Topochemical Fluorination of Epitaxial Thin Films of Barium-Doped Bismuth Iron Oxyfluoride
Mizuho Sano, Akiko Kamigaito, Yusuke Wakayama, Kei Shigematsu, Tsukasa Katayama, Yasushi Hirose, Akira Chikamatsu
Crystal Growth & Design, 24, 22, 9344, 9349, American Chemical Society (ACS), 07 Nov. 2024
Scientific journal

Ferroelectric BaTiO3 Freestanding Sheets for an Ultra-High-Speed Light-Driven Actuator
Lizhikun Gong, Atsushi Taguchi, Weikun Zhou, Ren Mitsuya, Hiromichi Ohta, Tsukasa Katayama
ACS Applied Materials & Interfaces, 09 Oct. 2024
Scientific journal

Unusual Crystal Orientation in Hexagonal HoFeO3 Multiferroic Films and the Effect on Magnetism
Binjie Chen, Jinghuang Lin, Bin Feng, Yuichi Ikuhara, Hiromichi Ohta, Tsukasa Katayama
Crystal Growth & Design, 03 Oct. 2024
Scientific journal

High-concentration doping effects of aliovalent Al and Ga on ferroelectric properties of BaTiO3 Films
Ko Marunouchi, Lizhikun Gong, Hiromichi Ohta, Tsukasa Katayama
Thin Solid Films, 796, 140339, 140339, Elsevier BV, May 2024
Scientific journal

Magnetic Phase Transition-Induced Modulation of Ferroelectric Properties in Hexagonal RFeO₃ (R = Tb and Ho)
Yaoming Liu, Binjie Chen, Yosuke Hamasaki, Lizhikun Gong, Hiromichi Ohta, Tsukasa Katayama
ACS Applied Materials & Interfaces, 16, 14, 17832, 17837, American Chemical Society (ACS), 01 Apr. 2024
Scientific journal

Cation-placement control in double-perovskite GdBaCo₂O₆ and its impact on magnetism via spin-state modification
Tsukasa Katayama, Kento Magara, Shiro Sakai, Yijie Zeng, Akira Chikamatsu, Tetsuya Hasegawa
Journal of Materials Chemistry C, 12, 28, 10428, 10436, Royal Society of Chemistry (RSC), 2024
Scientific journal, A-site-ordered and -disordered GdBaCo₂O₆ films were synthesized through topotactic oxidation. These films exhibit distinct magnetic and conductive properties, attributed to variations in Co spin states driven by changes in ionic arrangement.

Large tensile-strained BaTiO3 films grown on lattice-mismatched La-doped BaSnO3 bottom electrode
Lizhikun Gong, Ko Marunouchi, Akira Chikamatsu, Hiromichi Ohta, Tsukasa Katayama
CrystEngComm, Royal Society of Chemistry (RSC), 2024
Scientific journal, Perovskite BaTiO3 has been widely studied and utilized in various applications owing to its high permittivity, ferroelectricity, non-toxicity, and stability. However, its low ferroelectric-paraelectric phase transition temperature (TC, 120 °C)...

Ferroelectricity, High Permittivity, and Tunability in Millimeter-Size Crack-Free Ba_1-xSr_xTiO₃ Flexible Epitaxial Sheets
Yu, R., Gong, L., Ohta, H., Katayama, T.
ACS Applied Electronic Materials, 5, 9, 26 Sep. 2023
Scientific journal

Atomic Layer Deposition of HfO₂ Films Using Tetrakis(1-(N,N-dimethylamino)-2-propoxy)hafnium [Hf(dmap)₄] for Advanced Gate Dielectrics Applications
Akihiro Nishida, Tsukasa Katayama, Yasutaka Matsuo
ACS Applied Nano Materials, American Chemical Society (ACS), 15 Sep. 2023
Scientific journal

Anisotropic proton conduction in double-perovskite GdBaCo₂O_5.5
Katayama, T., Magara, K., Chikamatsu, A., Hasegawa, T.
Applied Physics Letters, 123, 1, 03 Jul. 2023
Scientific journal

Solid phase epitaxy of perpendicular magnetic BaFe₁₂O₁₉ flexible sheets on a mica substrate
Tsukasa Katayama, Shishin Mo, Akira Chikamatsu, Tetsuya Hasegawa
Japanese Journal of Applied Physics, 62, 6, 065505, 065505, IOP Publishing, 01 Jun. 2023
Scientific journal, Abstract

Epitaxial M-type BaFe₁₂O₁₉ flexible sheets, exhibiting RT perpendicular magnetization, were synthesized on mica substrates through the solid phase epitaxy method. The [110] and [001] directions of the BaFe₁₂O₁₉ layer were parallel to the [010] and [001] directions of the mica, respectively. In the synthesis process, amorphous BaFe₁₂O₁₉ films were firstly prepared on Al₂O₃-buffered mica (001) substrates and then annealed at 800 °C–900 °C in air for single crystallization. During optimization of the synthesis, we found that preparation of the buffer layer and controlling annealing temperature are important for obtaining the epitaxial sheet. The sheets exhibited perpendicular ferrimagnetism with large saturated magnetization (60 Am² kg⁻¹) and magnetic anisotropy coefficient (2.1 × 10⁵ J m⁻³) at 300 K, together with flexibility.

Half-Metallicity and Magnetic Anisotropy in Double-Perovskite GdBaCo₂O₆ Films Prepared via Topotactic Oxidation
Tsukasa Katayama, Shishin Mo, Akira Chikamatsu, Yuji Kurauchi, Hiroshi Kumigashira, Tetsuya Hasegawa
Chemistry of Materials, American Chemical Society (ACS), 18 Jan. 2023
Scientific journal

Atomic layer deposition of Y₂O₃ films using a novel liquid homoleptic yttrium precursor tris(sec-butylcyclopentadienyl)yttrium [Y(^sBuCp)₃] and water
Akihiro Nishida, Tsukasa Katayama, Yasutaka Matsuo
RSC Advances, 13, 39, 27255, 27261, Royal Society of Chemistry (RSC), 2023, [Peer-reviewed]
Scientific journal, Atomic layer deposition of Y₂O₃ films was demonstrated using a novel liquid homoleptic yttrium precursor Y(^sBuCp)₃ and water. The deposited Y₂O₃ films exhibit high purity, high crystallinity, a smooth surface, and high electronic performance.

Grain engineered polar-axis-oriented epitaxial Mn₂Mo₃O₈ films with enhanced magnetic transition temperature
Shishin Mo, Tsukasa Katayama, Akira Chikamatsu, Tetsuya Hasegawa
Journal of Materials Chemistry C, Royal Society of Chemistry (RSC), 2023
Scientific journal, Magnetic transition temperature of polar-axis-oriented Mn₂Mo₃O₈ epitaxial films was enhanced by grain engineering.

Synthesis and transparent conductivity of crack-free La:BaSnO₃ epitaxial flexible sheets
Lizhikun Gong, Rui Yu, Hiromichi Ohta, Tsukasa Katayama
Dalton Transactions, Royal Society of Chemistry (RSC), 2023
Scientific journal, La-doped BaSnO₃ (LBSO), which exhibits both high electron mobility and visible-light transparency, is a promising transparent electrode/transistor material that does not require expensive elements such as indium.

Hexagonal RFeO₃ (R = Dy, Er, and Lu) Films Grown on Glass Substrates with Both Magnetic and Ferroelectric Orders
Chen, B., Katayama, T.
ACS Applied Electronic Materials, 5, 1, 2023
Scientific journal

Significant Suppression of Cracks in Freestanding Perovskite Oxide Flexible Sheets Using a Capping Oxide Layer
Gong, L., Wei, M., Yu, R., Ohta, H., Katayama, T.
ACS Nano, 16, 12, 27 Dec. 2022
Scientific journal

Photo-induced antiferromagnetic-ferromagnetic and spin-state transition in a double-perovskite cobalt oxide thin film
Yujun Zhang, Tsukasa Katayama, Akira Chikamatsu, Christian Schü{\ss}ler-Langeheine, Niko Pontius, Yasuyuki Hirata, Kou Takubo, Kohei Yamagami, Keisuke Ikeda, Kohei Yamamoto, Tetsuya Hasegawa, Hiroki Wadati
Communications Physics, 5, 1, Springer Science and Business Media {LLC}, Dec. 2022
English, Scientific journal, AbstractInvestigation of ultrafast dynamic behaviors can provide novel insights about the coupling mechanisms among multiple degrees of freedom in condensed matters, such as lattice, magnetism and electronic structure. Here we investigate both the ferromagnetic (FM) and antiferromagnetic (AFM) dynamics of a strongly correlated oxide system, GdBaCo2O5.5 thin film by time-resolved x-ray magnetic circular dichroism in reflectivity (XMCDR) and resonant magnetic x-ray diffraction (RMXD). A photo-induced AFM-FM transition characterized by an increase of the transient XMCDR (sensitive to FM order) beyond the unpumped value and a decay of RMXD (sensitive to AFM order) was observed. The photon-energy dependence of the transient XMCDR and reflectivity could be interpreted as a concomitant photo-induced spin-state transition (SST). The AFM-FM transition and SST couple with each other in the time domain, resulting in unusual dynamic behaviors of the magnetism.

Epitaxial growth of hexagonal GdFeO3 thin films with magnetic order by pulsed laser deposition
Jun Kasahara, Tsukasa Katayama, Akira Chikamatsu, Yosuke Hamasaki, Tetsuya Hasegawa
Thin Solid Films, 757, 139409, 139409, Elsevier BV, Sep. 2022
Scientific journal

Ferroelectric and Magnetic Properties of Hexagonal ErFeO₃Epitaxial Films
Chen, B., Ohta, H., Katayama, T.
ACS Applied Electronic Materials, 4, 9, American Chemical Society (ACS), 23 Aug. 2022
Scientific journal

Ionic order and magnetic properties of double-perovskite GdBaCo₂O_5.5 films on SrTiO₃ substrates
Tsukasa Katayama, Akira Chikamatsu, Tetsuya Hasegawa
Journal of the Ceramic Society of Japan, 130, 7, 429, 431, Ceramic Society of Japan, 01 Jul. 2022
English, Scientific journal

Enhancement of room-temperature magnetization in GaFeO₃-type single crystals by Al and Sc doping
Wang, L., Katayama, T., Wang, C., Li, Q., Shi, Y., Fang, Y., Huang, F., Zhu, Y., Li, H.-F., Yasui, S., Huang, X., Yu, J.
AIP Advances, 12, 6, 065015, 065015, AIP Publishing, 01 Jun. 2022
Scientific journal, GaFeO₃-type oxides are promising multiferroic materials due to the coexistence of spontaneous magnetization and ferroelectric polarization properties at room temperature. As these ferroic properties feature a large anisotropy, single crystals are required. However, the magnetization of GaFeO₃-type single crystals remains low at room temperature. In this study, we largely enhanced the magnetization at room temperature of GaFeO₃-type single crystals by increasing the Fe content and co-doping Sc³⁺ and Al³⁺. Single crystals of Al _xSc_{0.1− x}Ga_0.6Fe_1.3O₃ ( x = 0.01–0.04) were prepared using the optical floating-zone method. The single crystals were rod-shaped, with a diameter and length of ∼6 mm and 7 cm, respectively. X-ray diffraction measurements confirmed the ferroelectric polarization of the crystals. In addition, they exhibited room-temperature ferrimagnetism, with Curie temperature in the range of 326–338 K; the crystals exhibit magnetic anisotropy along the a-axis. The magnetization of the single crystal at 300 K and 0.3 kOe was 13 emu g⁻¹, which is over ten times larger than those of previously reported single crystals with GaFeO₃-type crystal structure.

Crystal structure and electronic property modification of Ca2RuO4 thin films via fluorine doping
Shota Fukuma, Akira Chikamatsu, Tsukasa Katayama, Takahiro Maruyama, Keiichi Yanagisawa, Koji Kimoto, Miho Kitamura, Koji Horiba, Hiroshi Kumigashira, Yasushi Hirose, Tetsuya Hasegawa
Physical Review Materials, 6, 3, American Physical Society ({APS}), 23 Mar. 2022
English, Scientific journal, Layered ruthenium oxyfluorides have various crystal structures and Ru oxidation states and exhibit unique physical properties. While various layered ruthenates have reportedly been topochemically fluorinated with Sr as A sites, investigation of the fluorination of layered ruthenates containing smaller Ca ions is lacking. In this paper, we fabricated phase-pure and single-crystalline thin films of Ca2RuO2.5F2 on LaSrAlO4 (001) substrates via topochemical fluorination of the Ca2RuO4 precursor using polyvinylidene fluoride. The obtained fluorinated thin films had a chemical composition of Ca2RuO2.5F2 with the Ru3+ state, as determined by energy-dispersive x-ray spectroscopy and x-ray photoemission spectroscopy, whereas the film prepared via Sr2Ru4+O4 fluorination had a composition of (Sr2Ru4+O3F2). Scanning transmission electron microscopy revealed that Ca2RuO2.5F2 has only 1 F- site in CaO rock-salt blocks, whereas Sr2RuO3F2 has two inequivalent F- sites in the SrO layers. The Ca2RuO2.5F2 film was insulating, with a resistivity (rho) of 8.6 x 10(-2) Omega cm at 300 K. Moreover, the temperature behavior of rho was well described by the two-dimensional variable range hopping model. These results demonstrate that local distortion is an important factor that governs the topochemical fluorination of ruthenates and affects the crystal and electronic structures of the reactants.

Negative magnetoresistance in different nitrogen content EuNbO_3−xN_x single-crystalline thin films
Maruyama, T., Hirose, Y., Katayama, T., Sugisawa, Y., Sekiba, D., Hasegawa, T., Chikamatsu, A.
Journal of Materials Chemistry C, 10, 39, Royal Society of Chemistry (RSC), 2022
Scientific journal, In EuNbO_3−xN_x single-crystalline thin films, the transport properties gradually changed from metallic to semiconducting and the negative magnetoresistance at 2 K increased from 15% to 98% with x.

Antiferroelectric-to-ferroelectric phase transition in hexagonal rare-earth iron oxides
Chen, B., Hasegawa, T., Ohta, H., Katayama, T.
Journal of Materials Chemistry C, 10, 14, 5621, 5626, Royal Society of Chemistry ({RSC}), 2022
Scientific journal

Improvement of electric insulation in dielectric layered perovskite nickelate films via fluorination
Takuma Nishimura, Tsukasa Katayama, Shishin Mo, Akira Chikamatsu, Tetsuya Hasegawa
Journal of Materials Chemistry C, Royal Society of Chemistry ({RSC}), 2022
Scientific journal, Epitaxial films of La_3/2Sr_1/2NiO_xF_y with a wide range of fluorine content (y), 0.4–3, were prepared. The fluorinated film exhibited high electric insulation due to the large and random bond distortions provided by the fluorination.

Ferroelectric and magnetic properties in ε-Fe2O3 epitaxial film
Yosuke Hamasaki, Shintaro Yasui, Tsukasa Katayama, Takanori Kiguchi, Shinya Sawai, Mitsuru Itoh
Applied Physics Letters, 119, 18, 182904, 182904, AIP Publishing, 01 Nov. 2021
Scientific journal

Epitaxial-Strain-Induced Spontaneous Magnetization in Polar Mn2Mo3O8
Shishin Mo, Tsukasa Katayama, Akira Chikamatsu, Miho Kitamura, Koji Horiba, Hiroshi Kumigashira, Tetsuya Hasegawa
Chemistry of Materials, 33, 19, 7713, 7718, American Chemical Society ({ACS}), 12 Oct. 2021
Scientific journal

Single-Crystal Synthesis of ϵ-Fe₂O₃-Type Oxides Exhibiting Room-Temperature Ferrimagnetism and Ferroelectric Polarization
Zhang, Y., Wang, H., Tachiyama, K., Katayama, T., Zhu, Y., Wu, S., Li, H.-F., Fang, J., Li, Q., Shi, Y., Wang, L., Fu, Z., Xu, F., Yu, J., Yasui, S., Itoh, M.
Crystal Growth and Design, 21, 9, American Chemical Society ({ACS}), 29 Jul. 2021
Scientific journal

Ionic Order Engineering in Double-Perovskite Cobaltite
Tsukasa Katayama, Akira Chikamatsu, Yujun Zhang, Shintaro Yasui, Hiroki Wadati, Tetsuya Hasegawa
Chemistry of Materials, 33, 14, 5675, 5680, American Chemical Society ({ACS}), 27 Jul. 2021
Scientific journal, Double-perovskite GdBaCo2O5.5 (GBCO) exhibits various fascinating features, including temperature- and magnetic-field-induced ferromagnetic to antiferromagnetic phase transition associated with the two ionic orders: an A-site Gd/Ba order along the c-axis, and an octahedral CoO6/pyramidal CoO5 order along the b-axis. However, the fine control of ionic orders remains a challenging issue. Herein, we demonstrate the engineering of ionic orders in GBCO films through appropriate selection of substrates. Specifically, we prepared four kinds of films comprising only the Gd/Ba order [on SrTiO3(001)], only the CoO6/CoO5 order [on LaSrGaO4(001)], and both the Gd/Ba and CoO6/CoO5 orders [on NdGaO3(110) and LaSrAlO4(001) (LSAO)]. In addition to the selective ionic orders through substrate selection, the GBCO film on LSAO also contained multiple domains with domain boundaries of [CoOx]-[CoOy] stacking. Magnetic, magnetoresistance, and resonant X-ray magnetic diffraction measurements revealed that the magnetic properties of the above GBCO films were widely modulated. With the ionic order control of the GBCO film, different magnetic phase transition behaviors and magnetic anisotropy switching from the in-plane to perpendicular direction were observed.

Synthesis and magnetism of MoCo2O4 spinel thin films
Tsukasa Katayama, Shishin Mo, Yuji Kurauchi, Akira Chikamatsu, Tetsuya Hasegawa
Thin Solid Films, 728, 138696, 138696, Elsevier {BV}, Jun. 2021
Scientific journal

Room-Temperature Antiferroelectricity in Multiferroic Hexagonal Rare-Earth Ferrites
Jun Kasahara, Tsukasa Katayama, Shishin Mo, Akira Chikamatsu, Yosuke Hamasaki, Shintaro Yasui, Mitsuru Itoh, Tetsuya Hasegawa
ACS Applied Materials & Interfaces, 13, 3, 4230, 4235, American Chemical Society ({ACS}), 27 Jan. 2021, [International Magazine]
English, Scientific journal, The antiferroelectric (AFE) phase, in which nonpolar and polar states are switchable by an electric field, is a recent discovery in promising multiferroics of hexagonal rare-earth manganites (ferrites), h-RMn(Fe)O3. However, this phase has so far only been observed at 60-160 K, which restricts key investigations into the microstructures and magnetoelectric behaviors. Herein, we report the successful expansion of the AFE temperature range (10-300 K) by preparing h-DyFeO3 films through epitaxial stabilization. Room-temperature scanning transmission electron microscopy reveals that the AFE phase originates from a nanomosaic structure comprising AFE P3̅c1 and ferroelectric P63cm domains with small domain sizes of 1-10 nm. The nanomosaic structure is stabilized by a low c/a ratio derived from the large ionic radius of Dy3+. Furthermore, weak ferromagnetism and magnetocapacitance behaviors are observed. Below 10 K, the film exhibits an M-shaped magnetocapacitance versus magnetic field curve, indicating unusual magnetoelectric coupling in the AFE phase.

Large Polarization Switching and High-Temperature Magnetoelectric Coupling in Multiferroic GaFeO3 Systems
Hui Wang, Yang Zhang, Koki Tachiyama, Zhaoyang Xia, Jinghong Fang, Qin Li, Guofeng Cheng, Yun Shi, Ji, ing Yu, Tsukasa Katayama, Shintaro Yasui, Mitsuru Itoh
Inorganic Chemistry, 60, 1, 225, 230, American Chemical Society ({ACS}), 04 Jan. 2021
English, Scientific journal, GaFeO3-type iron oxides are promising multiferroics due to the coexistence of large spontaneous magnetization and polarization near room temperature. However, the high leakage current and difficulties associated with synthesizing single crystals E-induced make it difficult to achieve two important features in the system: a P switching large ferroelectric polarization switching and magnetoelectric coupling at a high-temperature region. Herein, we report successful achievement of these features by preparing high-quality Sc-doped GaFeO3 single crystals (ScxGa1-x/2Fe1-x/2O3 with x= 0-0.3) using the floating zone method. The x >= 0.05 crystals exhibit a leakage current 10(4) times lower than the x = 0 crystals, highlighting the importance of Sc doping. Because of the reduced leakage current, the Sc-doped crystals exhibit large ferroelectric polarization switching along the c-axis with a remanent polarization of 22-25 mu C/cm(2), which is close to the theoretically predicted polarization value of 25-28 mu C/cm(2). In addition, the Sc-doped crystals exhibit ferrimagnetism with magnetic anisotropy along the a-axis. Furthermore, a magnetic-field-induced modulation of polarization is observed in the x = 0.15 crystal even at a relatively high temperature, i.e., 100 K.

Investigation of the electronic states of A-site layer-ordered double perovskite YBaCo2Ox (x = 5.3 and 6) thin films by x-ray spectroscopy
Akira Chikamatsu, Tsukasa Katayama, Takahiro Maruyama, Miho Kitamura, Koji Horiba, Hiroshi Kumigashira, Hiroki Wadati, Tetsuya Hasegawa
Applied Physics Letters, 118, 1, 012401, 012401, {AIP} Publishing, 04 Jan. 2021
Scientific journal, In this study, the electronic structures of A-site layer-ordered double perovskite YBaCo2O6 and YBaCo2O5.3 epitaxial thin films were investigated by x-ray photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) measurements. The valence band PES spectrum of the YBaCo2O6 thin film exhibited a finite density of states (DOS) at the Fermi energy (EF), while that of the YBaCo2O5.3 thin film exhibited no DOS at EF, corresponding well with the metallic and insulating transport properties, respectively. The Co L-edge XAS measurements revealed that the Co ions in YBaCo2O6 are in a mixed valence state of high-spin (HS) Co3+ (50%) and intermediate spin (IS) Co4+ (50%), while HS Co3+ (80%) and HS Co2+ (20%) coexist in YBaCo2O5.3. The XMCD measurements led to the conclusion that both HS Co3+ and IS Co4+ contribute to the ferromagnetism of YBaCo2O6.

Simple Method to Obtain Large‐Size Single‐Crystalline Oxide Sheets
Ke Gu, Tsukasa Katayama, Shintaro Yasui, Akira Chikamatsu, Sou Yasuhara, Mitsuru Itoh, Tetsuya Hasegawa
Advanced Functional Materials, 30, 28, 2001236, 2001236, Wiley, 25 Jul. 2020
Scientific journal, Synthetic techniques to prepare large-size, flexible, and high-quality single-crystalline sheets of transition metal oxides are crucial to developing low-energy consumption devices. One promising way is a lift-off and transfer technique using a heterostructure of polymer supporting oxide and Sr3Al2O6 (SAO) layers grown on a single-crystalline substrate. By removing the water-soluble SAO and the supporting layers, the oxide sheet is obtained. Although some ferroelectric flexible sheets are prepared by this method, a simpler method for obtaining large-size sheets is required. Herein, a lift-off and transfer method is proposed without a supporting layer. With this simple method, single-crystalline SrRuO3 and BaTiO3 flexible sheets with a lateral size of a few millimeters are successfully prepared. The SrRuO3 sheet exhibits high crystallinity and conductivity. Meanwhile, the ferroelectricity of the BaTiO3 sheet is successfully observed via polarization hysteresis loop measurements. In addition to the simplicity, this method has low costs and the substrate is reusable. Accordingly, the proposed method could enhance the development of various kinds of large-size functional oxide sheets.

Modulating the Structure and Magnetic Properties of ε-Fe2O3 Nanoparticles via Electrochemical Li+ Insertion
Tsukasa Katayama
Inorganic Chemistry, 59, 7, 4357, 4365, AMER CHEMICAL SOC, 06 Apr. 2020
English, Scientific journal, epsilon-Fe2O3, a metastable phase of iron oxide, is widely known as a room-temperature multiferroic material or as a superhard magnet. Element substitution into epsilon-Fe2O3 has been reported in the literature; however, the substituted ions have a strong site preference depending on their ionic radii and valence. In this study, in order to characterize the crystal structure and magnetic properties of epsilon-Fe2O3 in the Fe2+/Fe3+ coexisting states, Li+ was electrochemically inserted into epsilon-Fe2O3 to reduce Fe3+. The discharge and charge of Li+ into/from epsilon-Fe2O3 revealed that Li+ insertion was successful. X-ray magnetic circular dichroism results indicated that the reduced Fe did not exhibit site preference. Increasing the Li+ content in epsilon-Fe2O3 resulted in decreased saturation magnetization and irregular variation of the coercive field. We present a comprehensive discussion of how magnetic properties are modified with increasing Li+ content using transmission electron microscopy images and considering the L+ diffusion coefficient. The results suggest that inserting L+ into crystalline epsilon-Fe2O3 is a useful tool for characterizing crystal structure, lithiation limit, and magnetic properties in the coexistence of Fe2+/Fe3+.

Redox-based multilevel resistive switching in AlFeO3 thin-film heterostructures
Tsukasa Katayama
ACS Applied Electronic Materials, 2, 4, 1065, 1073, American Chemical Society (ACS), 29 Mar. 2020
English, Scientific journal

Synthesis of multiferroic hexagonal GdFeO₃ film
Kasahara Jun, Tsukasa Katayama, Shishin Mo, Chikamatsu Akira, Yasui Shintaro, Itoh Mitsuru, Hasegawa Tetsuya
JSAP Annual Meetings Extended Abstracts, 2020.1, 1354, 1354, The Japan Society of Applied Physics, 28 Feb. 2020
Japanese

Electronic properties of perovskite strontium chromium oxyfluoride epitaxial thin films fabricated via low-temperature topotactic reaction
Akira Chikamatsu, Takahiro Maruyama, Tsukasa Katayama, Yu Su, Yoshihiro Tsujimoto, Kazunari Yamaura, Miho Kitamura, Koji Horiba, Hiroshi Kumigashira, Tetsuya Hasegawa
Physical Review Materials, 4, 2, 18 Feb. 2020
Scientific journal, Perovskite chromium oxyfluoride SrCrO2.6F0.4 epitaxial thin films were fabricated via low-temperature topotactic fluorination of SrCrO2.8 precursor with polyvinylidene fluoride as a fluorine source. The obtained SrCrO2.6F0.4 thin film had a different chemical composition than that of bulk polycrystalline SrCrO2.8F0.2, possibly due to the higher reactivity of such thin film. Both the precursor and fluorinated thin films containing Cr3.6+ exhibited insulating behavior with band gaps of ∼0.6 eV, in contrast to metallic SrCrO3 with Cr4+ at 300 K. The experimentally observed valence and conduction bands of the SrCrO2.8, SrCrO3, and SrCrO2.6F0.4 thin films suggested that the spectral weight transfer from the coherent part to the incoherent part with the reduction of Cr valences was caused by strong electron correlation effects. This study offers a fundamental understanding of the fluorine doping effects on a crystalline structure and the correlated electronic performance of chromium oxides.

Switchable third ScFeO3 polar ferromagnet with YMnO3-type structure
Yosuke Hamasaki, Tsukasa Katayama, Shintaro Yasui, Takahisa Shiraishi, Akihiro Akama, Takanori Kiguchi, Tomoyasu Taniyama, Mitsuru Itoh
Journal of Materials Chemistry C, 8, 13, 4447, 4452, Royal Society of Chemistry ({RSC}), 2020
Scientific journal,

We prepared a third ScFeO₃ polar ferromagnet with YMnO₃-type structure using a heteroepitaxial film technique and characterized its physical properties.

Influence of fluorination on electronic states and electron transport properties of Sr2IrO4 thin films
Takahiro Maruyama, Akira Chikamatsu, Tsukasa Katayama, Kenta Kuramochi, Hiraku Ogino, Miho Kitamura, Koji Horiba, Hiroshi Kumigashira, Tetsuya Hasegawa
Journal of Materials Chemistry C, 8, 24, 8268, 8274, Royal Society of Chemistry ({RSC}), 2020
Scientific journal, We fabricated layered-perovskite Sr2IrO4-xF2x thin films by combining pulsed-laser deposition with topotactic fluorination and investigated their structures, electronic states, and electron transport properties. In the fluorination process, the insertion of fluorine into SrO rock-salt layers and the partial removal of oxygen occurred simultaneously while keeping Ir4+. The fluorine amount was evaluated to be 2x ≈ 3, which was much larger than the bulk value. Optical and photoemission measurements revealed that the effective total angular momentum Jeff = 3/2 is stabilized upon fluorination owing to the large electronegativity of fluorine. The Sr2IrO4-xF2x film exhibited a semiconducting behavior described by Efros-Shklovskii variable-range hopping with ρ(T) ≈ T-1/2. These results will be useful for modifying electronic states by anion doping to explore unprecedented physical properties in Ruddlesden-Popper-type iridates. This journal is

Investigation of ferrimagnetism and ferroelectricity in AlxFe2−xO3 thin films
Badari Narayana Rao, Shintaro Yasui, Tsukasa Katayama, Ayako Taguchi, Hiroki Moriwake, Yosuke Hamasaki, Mitsuru Itoh
Journal of Materials Chemistry C, 8, 2, 706, 714, Royal Society of Chemistry ({RSC}), 2020
English, Scientific journal, AlxFe2-xO3 (x-AFO) thin films, belonging to the kappa-Al2O3 family, are interesting because they show room temperature ferrimagnetism and have a polar crystal structure. These types of materials are being studied to observe simultaneous ferrimagnetism and ferroelectricity, and the possibility of coupling between the two. However, it is difficult to realise ferroelectric properties at room temperature, due to the low resistivity of the films. In this work, we have optimized the synthesis conditions to obtain x-AFO (0.5 <= x <= 1) thin films with high resistance. While magnetic measurements confirmed room temperature ferrimagnetism of the films, the maximum magnetization was observed for the composition x = 0.8. In addition, the Curie temperature was found to be influenced by oxygen pressure during deposition. Ferroelectric measurements on the films showed small remnant polarization (similar to 0.5-2 mu C cm(-2)). In contrast, the predicted polarization from first principles calculations was calculated to be between 21 and 26 mu C cm(-2). The analysis also suggested that ferroelectric domain-switching occurs through shearing of in-plane oxygen layers. The presence of multiple in-plane domains, which oppose polarization switching of adjacent domains, is suggested to be the cause of the small observed polarization. The magnetocapacitance measurements showed weak magnetic coupling with the capacitance.

Two-Dimensional Fluorine Distribution in a Heavily Distorted Perovskite Nickel Oxyfluoride Revealed by First-Principles Calculation
Kurauchi, Y., Katayama, T., Chikamatsu, A., Hasegawa, T.
Journal of Physical Chemistry C, 123, 51, 31190, 31195, American Chemical Society ({ACS}), 26 Dec. 2019
Scientific journal

Theoretical Investigation of the Role of the Nitride Ion in the Magnetism of Oxynitride MnTaO₂N
Mo, S., Kurauchi, Y., Katayama, T., Hirose, Y., Hasegawa, T.
Journal of Physical Chemistry C, 123, 41, 25379, 25384, American Chemical Society ({ACS}), 17 Oct. 2019
Scientific journal

Ferroelectric and ferrimagnetic properties of ε-RhxFe2−xO3 thin films
Tsukasa Katayama
Journal of the Ceramic Society of Japan, 127, 7, 474, 477, CERAMIC SOC JAPAN-NIPPON SERAMIKKUSU KYOKAI, 01 Jul. 2019
English, Scientific journal, RhxFe2-xO3 solid-solution epitaxial thin films were prepared by pulsed laser deposition. The ferroelectricity of Rh0.15Fe1.85O3 thin film was presented by the result of piezoelectric force microscopy phase and amplitude loops measured at room temperature. Ferrimagnetism at room temperature is confirmed by the magnetic hysteresis loop. The coercive field of Rh0.15Fe1.85O3 thin film at 5K is 12 kOe, which is larger than that of epsilon-Fe2O3. Rh0.15Fe1.85O3 is confirmed to be a room temperature multiferroic material. (C) 2019 The Ceramic Society of Japan. All rights reserved.

Improved crystalline quality and electric conductivity in infinite-layer SrFeO₂ films through Sm substitution
Katayama, T., Chikamatsu, A., Kumigashira, H., Hasegawa, T.
Applied Physics Letters, 114, 23, 10 Jun. 2019
Scientific journal

P-Type Conductivity and Room-Temperature Ferrimagnetism in Spinel MoFe ₂ O ₄ Epitaxial Thin Film
Katayama, T., Kurauchi, Y., Mo, S., Gu, K., Chikamatsu, A., Galiullina, L., Hasegawa, T.
Crystal Growth and Design, 19, 2, 902, 906, American Chemical Society ({ACS}), 06 Feb. 2019
Scientific journal

Selective fluorination of perovskite iron oxide/ruthenium oxide heterostructures: Via a topotactic reaction
Chikamatsu, A., Suzuki, Y., Maruyama, T., Onozuka, T., Katayama, T., Ogawa, D., Hasegawa, T.
Chemical Communications, 55, 17, Royal Society of Chemistry ({RSC}), 2019
Scientific journal

Fabrication and Characterization of Multiferroic Al0.5Fe1.5O3 Epitaxial Thin Films
Tsukasa Katayama
MRS Advances, 4, 9, 539, 544, Springer Science and Business Media LLC, 2019
English, Scientific journal, ABSTRACTSingle-phase multiferroic materials have attracted considerable attention among scientists, due to the strong drive in industry towards device miniaturization, addition of new functionalities, etc. Currently, most of the discovered materials have at-least one ferroic order active only at low temperatures, thereby hindering their induction into practical devices. κ-Al2O3-type AlxFe2-xO3 (x-AFO) oxides belong to a new class of metastable multiferroic compounds (space group: Pna21), with relatively high Curie temperatures. The current work investigates the effect of thin film deposition conditions on the ferroelectric and ferrimagnetic properties of Al0.5Fe1.5O3 (0.5-AFO). Substrate temperature and oxygen partial pressure during deposition were found to be the critical parameters in obtaining high quality films. Optimizing the deposition conditions of 0.5-AFO enabled observation of both ferroelectricity and ferrimagnetism at room temperature.

Magnetic properties of Single Crystal GaFeO3
Tsukasa Katayama
MRS Advances, 4, 1, 61, 66, Springer Science and Business Media LLC, 2019
English, Scientific journal, Abstractκ-Al2O3-type GaFeO3 is a promising multiferroic material due to the coexistence of a large spontaneous magnetization and polarization near room temperature. In the current study, we present the magnetic properties of single crystalline GaFeO3 and compare it with that of ε-Fe2O3. Magnetic measurements revealed that spontaneous magnetization appears below 540 K in two steps, similar to that reported for ε−Fe2O3. Partial magnetic ordering takes place at 540 K (TN1), with Fe3+ ions in two distorted octahedral sites ordering antiparallel to one another. Upon further cooling at 200 K (TN2), the remaining Fe3+ ions in regular octahedra and tetrahedra order antiparallel to one another. Substitution of Ga for Fe in ε-Fe2O3 leads to a decrease in TN1 and TN2 from 850 to 540 K and from 480 to 200 K, respectively, caused by a dilution of magnetic Fe by nonmagnetic Ga and preferential site occupation of Ga.

Reactive solid phase epitaxy of layered aurivillius-type oxyfluorides Bi ₂ TiO ₄ F ₂ using polyvinylidene fluoride
Katayama, T., Mo, S., Maruyama, T., Chikamatsu, A., Hasegawa, T.
Dalton Transactions, 48, 16, Royal Society of Chemistry ({RSC}), 2019
Scientific journal

Epitaxial Growth of Orthorhombic GaFeO₃ Thin Films on SrTiO₃ (111) Substrates by Simple Sol-Gel Method.
Zhang, Minghui, Yasui, Shintaro, Katayama, Tsukasa, Rao, Badari Narayana, Wen, Haiqin, Pan, Xiuhong, Tang, Meibo, Ai, Fei, Itoh, Mitsuru
Materials (Basel, Switzerland), 12, 2, 2019, [International Magazine]
English, Scientific journal, A Sol-gel method assisted with spin-coating has been successfully used to grow orthorhombic GaFeO₃ epitaxial films on SrTiO₃ (111) substrates for the first time. The film with Pna2₁ crystal structure has been grown along the c-axis. The rocking curve of (004) reflection shows that the Full-Width at Half-Maximum (FWHM) value could be determined to be 0.230°, indicating good single crystallinity and high quality. X-ray Φ scan reveals a three-fold symmetry of the substrate and a six-fold symmetry of the film, respectively. The in-plane domains rotate 60° from each other in the film. Uniform film with dense structure, columnar grains with similar grain size was obtained. The thickness of the film was evaluated to be ~170 nm. The roughness value (RMS) measured by AFM was 4.5 nm, revealing a flat film. The in-plane Magnetization versus Magnetic field (M-H) curve at 5 K performs a typical ferri- or ferromagnetic hysteresis loop with a saturated magnetization (Ms) value of 136 emu/cm³. The Curie temperature could be determined to be 174 K. Compared to Pulsed Laser Deposition (PLD), the sol-gel method can prepare large area films with low cost. These new films show promising applications in multiferroic devices.

Synthesis of Single Crystalline SrRuO₃ Sheet
Gu Ke, Katayama Tsukasa, Yasui Shintaro, Chikamatsu Akira, Itoh Mitsuru, Hasegawa Tetsuya
JSAP Annual Meetings Extended Abstracts, 2018.2, 1505, 1505, The Japan Society of Applied Physics, 05 Sep. 2018
Japanese

Ferroelectric and Magnetic Properties in Room-Temperature Multiferroic Ga_xFe_2−xO₃ Epitaxial Thin Films
Katayama, T., Yasui, S., Hamasaki, Y., Shiraishi, T., Akama, A., Kiguchi, T., Itoh, M.
Advanced Functional Materials, 28, 2, 1704789, 1704789, Wiley-Blackwell, 27 Jan. 2018
Scientific journal

Fabrication of Fluorite-Type Fluoride Ba_0.5Bi_0.5F_2.5 Thin Films by Fluorination of Perovskite BaBiO₃ Precursors with Poly(vinylidene fluoride)
Chikamatsu, A., Kawahara, K., Shiina, T., Onozuka, T., Katayama, T., Hasegawa, T.
ACS Omega, 3, 10, 2018
Scientific journal

Effect of Cr substitution on ferrimagnetic and ferroelectric properties of GaFeO3 epitaxial thin films
Katayama, Tsukasa, Osakabe, Takuya, Yasui, Shintaro, Hamasaki, Yosuke, Rao, Badari Narayana, Zhang, Minghui, Itoh, Mitsuru
Applied Physics Letters, 113, 16, 162901, 162901, AIP Publishing, 2018
Scientific journal

Ferromagnetism with strong magnetocrystalline anisotropy in A-site ordered perovskite YBaCo2O6 epitaxial thin films prepared via wet-chemical topotactic oxidation
Katayama, Tsukasa, Chikamatsu, Akira, Hirose, Yasushi, Minohara, Makoto, Kumigashira, Hiroshi, Harayama, Isao, Sekiba, Daiichiro, Hasegawa, Tetsuya
Journal of Materials Chemistry C, 6, 13, 3445, 3450, ROYAL SOC CHEMISTRY, 2018
English, Scientific journal, A-site cation-ordered perovskite cobaltite, RBaCo2Ox (R = rare earth element), exhibits fascinating physical properties, such as spin-state ordering and high oxygen conductivity, because of the large tetragonal distortion of the Co orbital. However, the distorted coordination geometry prefers oxygen vacancies, resulting in a difficulty in obtaining the stoichiometric phase (x = 6). For example, x in YBaCo2Ox, which has largely distorted Co orbitals because of the small size of Y3+, has so far been limited to 5.52. To expand the available range of x, in this study, we performed a low-temperature topotactic oxidation of YBaCo2O5.3 epitaxial films using a strong oxidizing agent NaClO. The x value can be varied in a wide range of 5.3-6.0, maintaining the A-site cation-ordered perovskite structure, by changing the pH and temperature of NaClO. The single crystalline film with x = 6 exhibits large tetragonal distortion (c/a = 0.968) because of the small ionic radius of Y3+ and substrate-induced tensile strain. Unlike antiferromagnetic insulating YBaCo2O5.5, the fully oxidized film with x = 6 exhibits in-plane ferromagnetism and metallicity with a Curie temperature of 130 K possibly because of the double-exchange interaction between Co3+ and Co4+. Moreover, the YBaCo2O6 film exhibits huge magnetic anisotropy with a magnetic anisotropy constant of 1.5 x 10(8) erg cm(-3), demonstrating that the A-site cation-ordered perovskite structure is promising for obtaining high magnetocrystalline anisotropic materials.

Ferrimagnetism and Ferroelectricity in Cr-Substituted GaFeO3 Epitaxial Films
Tsukasa Katayama, Shintaro Yasui, Takuya Osakabe, Yosuke Hamasaki, Mitsuru Itoh
Chemistry of Materials, 30, 4, 1436, 1441, American Chemical Society ({ACS}), 2018
Scientific journal

Epitaxial thin film growth of garnet-, GdFeO3-, and YMnO3-type LuFeO3 using pulsed laser deposition
Tsukasa Katayama, Yosuke Hamasaki, Shintaro Yasui, Akiko Miyahara, Mitsuru Itoh
Thin Solid Films, 642, 41, 44, Elsevier {BV}, Nov. 2017
Scientific journal

Ferroelectricity and structure of κ-alumina-type ferroelectric
Itoh Mitsuru, Katayama Tsukasa, Yasui Shintaro, Taniyama Tomoyasu, Moriwake Hiroki, Konishi Ayako, Kiguchi Takanori, Shiraishi Takahisa, Akama Akihiro, Hamasaki Yosuke
Meeting Abstracts of the Physical Society of Japan, 72.2, 2321, 2321, The Physical Society of Japan, 2017
Japanese

Electric Transport Characteristics of Gallium Iron Oxide Epitaxial Thin Film
Katayama, Tsukasa, Yasui, Shintaro, Hamasaki, Yosuke, Itoh, Mitsuru
Mrs Advances, 2, 56, 2017
Scientific journal

Topotactic fluorination of perovskite strontium ruthenate thin films using polyvinylidene fluoride
Kawahara, K., Chikamatsu, A., Katayama, T., Onozuka, T., Ogawa, D., Morikawa, K., Ikenaga, E., Hirose, Y., Harayama, I., Sekiba, D., Fukumura, T., Hasegawa, T.
Crystengcomm, 19, 2, 313, 317, 2017
Scientific journal, We investigated the polyvinylidene fluoride (PVDF)-mediated topotactic fluorination of two perovskite ruthenate thin films with different crystallographic structures, non-layered perovskite SrRuO3 and layered perovskite Sr2RuO4. While the former ruthenate did not react with PVDF, the latter was fluorinated to form Sr2RuO3F2 with a structure that exhibited a largely expanded c-axis. These results indicate that fluorine ions are preferentially inserted into the SrO rocksalt blocks in perovskite ruthenates, and that the oxygen atoms in the precursor oxide were partially removed upon fluorination. Finally, we noted that the Sr2RuO3F2 film was insulating, with a resistivity of 4.1 × 10 Ω cm at 300 K. It should be noted that this resistivity is five orders of magnitude higher than that of the metallic Sr2RuO4 film (6.7 × 104 Ω cm).

Reversible Changes in Resistance of Perovskite Nickelate NdNiO3 Thin Films Induced by Fluorine Substitution
Onozuka, Tomoya, Chikamatsu, Akira, Katayama, Tsukasa, Hirose, Yasushi, Harayama, Isao, Sekiba, Daiichiro, Ikenaga, Eiji, Minohara, Makoto, Kumigashira, Hiroshi, Hasegawa, Tetsuya
Acs Applied Materials & Interfaces, 9, 12, 10882, 10887, AMER CHEMICAL SOC, 2017
English, Scientific journal, Perovskite nickel oxides are of fundamental as well as technological interest because they show large resistance modulation associated with phase transition as a function of the temperature and chemical composition. Here, the effects of fluorine doping in perovskite nickelate NdNiO3 epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiO3-xFx films is controlled with precision by varying the reaction time. The fully fluorinated film (x approximate to 1) is highly insulating and has a bandgap of 2.1 eV, in contrast to NdNiO3, which exhibits metallic transport properties. Hard X-ray photoelectron and soft X-ray absorption spectroscopies reveal the suppression of the density of states at the Fermi level as well as the reduction of nickel ions (valence state changes from +3 to +2) after fluorination, suggesting that the strong Coulombic repulsion in the Ni 3d orbitals associated with the fluorine substitution drives the metal-to-insulator transition. In addition, the resistivity of the fluorinated films recovers to the original value for NdNiO3 after annealing in an oxygen atmosphere. By application of the reversible fluorination process to transition-metal oxides, the search for resistance-switching materials could be accelerated.

First-Principles Calculations on the Crystal/Electronic Structure and Phase Stability of H-Doped SrFeO₂
Kurauchi, Y., Kamisaka, H., Katayama, T., Chikamatsu, A., Hasegawa, T.
Journal of Physical Chemistry C, 121, 13, 2017
Scientific journal

Control of crystal-domain orientation in multiferroic Ga_0.6Fe_1.4O₃ epitaxial thin films
Katayama, T., Yasui, S., Hamasaki, Y., Itoh, M.
Applied Physics Letters, 110, 21, 2017
Scientific journal

Chemical tuning of roomerature ferrimagnetism and ferroelectricity in ϵ-Fe₂O₃-type multiferroic oxide thin films
Katayama, T., Yasui, S., Hamasaki, Y., Osakabe, T., Itoh, M.
Journal of Materials Chemistry C, 5, 47, Royal Society of Chemistry ({RSC}), 2017
Scientific journal

Topotactic reductive synthesis of A-site cation-ordered perovskite YBaCo2Ox (x=4.5-5.5) epitaxial thin films
Katayama, Tsukasa, Chikamatsu, Akira, Fukumura, Tomoteru, Hasegawa, Tetsuya
Japanese Journal of Applied Physics, 55, 4, 04EJ05, Institute of Physics, 2016
English, Scientific journal, A-site cation-ordered perovskite YBaCo2Oxepitaxial films were synthesized by combining pulsed-laser deposition and topotactic reduction using CaH2. The oxygen contents (x) of the films could be controlled in a range of 4.5–5.5 by adjusting the reaction temperature. The c-axis length of the YBaCo2Oxfilms decreased with decreasing x when x ≥ 5.3 but drastically increased when x ∼ 4.5. In contrast, the in-plane lattice constants remained locked-in by the substrate after the reaction. The metal insulator transition observed in bulk YBaCo2O5.5was substantially suppressed in the present film, likely because of the epitaxial strain effect. The resistivity of the films was significantly enhanced by changing the x value from ∼5.5 to ∼4.5, reflecting the distortion of the CoOxlayers.

Formation of defect-fluorite structured NdNiO: _XH_y epitaxial thin films via a soft chemical route from NdNiO₃ precursors
Onozuka, T., Chikamatsu, A., Katayama, T., Fukumura, T., Hasegawa, T.
Dalton Transactions, 45, 30, 2016
Scientific journal

Experimental and theoretical investigation of electronic structure of SrFeO3-xFx epitaxial thin films prepared via topotactic reaction
Katayama, Tsukasa, Chikamatsu, Akira, Kamisaka, Hideyuki, Kumigashira, Hiroshi, Hasegawa, Tetsuya
Applied Physics Express, 9, 2, 25801, 25801, Institute of Physics, 2016
English, Scientific journal, We investigated the electronic structure of perovskite SrFeO3−xFx(0.6 ≤ x ≤ 1) films by optical absorption, photoemission, and X-ray absorption spectroscopies, as well as density functional theory (DFT)-based calculations. The optical bandgap expanded with x, yielding a wider direct bandgap for the SrFeO2F film than for the LaFeO3film. The DFT calculations suggested that the majority of FeO4F2octahedra in the SrFeO2F film had cis configurations and that the enlarged bandgap mainly originated from bond bending in the O–Fe–O chains. We experimentally observed the valence and conduction bands of the SrFeO2F film, and found them to be qualitatively consistent with the results of DFT-based calculations.

Epitaxial growth and electronic structure of oxyhydride SrVO₂H thin films
Katayama, T., Chikamatsu, A., Yamada, K., Shigematsu, K., Onozuka, T., Minohara, M., Kumigashira, H., Ikenaga, E., Hasegawa, T.
Journal of Applied Physics, 120, 8, 2016
Scientific journal

Topotactic synthesis of strontium cobalt oxyhydride thin film with perovskite structure
Katayama, T., Chikamatsu, A., Kamisaka, H., Yokoyama, Y., Hirata, Y., Wadati, H., Fukumura, T., Hasegawa, T.
AIP Advances, 5, 10, 2015
Scientific journal

Topotactic reductive fluorination of strontium cobalt oxide epitaxial thin films
Katayama, T., Chikamatsu, A., Hirose, Y., Fukumura, T., Hasegawa, T.
Journal of Sol-Gel Science and Technology, 73, 3, 2015
Scientific journal

Effects of Cr substitution on the magnetic and transport properties and electronic states of SrRu O3 epitaxial thin films
Kurita, K., Chikamatsu, A., Shigematsu, K., Katayama, T., Kumigashira, H., Fukumura, T., Hasegawa, T.
Physical Review B - Condensed Matter and Materials Physics, 92, 11, 2015
Scientific journal

Topotactic fluorination of strontium iron oxide thin films using polyvinylidene fluoride
Katayama, T., Chikamatsu, A., Hirose, Y., Takagi, R., Kamisaka, H., Fukumura, T., Hasegawa, T.
Journal of Materials Chemistry C, 2, 27, 2014
Scientific journal

Metallic conductivity in infinite-layer strontium iron oxide thin films reduced by calcium hydride
Katayama, T., Chikamatsu, A., Hirose, Y., Kumigashira, H., Fukumura, T., Hasegawa, T.
Journal of Physics D: Applied Physics, 47, 13, 2014
Scientific journal

Electronic and transport properties of Eu-substituted infinite-layer strontium ferrite thin films
Chikamatsu, A., Matsuyama, T., Katayama, T., Hirose, Y., Kumigashira, H., Oshima, M., Fukumura, T., Hasegawa, T.
Journal of Crystal Growth, 378, 2013
Scientific journal

水溶性Sr₃Al₂O₆薄膜を用いたフリースタンディングPSCTO単結晶薄膜の作製とその蛍光特性
太宰卓朗, 安井伸太郎, 片山司, 谷山智康, 谷山智康, 伊藤満, 応用物理学会春季学術講演会講演予稿集(CD-ROM), 66th, 2019

マルチフェロイックh-DyFeO₃薄膜の誘電・磁気特性
笠原淳, 片山司, MO S., 近松彰, 安井伸太郎, 伊藤満, 長谷川哲也, 応用物理学会秋季学術講演会講演予稿集(CD-ROM), 80th, 2019

マルチフェロイックFe₂Mo₃O₈薄膜における磁気転移温度の向上
MO S., 片山司, 近松彰, 芝田悟朗, 芝田悟朗, 藤森淳, 安井伸太郎, 伊藤満, 長谷川哲也, 応用物理学会秋季学術講演会講演予稿集(CD-ROM), 80th, 2019

GaFeO₃単結晶基板を用いたSc_0.5Fe_1.5O₃薄膜の単ドメイン化
立山昂輝, 片山司, 越阪部拓也, 太宰卓朗, YU Jianding, HE Huan, WANG Hui, 濱嵜容丞, 安井伸太郎, 谷山智康, 伊藤満, セラミックス基礎科学討論会講演要旨集, 56th, 2018

パルスレーザー堆積法によるFe₂Mo₃O₈エピタキシャル膜の合成
毛司辰, 片山司, 近松彰, 安井伸太郎, 伊藤満, 長谷川哲也, 応用物理学会秋季学術講演会講演予稿集(CD-ROM), 79th, 2018

κ-Al₂O₃型(A,Fe)₂O₃(A=Al,Ga,Fe,Sc,Rh,In)酸化物の強誘電性と磁性
伊藤満, 濱嵜容丞, 片山司, 越阪部拓也, 安井伸太郎, 谷山智康, 日本物理学会講演概要集(CD-ROM), 72, 1, 2017

κ-Al₂O₃型(A,Fe)₂O₃(A=Al,Ga,Fe,Rh,Sc,In)薄膜の磁性と強誘電性
伊藤満, 濱嵜容丞, 片山司, 安井伸太郎, 谷山智康, 日本化学会春季年会講演予稿集(CD-ROM), 97th, 2017

元素置換サイト設計による室温マルチフェロイックGaFeO₃型酸化物薄膜の磁気誘電特性制御
片山司, 安井伸太郎, 浜嵜容丞, 越阪部拓也, 伊藤満, 応用物理学会春季学術講演会講演予稿集(CD-ROM), 64th, 2017

マルチフェロイックGaFeO₃薄膜の磁気及び強誘電特性へのCr置換効果
越阪部拓也, 片山司, 安井伸太郎, 谷山智康, 伊藤満, 応用物理学会秋季学術講演会講演予稿集(CD-ROM), 78th, 2017

GaFeO₃型マルチフェロイック材料のドメイン構造
片山司, 安井伸太郎, 浜嵜容丞, 森分博紀, 小西綾子, 木口賢紀, 白石貴久, 赤間章裕, 伊藤満, 応用物理学会秋季学術講演会講演予稿集(CD-ROM), 78th, 2017

ε-Fe₂O₃薄膜の構造と磁気特性へのRh置換効果
越阪部拓也, 浜嵜容丞, 片山司, 安井伸太郎, 谷山智康, 伊藤満, 応用物理学会秋季学術講演会講演予稿集(CD-ROM), 77th, 2016

マルチフェロイックGa_xFe_2-xO₃薄膜の磁気誘電特性
片山司, 浜嵜容丞, 安井伸太郎, 伊藤満, 応用物理学会秋季学術講演会講演予稿集(CD-ROM), 77th, 2016

Development of charge and spin functionality via domain engineering
Grants-in-Aid for Scientific Research
01 Apr. 2020 - 31 Mar. 2024
片山 司
遷移金属酸化物は遷移金属元素の種類と結晶構造の組み合わせにより、光触媒・高温超伝導・巨大磁気抵抗・強誘電などの多種多様な物性を示すため、基礎研究から応用開発まで精力的な新奇物性探索が展開されている。中でも最近注目され始めたのが、ナノドメイン構造制御による機能拡張である。例えば、ドメイン境界を活用することでバンドギャップ以上のエネルギーの光起電力を得ることや、磁気スピン秩序を反強磁性から強磁性に変調すること、さらに二次元超伝導の発現など様々な新奇物性が報告されている。この様なドメイン由来の新奇物性を広く活用する上で、高密度ドメインを酸化物中に形成することが重要となる。しかしながら、これまで酸化物薄膜でのドメイン密度(単位面積当たりのドメイン数)は小さく、マクロ物性への影響は限られていた。2021年度、申請者はマルチフェロイック材料の六方晶希土類酸化物(h-RFeO3)系材料に注目し、研究を進めた。申請者は希土類イオンの大きさを制御することで、h-RFeO3薄膜内にP63cm構造とP-3C構造から成る高密度ドメインを膜内に形成し、この系で初めての室温反強誘電特性の発見に成功した。さらに、膜厚を系統的に変化させることで強誘電―反強誘電相転移を温度変化により引き起こすことに成功した[B. Chen, T. Katayama* et al., J. Mater. Chem. C 10, 5621 (2022).]。またこの反強誘電-強誘電転移温度はc/a値の制御により変化させることができた。さらに申請者はダブルペロブスカイトRBaCo2O5.5を対象に研究を進め、結晶ドメインを導入することで磁化の変調を可能にした[T. Katayama* et al., Chem. Mater. 33, 5675 (2021).]。
Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B), 20H02614

Investigation of single crystalline oxide sheet
Grants-in-Aid for Scientific Research
01 Apr. 2018 - 31 Mar. 2020
Katayama Tsukasa
I fabricated single-crystalline SrRuO3 and BaTiO3 flexible sheets with a few millimeters lateral size. The SrRuO3 sheet exhibits high crystallinity and conductivity. Meanwhile, ferroelectricity of the BaTiO3 sheet was successfully observed by the polarization hysteresis loop measurements. In addition to the simplicity, this method is low cost as a substrate is reusable. Accordingly, the proposed method could enhance a development of various kinds of large-size functional oxide sheets.
Japan Society for the Promotion of Science, Grant-in-Aid for Early-Career Scientists, The University of Tokyo, 18K14122

Investigation of multiferroic GaFeO3-type oxide thin films
Grants-in-Aid for Scientific Research
26 Aug. 2016 - 31 Mar. 2018
Katayama Tsukasa
I investigated chemical substitution effect on magnetic and ferroelectric properties of multiferroic GaFeO3-type oxide thin films. I found that leakage current significantly decreases with Sc doping, resulting in clear ferroelectric hysteresis loops. Furthermore, I found that the magnetic properties are widely controllable via chemical substitution.
Japan Society for the Promotion of Science, Grant-in-Aid for Research Activity Start-up, 16H06794

Design of perpendicularly magnetized multiferroic
Grants-in-Aid for Scientific Research
01 Apr. 2015 - 31 Mar. 2018
ITOH MITSURU, YASUI Shintaro, TANIYAMA Tomoyasu, HAMASAKI Yosuke, MORIWAKE Hiroki, KATAYAMA Tsukasa
This study was conducted for the purpose of the materials design and structural design of the meultiferroelectric device in order to control of their magnetization by the external electric field. First, thin films of the materials with κ-alumina-type structure were fabricated to investigate ferroelectric and ferrimagnetic properties. Ferroelectricity was confirmed for the all the prepared films. Secondary, ScFeO3 film with YMnO3-type structure, in magnetization is aligned along the perpendicular axis of the film, was fabricated for the fist time. Perpendicular magnetization and ferroelectricity were confirmed at the room temperature. Multiferroic κ-akumina-type this films could be also prepared by the sole-gel and ALD technique.
Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Tokyo Institute of Technology, 15H02292