Shin-ichi Ohkoshi

Light-induced spin-crossover magnet

The light-induced phase transition between the low-spin (LS) and high-spin (HS) states of some transition-metal ions has been extensively studied in the fields of chemistry and materials science. In a crystalline extended system, magnetically ordering the HS sites of such transition-metal ions by irradiation should lead to spontaneous magnetization. Previous examples of light-induced ordering have typically occurred by means of an intermetallic charge transfer mechanism, inducing a change of valence of the metal centres. Here, we describe the long-range magnetic ordering of the extended Fe(II)(HS) sites in a metal-organic framework caused instead by a light-induced excited spin-state trapping effect. The Fe-Nb-based material behaves as a spin-crossover magnet, in which a strong superexchange interaction (magnetic coupling through non-magnetic elements) between photo-produced Fe(II)(HS) and neighbouring Nb(IV) atoms operates through CN bridges. The magnetic phase transition is observed at 20 K with a coercive field of 240 Oe.

High proton conduction in a chiral ferromagnetic metal-organic quartz-like framework.

A complex-as-ligand strategy to get a multifunctional molecular material led to a metal-organic framework with the formula (NH(4))(4)[MnCr(2)(ox)(6)]·4H(2)O. Single-crystal X-ray diffraction revealed that the anionic bimetallic coordination network adopts a chiral three-dimensional quartz-like architecture. It hosts ammonium cations and water molecules in functionalized channels. In addition to ferromagnetic ordering below T(C) = 3.0 K related to the host network, the material exhibits a very high proton conductivity of 1.1 × 10(-3) S cm(-1) at room temperature due to the guest molecules.

Large Magnetization-Induced Second Harmonic Generation in an Enantiopure Chiral Magnet

The absence of centrosymmetry in the enantiopure chiral magnet [N(CH(3))(n-C(3)H(7))(2)(C*H(CH(3))C(2)H(5))][Mn(II)Cr(III)(ox)(3)] allows the observation of bulk second harmonic generation (SHG) in this material. At low temperature, the onset of magnetization gives birth to a magnetization-induced SHG (MSHG) contribution. With an angular shift of 13.1 degrees upon magnetization reversal, the MSHG effects appear to be much larger than the corresponding linear magneto-optical effects. Thanks to the single-crystalline state of the sample, the variation of the signal with the orientation of the magnetic field and/or the angle between the polarization of the incident radiation and the outgoing SHG signal in the paramagnetic and ferromagnetic phases is reproduced and well-understood through the use of a symmetry-based analysis of the nonlinear susceptibility tensor.

Photoinduced magnetic pole inversion in a ferro–ferrimagnet: (Fe0.40IIMn0.60II)1.5CrIII(CN)6

We tried to design the magnet exhibiting magnetic pole (N and S) inversion by photostimuli. The magnetization of Fe1.5IICrIII(CN)6⋅7.5H2O was changed in a photon mode by visible light. A ferro-ferrimagnet (Fe0.40IIMn0.60II)1.5CrIII(CN)6⋅7.5H2O mixed by ferromagnetic (Fe–Cr system showing the change of magnetization by optical stimuli) site and ferrimagnetic (Mn–Cr system showing no optical response) site showed negative magnetization at the temperature lower than compensation temperature (Tcomp=19 K). In this mixed metal cyanide magnet we have succeeded in demonstrating a novel magnetic behavior “photoinduced magnetic pole inversion.” Moreover, the magnetic pole inversion can be induced repeatedly by alternate optical and thermal stimulations.

First Observation of Phase Transformation of All Four Fe2O3 Phases (γ → ε → β → α-Phase)

Iron oxide (Fe(2)O(3)) has four crystal structures: gamma-, epsilon-, beta-, and alpha-Fe(2)O(3). Until now, routes of the phase transformations among the four Fe(2)O(3) phases have not been clarified because a systematic synthesis that yields all four Fe(2)O(3) phases has yet to be reported. Herein we report the synthesis of a series of Fe(2)O(3) nanoparticles using mesoporous SiO(2). The crystal structures of the Fe(2)O(3) nanoparticles change in the order of gamma-Fe(2)O(3) --> epsilon-Fe(2)O(3) --> beta-Fe(2)O(3) --> alpha-Fe(2)O(3) as the particle size increases. Threshold sizes were estimated as gamma --> epsilon at 8 nm, epsilon --> beta at 30 nm, and beta --> alpha at 50 nm in the synthesis using FeSO(4) as a precursor. The phase transformations among the four Fe(2)O(3) phases have been observed for the first time.

High Proton Conductivity in Prussian Blue Analogues and the Interference Effect by Magnetic Ordering

We observed high proton conductivities of 1.2 x 10(-3) and 1.6 x 10(-3) S cm(-1) on Co[Cr(CN)(6)](2/3).zH(2)O and V[Cr(CN)(6)](2/3).zH(2)O, respectively, and an interference effect between magnetic ordering and ionic conduction below the magnetic phase transition temperature.

Synthesis of a metal oxide with a room-temperature photoreversible phase transition

Photoinduced phase-transition materials, such as chalcogenides, spin-crossover complexes, photochromic organic compounds and charge-transfer materials, are of interest because of their application to optical data storage. Here we report a photoreversible metal-semiconductor phase transition at room temperature with a unique phase of Ti(3)O(5), lambda-Ti(3)O(5). lambda-Ti(3)O(5) nanocrystals are made by the combination of reverse-micelle and sol-gel techniques. Thermodynamic analysis suggests that the photoinduced phase transition originates from a particular state of lambda-Ti(3)O(5) trapped at a thermodynamic local energy minimum. Light irradiation causes reversible switching between this trapped state (lambda-Ti(3)O(5)) and the other energy-minimum state (beta-Ti(3)O(5)), both of which are persistent phases. This is the first demonstration of a photorewritable phenomenon at room temperature in a metal oxide. lambda-Ti(3)O(5) satisfies the operation conditions required for a practical optical storage system (operational temperature, writing data by short wavelength light and the appropriate threshold laser power).

Synthesis of an Electromagnetic Wave Absorber for High-Speed Wireless Communication

Millimeter waves (30-300 GHz) are starting to be used in next generation high-speed wireless communications. To avoid electromagnetic interference in this wireless communication, finding a suitable electromagnetic wave absorber in the millimeter wave range is an urgent matter. In this work, we prepared a high-performance millimeter wave absorber composed of a series of aluminum-substituted epsilon-iron oxide, epsilon-Al(x)Fe(2-x)O(3), nanomagnets (0 < or = x < or = 0.40) with a particle size between 25 and 50 nm. The materials in this series have an orthorhombic crystal structure in the Pna2(1) space group, which has four nonequivalent Fe sites and Al ion that predominantly occupies the tetrahedral [FeO(4)] site. The field-cooled magnetization curves showed that the T(C) values were 448, 480, and 500 K for x = 0.40, 0.21, and 0, respectively. The magnetization versus external magnetic field showed that the coercive field H(c) values at 300 K were 10.2, 14.9, and 22.5 kOe for x = 0.40, 0.21, and 0, respectively. The millimeter wave absorption properties were measured at room temperature by terahertz time domain spectroscopy. The frequencies of the absorption peaks for x = 0.40, 0.30, 0.21, 0.09, 0.06, and 0 were observed at 112, 125, 145, 162, 172, and 182 GHz, respectively. These absorptions are due to the natural resonance achieved by the large magnetic anisotropies in this series. Such frequencies are the highest ones for magnetic materials. Because aluminum is the third most abundant atom, aluminum-substituted epsilon-iron oxide is very economical, and thus these materials are advantageous for industrial applications.

Multiferroics by Rational Design: Implementing Ferroelectricity in Molecule‐Based Magnets

Multiferroics (MF) are materials that exhibit simultaneouslyseveral ferroic order parameters. Among the multiferroicmaterials, those combining antiferro- or ferroelectricity (FE)and antiferro-, ferri-, or ferromagnetism (FM) within thesame material are highly desirable: the coexistence of thepolar and magnetic orders paves the way towards four-levelmemories while their interactions through the magnetoelec-tric effect makes it possible to control the magnetization byelectric fields and hence to develop electronically tuneablemagnetic devices, which are an essential feature for spin-tronics.

Photo-magnetic and magneto-optical effects of functionalized metal polycyanides

Abstract This article deals with novel opto-magnetic and magneto-optical phenomena. These phenomena are demonstrated with metal polycyanides, which are classified as molecule-based magnets. Cobalt hexacyanoferrate and copper octacyanomolybdate solids exhibit a photo-reversible enhancement of magnetization. These photo-magnetic phenomena are based on the photo-induced electron transfer between diamagnetic (or paramagnetic) state and their valence isomers exhibiting ferrimagnetism (or ferromagnetism). By a combination of photo-magnetism and mixed ferro-ferrimagnetism, a novel photo-induced magnetic pole inversion is demonstrated with an iron–manganese hexyacyanochromate. Each of these photo-magnetic phenomenon is the first known example in the whole field of magnetically ordered materials. Furthermore, Faraday spectra of molecule-based magnets were observed for the first time with electrochemically synthesized vanadium hexacyanochromate films. This Faraday effect originates from the intervalence charge-transfer band between vanadium and chromium ions in the visible region.