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Magnetic properties of Fe_1-xCo_xCr₂S₄

Ryuichiro Noda, Yoichi Kamihara, and Masanori Matoba

Journal of Applied Physics 99 08F712-08F714 2006年04月

研究論文（学術雑誌）, 共著, 査読有り

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.

Electronic nature of layered oxysulfide Sr_2-xCa_xCu₂CoO₂S₂ with CoO₂ planes

Kana Ito, Taro Endo, Takashi Kubodera, Yoichi Kamihara, Masanori Matoba, and Hijiri Kito

Journal of Applied Physics 99 08F705-08F707 2006年04月

研究論文（学術雑誌）, 共著, 査読有り

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.

Ni substitution effect on magnetic and transport properties in metallic ferromagnet Co₃Sn₂S₂

Takashi Kubodera, Hirotaka okabe, Yoichi Kamihara, and Masanori Matoba

Physica B 378-380 1142-1143 2006年02月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

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.

Low-temperature magnetoresistance of layered cobalt oxides Na_xCoO₂

Hirotaka Okabe, Jun Akimitsu, Takashi Kubodera, Masanori Matoba, Toru Kyomen and Mitsuru Itoh

Physica B 378-380 863-864 2006年02月

研究論文（学術雑誌）, 共著, 査読有り

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.

Metal-nonmetal transition in Ge doped spinel-type CuCr₂S₄

Yoichi Kamihara, Masanori Matoba, Toru Kyomen and Mitsuru Itoh

Physica B 378-380 1120-1121 2006年02月

研究論文（学術雑誌）, 共著, 査読有り

Transport properties of Fe_1-xMn_xCr₂S₄

Yoichi Kamihara, Yukinori Takeshita, Masanori Matoba, Toru Kyomen, and Mitsuru Itoh

Physica B 359-361 1207-1209 2005年05月

研究論文（学術雑誌）, 共著, 査読有り

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_.

Transport Properties and Electronic States in the Layered Thermoelectric Rhodate (Bi_1-xPb_x)_1.8Ba₂Rh_1.9O_y

Satoshi Okada, Ichiro Terasaki, Hirotaka Okabe and Masanori Matoba

Journal of the Physical Society of Japan 74 1525-1528 2005年05月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

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.

Magnetoresistance in spinel-type Mg_xCu_1-xCr₂S₄

Yoichi Kamihara, Masanori Matoba, Toru Kyomen, and Mitsuru Itoh

Solid State Communications 132 247-251 2004年09月

研究論文（学術雑誌）, 共著, 査読有り

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_.

Dielectric responses of the layered cobalt oxysulfide Sr₂Cu₂CoO₂S₂ with CoO₂ square planes

Satoshi Okada, Ichiro Terasaki, Hiromasa Ooyama, and Masanori Matoba

Journal of Applied Physics 95 6816-6818 2004年06月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

We have studied the dielectric responses of the layered cobalt oxysulfide Sr_2_Cu_2_CoO_2_S_2_ with the CoO_2_ square planes. With decreasing temperature below the Néel temperature, the resistivity increases like a semiconductor, and the thermopower decreases like a metal. The dielectric constant is highly dependent on temperature, and the dielectric relaxation is systematically changed with temperature, which is strongly correlated to the magnetic states. These behaviors suggest that carriers distributed homogeneously in the paramagnetic state at high temperatures are expelled from the antiferromagnetically ordered spin domain below the Néel temperature.

First-principles study of electronic and magnetic structure in layered cobaltite Na_0.75CoO₂

Hirotaka Okabe, Masanori Matoba, Toru Kyomen, and Mitsuru Itoh

Journal of Applied Physics 95 6831-6833 2004年06月

研究論文（学術雑誌）, 共著, 査読有り

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.

スピネル型硫化物Fe_1-xMn_xCr₂S₄における巨大磁気抵抗効果

神原陽一，竹下之典, 的場正憲，京免徹，伊藤満

日本応用磁気学会誌 28 347-350 2004年03月

研究論文（学術雑誌）, 共著, 査読有り

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.

Magnetic Phase Transition in Pd_xCo_yO₂

Hirotaka Okabe, Masanori Matoba, and Mitsuru Itoh

Physica B 329-333 948-949 2003年05月

研究論文（学術雑誌）, 共著, 査読有り

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.

Magnetic property and electronic structure of itinerant Pd_xCo_yO₂ magnets

Hirotaka Okabe, Masanori Matoba, Toru Kyomen, and Mitsuru Itoh

Journal of Applied Physics 93 7258-7260 2003年05月

研究論文（学術雑誌）, 共著, 査読有り

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<T_f_.

Doping effect on transport properties of layered oxysulfide Sr₂Cu₂CoO₂S₂ with CoO₂ square planes

Satoshi Okada, Hirotaka Okabe and Masanori Matoba

Physica B 329-333 916-917 2003年05月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

Layered cobalt oxysulfide Sr_2_Cu_2_CoO_2_S_2_ crystallizes in an unusual intergrowth structure which represents a combination of SrCu_2_S_2_ and SrCoO_2_. Sr_2_Cu_2_CoO_2_S_2_ is a p-type antiferromagnetic semiconductor with T_N_(=200 K). And below T_t_(=125 K), the antiferromagnetic nature is changed from 2D to 3D. We studied Seebeck coefficient of Sr_2_Cu_2_CoO_2_S_2_ and its Cu-doped compounds. Their Seebeck coefficients near Tt tend to enhance by the Cu-doping into CoO_2_ square planes. The curves of the temperature dependence of Sr_2_Cu_2_Cu_x_Co_1-x_O_2_S_2_ is changed near T_N_, 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 Sr_2_Cu_2_CoO_2_S_2_ is affected by their magnetism.

層状Mn酸化硫化物Sr₂CuMnO₃S及びSr₄Cu₂Mn₃O_7.5S₂における不純物置換効果

神原陽一, 長沼順子，安仲健太郎，石井守，上野広伸，的場正憲

日本応用磁気学会誌 27 666-669 2003年04月

研究論文（学術雑誌）, 共著, 最終著者, 査読有り

Sr_4-x_La_x_Cu_2_Mn_3_O_7.5_S_2_ with alternating Cu_2_S_2_ and perovskite oxide layers have been synthesized and characterized. The perovskite oxide layers in Sr_2_CuMO_3_S (M=Fe and Ga) and A_4_Cu_2_Mn_3_O_7.5_S_2_ (A=Sr and La) have the stacking sequence with -MO_2_-SrO-SrO-MO_2_- and -MnO_2_-AO-Mn_1.5_-AO-MnO_2_-, respectively. Sr_2_CuFe_1-x_Ga_x_O_3_S (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. Sr_4_Cu_2_Mn_3_O_7.5_S_2_ exhibits a phase transition from paramagnetic to antiferromagnetic states below 100K. In
Sr_4-x_La_x_Cu_2_Mn_3_O_7.5_S_2_ (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.

Pressure effect on magnetic nature of Sr₂Cu₂CoO₂S₂ with strongly correlated CoO₂ square-planes

Satoshi Okada, Masanori Matoba, Hajime Yoshida, Kenji Ohoyama, and Yasuo Yamaguchi

Journal of Physics and Chemistry of Solids 63 983-985 2002年08月

研究論文（学術雑誌）, 共著, 査読有り

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.

Magnetic properties of novel layered oxysulfide Sr₂Cu₂(Co,Cu)O₂S₂ with Cu-doped CoO₂ square-planes

Satoshi Okada, Masanori Matoba, Yoichi Kamihara, Tadamasa Takeuchi, Satoshi Fukumoto, Shunji Soyano, Hajime Yoshida, Kenji Ohoyama, and Yasuo Yamaguchi

Journal of Applied Physics 91 8861-8863 2002年08月

研究論文（学術雑誌）, 共著, 査読有り

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.

Magnetic and electronic nature of novel layered manganese oxysulfide Sr₂CuMnO₃S

Yoichi Kamihara, Masanori Matoba, Toru Kyomen, and Mitsuru Itoh

Journal of Applied Physics 91 8864-8866 2002年08月

研究論文（学術雑誌）, 共著, 査読有り

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.

Magnetic Anomalies in Na_xCoO₂ (x=0.75)

Tadamasa Takeuchi, Masanori Matoba, Tomoko Aharen, and Mitsuru Itoh

Physica B 312-313 719-720 2002年07月

研究論文（学術雑誌）, 共著, 査読有り

Magnetic Dilution and Electronic Nature in La_0.7Pb_0.3Mn_1-yZn_yO₃

Tadahiko Kanoh and Masanori Matoba

Journal of Physics and Chemistry of Solids 63 252 2002年07月

研究論文（学術雑誌）, 共著, 査読有り

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.

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