K. Nenkov

Leibniz Institute for Solid State and Materials Research Dresden, Dresden, Saxony, Germany

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Publications (268)432.41 Total impact

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    ABSTRACT: MgB2 precursor wires were prepared using powder in tube technique by Institute of High Pressure PAS in Warsaw. All samples were annealed under isostatic pressure generated by liquid Argon in the range from 0.3 GPa to 1 GPa. In this paper, we show the effects of different processing routes, namely, cold drawing (CD), cold rolling (CR), hot isostatic pressure (HIP) and doping on critical current density (Jc ), pinning force (Fp ), irreversible magnetic-field (Birr ), critical temperature (Tc ), n value, and dominant pinning mechanism in MgB2/Fe wires with ex situ MgB2 barrier. The results show that medium pressures (∼0.35 GPa) lead to high Jc in low and medium magnetic fields (0 T - 9 T). On the other hand, higher pressures (∼1 GPa) lead to enhanced Jc in high magnetic fields (above 9 T). Transport measurements show that CD, CR, and HIP have small effects on Birr and Tc , but CD, CR, HIP, and doping enhance Jc and Fp in in situ MgB2 wires with ex situ MgB2 barrier. Transport measurements on in situ undoped MgB2 wire with ex situ MgB2 barrier yield a Jc of about 100 A/mm2 at 4.2 K in 6 T, at 10 K in 4 T and at 20 K in 2 T. The results also show that cold drawing causes increase of n value.
    Journal of Applied Physics 05/2015; 117(17):173908. DOI:10.1063/1.4919364 · 2.19 Impact Factor
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    ABSTRACT: The modification of structural properties and magnetic behaviour of the ErFe2−xAlx Laves phase intermetallic compounds have been studied while changing the Al content in the range 0.36 ≤ x ≤ 1.5. Powder X-ray diffraction study at room temperature showed the formation of the cubic C15 structure, in the samples with Al content of 0.36 ≤ x < 0.75 and 1.4 ≤ x ≤ 2.0. For the intermediate compositions with 0.75 ≤ x ≤ 1.4, the hexagonal C14 structure is observed. Magnetic and magnetocaloric properties of polycrystalline ErFe2−xAlx intermetallic compounds were investigated experimentally using magnetic and heat capacity measurements. The Curie temperature TC decreases from 275 to 28 K as the Al content increases from x = 0.36 to x = 1.5, respectively. Magnetization measurements in strong magnetic fields showed the complex mechanism of magnetic saturation in the studied compounds. Under an external field change from 0 to 2 T, the maximum entropy change is 6 J/kg K at T = 28 K for the composition with x = 1.5. The minimum in the concentration dependence of the magnetic entropy change is observed in the vicinity of x equal to 1.25. Regularities of variations of magnetocaloric properties of ErFe2−xAlx intermetallic compounds with increasing Al content are discussed from the point of view of their both structural and magnetic states.
    Journal of Applied Physics 03/2015; 117(12):123912. DOI:10.1063/1.4916353 · 2.19 Impact Factor
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    ABSTRACT: The superconducting properties of mm-sized Fe1.02Se crystals grown by a flux method are investigated. The structural and morphological features are studied by x-ray diffraction (XRD) and by scanning electron microscopy-energy dispersive x-ray spectroscopy SEM-EDX analysis, which identified a co-growth of a dominant superconducting tetragonal phase, with the minority of a non-superconducting hexagonal phase. The ac magnetic response is analyzed using a combined method of the fundamental and the 3rd harmonic ac magnetic susceptibility as a function of the temperature at different ac magnetic field amplitudes and frequencies and with various superimposed dc fields. The variation of the ac magnetic field and frequency in different ranges especially affects the 3rd harmonic components, which are more sensitive to the changes in the flux dynamic regimes. This allows a fine observation of the evolution of the different linear and non-linear processes responsible for the ac magnetic response of the Fe1.02Se crystals. At low enough ac amplitudes and frequencies, and even in high imposed dc magnetic fields, the Fe1.02Se crystals show a typical critical state behavior, marking a high stability of the pinning, with very small influence of the vortex dynamical processes. With the change of ac field amplitude and frequency a gradual crossover is observed from the initial stable pinning state through the domination of the intermediate regimes as flux creep and finally to the complete dominance of flux flow. The ac magnetic response is also influenced by geometric edge barrier effects arising from the plate-like geometry of the Fe1.02Se crystals. The changes of the dominant irreversible (non-linear) mechanism from surface pinning to bulk pinning or to prevailing dynamical regimes is also identified by analyzing the behavior of the 3rd harmonic components.
    Superconductor Science and Technology 02/2015; 28(3). DOI:10.1088/0953-2048/28/3/035009 · 2.80 Impact Factor
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    ABSTRACT: Deformation of an austenitic Fe-24Mn-7Ni-8Cr (mass-%) alloy results in distinct changes of its antiferromagnetic (afm) properties. With rising degree of deformation these are: a decrease of the apparent Néel temperature as characterised by the maximum of the magnetization-vs.-temperature curves measured in field, a broadening of the paramagnetic to afm transition and a splitting between magnetization-vs.-temperature curves measured during field cooling (FC) below the Néel temperature and those measured at the same field after zero-field cooling (ZFC). It is shown further that this splitting corresponds to the appearance of a thermoremanent magnetisation (TRM) for a given cooling field and that the magnitude of the TRM scales with the degree of deformation as well as the cooling field. The ability of the deformed afm alloy to become thermoremanently magnetised is attributed to the deformation induced defects which act as a source of uncompensated magnetic moments. These uncompensated moments are oriented during FC and interact strongly with the bulk afm moments. Support for this interpretation is given by the fact that the net magnetic moments vanish above the Néel temperature. Neither a saturation of the magnitude of the TRM with applied field during cooling nor its switching in field cooled samples could be observed in magnetic fields up to +/- 7 T. Consequently, a stable shift of corresponding magnitude is observed in magnetisation-vs.-field curves. It is concluded that the uncompensated moments are strongly exchange coupled to the afm matrix surrounding them. For the maximum cooling field of 7 T applied in the course of this experimental investigation the highest TRM reached after FC corresponds to about 4 e-4 Bohrmagneton/atom of the afm sample with the highest degree of deformation.
    Materials Science and Engineering (MSE) congress 2014, Darmstadt, Germany; 09/2014
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    ABSTRACT: We investigated the superconducting transition and the pinning properties of undoped and Ag-doped FeSe0.94 at magnetic fields up to 14 T. It was established that due to Ag addition the hexagonal phase formation in melted FeSe0.94 samples is suppressed and the grain connectivity is strongly improved. The obtained superconducting zero-field transition becomes sharp (with a transition width below 1 K), Tc and the upper critical field were found to increase, whereas the normal state resistivity significantly reduces becoming comparable with those of FeSe single crystals. In addition, a considerable magnetoresistance was observed due to Ag doping. The resistive transition of undoped and Ag-doped FeSe0.94 is dominated by thermally activated flux flow. From the activation energy U vs H dependence, a crossover from single-vortex pinning to a collective creep pinning behavior was found with increasing the magnetic field.
    Superconductor Science and Technology 07/2014; 28(2). DOI:10.1088/0953-2048/28/2/025013 · 2.80 Impact Factor
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    ABSTRACT: Samples with nominal compositions HoBa2Cu3Oy and (Ho0.97Sn0.03)Ba2Cu3Oy are obtained by the melt-texture-growth (MTG) method and their superconducting properties are investigated. It is shown that the Sn-doping increases the irreversibility field and the upper critical field of the pristine compound. Both the undoped and Sn-doped samples undergo a vortex glass-vortex liquid transition. The observed phenomena are discussed.
    Journal of Superconductivity and Novel Magnetism 03/2014; 27(3). DOI:10.1007/s10948-013-2378-y · 0.93 Impact Factor
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    ABSTRACT: The magnetic properties of bi-, tri- and multicrystals of 3D topological insulator Bi1−x1−x Sb(x0.07<x<0.22) was studied in the temperature range 1.6 - 300 K and fields up to 70 kOe using a SQUID magnetometer. Two superconducting phases associated with adjacent and central layers of the crystallite interfaces of the bicrystals are identified. It has been found that due to the different stress structure the transition temperature TcTc for one superconducting phase changes considerably, from 8.3 to 36 K, while for second superconducting phase, TcTc remains within the range 3.7 - 4.6 K. In tricrystals and bicrystals with high contents of structural disorder and topological defects, ferromagnetic hysteresis loops and magnetic field expulsion have been observed simultaneously.
    Physics Letters A 03/2014; 378(16-17). DOI:10.1016/j.physleta.2014.02.033 · 1.63 Impact Factor
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    ABSTRACT: We present study of derivatives of current-voltage I(V) characteristics of point-contacts (PCs) based on Ba{1-x}Na{x}Fe2As2 (x=0.25) in the normal and superconducting state. The detailed analysis of dV/dI(V) data (also given in Appendix A) shows that the thermal regime, when temperature increases with a voltage at a rate of about 1.8 K/mV, is realized in the investigated PCs at least at high biases V above the superconducting (SC) gap \Delta. In this case, specific resistivity \rho (T) in PC core is responsible for a peculiar dV/dI(V) behavior, while a pronounced asymmetry of dV/dI(V) is caused by large value of thermopower in this material. A reproducible zero-bias minima detected on dV/dI(V) at low biases in the range \pm(6--9)mV well below the SC critical temperature T_c could be connected with the manifestation of the SC gap \Delta. Evaluation of these Andreev-reflection-like structures on dV/dI(V) points out to the preferred value of 2\Delta/kT_c \approx 6. The expected second gap features on dV/dI(V) are hard to resolve unambiguously, likely due to impurity scattering, spatial inhomogeneity and transition to the mentioned thermal regime as the bias further increases. Suggestions are made how to separate spectroscopic features in dV/dI(V) from those caused by the thermal regime.
    Physical Review B 02/2014; 89(10). DOI:10.1103/PhysRevB.89.104512 · 3.66 Impact Factor
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    ABSTRACT: We present a systematic study of the electronic specific-heat jump (ΔCel) at the superconducting transition temperature Tc of K1-xNaxFe2As2. Both Tc and ΔCel monotonously decrease with increasing x. The specific heat jump scales approximately with a power law, ΔCel∝Tcβ, with β ≈2 determined by the impurity scattering rate, in contrast to most iron-pnictide superconductors, where the remarkable Bud'ko-Ni-Canfield (BNC) scaling ΔCel∝T3 has been found. Both the T dependence of Cel(T ) in the superconducting state and the nearly quadratic scaling of ΔCel at Tc are well described by the Eliashberg theory for a two-band d-wave superconductor with weak pair breaking due to nonmagnetic impurities. The disorder induced by the Na substitution significantly suppresses the small gaps, leading to gapless states in the slightly disordered superconductor, which results in a large observed residual Sommerfeld coefficient in the superconducting state for x >0.
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    ABSTRACT: Since the discovery of superconductivity (SC) in the tetragonal FeSe phase this compound is an intensively investigated iron based superconductor. In this study we examined the influence of Ag addition on the superconducting properties of selenium deficient polycrystalline FeSe0.94, which is almost free of non-superconducting hexagonal phase. The samples were obtained by solid state reaction and melting methods. Their characteristics were investigated by fundamental and third harmonic AC magnetic susceptibility. EDX analysis shows inhomogeneous Ag distributions in the grains. In spite of that intergranular critical current in Ag doped sample is several times higher at approximately the same temperatures. Intragranular current is field independent up to almost 1000 Oe. Using the temperature dependence of third harmonic AC magnetic susceptibility at different DC magnetic fields, the irreversibility lines were obtained for all samples. It was found that Ag addition increased the irreversibility field in comparison with undoped melted and powder sintered samples. All results show that the Ag addition in selenium deficient (FeSe0.94) samples leads to improved superconducting properties.
    Journal of Superconductivity and Novel Magnetism 01/2014; 28(3). DOI:10.1007/s10948-014-2821-8 · 0.93 Impact Factor
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    ABSTRACT: The role of elastic strain for magnetoelectric materials and devices is twofold. It can induce ferroic orders in thin films of otherwise non-ferroic materials. On the other hand, it provides the most exploited coupling mechanism in two-phase magnetoelectric materials and devices today. Complex oxide films (perovskites, spinels) are promising for both routes. The strain control of magnetic order in complex oxide films is a young research field, and few ab initio simulations are available for magnetic order in dependence on lattice parameters and lattice symmetry. Here, an experimental approach for the evaluation of how elastic strain in thin epitaxial films alters their magnetic order is introduced. The magnetic films are grown epitaxially in strain states controlled by buffer layers onto piezoelectric substrates of 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3(001). As an example, the strain dependence of the ordered magnetic moment of SrRuO3 has been investigated. At a tensile strain level of approximately 1%, SrRuO3 is tetragonal, and biaxial elastic strain induces a pronounced suppression of the ordered magnetic moment. As a second example, a strain-driven transition from a ferromagnetic to a magnetically disordered phase has been observed in epitaxial La0.8Sr0.2CoO3 films.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 01/2014; 372(2009):20120441. DOI:10.1098/rsta.2012.0441 · 2.86 Impact Factor
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    ABSTRACT: Bulk superconducting MgB2 samples, 20 mm in diameter, were prepared by hot-pressing of ball-milled Mg and B powders using fine-grained boron powders. High maximum trapped fields of B0 = 5.4 T were obtained at 12 K in one of the investigated trapped field magnets (height 8 mm) at the centre of the bulk surface. Investigating the temperature dependence of the trapped field for short MgB2 samples (height ≤1.6 mm), trapped fields of up to B0 = 3.2 T at 15 K were achieved. These high trapped fields are related to extremely high critical current densities of up to 106 A cm-2 at 15 K, indicating strong pinning due to nanocrystalline MgB2 grains. Expected trapped field data for long trapped field magnets prepared from the available MgB2 material are estimated.
    Superconductor Science and Technology 12/2013; 26(12):2002-. DOI:10.1088/0953-2048/26/12/122002 · 2.80 Impact Factor
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    ABSTRACT: SrRuO_{3} films have been grown in the tetragonal, structurally single-domain state under 1% of biaxial tensile strain. The angular dependencies of the magnetization and the magnetoresistance reveal an upright orientation of the tetragonal unit cell and biaxial magnetic in-plane anisotropy with 110_{t} easy axes. Reversible biaxial strain from piezoelectric Pb(Mg_{1/3}Nb_{2/3})_{0.72}Ti_{0.28}O_{3} (PMN-PT) substrates has been applied to probe the direct strain response of the magnetization and the electrical resistance. At 1% tensile strain, the Curie temperature (T_{C}) and the ordered magnetic moment (m_{S}) at low temperatures are found to substantially decrease with further growing tensile strain. This suggests a suppression of m_{S} resulting from distortions of the RuO_{6} octahedra, in line with reported density-functional calculations. Reversible strain has also been applied to a film under weak tensile strain revealing the opposite response, i.e., an enhancement of T_{C} and m_{S} with tensile strain. Structural and magnetic properties of SrRuO_{3} films in several static strain states (compressive, weak and strong tensile strain) are compared.
    Physical Review B 10/2013; 88(14). DOI:10.1103/PhysRevB.88.144412 · 3.66 Impact Factor
  • J. Ćwik, N. Kolchugina, K. Nenkov
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    ABSTRACT: Structural, magnetic and magnetocaloric properties and heat capacity of polycrystalline Dy1−xHoxCo2 (0.1 ⩽ x ⩽ 0.5) solid solutions were investigated experimentally using by X-ray diffraction, magnetic and heat capacity measurements; the Landau theory was applied also to clarify peculiarities of magnetic phase transition in the compounds.X-ray diffraction patterns taken at room temperature exhibit the existence of the C15 cubic Laves phase structure for all samples under study. The lattice parameter decreased from 0.7186 for x = 0.1–0.7179 nm for x = 0.5 and the Curie temperature TC decreases from 130 to 109 K as the Ho content increases to x = 0.5. The isothermal entropy change −ΔSmag was calculated according to magnetic measurements using thermodynamic Maxwell’s relation. A large entropy change has been observed for all Ho concentrations. Under an external field change from 0 to 3 T, the maximum entropy change fluctuates within 9 J/kg K. The maximum ΔTad value varies within 3 and 2 K for a magnetic field change of 2 and 1 T, respectively. The effect of increasing Ho amount in the Dy1−xHoxCo2 solid solutions on their magnetic and magnetocaloric properties is discussed.
    Journal of Alloys and Compounds 05/2013; 560:72-79. DOI:10.1016/j.jallcom.2013.01.114 · 2.73 Impact Factor
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    ABSTRACT: High-quality KFe2As2 (K122) single crystals synthesized by different techniques have been studied by magnetization and specific heat (SH) measurements. There are 2 types of samples both affected by disordered magnetic phases: (i) cluster-glass (CG) like or (ii) Griffiths phase (G) like. For (i) at low applied magnetic fields the T-dependence of the zero field cooled (ZFC) linear susceptibility (chi_l) exhibits an anomaly with an irreversible behavior in ZFC and field cooled (FC) data. This anomaly is related to the freezing temperature T_f. The extrapolated T_f to B=0 varies between 50 K and 90 K. Below T_f we observed a magnetic hysteresis in the field dependence of the isothermal magnetization (M(B)). The frequency shift of the freezing temperature delta T_f=Delta T_f/[T_f\Delta(\ln \nu)]\sim 0.05$ has an intermediate value, which provides evidence for the formation of a CG-like state in the K122 samples of type (i). The frequency dependence of their T_f follows a conventional power-law divergence of critical slowing down: tau=tau_0 [T_f(nu)/T_f(0)-1]^{-z\nu^{'}} with the critical exponent z\nu^{'}=10(2) and a relatively long characteristic time constant tau_0 =6.9 x10^{-11}$s also supporting a CG behavior. The large value of the Sommerfeld coefficient was related to magnetic contribution from a CG. Samples from (ii) did not show a hysteresis behavior for chi_l(T) and M(B). Below a crossover temperature T^* sim 40 K a power-law dependence in the chi_l propto T^[lambda_G-1}], with a non-universal lambda_G was observed, suggesting a quantum G-like behavior. In this case chi_l and M(B) can be scaled using the scaling function M_s(T,B)= B^{1-\lambda_{\tiny G}}Y(mu B/k_BT) with the scaling moment mu of the order of 3.5mu_b. The same non-universal exponent was found also in SH measurements, where the magnetic contribution C/T propto T^(lambda_G-1).
    physica status solidi (b) 03/2013; 250(3). DOI:10.1002/pssb.201200805 · 1.61 Impact Factor
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    ABSTRACT: We present the result of high-field (up to 40 T) galvanomagnetic study of bismuth bicrystals with nano-width crystallite interfaces (∼100 nm). At B>2 T directed along the interface plane in the quantum oscillations spectrum of the longitudinal Hall effect, two new harmonics have been detected. Their periods of oscillation characterise the much larger cross-sectional areas of Fermi surface of charge carriers than it is in single crystalline bismuth. At the same time, in small disorientation angle bicrystals of an inclination type, a number of Hall quasi-plateaus were observed, which vanish by reversing the magnetic field. We conclude that in bicrystals there are spin-oriented carriers that are located in spectrum of Landau levels at the particular orientation of the magnetic field.
    Solid State Communications 03/2013; 158:9–12. DOI:10.1016/j.ssc.2012.12.018 · 1.70 Impact Factor
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    ABSTRACT: We report AC loss data of superconducting pancake coils (PC) wound from several non-transposed YBCO tapes connected in parallel. Due to coupling losses, their magnetization AC losses in AC fields applied parallel to the tape surface are significantly higher than for single-tape PCs. It was experimentally demonstrated that these coupling losses can be effectively reduced by using double-PCs in which the parallel tapes between the two PCs are connected in reversed order. This special connection between PCs imitates a twisting of parallel tapes and results in their decoupling. An AC current applied additionally with an external AC magnetic field to the double-PCs does not impede the decoupling of the tapes. We propose a simple model to calculate the magnetization AC losses of PCs by approximating the coils by face-to-face stacks of YBCO tapes.
    Superconductor Science and Technology 01/2013; 26(3):035002. DOI:10.1088/0953-2048/26/3/035002 · 2.80 Impact Factor
  • N. Balchev, K. Nenkov, V. Antonov
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    ABSTRACT: We investigated the vortex pinning energies and H(T) characteristic lines of samples with nominal composition MoSr2YCu2O8−δ . The vortex pinning energies were obtained using two approximations of the resistive transition. The characteristic lines H c2(T) and H irr(T) were obtained using resistivity, susceptibility and magnetization measurements. The values of H c2(0) and different irreversibility field values are discussed.
    Journal of Superconductivity and Novel Magnetism 01/2013; 26(1). DOI:10.1007/s10948-012-1704-0 · 0.93 Impact Factor

Publication Stats

2k Citations
432.41 Total Impact Points

Institutions

  • 2000–2015
    • Leibniz Institute for Solid State and Materials Research Dresden
      • • Institute for Solid State Research
      • • Institute for Metallic Materials
      Dresden, Saxony, Germany
    • Universitätsmedizin Göttingen
      Göttingen, Lower Saxony, Germany
    • Russian Academy of Sciences
      • Institute of High Pressure Physics
      Moskva, Moscow, Russia
  • 1983–2012
    • Bulgarian Academy of Sciences
      • • Institute of Catalysis
      • • Institute of General and Inorganic Chemistry
      Ulpia Serdica, Sofia-Capital, Bulgaria
  • 2005–2010
    • Leibniz Institute of Polymer Research Dresden
      Dresden, Saxony, Germany
  • 2004
    • Max Planck Institute for Chemical Physics of Solids
      Dresden, Saxony, Germany
  • 2002
    • Kiel Institute for the Word Economy
      Kiel, Schleswig-Holstein, Germany
  • 2001
    • Moscow State Forest University
      Mytishi, Moskovskaya, Russia
  • 1999
    • Institute of Electronic Materials Technology
      Warszawa, Masovian Voivodeship, Poland
  • 1997
    • Polish Academy of Sciences
      Warszawa, Masovian Voivodeship, Poland