A. S. Panfilov

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.

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.

Effect of pressure on the magnetic properties of YNi5, LaNi5, and CeNi5

The effect of pressure on the electronic structure and magnetic properties of the compounds YNi5, LaNi5, and CeNi5 is studied. Large magnetovolume effects of magnitude dlnχ∕dlnV≃4–7 are found for the magnetic susceptibility χ of these systems at low temperatures. Experimental data and “first principles” calculations of the electronic structure and paramagnetic contributions to the susceptibility indicate that YNi5, LaNi5, and CeNi5 are close to a quantum critical point. It is found that the van Vleck orbital contribution to the magnetic susceptibility is 15–20% in these exchange-enhanced paramagnetic materials and should be taken into account in analyzing experimental data on χ and dlnχ∕dlnV. Calculations of the magnetic-field induced moments for atoms in the unit cell of YNi5 reveal an inhomogeneous distribution of the magnetization density and a nontrivial competition between the spin and orbital moments.

Effect of pressure on the magnetic susceptibility of CeCo2

Abstract The magnetic susceptibility, χ, of CeCo2 compound has been measured under pressure up to 2 kbar at T=78 and 300 K , and a pronounced pressure effect, d ln χ/ d P≃−5 Mbar −1 , is revealed. In order to analyze the experimental magnetovolume effect, d ln χ/ d ln V , the volume-dependent electronic structure of CeCo2 has been calculated ab initio by employing a relativistic full-potential LMTO method. The calculations have brought out strong hybridization of the itinerant 4f (Ce) and 3d (Co) states, which leads to a filling of the peculiar bonding band. It is demonstrated that the magnetic properties of CeCo2 are consistent with itinerant behavior of the 4f electrons of cerium. The large volume derivative of the effective electron–electron interaction parameter, d ln I/ d ln V≃−1.7 , implies a predominant role of the short-range correlations, in accordance with the Hubbard model.

Anisotropy of the magnetic properties and the electronic structure of transition-metal diborides

The temperature dependences of the magnetic susceptibility χ and its anisotropy Δχ=χ∥−χ⊥ have been measured for hexagonal single crystals of transition-metal diborides MB2 (M=Sc,Ti,V,Zr,Hf) in the temperature interval 4.2–300K. It is found that the anisotropy Δχ is weakly temperature-dependent, a nonmonotonic function of the filling of the hybridized p−d conduction band, and largest for group-IV transition metals. First-principles calculations of the electronic structure of diborides and the values of the paramagnetic contributions (spin and Van Vleck) to their susceptibility show that the behavior of the magnetic anisotropy is due to the competition between Van Vleck paramagnetism and orbital diamagnetism of the conduction electrons.

Pressure effect on electronic structure and magnetic properties of RNi5

Abstract Magnetic susceptibility of RNi 5 single crystals (R  Gd, Tb, Dy) was studied under uniform and uniaxial pressures at temperatures above T C . The uniform pressure derivatives of the paramagnetic Curie temperature ϑ and the crystalline electric field parameter B 2 0 appear to be small. On the other hand, ϑ and B 2 0 parameters display a large sensitivity to the uniaxial pressure due to related changes of c / a ratio. Ab initio electronic structure calculation was performed for GdNi 5 to evaluate the volume derivative of T C .

The effect of pressure on the magnetic susceptibility of alloys

A transition from the Kondo state to an intermediate-valence state takes place in the quasibinary alloys with cubic -type structure. The effect of pressure P up to 2 kbar on the magnetic susceptibility of this system was measured at 78, 150, and 295 K. The value of obtained is well above those observed for similar RE compounds with stable valences. This derivative tends to decrease with temperature, and to increase (but not monotonically) with the tin content x. The experimental data on were analysed in terms of the changes in the spin-fluctuation temperature .

Pressure effects on the magnetic susceptibility of FeTex (x\simeq 1.0 )

The magnetic susceptibility χ of FeTe(x) compounds (x approximately 1.0) was studied under hydrostatic pressure up to 2 kbar at fixed temperatures of 55, 78 and 300 K. Measurements were taken both for polycrystalline and single crystalline samples. At ambient pressure, with decreasing temperature a drastic drop in χ(T) was confirmed at T approximately 70 K, which appears to be closely related to antiferromagnetic ordering. The obtained results have revealed a puzzling growth of susceptibility under pressure, and this effect is enhanced by lowering the temperature. To shed light on the pressure effects in the magnetic properties of FeTe, ab initio calculations of its volume dependent band structure and the exchange enhanced paramagnetic susceptibility were performed within the local spin density approximation.