R. Puźniak

Growth conditions, structure and superconductivity of pure and metal-doped FeTe1 − xSex single crystals

Superconducting single crystals of pure FeTe1 xSex and FeTe0.65Se0.35 doped with Co, Ni, Cu, Mn, Zn, Mo, Cd, In, Pb, Hg, V, Ga, Mg, Al, Ti, Cr, Sr or Nd into Fe ions site have been grown applying Bridgmans method. It has been found that the sharpness of transition to the superconducting state in FeTe1 xSex is evidently inversely correlated with crystallographic quality of the crystals. Among all of the studied dopants only Co, Ni and Cu substitute Fe ions in FeTe0.65Se0.35 crystals. The remaining examined ions do not incorporate into the crystal structure. Nevertheless, they form inclusions together with selenium, tellurium and/or iron, what changes the chemical composition of host matrix and therefore influences Tc value. Small disorder introduced into magnetic sublattice, by partial replacement of Fe ions by slight amount of nonmagnetic ions of Cu (~ 1.5 at%) or by magnetic ions of Ni (~ 2 at%) and Co (~5 at%) with spin value different than that of Fe ion, completely suppresses superconductivity in FeTe1 xSex system. This indicates that even if superconductivity is observed in the system containing magnetic ions it can not survive when the disorder in magnetic ions sublattice is introduced, most likely because of magnetic scattering of Cooper pairs.

Microstructural magnetic phases in superconducting FeTe0.65Se0.35

In this paper, we address a number of outstanding issues concerning the nature and the role of magnetic inhomogeneities in the iron chalcogenide system FeTe1?xSex and their correlation with superconductivity in this system. We report morphology of superconducting single crystals of FeTe0.65Se0.35 studied with transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy and their magnetic and superconducting properties characterized with magnetization, specific heat and magnetic resonance spectroscopy. Our data demonstrate the presence of nanoscale hexagonal regions coexisting with a tetragonal host lattice, a chemical disorder demonstrating a nonhomogeneous distribution of host atoms in the crystal lattice, as well as iron-deficient bands hundreds of nanometres in length. From the magnetic data and ferromagnetic resonance temperature dependence, we attribute magnetic phases in Fe?Te?Se to Fe3O4 inclusions and to hexagonal symmetry nanoscale regions with a structure of the Fe7Se8 type. Our results suggest that a nonhomogeneous distribution of host atoms might be an intrinsic feature of superconducting Fe?Te?Se chalcogenides and we find a surprising correlation indicating that a faster grown crystal of inferior crystallographic properties is a better superconductor.

Mössbauer studies of powdered single crystals of FeTe0.5Se0.5

Mossbauer measurements performed on powdered single crystals of FeTe0.5Se0.5 (Tc ~ 14.7 K) reveal minor content of two impurity phases, identified as Fe3O4 and Fe7Se8, among the major tetragonal phase. From the shape of the impurity subspectra it follows that Fe7Se8 behaves in a superparamagnetic manner, most likely because of randomly distributed Fe vacancies in the lattice structure of Fe–Te–Se. The magnetite content in the powdered absorber exposed to ambient air conditions remains unchanged during a period of 16 months. Ageing effects are observed for the samples stored under argon atmosphere and a small increase of the isomer shift of the doublet is detected. The presented temperature dependence of the hyperfine parameters can be explained as due to possible orthorhombic distortion or to temperature behaviour of the impurity phases Fe3O4 and Fe7Se8. A strong tendency for the formation of crystalline texture of the powdered sample is observed.

Electronic origin of the step-like character of the discharge curve for NaxCoO2-y cathode

This paper shows electronic approach to an explanation of the step-like character of the discharge/charge curve of Na/Na+/NaxCoO2-y battery. On a basis of comprehensive experimental studies of physicochemical properties of NaxCoO2-y cathode material (XRD, electrical conductivity, thermoelectric power, electronic specific heat) supported by calculations performed using the Korringa–Kohn–Rostoker method with the coherent potential approximation to account for chemical disorder, it has been shown that the observed step-like character of the discharge curve reflects the variation of the chemical potential of electrons (Fermi level) in the density of states of NaxCoO2-y, which is anomalously perturbed by a presence of the oxygen vacancy defects and sodium ordering.

Magnetic properties of Bi0.5Sr0.5FexMn1−xO3(0⩽x⩽0.7)

Fe-substituted manganites Bi0.5Sr0.5FexMn1−xO3 (0⩽x⩽1.0) were synthesized by a solid-state reaction. X-ray powder diffraction and neutron diffraction investigations performed at room temperature show that the crystal structure is orthorhombic for x⩽ 0.3 and cubic for x>0.4. The unit cell volume increases with x. Magnetic properties were studied between 5 and 1000K in a magnetic field up to 16kOe for the compounds with x⩽0.7. A charge/orbital ordering feature was only observed in magnetization curves for the compound with no Fe. The magnetic state of the compounds with different Fe contents changes from paramagnetic to weak antiferromagnetic below the temperature TN varying in the range of 115–155K. Unusual thermomagnetic behavior in the paramagnetic region was observed, which could be related to a coexistence of antiferromagnetic and ferromagnetic fluctuations far above TN. The typical thermomagnetic zero field cooled/field cooled hysteresis below about 40K can be related to the formation of small ferroma...

High field susceptibility of Co-based amorphous ferromagnets

Abstract High field magnetization measurements of Co-based amorphous ferromagnets have been presented. The contributions to high field magnetic susceptibility for itinerant systems have been calculated. It was shown that the significant contribution to M ( T,H ) resulting from the dispersion of the exchange integral should be taken into account.

Noise signatures of metastable resistivity states in ferromagnetic insulating manganite

Pronounced noise signatures enabling one to discriminate metastable resistivity states in La0.86Ca0.14MnO3 single crystals have been observed. The normalized noise spectra for metastable resisitivity differ both in shape and magnitude, indicating that the metastable state is associated with transition of the electronic system into another local minimum of the potential landscape. Such scenario is consistent with freezing of the electronic system into a Coulomb glass state.

Magnetism and superconductivity in RBaSrCuO (R=Gd, Dy)

Abstract The influence of substitution of Ba 2+ with Sr 2+ on superconducting and magnetic properties of ceramic RBa 2− x Sr x Cu 3 O 7−δ (R  Gd, Dy; x = 0.5, 1.0) samples with different oxygen content has been investigated. In contrast to YBCO, RBa 2− x Sr x Cu 3 O 7−δ becomes superconducting in a narrow range of δ between 0 and 0.35. Magnetic phase diagrams were constructed for these compounds.

High frequency cut-off in 1/f conductivity noise of hole-doped La1−x Ca x MnO3 manganite single crystals

High frequency bias and temperature-dependent Lorentzian cut-off has been observed in the 1/f spectra of the conductivity fluctuations in low hole-doped ferromagnetic insulating La1−x Ca x MnO3 manganite at low temperatures. The cut-off frequency depends on dc current bias and temperature. The high frequency cut-off has been tentatively associated with intrinsic limits of the appearance of 1/f noise in the hopping regime of the Coulomb glass state. The assumption is validated by the fact that the Efros–Shklovskii temperature , estimated from the fit of the model to the experimentally measured temperature dependence of the cut-off frequency, has the same value as the temperature evaluated independently from the temperature dependence of the resistivity in the corresponding temperature range.

Anomalies of the dependence of the magnetic properties on the chemical composition in amorphous Fe-Si-B alloys

Abstract The magnetic properties of Fe x Si 90− x B 10 metallic glasses (68⩽ x ⩽83) have been studied. In that group of glassy alloys anomalies of the dependence of the magnetic properties with the Fe content are found. There occurs a maximum of the Curie temperature in the vicinity of x = 75. The saturation magnetic moment varies linearly up to x = 75. For higher Fe content changes of the magnetic moment are small. The spinware stiffness constant is almost constant for x ⩽ 75 whereas for x > 75 it decreases strongly. Among the curves describing the reduced saturation magnetization as a function of the reduced temperature the plot for x = 75 is located highest. It is suggested that there is a gradual transition from normal to Invar-like behaviour in that system.