G. E. Grechnev

Electronic structure and bulk properties of MB6 and MB12 borides

Ab initio band structure calculations are carried out for the higher borides MB6 and MB12. High-precision measurements of the elastic constants are performed for the compounds ZrB12, HoB12, ErB12, TmB12, LuB12, YB6 and LaB6 at low temperatures. The bulk properties of the borides are analyzed on the basis of the calculated equations of states and balanced crystal orbital overlap populations. Our calculations indicate that hexaborides with divalent metals, CaB6, SrB6, BaB6, and YbB6, are semiconductors with small energy gaps. The metallic MB6 hexaborides with trivalent M atoms are found to possess larger bulk moduli values. For dodecaborides the bulk moduli are found to be higher for MB12 with increased filling of the conduction band (ZrB12, HfB12, UB12) in comparison with M3+B12 compounds. The total energy calculations for different magnetic configurations in YbB12 point to the possibility of antiferromagnetic coupling between Yb3+ ions.

Structure and magnetic properties of multi-walled carbon nanotubes modified with iron

Magnetic properties of multi-walled carbon nanotubes modified with iron MWCNT+Fe are studied in detail in the temperature range 4.2–300 K. Carbon encapsulated Fe nanoparticles were produced by chemical vapor deposition. Low-temperature SQUID magnetization measurements are supplemented by structural studies employing thermogravimetric TG analysis, transmission electron microscopy TEM , x-ray diffraction spectroscopy XRD , and scanning electron microscopy SEM . The magnetic susceptibility of MWCNT+Fe was also studied above room temperature to provide a complete picture of its magnetic phase transitions.

Magnetic and superconducting properties of FeSe1−xTex (x∼0, 0.5, and 1.0)

The magnetization of FeSe1−xTex (x∼0, 0.5, and 1.0) compounds has been studied in magnetic fields up to 50kOe and at temperatures of 2–300K. The superconducting transition was observed at Tc∼8K and 13.6–14.2K in FeSe0.963 and FeSe0.5Te0.5, respectively. For most of the samples, nonlinearity of the magnetization curves in the normal state gives evidence of a common, substantial presence of ferromagnetic impurities in these compounds. By taking these impurity effects into account, the intrinsic magnetic susceptibility χ of FeSe0.963, FeSe0.5Te0.5, and FeTe was estimated to increase gradually with Te content. For FeTe a drastic drop in χ(T) with decreasing temperature was found at TN∼70K, which is presumably related to antiferromagnetic ordering. To shed light on the observed magnetic properties, ab initio calculations of the exchange enhanced magnetic susceptibility are performed for FeSe and FeTe in the local spin density approximation.

Conduction-electron-mediated exchange coupling in heavy rare earth metal compounds RM and RM3

Abstract The effect of pressure on the magnetic susceptibility of rare earth compounds GdM (MCu, Ag, Mg) and RIn 3 (RGd, Tb, Dy) has been investigated. The experimental data and results of ab initio calculations of the volume derivatives of the band structure and the exchange parameters for the corresponding series of compounds have been used to analayze the nature of the f-f interactions.

Exchange coupling in GdM compounds

Abstract The magnetic susceptibility of ferromagnetic GdM alloys (M = Cu 1− x Ga x Mg, Zn) has been investigated under helium gas pressure for temperatures above T c . The evaluated pressure derivatives of the paramagnetic Curie temperature, d ln Θ d P . appeared to be remarkably different for isovalent GdMg and GdZn compounds (−11.2 Mbar 1 and −0.1 Mbar 1 respectively). Analysis of the obtained d ln Θ d P values for GdCu 1− x Ga x alloys and results of ab initio electronic structure calculations have revealed the essential role of 5d electrons as the mediators of exchange coupling in ferromagnetic GdM compounds. The pressure derivatives of T c were calculated in the frameworks of modern mean-field theories by using volume-dependent band and exchange parameters, and including spin fluctuations effects. As a result, good agreement is found with the experimental values of d ln Θ d P .

Magnetic properties of superconducting FeSe in the normal state

A detailed magnetization study for the novel FeSe superconductor is carried out to investigate the behavior of the intrinsic magnetic susceptibility χ in the normal state with temperature and under hydrostatic pressure. The temperature dependences of χ and its anisotropy Δχ = χ([parallel]) - χ([perpendicular]) are measured for FeSe single crystals in the temperature range 4.2-300 K, and a substantial growth of susceptibility with temperature is revealed. The observed anisotropy Δχ is very large and comparable to the averaged susceptibility at low temperatures. For a polycrystalline sample of FeSe, the significant pressure effect on χ is determined to be essentially dependent on temperature. Ab initio calculations of the pressure-dependent electronic structure and magnetic susceptibility indicate that FeSe is close to magnetic instability, with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and especially on the height Z of chalcogen species from the Fe plane. The change of Z under pressure determines a large positive pressure effect on χ, which is observed at low temperatures. It is shown that the literature experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with the calculated behavior of the density of electronic states at the Fermi level.

Magnetic-field-induced effects in the electronic structure of itinerant d- and f-metal systems

A paramagnetic response of transition metals and itinerant d- and f-metal compounds in an external magnetic field is studied by employing ab initio full-potential LMTO method in the framework of the local spin density approximation. Within this method the anisotropy of the magnetic susceptibility in hexagonal close-packed transition metals is evaluated for the first time. This anisotropy is owing to the orbital Van Vleck-like paramagnetic susceptibility, which is revealed to be substantial in transition-metal systems due to hybridization effects in the electronic structure. It is demonstrated that compounds TiCo, Ni3Al, YCo2, CeCo2, YNi5, LaNi5, and CeNi5 are strong paramagnets close to the quantum critical point. For these systems the Stoner approximation underestimates the spin susceptibility, whereas the calculated field-induced spin moments provide a good description of the large paramagnetic susceptibilities and magnetovolume effects. It is revealed that an itinerant description of hybridized f elect...

The Fermi surface of ErGa3

The Fermi surface geometry and cyclotron masses of the antiferromagnet ErGa3 in a magnetic-field-induced paramagnetic phase are determined by using the de Haas-van Alphen effect method. The results are analysed on the basis of ab initio band-structure calculations and compared to corresponding results for the isostructural compounds TmGa3 and LuGa3.

Magnetoresistance of nanocarbon materials based on carbon nanotubes

The results of experimental investigations of magnetoresistance in nanocarbon material (NCM) containing carbon nanotubes in magnetic field up to 5 T and at temperature up to 0.54 K are reported. The obtained experimental magnetoresistance curves of NCM are described satisfactorily within the framework of the shrinkage effect of wave function of localized state in a magnetic field along with the spin-polarization mechanism.

Electronic structure and magnetic properties of RNi5−xCux alloys (R=Y, La, Ce)

A study is made of the electronic structure and magnetic properties of RN5−xCux alloys (R=Y, La, Ce). The influence of substitution of nickel by copper on the features of the band structure and behavior of the magnetic susceptibility of these alloys is investigated. An analysis is made of the electronic states and interactions responsible for the magnetic properties of RNi5−xCux alloys.