A theory of magnetism for semi-metallic uranium compounds

A theoretical study is carried out to explain the magnetic properties of uranium compounds of NaCl, anti-Cu₂Sb and Th₃P₄ type. The electronic model used is a modified free electron model along the lines of the band structure for NaCl type uranium compounds previously reported. An exchange interaction via the conduction electrons is assumed for the mechanism of magnetic ordering. The molecular field approximation is used to calculate θ, T_N and the exchange interaction constants. The stability range of various magnetic ordering states and the variation of θ, T_N and exchange interaction constants are obtained as functions of lattice parameter. These results well explain the experimental data.     

Theory of knight shift due to indirect nuclear hyperfine interactions

We derive a theory of Knight shift (K) in solids including the effects of periodic potential, spin-orbit interaction, magnetic hyperfine interactions and indirect nuclear hyperfine interaction. We use a temperature dependent Green's function technique to evaluate the thermodynamic potential which is then used to obtain a general expresion for the Knight shift. Our formula for K is expressed as a sum of contributions due to conduction electrons and localized electrons of either d- or f-type: K_cond and K_loc. While K_cond is the same as our previous expression for K derived in the absence of localized magnetic moments, K_loc is a new contribution and is due to the hybridization of conduction and localized electron magnetic moments. We also briefly discuss the many-body effects on the different contributions to K. Finally, the importance of the present theory in possible applications to metals, alloys and compounds containing transition and rare-earth elements, and magnetic semiconductors is discussed.     

Itinerant magnetism in metallic glasses

It is argued that when a ferromagnetic transition metal T (e.g. Fe) is mixed with a metalloid M to form a metallic glass (having a narrow band of conduction electrons) the competing effects of the 3-d intra-atomic Coulomb repulsion and of the interaction between conduction and d-electrons make the magnetic moment of T-atoms to vanish when the concentration x of T-atoms is smaller than a critical concentration x_c. Experimental data for the magnetic moment of iron atoms in Fe_xB_1-x is analyzed along this line and a satisfactory fit is obtained. Inclusion of structural as well as compositional disorder effects could result in a better agreement between theory and experiment.     

Current-induced effect on resistivity and magnetoresistance of La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La_0.67Ba_0.33MnO₃ bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature (T_IM) and the change in MR around the ferromagnetic transition temperature (T_C). The decrease in MR around T_C at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around T_IM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons.     

Anomalies of magnetoresistance of compounds with atomic clusters RB12 (R = Ho, Er, Tm, Lu)

The magnetoresistance and magnetization of single-crystal samples of rare-earth dodecaborides RB₁₂ (R = Ho, Er, Tm, Lu) have been measured at low temperatures (1.8–35 K) in a magnetic field of up to 70 kOe. The effect of positive magnetoresistance that obeys the Kohler’s rule Δρ/ρ = f(ρ(0, 300 K)H/ρ(0, T)) is observed for the nonmagnetic metal LuB₁₂. In the magnetic dodecaborides HoB₁₂, ErB₁₂, and TmB₁₂, three characteristic regimes of the magnetoresistance behavior have been revealed: the positive magnetoresistance effect similar to the case of LuB₁₂ is observed at T > 25 K; in the range T _N ≤ T ≤ 15 K, the magnetoresistance becomes negative and depends quadratically on the external magnetic field; and, finally, upon the transition to the antiferromagnetic phase (T < T _N ), the positive magnetoresistance is again observed and its amplitude reaches 150% for HoB₁₂. It has been shown that the observed anomalies of negative magnetoresistance in the paramagnetic phase can be explained within the Yosida model of conduction electron scattering by localized magnetic moments. The performed analysis confirms the formation of spin-polaron states in the 5d band in the vicinity of rare-earth ions in paramagnetic and magnetically ordered phases of RB₁₂ and makes it possible to reveal a number of specific features in the transformation of the magnetic structure of the compounds under investigation.     

Ferromagnetic transition in ordered double perovskites and related alloys

We study the electronic and magnetic properties of ordered perovskites Sr ₂FeMO₆ (where M is a transition metal), among which some compounds, like Sr₂FeMoO₆ are half-metallic with high Curie temperature while Sr₂FeWO₆ is an antiferromagnetic insulator. Using a double exchange type model with an interaction between localized spins and conduction electrons together with a tight-binding Hamiltonian and the renormalized perturbation expansion method, we study the behavior of T_c as a function of the number of conduction electrons and also as a function of the Fe-M energy difference. We also consider the Sr₂FeMo_xW_1-xO₆ compounds which present a magnetic and metal-insulator transition as a function of doping.     

Strong coupling in the Kondo problem in the low-temperature region

The magnetic field dependence of the average spin of a localized electron coupled to conduction electrons with an antiferromangetic exchange interaction is found for the ground state. In the magnetic field range μH∼0.5T _c (T _c is the Kondo temperature) there is an inflection point, and in the strong magnetic field range μH≫T _c , the correction to the average spin is proportional to (T _c /μ H)². In zero magnetic field, the interaction with conduction electrons also leads to the splitting of doubly degenerate spin impurity states. Zh. éksp. Teor. Fiz. 115, 1263–1284 (April 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Hybridization effect on the Falicov-Kimball model

The extended Falicov-Kimball model which includes the hybridization effect between the localized electron state and the conduction band is treated within the Hartree-Fock approximation at T=0 K. The occupation number n_e of the conduction band is calculated as a function of the localized level. The both discontinuous and continuous changes of n_e appear in a certain region of parameters. This result may correspond to the experimental facts of samarium-monochalcogenides under pressure.     

Magnetic hyperfine interactions in GdAl3

We have studied the magnetic hyperfine interactions in GdAl₃ using¹⁵⁵Gd Mössbauer spectroscopy between the temperatures of 4K and 90K. Previous studies on GdAl₃ have shown that antiferromagnetic ordering occurs at 18K, and that a fit of the susceptibility to 1/(T-θ_p) yields a θ_p value of ?89K. The large ratio of θ_p to T_N is indicative of magnetic frustration between competing ferro-and antiferromagnetic interactions, which may be due to a combination of the oscillatory nature of the RKKY interaction and the geometry of the hexagonal lattice. Our studies show that the saturation magnetic hyperfine field at the Gd site is ?24.0 T, with the moments lying in the basal plane. The efg at the gadolinium site is 2.55(1)×10¹⁷V cm^?2 which is considerably larger than the value predicted by a point charge calculation. This difference may indicate that there is a conduction electron contribution. A helical magnetic structure has been calculated from RKKY theory.     

Electrical and magnetic properties of ε-Fe3N nanoparticles synthesized by chemical vapor condensation process

ε-Fe₃N nanoparticles synthesized by chemical vapor condensation (CVC) are covered with shells of disordered Fe₃O₄ phase, as observed by a transmission electron microscopy. The zero-field cooling and field cooling temperature dependence of magnetization, ac susceptibility as a function of frequency, magnetic hysteresis loops, and the temperature dependence of resistivity of the ε-Fe₃N nanoparticles are systematically studied. The results indicate the existence of complex magnetic properties, such as superparamagnetic behavior, exchange bias, magnetic dipole interaction, and the possible coexistence of ferromagnetic and spin-glass-like states and/or disordered surface spins of the shells at low temperatures. The temperature dependence of resistivity ρ(T) for compacted ε-Fe₃N nanoparticles in a temperature range of 110 K< T< 300 K can be well described by the mechanism of fluctuation-induced tunneling conduction, while that below 110 K can be ascribed to conducting electrons scattered by localized magnetic moments and impurity as well as the influence of freezing of spin-glass-like moments and/or disordered surface spins of the shells.     

An anomaly in the magnetic susceptibility of the Mg1−x CuxO solid solutions with 0.01≤x≤0.20

The problem of localized superconductivity has motivated the preparation of Mg_1−x Cu_xO solid solutions with NaCl structure and 0.01≤x≤0.20, as well as a study of the magnetization and magnetic susceptibility χ in the 2–400 K temperature range and in magnetic fields of up to 5 T. The temperature dependence of χ is described for all compositions by the Curie-Weiss law, χ = C/(T − θ), where the constant C is close to the value calculated for each composition for μ_eff = 1.7–1.9μ_B, and θ is close to zero. For T < 30 K, χ(T) deviates for all compositions toward lower χ, which can be attributed to magnetic ordering of exchange-coupled clusters in the solid solution. At T∼320–330 K, an anomaly of a diamagnetic type, i.e., a decrease of χ by 6–30% of its paramagnetic value, has been observed for all compositions against the background of the generally paramagnetic χ(T). A discussion is presented of alternative reasons for this anomaly and of its possible connection with localized superconductivity. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 4, 2000, pp. 701–703. Original Russian Text Copyright ? 2000 by Samokhvalov, Arbuzova, Viglin, Naumov, Smolyak, Korolev, Lobachevskaya.     

Spin resonance of conduction electrons and EPR of localized moments in a low-dimensional organic conductor [Pd(dddt)2]Ag1.5Br3.5

The EPR spectra of a quasi-two-dimensional organic metal [Pd(dddt)₂]Ag_1.5Br_3.5 contain signals due to the spin resonance of conduction electrons (CESR) and signals due to the localized magnetic moments of Ag²⁺. The system of Ag²⁺ ions exhibits a strong indirect antiferromagnetic exchange interaction characterized by the Weiss constant Θ=−280(25) K, which leads to magnetic ordering at a temperature of T ₀=70(10) K. The same temperature T ₀ corresponds to a strong anomaly in the CESR linewidth. The observed anomaly in the CESR linewidth, as well as some features of the temperature dependence of conductivity in the system studied, are explained by the interaction between conduction electrons and Ag²⁺ ions localized in the anion layers (π-d interaction) and by antiferromagnetic ordering of the spins of Ag²⁺ magnetic ions.     

Crystal structure and magnetic properties of pseudoternary TbRh2−xMxSi2(M = Ru,Ir) compounds

The structural and magnetic characteristics of the pseudoternary TbRh_2−xM_xSi₂(M = Ru, Ir) compounds were studied. The compounds crystallize in the tetragonal ThCr₂Si₂-type structure. The magnetic data were collected in the temperature range 70–300 K. Their magnetic susceptibilities satisfy the Curie-Weiss law in the temperatures higher than 130 K. The magnetic moment of the rare earth atom is larger than of the free Tb³⁺ ion. A modified RKKY theory with included interaction between the conduction electrons was applied to explain the variation of properties of the compounds.     

Antiferromagnet-ferromagnet transition in α-MnxS manganese sulfides

Off-stoichiometric manganese monosulfides α-Mn_xS (1≤x≤1.25) are synthesized, and their crystal structure and magnetic properties are studied in the 4.2-to 300-K range. The compounds have a NaCl fcc lattice. Increasing the manganese ion concentration x in the antiferromagnetic semiconductors α-Mn_xS is found to result in concentration-(x _c ～ 1.05) and temperature-driven (T _c ～ 50 K) magnetic transitions from the antiferromagnetic to ferromagnetic state, with the cubic structure remaining unchanged.     

High field magnetoconductivity of Si inversion layers

The transverse magnetoconductivity (σ_χχ) of electron inversion layers on (100) Si is measured in magnetic fields up to 220 kG at temperatures from 4.2 to 1.6 K. The dependence of σ_χχ on T, H, and the electric field along the inversion layer suggests that immobile electrons between two Landau subbands are to a large extent localized out of the top of the lower subband. Fine structure, which may be indicative of inhomogeneities of electronic origin, is observed in σ_χχ vs electron density.     

Influence of Ce doping on electrical and thermal properties of La0.7−xCexCa0.3MnO3 (0.0≤x≤0.7) manganites

The effect of Ce-doping on structural, magnetic, electrical and thermal transport properties in hole-doped manganites La_0.7−xCe_xCa_0.3MnO₃ (0.0≤x≤0.7) is investigated. The structure of the compounds was found to be crystallized into orthorhombically distorted perovskite structure. dc Susceptibility versus temperature curves reveal various magnetic transitions. For x≤0.3, ferromagnetic regions (FM) were identified and the magnetic transition temperature (T_C) was found to be decreasing systematically with increasing Ce concentration. The electrical resistivity ρ(T) separates the well-define metal-semiconducting transition (T_MS) for low Ce doping concentrations (0.0≤x≤0.3) consistent with magnetic transitions. For the samples with 0.4≤x≤0.7, ρ(T) curves display a semiconducting behavior in both the high temperature paramagnetic (PM) phase and low temperature FM or antiferromagnetic phase. The electron–phonon and electron–electron scattering processes govern the low temperature metallic behavior, whereas small polaron hopping model is found to be operative in PM phases for all samples. These results were broadly corroborated by thermal transport measurements for metallic samples (x≤0.3) in entire temperature range we investigated. The complicated temperature dependence of Seebeck coefficient (S) is an indication of electron–magnon scattering in the low temperature magnetically ordered regime. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The observation of a significant difference between ρ(T) and S(T) activation energies and a positive slope in thermal conductivity κ(T) implying that the conduction of charge carriers were dominated by small polaron in PM state of these manganites.     

Preexponential factor in variable-range hopping conduction in CuInTe2

We study the temperature dependence of the electrical resistivity in a single crystal of p-type uncompensated CuInTe₂ on the insulating side of the metal-insulator transition down to 0.4 K. We observe a crossover from Mott to Efros-Shklovskii variable-range hopping conduction. In Efros-Shklovskii-type conduction, the resistivity is best described by explicitly including a preexponential temperature dependence according to the general expression ρ=ρ₀T^αexp(T_ES/T)^1/2, with α≠0. A theory based on the resistor network model was developed to derive an explicit relation between α and the decay of the wavefunction of the localized states. A consistent correspondence between the asymptotic extension of the wavefunction and the conduction regime is proposed. The results indicate a new mechanism for a local resistivity maximum in insulators, not involving magnetic effects.     

Hyperfine interactions at R and In sites in RNiIn (R = Gd, Tb, Dy, Ho) compounds measured by perturbed angular correlation spectroscopy

Perturbed gamma–gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (mhf) in RNiIn (R = Gd, Dy, Tb, Ho) intermetallic compounds using the ¹¹¹In→¹¹¹Cd and ¹⁴⁰La→¹⁴⁰Ce probe nuclei. The PAC spectra for ¹¹¹Cd measured above magnetic transition temperature show a major fraction with a well defined quadrupole interaction for all compounds except GdNiIn where a single frequency was observed. PAC measurements below T _C showed a combined electric quadrupole plus magnetic dipole interaction for ¹¹¹Cd probe at In sites, and a pure magnetic interaction for ¹⁴⁰Ce at R sites. The temperature dependence of mhf measured with ¹⁴⁰Ce at R sites shows that the values of fields drop to zero at temperatures around the expected T _C for each compound. However, in the measurements with ¹¹¹Cd at In sites, the mhf values become zero at temperatures which are smaller than T _C . The difference between the temperatures at which mhf is zero for ¹⁴⁰Ce and ¹¹¹Cd probes correlates with T _C . For each compound this difference decreases with T _C . The results are discussed in terms of the RKKY model for magnetic interactions and the existence of two magnetic systems, with distinct exchange interaction energies due to different types of atomic layers in these compounds.     

Magnetic polaron structures in the one-dimensional double and super-exchange model

An analytical and numerical study of the one-dimensional double and super-exchange model is presented. A phase separation between ferromagnetic and anti-ferromagnetic phases occurs at low super-exchange interaction energy. When the super-exchange interaction energy gets larger, the conduction electrons are self-trapped within separate small magnetic polarons. These magnetic polarons contain a single electron inside two or three sites depending on the conduction electron density and form a Wigner crystallization. A new phase separation is found between these small polarons and the anti-ferromagnetic phase. Our results could explain the spin-glass-like behavior observed in the nickelate one-dimensional compound Y _2?xCa_xBaNiO₅.