Published papers

CuCr_2_S_4_ shows a p-type metallic conductivity and ferromagnetism with Curie temperatures T_c_ of 375 K, and exhibit magnetic anomaly at 84 K (=T_m_). CuCr_2_S_4_ exhibits the coexistence of both the ferromagnetism and spin-glass with the irreversible magnetic effect below T_m_. The partial 20% Mg-substitution for Cu in CuCr_2_S_4_ slightly reduces the T_c_ and T_m_. Magnetorsistance of Mg_x_Cu_1-x_Cr_2_S_4_ can be expressed as Δρ=aH^2^-bH^n^ below T_m_. The parameters a, b, and n are temperature-dependent adjustable parameters. These unique MR behavior would be attributed to the coexistence of the ferromagnetism and spin-glass below T_m_. Mg-substitution decreased the conducting carrier, which forms the conduction band whose orbital motion gave the positive MR contribution. Therefore, Mg-substitution gave smaller positive MR contribution and consequently gave larger negative MR (max. 7% at 10 K) in Mg_0.2_Cu_0.8_Cr_2_S_4_. These results suggest that Cr 3d band filling control can be expected to give functional magnetoelectric transport properties in CuCr_2_S_4_.

FeCr_2_S_4_ has a spin-glass phase below 60K (=T_g_) and a ferrimagnetic phase in the temperature range 60–170K (T_C_=170 K). Mn doping gives an increase of a paramagnetic phase which coexists with a ferrimagnetic phases, and shift of magnetic transition temperature (T_g_=45K, T_C_=130K for Fe_0.5_Mn_0.5_Cr_2_S_4_). The conduction property changes into variable range hopping (VRH) from thermal activated transport below T_g_ for Fe_1-x_Mn_x_Cr_2_S_4_. This conducting transition would be associated with the correlated Fe 3d orbital ordering below T_g_.

Resistivity, thermopower and Hall coefficient have been systematically measured for polycrystalline samples of (Bi_1-x_Pb_x_)_1.8_Ba_2_Rh_1.9_O_y_. An upturn of the resistivity and the Hall coefficient below 50 K is ascribed to decrease in the density of states due to a pseudogap formation. The thermopower and Hall coefficient are nearly identical to those of the thermoelectric oxide Bi_2_Sr_2_Co_2_O_y_. This suggests a nearly identical density of states between the two oxides, which is also verified from a first-principle calculation, where a broader 4d orbital and a larger rhodium-rhodium distance along the in-plane direction accidentally give nearly the same density of states as those of the layered cobalt oxides.

CuCr_2_S_4_ shows a p-type metallic conductivity and ferromagnetism with Curie temperatures T_c_ of 375 K, and exhibits magnetic anomaly at 84 K (=T_m_). The partial Ge substitution for Cu in CuCr_2_S_4_ greatly reduces T_c_. Moreover Ge-substitution gives metal–nonmetal transition at x=1/8 in Cu_1-x_Ge_x_Cr_2_S_4_. These results suggest that Cr 3d band filling control can be expected to give functional magnetoelectric transport properties in CuCr_2_S_4_.

We measured transport properties in layered cobalt oxides Na_x_CoO_2_, that are parent materials of the superconductor Na_x_CoO_2_·yH_2_O, in order to reveal so-far unsettled origin of these electronic and magnetic features. Various sodium nonstoichiometric samples Na_x_CoO_2_ (x=0.47,0.62,1.0) have been carefully synthesized to control the carrier density and the electronic state of cobalt ions. We found a large negative magnetoresistance (MR) effect in the vicinity of x~0.5 and 1.0. The x=0.47 and 1.0 samples show negative MR, -45% and -10%, respectively, at 90 kOe and 4 K. These negative MR suggest the pseudogap formation due to the charge ordering in CoO_2_ layers, such as Ca_3_Co_4_O_9_ and Bi2-x_Pb_x_Sr_2_Co_2_O_y_, all of which exhibit similar behaviors. The suppression of the pseudogap by a magnetic field causes the large negative magnetoresistance.

We investigated the magnetic and transport properties of polycrystalline (Co_1-x_Ni_x_)_3_Sn_2_S_2_(0≦x≦1) to ascertain the magnetism of the new metallic ferromagnet Co_3_Sn_2_S_2_. In Co_3_Sn_2_S_2_ magnetization does not saturate up to 5.5 T at 10 K, and the estimated saturation moment (p_s_) is small (0.2μ_B_ per Co atom). In (Co_1-x_Ni_x_)_3_Sn_2_S_2_, the electrical resistivity shows metallic behavior without a hump but has a kink at T_C_. The T_C_ and magnetic susceptibility gradually decrease with increasing x, and there is no antiferromagnetic phase throughout the full range of composition. These results indicate that Co_3_Sn_2_S_2_ is a weak itinerant ferromagnet; while, the same order of the Rhodes–Wohlfarth p_c_/p_s_ value as CoS_2_ suggests the existence of a localized moment.

We have studied the electronic nature of the layered oxysulfide Sr_2-x_Ca_x_Cu_2_CoO_2_S_2_ which has strongly correlated CoO_2_ square planes. These materials exhibit narrow-gap semiconducting behavior and the conduction mechanism depends on the temperature. Valence band x-ray photoemission spectroscopy spectra show that Ca doping and the production of Sr defects cause the generation of a quasiparticle band near the Fermi level. These phenomena suggest that the materials are changing from correlated semiconductors to bad metals.

We investigated electronic and magnetic nature of A_2_Co_3_S_2_ (A=Sn,In) to reveal two different characteristics of metallic Co–Sn and strongly correlated electronic Co–S bondings. Co 2p X-ray photoelectron spectroscopy (XPS) shows a complicated and mixed-valence like feature with strong satellite. This origin can be explained by two different characteristics of Co–Sn/Co–S bonding nature. This feature is consistent with characteristic electron-density distributions obtained by the maximum-entropy method (MEM)-based pattern fitting analysis and the LSDA+U band calculation.

We designed a series of samples with Co substitution (Fe_1-x_Co_x_Cr_2_S_4_, 0≦x≦1) and studied the Co doping effect in FeCr_2_S_4_. Spin glass transitions are observed in lower temperature regions and ferrimagnetic transitions are observed in higher temperature regions. The paramagnetic-ferrimagnetic transition temperature (T_C_) increases with the amount of Co substitution. In order to clarify the magnetic character at the transition point, a critical exponent was obtained at about T_C_. The exponent changes from Ising to Heisenberg as x increases.

層状ビスマス硫化物 EuBiS2Fの電気的熱的輸送現象を報告する。この物質において、Eu 4f-Bi6p軌道混成とそれに起因する興味深い輸送現象が予想されている。しかしながら300 K以上の温度領域では、EuBiS2Fで観察される輸送現象は4f軌道が電気的性質に寄与しないLaBiS2Oと同様であり、Eu 4f-Bi 6p軌道混成の影響は定かで無い。EuBiS2Fの格子熱伝導率はSrBiS2Fに比べて小さい。 これはEuがSrに比べ、質量数が大きいことに起因する。

混合アニオン層状化合物(mixed anion layered compound, MALC)の一つであるSr2VFeAsO3の超伝導線材の作製に, Powder-in-tube法を用いて世界で初めて成功した. 熱処理は, 900℃で1-4時間とした. ゼロ抵抗が観測され, 低温外挿による絶対零度での超伝導電流密度(Jc)は, 45.3 Acm-2であった.

Mn3O4は15 GPa 以上の高圧下で、スピネル構造からポストスピネル構造に転移し、低温から室温にかけて、逐次結晶構造相転移が生じることがHiraiらにより確かめられている。本論文は、粉末の反射率分光法、及び密度汎関数理論を用いてポストスピネル型Mn3O4の電子磁気構造を、実験的、理論的に明らにする。Mn3O4は絶縁体であり、伝導体に存在するエネルギーの異なる２つの状態密度の存在は、実験的にも示された。

We report the results of 31P-nuclear magnetic resonance (NMR) measurements on the layered cobalt oxyphosphide Sr2Sc(Co1-xFe x )PO3 in order to investigate the magnetic properties at low temperatures from a microscopic view point. The 31P-Knight shifts measured at the resonance peak maximum of Sr2Sc(Co1-xFe x )PO3 have positive values and are T-independent in an entire temperature range, and the absolute value decreases with increasing Fe content. Also, the nuclear spin-lattice relaxation rate 1/T 1 is almost proportional to the temperature at low temperatures. The magnitude of 1/T 1 T decreases with increasing the Fe content, which suggests the decrease of the density of states around the Fermi level.

Polycrystalline samples of novel oxypnictides, Sr 2 ScCo 1-x Fe x PO 3 , with Fe-doped Co 1-x Fe x P layer were synthesized, and their resistivities and Seebeck coefficients were measured. These materials, Sr 2 ScCo 1-x Fe x PO 3 , crystallize in a layered structure which represent an alternative stack of ThCr 2 Si 2 type Co 1-x Fe x layers and K 2 NiF 4 type Sr 2 ScO 3 layers. The resistivities (ρ) decrease with decreasing temperature like a metal. The Seebeck coefficients (S) show negative values at room temperature, while the S show positive values for x = 0.04-0.7 at low temperature.

We have studied the magnetic properties of Ba-doped layered cobalt oxysulfide (Sr,Ba)_2_Cu_2_CoO_2_S_2_ with CoO_2_ squre-planes. The antiferromagnetic nature was considered to originate from the CoO_2_ layer dominantly. However, the effect of the inter-layer of CoO_2_ planes has not been clear. We therefore synthesized polycrystalline (Sr,Ba)_2_Cu_2_CoO_2_S_2_, and investigated the effect of inter-layer distance on the magnetic properties in detail. When Sr was substituted by Ba, the difference between zero-field-cooled and field-cooled magnetization was found to decrease clearly, and such difference was ascribed to reflect the increase of the distance between CoO_2_ layers owing to the difference of the ionic radii between smaller Sr and lager Ba. There is a possibility that two dimensional magnetic order has strengthened.

To find good sheath materials that react minimally with the superconducting core of iron-based superconducting wires, we investigated the reaction between polycrystalline SmFeAsO_0.92_F_0.08_ and the following seven metals: Cu, Fe, Ni, Ta, Nb, Cr and Ti. Each of the seven metals was prepared as a sheath-material candidate. The interfacial microstructures of SmFeAsO_0.92_F_0.08_ and these metal-sheath samples were analysed by an electron probe microanalyzer after annealing at 1000◦C for 20 h. Amongst the seven metal-sheath samples, we found that Cu was the best, because it reacted only very weakly with polycrystalline SmFeAsO_0.92_F_0.08_. Moreover, Cu is essential for superconducting wires as a stabilizing material. Metal sheaths made of Fe and Ni do not give rise to reaction layers, but large interdiffusion between these metals and polycrystalline SmFeAsO0.92F0.08 occurs. In contrast, metal sheaths made of Ta, Nb, Cr and Ti do form reaction layers. Their reaction layers apparently prevent electric current from flowing from the sheath material to the superconducting core. In general, through this research, Cu will be expected to be suitable not only as a stabilizing material but also as a sheath material for superconducting Sm-1111 wire fabricated by the ex situ PIT method.