Published papers

We have studied pressure effects on the magnetic and electronic nature of layered oxysulfide Sr_2_Cu_2_CoO_2_S_2_ which crystallizes in an unusual intergrowth structure. The derivative permeability dμ/dT curves have a kink near T_t_ at which the antiferromagnetic nature is changed from 2D to 3D; T_t_ extremely rises with increasing pressure (dT_t_/dP=13.5 K/kbar), indicating that 3D antiferromagnetic state is more stable under pressure. The T_t_ in the dμ/dT curves correspond to the bending point of the resistivity, and above T_t_ the nearest-neighbor hopping conduction occurs. These features would be discussed from the viewpoint of the coupled nature of charge carriers and 2D antiferromagnetic spin ordering.

We have studied the magnetic properties of layered oxysulfide Sr_2_Cu_2_(Co,Cu)O_2_S_2_ with the (Co,Cu)O_2_ planes. Magnetic susceptibility shows a broad maximum indicative of two-dimensional antiferromagnetic nature above 140K. Below 80K, the divergence between zero-field cooled and field cooled magnetizations is seen; long time decay is observed at 10 and 75K, while it is not observed at 45K. These feature is discussed from the viewpoint of the strongly and frustrated spin system which is different from the traditional spin glass.

We have synthesized and investigated the magnetic and electronic nature of layered manganese oxysulfide Sr_2_CuMnO_3_S which crystallizes in an unusual intergrowth structure with Cu_2_S_2_ and pyramidal MnO_5_ layers separated by Sr ions. A Cu 2p x-ray photoemission spectroscopy (XPS) spectrum of Sr_2_CuMnO_3_S indicates that the Cu ion is in the monovalent state, or Cu^+^. This feature of the Mn 2p XPS spectrum of Sr_2_CuMnO_3_S is similar to that of the parent compound La^3+^Mn^3+^O_3_ of colossal magnetoresistance materials. The MnO_5_ cluster model analysis suggests that the electronic nature of the pyramidal MnO_5_ layers belongs to a strongly correlated electron system, and the system is the so-called Mott insulator resulting from the strongly correlated electron nature. Magnetic susceptibility exhibits low-dimensional antiferromagnetic ordering of Mn^3+^ in pyramidal MnO_5_ layers. Fitting the data to Curie–Weiss law provides the evidence of strong second neighbor Mn–Mn interaction.

We have studied the magnetic dilution and electronic nature of Zn doping on the Mn site in the colossal magnetoresistant material La_0.7_Pb_0.3_MnO_3_ (x≦0.3). Small non-magnetic Zn^2+^ doping tends to separate the system into ferromagnetic clusters to weaken the long-range ferromagnetic order and to reduce the Curie temperature. The spin polarizability of the x=0〜0.3 samples is estimated to be 0.97〜1.00, indicating that the x=0〜0.3 samples are the spin polarized materials in which the conductivity is dominated by single-spin charge carriers. Small doping (x≧0.1) induces the metal–insulator transition and destroys the metallic state with long-range ferromagnetic order.

Magnetic properties in Na_0.75_CoO_2_ (space group P6_3_/mmc) are investigated by DC magnetic measurements from 2 to 300 K. The temperature-dependent magnetic susceptibility data show Curie–Weiss behaviour down to 15 K. Spin glass like characteristics are also found; the cusp anomaly seen at 3 K, irreverisbility between zero-field and field cooled magnetic susceptibility below 13 K.

The electronic structure of the delafossite-type compound PdCoO_2_ is calculated by the full potential linear muffin-tin orbital method and the magnetic property of the composition modified (defect injection introduced) compound Pd_0.72_Co_0.68_O_2_ was investigated. The calculation reveals that the density of states of PdCoO_2_ around the Fermi level is mainly composed of Pd 4d and Co 3d and O 2p orbitals, and the Fermi level is located just at the depression of the density of states. The magnetic nature of the composition modified Pd_0.72_Co_0.68_O_2_ compound is quite complex. The slow spin dynamics found below T_f_ can be interpreted by considering that the weak ferromagnetic moment in each CoO_6_ plane is itself disordered. This behavior is explained by the short-range character of the randomness which leads to a finite-size magnetized cluster, and suggests that the presence of these clusters would induce a spin-glass like feature for T

The nonstoichiometric compound Pd_x_Co_y_O_2_ (x=0.72,y=0.68) were carefully prepared by the metathetical reaction and characterized by the measurements of the powder X-ray diffraction, the inductively coupled plasma spectrum, the DC magnetic susceptibility, and the specific heat. The stoichiometric PdCoO_2_ exhibits Pauli-paramagnetism, while two magnetic phase transitions were indeed observed in Pd_0.72_Co_0.68_O_2_. The deviation of field-cooled from zero-field-cooled curves appears below 55 K, indicating a certain kind of antiferromagnetic phase transition. The well-defined cusp is seen with a peak at T_g_=4.5 K, indicating the existence of spin-glass like transition.

Optical reflectivity spectra of the hexagonal Ni_1-δ_S have been measured to study its electronic structures, in particular those associated with the metal–nonmetal transition in this compound. Samples with δ=0.002 and 0.02 are studied, which have transition temperatures T_t_ = 260 K and 150 K, respectively. Upon the transition, a pronounced dip appears in the infrared region of the reflectivity spectra. The optical conductivity spectra suggest that the nonmetallic phase is a carrier-doped semiconductor with an energy gap of 0.2–0.3 eV. The spectra also show that the gap becomes larger with decreasing temperature, and smaller with increasing . It is found that the overall spectrum in the nonmetallic phase can be explained in terms of a charge-transfer semiconductor, consistent with recent theoretical and photoemission studies of NiS.

Cu_4_(Sn_1-x_Ge_x_)S_4_に関して，室温付近での熱電性能指数の大きい物質を探索するために, 表記の系の物質の熱起電力と電気抵抗率を測った。ｘ増大により高抵抗相(低温)-低抵抗相(高温)の転移温度が室温を切るx＝0.7付近で抵抗率が小さくまた，熱起電力の大きい物質を得た。

We have extended the single-impurity cluster model to include the effects of charge transfer to the conduction band. It is found that these effects can have a considerable influence on the line shapes of core-level spectra of small-gap transition-metal compounds. Using a few additional, well-defined parameters to describe the interactions between the local cluster and the conduction band, and retaining the original intracluster parameters from previous single-impurity models, we find an improved agreement for the Ni 2p spectrum of NiS. The asymmetric line shape is well reproduced, keeping the correct satellite to main peak ratio, which is not possible using the three-peak structure of previous models. The new model is less successful in explaining effects beyond standard single-impurity models in more ionic compounds, such as the double-peak stucture of the main line of the core-level spectrum of NiO, where the interactions between the metal d site and the conduction band are expected to play a lesser role.

The electronic structures of the itinerant ferromagnets Cr_1-x_Te (x=0.05, 0.25, and 0.375) have been studied by photoemission spectroscopy. The valence-band spectra are compared with the density of states given by band structure calculations. In spite of the itinerant nature of the d electrons, disagreement between the photoemission spectra and the band-structure calculations exists in the magnitude of the d-band exchange splitting and the spectral weight at the Fermi level and 2–4 eV below it: The occupied d band for x=0.05 is shifted away from the Fermi level; the observed spectral weight at the Fermi level is significantly suppressed compared with the band structure calculations for x=0.05 and 0.375, where the nominal d-electron numbers are close to integers 4 and 3, respectively. Configuration-interaction cluster-model calculations have been made for x=0.05 and 0.375 to explain the spectral weight distribution in the high-binding-energy (2–4 eV) region in terms of electron-correlation effects. The d-d on-site Coulomb energy is estimated to be significant, U=2 eV, and nearly equal to or smaller than the charge-transfer energy Δ=2–3 eV.

We report the Mn doping effect in FeCr_2_S_4_, which does not possess heterovalency, or static Jahn-Teller distortion, but exhibits large magnetoresistance. In order to see the effects of substituting Fe^2+^ for Mn^2+^ on the conduction, magnetism, and magnetoresistance, we carried out an experimental study of the electrical and magnetic properties of Fe_0.5_Mn_0.5_Cr_2_S_4_. FeCr_2_S_4_ has a spin-glass phase below 60 K (=T_g_) and a ferrimagnetic phase in the temperature range 60-170 K (T_c_ = about 170 K). In Fe_0.5_Mn_0.5_Cr_2_S_4_, both the spin-glass and ferrimagnetic transitions are observed in lower temperature regions (T_g_=45 K, T_c_=140 K). FeCr_2_S_4_ is a semiconductor, but exhibits metallic-like behavior in the temperature range 150-170 K. Fe_0.5_Mn_0.5_Cr_2_S_4_ does not exhibit such metallic behavior, but anomalous transition of the temperature dependence of resistivity is observed in the temperature range 100-180 K. Fe_0.5_Mn_0.5_Cr_2_S_4_ exhibits a larger maximum negative magnetoresistance (MR) than FeCr_2_S_4_. Thses experimental data can be well explained on the basis of the polaron model.

The electronic structure of Na_0.75_CoO_2_ has been calculated by the LDA+U method, considering the location of Na ions. The magnetic moments of Co ions are strongly influenced by the bond distances between the Co and Na ions, and are unusually distributed. This implies the occurrence of a charge disproportionation state (2Co^+3.25^+Co^+3.25+d^+Co^+3.25–d^) in this material. The magnetic ground state of Na_0.75_CoO_2_ is therefore a spin-glass-like, short-range ordered state resulting from randomly distributed magnetic Co ions.