Systematic investigation of electronic structure in BEDT-TTF based organic superconductors withT c above 10 K; κ-(BEDT-TTF)2 X (X=Cu(NCS)2, Cu[N(CN)2]Br,and Cu(CN)[N(CN)2])

The electronic structure of the title superconductors has been investigated by electrical resistivity, complex susceptibility, and electron paramagnetic resonance (EPR) measurements. The superconducting properties (pressure dependence ofT _c , magnetic penetration depth, upper critical field, and so on) of these three salts are similar to each other, while transport properties in the normal state have shown a large variety in the temperature dependence. In order to clarify the electronic structure in the normal state, the EPR parameters, the spin susceptibility (χ _spin), and the linewidth (ΔH _pp), are compared. An anomalous temperature dependence of theg-value has been observed below 150 K in the Cu(NCS)₂ and Cu(CN)[N(CN)₂] salts.     

Magnetic susceptibility of Al-Co-R (R = Ce, Dy) glass-forming alloys at high temperatures

The magnetic susceptibility of Al-Co-Ce and Al-Co-Dy amorphous alloys is investigated in a wide range of temperatures (t = 20?1700 °C) and magnetic fields (B = 0.3?1.3 T). A weak gC(B) dependence in the entire temperature range and an anomalous increase in susceptibility beginning above the melting temperature of Al₂R compound are revealed for all investigated compositions. The experimental data are used to calculate the parameters of the electronic structure of alloys and to estimate the size of super-paramagnetic inclusions. It follows from the experimental data that cobalt atoms are present in these alloys in the nonmagnetic state. It is found that the values of effective magnetic moments on cerium and dysprosium atoms are significantly lower than those for R³⁺ ions and independent of the chemical composition of alloys. The results are discussed assuming the existence in melts of microregions formed by quasi-molecules of Al₂R.     

Formation of local magnetic moments in (V,Ti)3S4

The system of V_1?xTi_xS_1.40 has been studied through magnetic susceptibility and NMR measurements. The Curie constant increases with quadratic dependence on the composition x within the V₃S₄ phase (0 ≤ x ≤ 0.6) and then, rapidly decreases toward the Ti₂S₃ phase (0.8 < x ≤ 1.0). The NMR absorption from V⁵¹ nuclei with itinerant delectrons rapidly decreases with increasing Ti content in the former phase. These phenomena may indicate that local magnetic moments are formed in this system under a particular environment of vanadium atoms proposed. Metal distribution of V and Ti is also discussed.     

Role of Ce sublattices in the thermal and magnetic properties of Ce7 X 3 (X=Ni, Ru, Rh, Ir, Pd, and Pt) compounds

The role of the Ce sublattices in the thermal and magnetic properties of the Ce₇ X ₃ (X=Ni, Ru, Rh, Ir, Pd, and Pt) family of compounds is studied by means of ac and dc magnetic susceptibility, magnetization and mainly specific-heat experiments in applied magnetic field. The experimental data show that in these compounds there is coexistence of magnetic order, heavyfermion and intermediate-valence behavior, which is interpreted in terms of the contribution of the three different sublattices present in the crystalline structure of Th₇Fe₃-type (denoted by 1Ce_I, 3Ce_II, and 3Ce_III). From the available volume of the Ce_III atoms in their crystallographic environment it is found that sublattice Ce_III has an intermediate-valence behavior, whereas from entropic considerations sublattices Ce_I and Ce_II are identified as responsible for the magnetic order or heavy-fermion behavior, depending on the Ce-ligand electronic structure. This systematics evidences a clear correlation between the thermal and magnetic properties of these compounds and the position of the respective Ce-ligands in the periodic table, through the particular sensitivity of Ce to the environmental conditions.     

Demagnetization due to interconfiguration fluctuations in the RE-Cu2Si2 compounds

Magnetic susceptibility, resistivity, specific heat, and thermoelectric power measurements on EuCu₂Si₂, YbCu₂Si₂, CeCu₂Si₂, and GdCu₂Si₂ are presented. The first three compounds show a demagnetization of the 4f shell, while the fourth was studied to determine the normal magnetic and electronic properties for the RE-Cu₂Si₂ system. YbCu₂Si₂, CeCu₂Si₂, and EuCu₂Si₂ are found to exhibit similar lattice, electronic, and magnetic properties: an intermediate lattice constant; a magnetic susceptibility that is large and paramagnetic but neither shows order nor a Curie divergence; a large electronic specific heat coefficient; strong deviations from a simple T law in the high-temperature resistivity; and a large maximum in the thermoelectric power. The data are interpreted in terms of interconfiguration fluctuations (ICF), and a simple model is presented which can quantitatively explain the magnetic susceptibility and semiquantitatively predicts the specific heat. In addition, the model is in qualitative agreement with the resistivity and thermoelectric power data.Supported by Air Force Grant No. AF-AFOSR-71-2073 and by U.S. Atomic Energy Commission Contract No. USAEC-AT-(04-3)-34.     

Investigation of Electronic and Magnetic Properties of Iron(II)-Bromide Compound by First Principle,Mean Field,Series Expansion Calculations and Monte Carlo Simulation

The self-consistent abinitio calculations, based on DFT (density functional theory) approach and using FLAPW (full-potential linear augmented plane wave) method, are performed to investigate both electronic and magnetic properties of the FeBr ₂ compound. Polarized spin and spin-orbit coupling are included in calculations within the framework of the ferromagnetic state between two adjacent Fe atoms. Magnetic moments considered to lie along (001) axes are computed. The antiferromagnetic and ferromagnetic energies of FeBr ₂ are estimated. Obtained data from abinitio calculations are used as input for the high-temperature series expansion (HTSE) calculations to compute other magnetic parameters. The exchange interactions between the magnetic atoms Fe–Fe in FeBr ₂ are established by using the mean field theory. The critical temperature T _C(K) is obtained by HTSEs combined with the Padé approximant method. The critical exponent γ associated with the magnetic susceptibility is established. The critical temperature and magnetic hysteresis cycle are obtained by Monte Carlo simulation.     

Electronic structure and magnetism in superconductor ZnNNi3: A comparative study with ZnCNi3 and ZnNi3

We have investigated the structural, electronic, and magnetic properties of a newly discovered antiperovskite superconductor ZnNNi₃ and related compounds ZnCNi₃ and ZnNi₃ by using full-potential linearized augmented plane-wave method with the generalized gradient approximation (GGA). It is found that the electronic structures of ZnNNi₃ and ZnCNi₃ are very similar, existing a characteristic density of states (DOS) peak just below the Fermi level, which is dominated by Ni d bands with a small contribution from N/C p states. Contrarily, the band structure and Fermi surface in ZnNi₃ is changed considerably. Based on the free electron model, the Sommerfeld coefficients and the molar Pauli paramagnetic susceptibility for these compounds are evaluated. The spin-polarized calculations and the fixed spin-moment calculations indicate that both ZnNNi₃ and ZnCNi₃ are not near magnetism, while ZnNi₃ shows a typically ferromagnetic behavior. Furthermore, we also investigated the influence of N/C-defect on the electronic and magnetic properties. We found that ZnCNi₃ is more sensitive to the defect than ZnNNi₃, which well explains the fact why superconductivity has not yet been observed in ZnCNi₃.     

Magnetization Process of the Spin-1/2 Two-Leg Ladder Compound (C5H12N)2 CuBr4: The Jordan-Wigner Approach

We have studied the magnetization process in the spin S = 1/2 antiferromagnetic two-leg ladder compound (C₅H₁₂N)₂ CuBr₄ by using the analytical fermionization technique. The magnetization and the susceptibility are calculated as functions of the temperature and the external magnetic field. The magnetization of the system shows various behaviors in different regions of the magnetic field. The field-dependence of the susceptibility is almost symmetric about the average of quantum critical fields in the low-temperature region leading to a complete agreement with the experimental results.     

Ab Initio,Mean Field and Series Expansions Calculations Study of Structural,Electronic and Magnetic Properties of MnAs

The self-consistent ab initio calculations, based on density functional theory (DFT) approach and using full-potential linear augmented plane wave (FLAPW) method, are performed to investigate both electronic and magnetic properties of the MnAs compounds. Polarized spin and spin-orbit coupling are included in calculations within the framework of the ferromagnetic state between two adjacent Mn atoms. The ferromagnetic and antiferromagnetic energies of MnAs systems are obtained. Magnetic moment considered to lie along (001) axes is computed. The obtained data from ab initio calculations are used as input for the high-temperature series expansion (HTSE) calculations to compute other magnetic parameters. The exchange interactions between the magnetic atoms Mn-Mn in MnAs are given using the mean field theory. The HTSEs of the magnetic susceptibility with the magnetic moments in MnAs (m _Mn) through the Ising model is given up to the tenth order series in x=J ₁(Mn-Mn)/ k _B T. The Néel temperature T _C is obtained by HTSEs of the magnetic susceptibility series combined with the Padé approximant method. The critical exponent γ associated with the magnetic susceptibility is deduced as well.     

Proximity effects on the spin density waves in X/Cr(001) multilayers (X = Sn, V, and Mn)

We present ab initio density functional calculations of the electronic structure and magnetic properties of X₂/Cr₃₆(001) and X₁/Cr₃₇(001) multilayers, with X = Sn, V and Mn, to investigate the impact of the proximity effects of the X layers on the spin density waves of the Cr slab. We find different magnetic profiles corresponding to the spin density wave and to the layered antiferromagnetic configurations. The nature of the different magnetic solutions is discussed in terms of the different interfacial environments in the proximity of Sn, V or Mn. The magnetic behavior at the interface is discussed in connection with the electronic structure through the density of electronic states projected at the interfacial X and Cr sites. We compare the results with those previously obtained for Fe₃/X₁/Cr₃₇/X₁(001) multilayers to analyze the role played by the ferromagnetic iron slab.     

Electronic and magnetic properties of dilute transition clusters in noble (or transition) metals

Using a realistic band structure for the host (noble or transition metal), we present a detailed study of the electronic and magnetic properties of transitional impurity clusters without interactions between the clusters. For the calculation of the one-electron properties, the Hartree-Fock environment effects are self-consistently taken into account by Friedel's rule. The impurity potentials and densities of states are very sensitive to the impurity-impurity interactions inside the clusters. For the calculation of the magnetic properties, the electron-electron interactions are taken into account in the random phase approximation, which allows one to obtain simple expressions for the specific heat and the low-temperature-dependent resistivity. These properties are expressed in terms of three characteristic temperatures T _f(1), T _f(2) ⁺ , and T _f(2) ^– corresponding to three different modes of spin fluctuations for the considered model. We present numerical results from the study of the local magnetic susceptibility, which depends on local environments via several combined effects. Our conclusions on nearly magnetic copper-based alloys are in good agreement with the experimental data. In particular, our calculations confirm that a magnetic moment appears on a nickel atom when it is surrounded by approximately eight nearest-neighboring Ni atoms.This publication is a part of a thesis to be presented at the University Louis Pasteur.     

Magnetic, electrical, and optical properties of (HgS)1.5(Al2S3)0.5?Mn? and (HgS)1.5(In2S3)0.5?Mn? crystals

We have studied the magnetic, electrical, and optical properties of (HgS)_1.5(Al₂S₃)_0.5〈Mn〉 and (HgS)_1.5(In₂S₃)_0.5〈Mn〉 crystals and determined their magnetic and band structure parameters. The magnetic (Faraday method) and transport (four-probe method) properties of the (HgS)_1.5(Al₂S₃)_0.5〈Mn〉 and (HgS)_1.5(In₂S₃)_0.5〈Mn〉 crystals were investigated in the ranges T = 77−300 K and H = 20−320 kA/m. The magnetic susceptibility data suggest that the magnetic properties of the crystals can be understood in terms of Mn-S-Mn-S clusters. The Hall coefficient of the crystals is temperature-independent. The electrical conductivity of (HgS)_1.5(Al₂S₃)_0.5〈Mn〉 varies little with temperature and has a maximum, and that of (HgS)_1.5(In₂S₃)0.5〈Mn〉 shows metallic behavior and is a nearly linear function of temperature. The thermopower of the crystals increases with temperature. Optical data attest to direct allowed interband optical transitions. The magnetic and band structure parameters of the crystals are determined.     

Electronic properties of M2InV3O11 (M(II) = Zn(II) and Co(II)) compounds

The electronic properties of multicomponent vanadate oxides M₂InV₃O₁₁ (M(II) = Zn(II) and Co(II)) were investigated by electrical resistivity and electron paramagnetic resonance (EPR) measurements. Replacement of non-magnetic Zn(II) cations with magnetic Co(II) ions resulted in a significant drop in the electrical conductivity and an increase in the activation energy. The EPR spectroscopy revealed the presence of VO²⁺ vanadyl ions in both compounds, while the presence of divalent cobalt ions was identified in the Co₂InV₃O₁₁ oxide at low temperatures. The concentration of VO²⁺ vanadyl ions was found to be about one order higher for the vanadate oxide without magnetic ions. It is suggested that the increased concentration of VO²⁺ ions could be responsible for the enhanced conductivity of Zn₂InV₃O₁₁.     

Electronic and Magnetic Properties of MnSb Compounds

The self-consistent ab initio calculations, based on density functional theory (DFT) approach and using full-potential linear augmented plane wave (FLAPW) method, are performed to investigate both electronic and magnetic properties of the MnSb compounds. Polarized spin and spin-orbit coupling are included in calculations within the framework of the ferromagnetic state between two adjacent Mn atoms. The ferromagnetic energy of MnSb systems is obtained. Magnetic moment considered to lie along the (001) axes are computed. The exchange interactions between the magnetic atoms Mn–Mn in MnSb are given using the mean field theory. Obtained data from ab initio calculations are used as input for the high-temperature series expansions (HTSEs) calculations to compute other magnetic parameters. The HTSEs of the magnetic susceptibility of the magnetic moments in MnSb (m _Mn) is given up to the tenth order series in (x = J ₁(Mn–Mn)/ k _B T). The critical temperature T _C is deduced by HTSEs of the magnetic susceptibility series combined with the Padé approximation method. The critical exponent γ associated with the magnetic susceptibility is deduced as well.     

Finite Ferromagnetic Clusters La1−xZnxMnO3 (x = 0.05 and 0.10)

Detailed magnetic and nonlinear susceptibility measurements of the La_1?x Zn _x MnO₃ system are reported. For x = 0.05 and 0.1, a typical ferromagnetic (FM) signal in the field-cooled curve is observed at 38 K. However, for the insulating antiferromagnetic (AFM) parent compound LaMnO₃ and for the x = 0.2 and 0.33 samples, the magnetization curves in the low temperature range, are smooth and flat. This observation confirms the coexistence of finite FM clusters in the insulating AFM region below the critical concentration (x _c = 0.16) for percolation, predicted by the percolation model.     

Intermediate valence behavior of ternary cerium and uranium transition metal borides

Low-temperature specific heat, electrical resistivity, ac magnetic susceptibility, and dc magnetization measurements were made on ternary cerium and uranium transition metal borides with the general formula CeT₃B₂ (T=Co, Ru, Rh, and Ir) and UT₃B₂ (T=Co, Ru, and Ir). The compound CeRu₃B₂ was found to exhibit superconductivity below 0.68 K. The values of the electronic specific heat coefficient range from 9.7 mJ/mole-K² for CeCo₃B₂ to 64 mJ/mole-K² for UIr₃B₂. The electrical resistivity versus temperature curves of all of the compounds exhibit negative curvature and are reminiscent of valence fluctuation behavior. In the case of CeIr₃B₂, the electrical resistivity attains a maximum value at 180 K, while the dc magnetic susceptibility has a temperature dependence that is typical of intermediate valence Ce compounds, approaching a finite value at zero temperature. The electrical resistivity of the ferromagnetic compound CeRh₃B₂ reveals a rapid decrease in spin disorder resistivity below 120 K. The dc magnetic susceptibility of this material can be described as the sum of a constant term and a Curie-Weiss contribution with an effective magnetic moment of 1.01 µ_B per formula unit and a Curie-Weiss temperature of 119 K. Magnetization measurements on CeRh₃B₂ yield a saturation magnetic moment of 0.37 µ_B per formula unit and a Curie temperature of 113 K.     

Magnetic behavior of Mn(NH4)2(SO4)2 · 6H2O in its Curie region in fields directed along theb-axis

The intensity of magnetization and the isentropic magnetic susceptibility of Mn(NH₄)₂(SO₄)₂ · 6H₂O have been determined along 13 isentropes (between 0.620 and 2.825 cal mole^–1 deg^–1) in fields up to 1670 Oe directed along theb-axis of a spherical single crystal. Eleven of the isentropes originated, in zero field, within the Curie region of the salt. Field increments of 10 to 15 Oe were used below 170 Oe, with larger steps above this. All isentropes originating within the Curie region show a common (high) susceptibility at zero field. In each case the susceptibility then drops sharply at a small field characteristic of the particular entropy. The drop in susceptibility was used to define the boundaries of the Curie region (critical fieldvs entropy). The change of temperature with magnetic field was determined along the isentropes and examples are given.     

Antiferromagnetically Spin Polarized Oxygen and Manganese in MnO Layers Investigated by First Principle and Series Expansions Calculations

Self-consistent ab initio calculations, based on DFT (Density Functional Theory) approach and using the FLAPW (Full potential Linear Augmented Plane Wave) method, are performed to investigate both electronic and magnetic properties of the MnO layers. Polarized spin and spin-orbit coupling are included in calculations within the framework of the antiferromagnetic state between two adjacent Mn layers. Magnetic moment considered to lie along (110) axes are computed. Obtained data from ab initio calculations are used as input for the high temperature series expansions (HTSEs) calculations to compute other magnetic parameters. The exchange integrals between the magnetic atoms Mn–Mn and Mn–O in the same layer and between the adjacent bilayers are given by using the mean field theory. The antiferromagnetic energies of MnO layers are obtained. The High Temperature Series Expansions (HTSEs) of the magnetic susceptibility on MnO antiferromagnetic layers through Heisenberg and XY models is given up to tenth order series in (x=J _Mn–Mn/k _B T). The reduced Néel temperature x _N is obtained by HTSEs of the magnetic susceptibility and by using the Padé approximant method. The critical exponent γ associated with the magnetic susceptibility is deduced.     

Superconductivity and Magnetism in R 2CaBa2Cu5O z (R=La, Pr, Nd and Eu)

A series of compounds R ₂CaBa₂Cu₅O _z (R=La, Pr, Nd and Eu) have been synthesized by conventional solid state reaction and characterized for their structural, superconducting and magnetic properties. All compounds crystallize with the tetragonal LaBa₂Cu₃O _z type structure, space group P4/mmm. Among the four compounds studied here, R=La, Nd and Eu are superconductors with superconducting transition temperatures (T _c ^R=0) of 72, 40 and 55 K, respectively. On the other hand, neither superconductivity nor magnetic ordering is seen for R=Pr down to 3 K. The effect of the magnetic field on the susceptibility and (magneto) resistance for the superconducting samples has been investigated. The superconductivity of compounds with magnetic rare-earth ions Nd and Eu exhibit a profound influence of the magnetic field, whereas the application of the magnetic field has a limited effect on the La compound. The Pr compound is paramagnetic and does not exhibit magnetic ordering either.