 and 149 K with second-order character. We moreover observed low-energy localized modes for both compounds, which is presumably tied to anomalous thermal motion, rattling, of the FA molecule.

The spin-driven component of electric polarization in a single crystal of multiferroic BiFeO3 was experimentally investigated in pulsed high magnetic fields up to 41 T. Sequential measurements of electric polarization for various magnetic field directions provide clear evidence of electric polarization normal to the hexagonal c axis (P-t) in not only the cycloidal phase, but also the field-induced canted antiferromagnetic phase. The direction of P-t is directly coupled with the ferromagnetic moment in the canted antiferromagnetic phase, and thus controlled by changing the direction of the applied magnetic field. This magnetoelectric coupling is reasonably reproduced by the metal-ligand hybridization model.

We investigated the fundamental physical properties in the ultra-quantum limit state of bismuth through measurements of magnetoresistance, magnetization, magnetostriction, and ultrasound attenuation in magnetic fields up to 60T. For magnetic fields applied along the bisectrix direction of a single crystal, a drastic sign reversal in magnetostriction was observed at approximately 39T, which could be ascribed to the complete valley polarization in the electron Fermi pockets. The application of magnetic fields along the binary direction presented an anomalous feature at approximately 50T only in the magnetoresistance. The emergence of a field-induced splitting of a valley was proposed as a possible origin of this anomaly.

The effects of high magnetic fields applied perpendicular to the spontaneous ferroelectric polarization on single crystals of BiFeO3 were investigated through magnetization, magnetostriction, and neutron diffraction measurements. The magnetostriction measurements revealed lattice distortion of 2 x 10(-5) during the reorientation process of the cycloidal spin order by applied magnetic fields. Furthermore, anomalous changes in magnetostriction and electric polarization at a larger field demonstrate an intermediate phase between cycloidal and canted antiferromagnetic states, where a large magnetoelectric effect was observed. Neutron diffraction measurements clarified that incommensurate spin modulation along the [110](hex) direction in the cycloidal phase becomes Q = 0 commensurate along this direction in the intermediate phase. Theoretical calculations based on the standard spin Hamiltonian of this material suggest an antiferromagnetic cone-type spin order in the intermediate phase.