Evidence of magnetic ordering in tellurium substituted FeSb2

We report on a⁵⁷Fe Mössbauer study of tellurium substituted FeSb₂, FeSb_2?x Tc _x (x=0.2, 0.4, 0.6), at temperatures between 4.2 K and 300 K. For all three alloys, the Mössbauer spectra at 4.2 K are characteristic of a magnetically ordered state. The hyperfine field at Fe site increases with increasing tellurium concentration. The magnetic character may be attributed to the existence of a very narrow band gap leading to fairly strong Coulomb and exchange interactions between holes in the valence band and electrons in the conduction band.     

Sb Magnetic Resonance as a Local Probe for the Gap Formation in the Correlated Semimetal FeSb2

We report on a comparative study of the narrow-band semimetals FeSb₂ and its structural homologue RuSb₂ by means of ^121,123Sb nuclear quadrupole (NQR) and nuclear magnetic resonance (NMR) spectroscopy. From NQR for both compounds two temperature regimes could be identified by use of ¹²³(1/T ₁) measurements. Above 40 K a conventional activated behavior (with Δ/k _B ? 400 K for FeSb₂) dominates in ¹²³(1/T ₁), whereas below 40 K in both systems an unconventional ¹²³(1/T ₁) behavior with a smooth maximum at around 10 K is observed. To analyze this behavior, we propose the presence of T-dependent in-gap states forming a narrow energy level of localized spins with S = ½ near the bottom of the conduction band. These states might have originated from an inherent Sb-deficiency in both compounds. This model enables us to fit the ¹²³(1/T ₁) data in the entire investigated temperature range (2–200 K) for FeSb₂. Ab initio band structure calculations reveal more than a factor of two larger Δ value for RuSb₂ as compared with FeSb₂. This results in dissimilar behavior of ¹²³(1/T ₁) in FeSb₂ and RuSb₂ above 40 K evidencing the inefficiency of thermal activation of electrons over the large energy gap at T ≤ 300 K in RuSb₂ and dominating of quadrupole relaxation channel in RuSb₂ in this temperature range caused by phonon relaxation involving two-phonon (Raman) scattering. In addition, extra wide range field-sweep NMR measurements are performed at various temperatures on FeSb₂ and RuSb₂. The complex broad spectra could be modeled and from the shift of the ¹²¹Sb central transition the 3d component of the shift K _3d (T) could be extracted.     

The semiconductor-to-ferromagnetic-metal transition in FeSb2

We propose FeSb₂ to be a nearly ferromagnetic small gap semiconductor, hence a direct analog of FeSi. We find that despite different compositions and crystal structures, in the local density approximation with on-site Coulomb repulsion correction (LDA+U) method magnetic and semiconducting solutions for U=2.6 eV are energetically degenerate similar to the case of FeSi. For both FeSb₂ and FeSi (FeSi_1-xGe_x alloys) the underlying transition mechanism allows one to switch from a small gap semiconductor to a ferromagnetic metal with magnetic moment ≈1 μ_B per Fe ion with external magnetic field.     

Comparative study of NiSb2 and FeSb2 as negative electrodes for Li-ion batteries

Crystalline FeSb₂ powder prepared by ceramic route is examined as negative electrodes for lithium-ion batteries. The complete reaction mechanism of FeSb₂ is investigated by ¹²¹Sb and ⁵⁷Fe Mössbauer spectroscopy as well as magnetic measurements and the results are correlated with a previous in situ XRD characterization. On the first discharge the reaction with Li proceeds through a biphasic process transforming FeSb₂ into a new Li_xFe_ySb₂ phase, and this ternary phase is then converted into fcc Li₃Sb and metallic Fe nanoparticles. The combination of Mössbauer spectroscopy and magnetic analyses leads i) to a better understanding of the FeSb₂ → ternary phase reaction and concomitantly allowed ii) to specify the stoichiometry of the new ternary phase. On charge, the extrusion of lithium includes the back conversion of the Li₃Sb/Fe mixture into both Li₄Fe_0.5Sb₂ and metallic Sb, which are the main active species for the following cycles, responsible for the poor cycling life of the FeSb₂ electrode. The nature of these resulting products is quite different from that previously observed for the isotype NiSb₂ electrode which is characterized by a highly reversible mechanism.     

XPS and thermomagnetic characterization of the CeNi4Cr compound

The magnetic, thermodynamic and electronic structure properties are discussed for the CeNi₄Cr compound. The X-ray photoemission spectra (XPS) provide an evidence of a mixed valence behavior with the occupancy of the f states n_f=0.89 and their hybridization with the conduction electrons Δ=30 meV. These values reproduce well the magnetic susceptibility χ(T=0), which is enhanced compared to similar CeNi₄M (M=Al, B, Cu) compounds. In combination with a slightly increased electronic specific heat coefficient (up to 100 mJ mol⁻¹ K⁻²), this compound can be classified as being on the border of the heavy fermion and mixed valence behavior. Using a small magnetic field in the χ(T) measurements reveals a presence of magnetically ordered impurity phase, which is easily damped by higher fields and it is shown that the contribution of this phase is minor. The question of the dependence of the electronic specific heat coefficient on the magnetic field is also addressed and the observations agree well with theoretical predictions based on the Anderson model.     

The influence of imperfection of the crystal lattice on the electrokinetic and magnetic properties of disordered titanium monoxide

The conductivity and magnetic susceptibility of disordered titanium monoxide TiO_y (0.920≤y≤1.262) containing vacancies in titanium and oxygen sublattices are investigated. For TiO_y monoxides with an oxygen content y≤1.069, the temperature dependences of the conductivity are described by the Bloch-Grüneisen function at a Debye temperature ranging from 400 to 480 K and the temperature dependences of the magnetic susceptibility are characterized by the contribution from the Pauli paramagnetism due to conduction electrons. The behavior of the conductivity and magnetic susceptibility of TiO_y monoxides with an oxygen content y≥1.087 is characteristic of narrow-gap semiconductors with nondegenerate charge carriers governed by the Boltzmann statistics. The band gap ΔE between the valence and conduction bands of TiO_y monoxides with y≥1.087 falls in the range 0.06–0.17 eV.     

Electrical conductivity and magnetic susceptibility of titanium monoxide

Conductivity and magnetic susceptibility of disordered cubic titanium monoxide TiO_y(0.920≤y≤1.262) are studied. Temperature dependences of the conductivity of TiO_y monoxides with y≤1.069 are described by the Bloch-Grüneisen function with Debye temperature 400–480 K, and temperature dependences of the susceptibility include Pauli paramagnetism of conduction electrons. The behavior of conductivity and susceptibility of TiO_y with y≥1.087 is typical of semiconductors with nondegenerate charge carriers obeying Boltzmann statistics. The band gap ΔE between the valence and conduction bands of TiO_y(y≥1.087) is 0.06–0.17 eV, and effective mass of charge carriers is equal to 7–14 electron masses.     

Magnetic susceptibility and valence fluctuations in Sm3S4

The magnetic susceptibility of Sm₃S₄ single crystals has been measured between 1.8 K and 300 K. At low temperatures the susceptibility is composed of χ_Sm²⁺ + 2χ_Sm³⁺. Near and above 100 K the susceptibility of the Sm³⁺ ions is quenched due to the increasing valence fluctuation rate.     

Spin susceptibility divergence in high-temperature superconductors

Neutron scattering experiments at low frequency reveal susceptibility peaks in La_2−xSr_xCuO₄ which diverge as the temperature is lowered toward the superconducting transition. This divergence at a nesting momentum stands out from the conventional Pauli susceptibility of this cuprate at long wavelengths. We explain the peak singularity by calculating many-body Coulomb correlations which originate from Fermi surface nesting and dominant vertex corrections. We sum the leading singularities using the parquet method and find that the susceptibility diverges as T^−α, where α depends on the Coulomb interaction. Our theory also explains a similar susceptibility surge in the metallic V₂O₃.     

X-ray photoemission and magnetometric studies of valence changes in Ce(Cu1−xNix)4Ga

The compounds Ce(Cu_1−xNi_x)₄Ga crystallize in the hexagonal CaCu₅-type structure for the whole doping range 0≤x≤1. The border compounds CeCu₄Ga and CeNi₄Ga represent a heavy fermion and fluctuating valence systems, respectively. We report on the studies of the valence evolution in Ce(Cu_1−xNi_x)₄Ga employing the X-ray photoemission spectroscopy (XPS) and magnetic susceptibility measurements. The photoemission of the Ce 3d peaks shows a gradual decrease of the occupation of the f states with Ni content. Simultaneously, the hybridization strength and the low temperature magnetic susceptibility are reduced. Within the valence band spectrum a transition from the dominance of the Cu 3d to the dominance of the Ni 3d states is well visible with the traces of the Ce 4f¹ states for up to x=0.5.     

Study of the valence transition in La- and Yb-substituted SmB6

Lattice parameter and magnetic susceptibility measurements have been made on Sm_1?xLa_xB₆ and Sm_1?yYb_yB₆ alloys. The results indicate that La³⁺ substitution for Sm in SmB₆ decreases the Sm valence, whereas substitution with Yb²⁺ increases the Sm valence. It is found that the contribution of Sm³⁺ to the susceptibility disappears for a wide range of composition.     

Electronic structure of the Sr2MgSi2O7:Eu persistent luminescence material

The electronic structures of the distrontium magnesium disilicate (Sr₂MgSi₂O₇(:Eu²⁺)) materials were studied by a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu²⁺ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu²⁺ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available.     

First-principles study of electronic structure and magnetism of cubic Al1−xErxN using the LSDA+U approach

First-principles calculations, by means of the full-potential augmented plane wave method using the LSDA+U approach (local spin density approximation with Hubbard-U corrections), have been carried out for the electronic structure of the Al_0.75Er_0.25N. The LSDA+U method is applied to the rare-earth 4? states. We have investigated the electronic and magnetic properties.The Al_0.75Er_0.25N is shown to be a semiconductor, where the filled ? states are located in the valence bands and the empty ones above the conduction band edge. The magnetic interaction of the rare-earth ion with the host states at the valence and conduction band edges has been investigated and discussed.     

Localization of conduction electrons and the magnetic properties of the molecular metals β″-(BEDT-TTF)4NH4[M(C2O4)3] · DMF (M = Cr3+, Fe3+)

Quasi-two-dimensional organic metals β″-(BEDT-TTF)₄NH₄[M(C₂O₄)₃] · DMF containing the oxalate complexes of Cr³⁺ or Fe³⁺ ions between the conducting organic layers of the BEDT-TTF molecules are studied by EPR spectroscopy, and the contributions of these metallic complexes, conduction electrons, and non-equilibrium lattice defects to the magnetic susceptibility are determined. An analysis of the temperature dependence of the magnetic susceptibility and the EPR line shape has revealed partial localization of conduction electrons at T < 20 K in the crystals with Cr³⁺ ions. The size of the localization region is close to the size of an individual BEDT-TTF molecule. The localization effect weakens as nonequilibrium defects disappear during long-term storage at room temperature. The localization of conduction electrons is found to be accompanied by the appearance of weak antiferromagnetic interaction between the Cr³⁺ ions at T < 20 K, which disappears when Fe³⁺ ions substitute for Cr³⁺ ions.     

High-temperature studies of the magnetic susceptibility of samarium and the Al<Subscript>2</Subscript>Sm compound

The magnetic susceptibility of metallic samarium and the Al₂Sm intermetallic compound has been experimentally studied by the Faraday method in the temperature range of 300–1800 K. It has been shown that the temperature dependences of the magnetic susceptibility of Sm and Al₂Sm in a crystalline state can be described in the framework of Van Vleck paramagnetism theory taking into account variable valence and the contribution from the conduction electrons. Using this theoretical interpretation of the data, the effective valence of samarium in the metallic state and in the Al₂Sm intermetallic compound has been estimated as a function of the temperature.     

Bethe-ansatz solution of a model for a mixed valent impurity with two magnetic configurations: II. Numerical solution of the thermodynamic equations

A mixed valence impurity with two magnetic configurations of total angular momentumJ ₂ andJ ₁=J ₂+1/2, respectively, coupled by conduction electrons with total angular momentum 1/2 via a hybridization matrix element is considered. The thermodynamic Bethe-ansatz equations derived previously are solved numerically for various values ofJ ₂. Thef-level occupation, the entropy, the magnetic susceptibility and the specific heat are obtained as a function of temperature for variousf-level positions. The magnetic field dependence is also discussed in the limit of integer valence (exchange model).Supported by the CONICET, Argentina     

57Fe and 151Eu Mössbauer studies of magnetoresistive Europium based cobalt perovskites

Eu_0.8Sr_0.2Fe _x Co_1?x O_3?z CMR perovskites with different iron concentrations (x?=?0, 0.025, 0.075, 0.15, 0.3) were investigated by X-ray diffraction, AC magnetic susceptibility, magnetotransport, as well as ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectrometry. The valence state of europium ions was found to be trivalent, independently of the iron concentration. ⁵⁷Fe Mössbauer spectra and magnetic susceptibility of the investigated perovskites presented complementary results for the magnetic transitions.     

Optical absorption due to paraexciton of Cu2O

In Cu₂O a new absorption line is observed at 97 cm^?1 below the n =1 of the yellow exciton (triply degenerate orthoexciton) under a strong magnetic field at 4.2 K. The line is assigned as a transition to a nondegenerate spin triplet state Γ⁺₂ (paraexciton). An analysis including the effects due to the n =1 of the green exciton yields 364 cm^?1 as the exchange energy, and 2.68 and ?1.02, or 1.02 and ?2.68 as the g-factors of the conduction and valence bands forming the yellow exciton.     

First evidence for a Sm<Superscript>3+</Superscript>-type contribution to the magnetic form factor in the quasielastic spectral response of intermediate valence SmB<Subscript>6</Subscript>

The momentum-transfer dependence of the magnetic form factor associated with the quasielastic spectral component in the dynamic magnetic response of intermediate valence SmB₆ has been measured using inelastic neutron scattering on a double-isotope (¹⁵⁴Sm, ¹¹B) single crystal. The experimental dependence differs qualitatively from those obtained earlier for the inelastic signals, as well as from the field-induced magnetic form factor of SmB₆ obtained by polarized neutron diffraction. This observation is interpreted by specifically considering the Curie-type contributions to the dynamic susceptibility, which arise from the mixing of 4 f⁵ and 4 f⁶J-multiplets into the intermediate valence state wavefunction.     

Mössbauer study of amorphous Fe-Sb alloys with large composition range

Amorphous Fe_x Sb_100-x alloys with large composition range of 3<-x<-80 have been investigated with⁵⁷Fe Mössbauer spectroscopy. The concentration dependence of quadrupole splitting and its distribution reveal a similar atomic packing scheme in the range of 3<-x<-50. Significantly, the quadrupole splitting of crystalline FeSb₂ is larger than that of amorphous FeSb₂ by a factor of 3. The isomer shifts have been analyzed based on the Miedema-van der Woude's model of isomer shift. The distribution of magnetic hyperfine field P(H) indicates that the alloys are rather magnetically inhomogeneous around x=55.