1. Electronic Structure of the Delafossite-type PdCoO2
The electronic structure of the delafossite-type compound PdCoO2 is calculated by the full potential linear muffin-tin orbital method and the magnetic property of the composition modified (defect injection introduced) compound Pd0.72Co0.68O2 was investigated. The calculation reveals that the density of states of PdCoO2 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 Pd0.72Co0.68O2 compound is quite complex. The slow spin dynamics found below Tf can be interpreted by considering that the weak ferromagnetic moment in each CoO6 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<Tf.
2. Magnetic Phase Transition in PdxCoyO2
The nonstoichiometric compound PdxCoyO2 (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 PdCoO2 exhibits Pauli-paramagnetism, while two magnetic phase transitions were indeed observed in Pd0.72Co0.68O2. 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 Tg=4.5 K, indicating the existence of spin-glass like transition.
3. Development of Sr2Cu(Fe,Ga)O3S and (Sr,La)4Cu2Mn3O7.5S2
New layered oxysulfides, Sr2CuFe1-xGaxO3S and Sr4-xLaxCu2Mn3O7.5S2 with alternating Cu2S2 and perovskite oxide layers have been synthesized and characterized. The perovskite oxide layers in Sr2CuMO3S (M=Fe and Ga) and A4Cu2Mn3O7.5S2 (A=Sr and La) have the stacking sequence with -MO2-SrO-SrO-MO2- and -MnO2-AO-Mn1.5-AO-MnO2-, respectively.
Sr2CuFe1-xGaxO3S (x=0, 0.05) is semiconducting. The electrical resistivity of the x=0.05 sample is ten times lower than that of the x=0 sample, indicating that the conduction path is in Ga-introduced iron oxide layer. Sr4Cu2Mn3O7.5S2 exhibits a phase transition from paramagnetic to antiferromagnetic states below 100K. In Sr4-xLaxCu2Mn3O7.5S2 (x=0, 0.1, 0.2, 0.4), the lattice parameters a and c decerease with increasing x. All samples are semiconducting. The electrical resistivity is lowered with increasing x; that of the x=0.4 sample is one-hundred times lower than that of the x=0 sample.
4. Pressure Effect on Magnetic Semiconductor Sr2Cu2CoO2S2
Layered cobalt oxysulfide Sr2Cu2CoO2S2 crystallizes in an unusual intergrowth structure which represents a combination of SrCu2S2 and SrCoO2. Sr2Cu2CoO2S2 is a p-type antiferromagnetic semiconductor with TN(=200 K). And below Tt(=125 K), the antiferromagnetic nature is changed from 2D to 3D. We studied Seebeck coefficient of Sr2Cu2CoO2S2 and its Cu-doped compounds. Their Seebeck coefficients near Tt tend to enhance by the Cu-doping into CoO2 square planes. The curves of the temperature dependence of Sr2Cu2CuxCo1-xO2S2 is changed near TN, and the change points shift to high temperature by Cu-doping. The behavior resembles in TN shift of the magnetic property. The transport properties of Cu-doped Sr2Cu2CoO2S2_is affected by their magnetism.
5. Giant Magnetoresistance of Spinel-Type Fe1?xMnxCr2S4
We report the Mn doping effect in FeCr2S4, which does not possess heterovalency, or static Jahn-Teller distortion, but exhibits large magnetoresistance. In order to see the e?ects of substituting Fe2+ for Mn2+ on the conduction, magnetism, and magnetoresistance, we carried out an experimental study of the electrical and magnetic properties of Fe0.5Mn0.5Cr2S4. FeCr2S4 has a spin-glass phase below 60 K ( = Tg) and a ferrimagnetic phase in the temperature range 60-170 K (Tc = about 170 K). In Fe0.5Mn0.5Cr2S4, both the spin-glass and ferrimagnetic transitions are observed in lower temperature regions (Tg=45 K, Tc=140 K). FeCr2S4 is a semiconductor, but exhibits metallic-like behavior in the temperature range 150-170 K. Fe0.5Mn0.5Cr2S4 does not exhibit such metallic behavior, but anomalous transition of the temperature dependence of resistivity is observed in the temperature range 100-180 K. Fe0.5Mn0.5Cr2S4 exhibits a larger maximum negative magnetoresistance (MR) than FeCr2S4. Thses experimental data can be well explained on the basis of the polaron model.
Hirotaka Okabe, Masanori Matoba, Toru Kyomen, and Mitsuru Itoh: “Magnetic property and electronic structure of itinerant PdxCoyO2 magnets”, Journal of Applied Physics 93 (2003) 7258-7260.
Hirotaka Okabe, Masanori Matoba, and Mitsuru Itoh: “Magnetic Phase Transition in PdxCoyO2”, Physica B329-333 (2003) 948-949.
神原陽一, 長沼順子，安仲健太郎，石井守，上野広伸，的場正憲: “層状Mn酸化硫化物Sr2CuMnO3S及びSr4Cu2Mn3O7.5S2における不純物置換効果”, 日本応用磁気学会誌 27 (2003) 666-669.
Satoshi Okada, Masanori Matoba, Hajime Yoshida, Kenji Ohoyama, and Yasuo Yamaguchi: “Pressure effect on magnetic nature of Sr2Cu2CoO2S2 with strongly correlated CoO2 square-planes”, Physica B329-333 (2003) 916-917.
神原陽一, 竹下之典, 的場正憲，京免徹，伊藤満: “スピネル型硫化物Fe1-xMnxCr2S4における巨大磁気抵抗効果”, 日本応用磁気学会誌 28 (2004) 347-350.
松井雅博：Crystallographic and Electronic Nature of Layered Oxysulfide Sr2Cu2MxZn1-xO2S2 (M=Cu,Mn)