Browsing by Author "Yamauchi, Hisao"

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  • Anisotropic thermoelectric properties associated with dimensional crossover in quasi-one-dimensional SrNbO3.4+d (d ~ 0.03)
    (2011) Kobayashi, W; Hayashi, Y; Matsushita, M; Yamamoto, Y; Terasaki, I; Nakao, A; Nakao, H; Murakami, Y; Moritomo, Y; Yamauchi, Hisao; Karppinen, Maarit
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have grown large single crystals of SrNbO3.4+d (d∼0.03) with n=5 in the homologous series SrnNbnO3n+2 by using a traveling solvent floating-zone method and measured resistivity, thermopower, and thermal conductivity along all crystallographic axes. The thermoelectric properties are found to be highly anisotropic, which reflects a quasi-one-dimensional electronic structure. In particular, the thermopower along the b axis is −170 μV/K at 300 K, which is 1 order of magnitude higher than the −15 μV/K along the a axis and −25 μV/K along the c axis. A possible origin of the high anisotropy in the thermopower is discussed in terms of dimensional crossover associated with a structural modification.
  • Atomic layer deposition of Al-doped ZnO thin films
    (2013) Tynell, Tommi; Yamauchi, Hisao; Karppinen, Maarit; Okazaki, Ryuji; Terasaki, Ichiro
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Atomic layer deposition has been used to fabricate thin films of aluminum-doped ZnO by depositing interspersed layers of ZnO and Al 2O3 on borosilicate glass substrates. The growth characteristics of the films have been investigated through x-ray diffraction, x-ray reflection, and x-ray fluorescence measurements, and the efficacy of the Al doping has been evaluated through optical reflectivity and Seebeck coefficient measurements. The Al doping is found to affect the carrier density of ZnO up to a nominal Al dopant content of 5 at. %. At nominal Al doping levels of 10 at. % and higher, the structure of the films is found to be strongly affected by the Al 2O3 phase and no further carrier doping of ZnO is observed.
  • Characterization of magnetic properties of Sr2CuWO6 and Sr2CuMoO6
    (2014) Vasala, Sami; Saadaoui, Hassan; Morenzoni, Elvezio; Chmaissem, Omar; Chan, Ting-Shan; Chen, Jin-Ming; Hsu, Ying-Ya; Yamauchi, Hisao; Karppinen, Maarit
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this work we examine the low-temperature magnetic properties of the two double-perovskite compounds Sr2CuWO6 and Sr2CuMoO6 using magnetic susceptibility, muon spin rotation and relaxation, and neutron powder diffraction measurements. Additionally, the most relevant spin exchange interaction constants are derived from ab initio electronic structure calculations, aided by x-ray absorption spectroscopy. The compounds exhibit quasi-two-dimensional magnetic properties, with broad maxima at Tmax = 83 and 95 K for Sr2CuWO6 and Sr2CuMoO6, respectively. However, three-dimensional long-range order takes place below TN = 24(1) and 28(2) K for Sr2CuWO6 and Sr2CuMoO6, respectively. Our results show that the low-dimensional magnetic correlations are mainly due to the significant next-nearest-neighbor interactions in the ab plane of the double-perovskite structure, whereas three-dimensional long-range magnetic order is caused by weaker next-nearest-neighbor interactions along the c axis. Next-nearest-neighbor interactions are also slightly frustrated by weaker nearest-neighbor interactions within the ab plane. Based on these results we predict the low-temperature magnetic structure in these compounds to be type-II antiferromagnetic order of the double-perovskite lattice.
  • Competition between intragranular and intergranular tunneling magnetoresistance in polycrystalline Sr2FeMoO6
    (2006) Huang, Yun-Hui; Yamauchi, Hisao; Karppinen, Maarit
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Polycrystalline Sr2FeMoO6 (SFMO) samples with various grain sizes and densities of Fe/Mo disorder defects were synthesized from sol-gel-derived precursors by means of a sample-encapsulation technique. The samples with perfect Fe/Mo ordering exhibited metallic transport behavior and a large magnetoresistance (MR) effect, whereas the samples with a high-disorder density and small grain size showed a metal-insulator transition and lower MR values. It is suggested that a competition between intragranular and intergranular tunneling MR effects exists in the SFMO system. To confirm this competition, homocomposite samples consisting of two or three single-phase SFMO components with different grain sizes were designed. Large enhancement in low-field magnetoresistance (LFMR) was observed in the homocomposites, which could be attributed to the enhanced intergranular effect. It is thus concluded that the intergranular effect is more important to the LFMR than the intragranular effect.
  • Hybrid inorganic–organic superlattice structures with atomic layer deposition/molecular layer deposition
    (2014) Tynell, Tommi; Yamauchi, Hisao; Karppinen, Maarit
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    A combination of the atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques is successfully employed to fabricate thin films incorporating superlattice structures that consist of single layers of organic molecules between thicker layers of ZnO. Diethyl zinc and water are used as precursors for the deposition of ZnO by ALD, while three different organic precursors are investigated for the MLD part: hydroquinone, 4-aminophenol and 4,4′-oxydianiline. The successful superlattice formation with all the organic precursors is verified through x-ray reflectivity studies. The effects of the interspersed organic layers/superlattice structure on the electrical and thermoelectric properties of ZnO are investigated through resistivity and Seebeck coefficient measurements at room temperature. The results suggest an increase in carrier concentration for small concentrations of organic layers, while higher concentrations seem to lead to rather large reductions in carrier concentration.
  • Magnetic and transport properties of the spin-state disordered oxide La0.8Sr0.2Co1-xRhxO3-[delta]
    (2011) Shibasaki, Soichiro; Terasaki, Ichiro; Nishibori, Eiji; Sawa, Hiroshi; Lybeck, Jenni; Yamauchi, Hisao; Karppinen, Maarit
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report measurements and analysis of magnetization, resistivity, and thermopower of polycrystalline samples of the perovskite-type Co/Rh oxide La0.8Sr0.2Co1-xRhxO3-[delta]. This system constitutes a solid solution for a full range of x, in which the crystal structure changes from rhombohedral to orthorhombic symmetry with increasing Rh content x. The magnetization data reveal that the magnetic ground state immediately changes upon Rh substitution from ferromagnetic to paramagnetic with increasing x near 0.25, which is close to the structural phase boundary. We find that one substituted Rh ion diminishes the saturation moment by 9 μB, which implies that one Rh3+ ion makes a few magnetic Co3+ ions nonmagnetic (the low-spin state) and causes disorder in the spin state and the highest occupied orbital. In this disordered composition (0.05⩽x⩽0.75), we find that the thermopower is anomalously enhanced below 50 K. In particular, the thermopower of x=0.5 is larger by a factor of 10 than those of x=0 and 1, and the temperature coefficient reaches 4 μV/K2, which is as large as that of heavy-fermion materials such as CeRu2Si2.
  • Magnetic and transport properties of the spin-state disordered oxide La0.8Sr0.2Co1−xRhxO3−δ
    (2011) Shibasaki, S.; Terasaki, I.; Nishibori, E.; Sawa, H.; Lybeck, J.; Yamauchi, Hisao; Karppinen, Maarit
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report measurements and analysis of magnetization, resistivity, and thermopower of polycrystalline samples of the perovskite-type Co/Rh oxide La0.8Sr0.2Co1−xRhxO3−δ. This system constitutes a solid solution for a full range of x, in which the crystal structure changes from rhombohedral to orthorhombic symmetry with increasing Rh content x. The magnetization data reveal that the magnetic ground state immediately changes upon Rh substitution from ferromagnetic to paramagnetic with increasing x near 0.25, which is close to the structural phase boundary. We find that one substituted Rh ion diminishes the saturation moment by 9 μB, which implies that one Rh3+ ion makes a few magnetic Co3+ ions nonmagnetic (the low-spin state) and causes disorder in the spin state and the highest occupied orbital. In this disordered composition (0.05⩽x⩽0.75), we find that the thermopower is anomalously enhanced below 50 K. In particular, the thermopower of x=0.5 is larger by a factor of 10 than those of x=0 and 1, and the temperature coefficient reaches 4 μV/K2, which is as large as that of heavy-fermion materials such as CeRu2Si2.
  • Structural transformation and magnetic competition in Yb(Mn1-xFex)O3
    (2007) Huang, Yun-Hui; Karppinen, Maarit; Imamura, Naoki; Yamauchi, Hisao; Goodenough, John B.
     School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Structural and magnetic properties of the Yb(Mn1−xFex)O3 (0⩽x⩽1) system have been systematically investigated. Initial samples were prepared via a sol-gel method. A pure hexagonal phase was only obtained for samples with x⩽0.5. With high-pressure annealing, a pure orthorhombic perovskite phase was achieved for all the compositions. The Fe57 Mössbauer spectrum for x=0.5 shows that only Fe3+ ions exist in the system; there was no evidence of chemical inhomogeneities. With increasing x, the Néel temperature TN increases for both hexagonal and orthorhombic phases. The orthorhombic Yb(Mn0.5Fe0.5)O3 shows an interesting weak ferromagnetic state in the temperature range of 239–298K, the ferromagnetism disappearing abruptly on cooling below Tt=239K. The transition at Tt appears to be a reorientation of the spin axis of a type-G antiferromagnetic order from the orthorhombic a axis to the b axis in the (010) plane.
  • Synthesis, Oxygen Non-Stoichiometry and Magnetic Properties of REn+1ConO3n+1 Ruddlesden-Popper Phases
    (2001) Matvejeff, Mikko
     Helsinki University of Technology | Master's thesis