Nanostructure-induced L10-ordering of twinned single-crystals in CoPt ferromagnetic nanowires.

L10-ordered ferromagnetic nanowires with large coercivity are essential for realizing next-generation spintronic devices. Ferromagnetic nanowires have been commonly fabricated by first L10-ordering of initially disordered ferromagnetic films by annealing and then etching them into nanowire structures using lithography. If the L10-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 L10-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 L10-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 L10-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 L10-ordering. The fabrication method for nanostructure-induced L10-ordered CoPt ferromagnetic nanowires with twinned single-crystals on Si/SiO2 substrates would be significant for future silicon-technology-compatible spintronic applications.

Reversible 3D-2D structural phase transition and giant electronic modulation in nonequilibrium alloy semiconductor, lead-tin-selenide.

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-x Sn x )Se alloy system, where Pb2+ and Sn2+ have similar ns2 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-x Sn x )Se alloy epitaxial films by using a nonequilibrium growth technique. Reversible giant electronic property change was attained at x ~ 0.5 originating in the abrupt band structure switch from gapless Dirac-like state to semiconducting state.

Capacitive detection of magnetostriction, dielectric constant, and magneto-caloric effects in pulsed magnetic fields.

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.

Structural and Thermal Properties in Formamidinium and Cs-Mixed Lead Halides.

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 FA1-xCsxPbI3 for x = 0 and 0.1 through X-ray diffraction and heat capacity measurements. Both α-FA0.9Cs0.1PbI3 as well as α-FAPbI3 crystallize in a cubic Pm3̅m structure with orientationally disordered FA molecules confined in the nondistorted Pb-I framework. In FAPbI3, we observed a second-order transition at 280 K and two first-order transitions at 141.2 and 130.2 K in between ß- and γ-phases instead of the previously known single ß-γ 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.

Thermodynamic evidence of magnetic-field-induced complete valley polarization in bismuth.

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.