arXiv subjects

I. Felner

Publications and source records attributed to I. Felner.

At least 37 records · Page 2Linked to original sources

arXiv2010

Magnetic ordering and dense Kondo behaviour in EuFe$_{2}$P$_{2}$

Ternary iron phosphide EuFe$_2$P$_2$ with ThCr$_2$Si$_2$-type structure has been systematically studied by the measurements of crystal structure, magnetization, Mössbauer effect, transport properties and specific heat. The structural refinement result confirms no direct P-P covalent bonding. The Mössbauer spectra indicate no magnetic moment for the Fe atoms and, that the Eu ions are divalent in the whole temperatures, carrying local moments of $S$=7/2. The Eu$^{2+}$ spins order ferromagnetically at $T_C$=29.5 K, followed by a possible helimagnetic ordering below $T_{HM}$=26 K, where the Eu$^{2+}$ moments tilt a little from the c-axis. External magnetic field increases $T_C$ gradually, but suppresses $T_{HM}$ rapidly. (Magneto)resistivity data indicate characteristic dense Kondo behaviour above the Curie temperature. The result is discussed in terms of the interplay between intersite RKKY and intrasite Kondo interactions.

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arXiv2010

Self doping effect and successive magnetic transitions in superconducting Sr$_2$VFeAsO$_3$

We have studied a quinary Fe-based superconductor Sr$_2$VFeAsO$_3$ by the measurements of x-ray diffraction, x-ray absorption, Mössbauer spectrum, resistivity, magnetization and specific heat. This apparently undoped oxyarsenide is shown to be self doped via electron transfer from the V$^{3+}$ ions. We observed successive magnetic transitions within the VO$_2$ layers: an antiferromagnetic transition at 150 K followed by a weak ferromagnetic transition at 55 K. The spin orderings within the VO$_2$ planes are discussed based on mixed valence of V$^{3+}$ and V$^{4+}$.

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arXiv2010

Large decrease in the critical temperature of superconducting LaFeAsO0.85 compounds doped with 3% atomic weight of nonmagnetic Zn impurities

We observed a large decrease of Tc by no more than 3 at.% of Zn doped to the optimized superconductor LaFeAsO0.85 (Tc = 26 K), confirmed by measurements of electrical resistivity, magnetic susceptibility, specific heat, Mössbauer spectroscopy, Hall coefficient, and an electron probe micro-analysis. The rate ~9 K/% is remarkably higher than observations regarding nonmagnetic impurities. The Tc suppression is likely due to pair-breaking caused by scatterings associated with highly localized electronic state of Zn doped into the Fe2As2 layer. If this is true, the Zn result well accords with the theoretical prediction that suggests a sign reversal s-wave pairing model for the Fe pnictide superconductors, unlike other nonmagnetic impurity results.

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arXiv2009

Drastic suppression of the superconductivity of LaFeAsO0.85 by a nonmagnetic impurity

We observed a drastic Tc suppression caused by no more than 3 at.% of Zn doped to the optimized superconductor LaFeAsO0.85 (Tc = 26 K). The electrical resistivity and magnetic susceptibility measurements suggested that it is likely due to impurity scatterings rather than losing the metallic nature, seeming to support the s+/- pairing model proposed for the Fe pnictide superconductor.

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arXiv2009

Characteristic crossing point ($T_{\ast}\approx 2.7$ K) in specific-heat curves of samples RuSr$_2$Gd$_{1.5}$Ce$_{0.5}$Cu$_2$O$_{10-δ}$ taken for different values of magnetic field

Magnetic properties of polycrystalline samples of RuSr$_2$(Gd$_{1.5}$Ce$_{0.5}$)Cu$_{2}$O$_{10-δ}$, as-prepared (by solid-state reaction) and annealed (12 hours at 845$^{\circ}$C) in pure oxygen at different pressure (30, 62 and 78 atm) are presented. Specific heat and magnetization were investigated in the temperature range 1.8--300 K with a magnetic field up to 8 T. Specific heat, $C(T)$, shows a jump at the superconducting transition (with onset at $T\approx 37.5$ K). Below 20 K, a Schottky-type anomaly becomes apparent in $C(T)$. This low-temperature anomaly can be attributed to splitting of the ground term ${^8}S_{7/2}$ of paramagnetic Gd$^{3+}$ ions by internal and external magnetic fields. It is found that curves $C(T)$ taken for different values of magnetic field have the same crossing point (at $T_{\ast}\approx 2.7$ K) for all samples studied. At the same time, $C(H)$ curves taken for different temperatures have a crossing point at a characteristic field $H_{\ast}\approx 3.7$ T. These effects can be considered as manifestation of the crossing-point phenomenon which is supposed to be inherent for strongly correlated electron systems.

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arXiv2008

Two dimensional frustrated magnetic state in superconducting RuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222)

In this paper we investigate the magnetic state and the role of the crystalline structure in RuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222). Measurements were made in the isomorphic series (Nb1-xRux)Sr2Eu1.5Ce0.5Cu2O10 [(Nb,Ru)-1222], with x between 0 and 1. 3D XY fluctuations above the magnetic transition were not observed in Ru-1222, suggesting a weak inter-plane coupling between the RuO2 layers. The compositional dependence of the magnetic susceptibility shows a rapid broadening with increasing Nb content, explained in terms of a cluster-glass state. The variation of several superconducting parameters as a function of Ru content is linear in the whole concentration range, with no jumps at the critical concentration for which percolation of long range order is expected. 3D Arrhenius and Vogel-Fulcher-type dependencies fail to describe the dynamic properties. Fitting of a generalized Vogel-Fulcher-type dependence, with ln(tau/tau0) = A(T-T0)^-B, yield B = 2.0, in excellent agreement with Monte Carlo simulations for 2D systems. The value deduced for T0 agrees well with the re-opening of hysteresis in the M(H) curves. The observed superconducting and magnetic features are explained in terms of a scenario of 2D magnetic islands at the RuO2 layers, with no long range magnetic order.

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arXiv2008

Superconductivity of Various Borides: The Role of Stretched c- Parameter

The superconductivity of MgB2, AlB2, NbB2+x and TaB2+x is inter-compared. The stretched c-lattice parameter (c = 3.52 A) of MgB2 in comparison to NbB2.4 (c = 3.32 A) and AlB2 (c = 3.25 A) decides empirically the population of their p and s bands and as a result their transition temperature, Tc values respectively at 39K and 9.5K for the first two and no superconductivity for the later. Besides the electron doping from substitution of Mg+2 by Al+3, the stretched c-parameter also affects the Boron plane constructed hole type Sigma-band population and the contribution from Mg or Al plane electron type Pi band. This turns the electron type (mainly Pi-band conduction) non-superconducting AlB2 to hole type (mainly s-band conduction) MgB2 superconductor (39 K) as indicated by the thermoelectric power study. Keeping this strategy in mind that stretching of c-parameter enhances superconductivity, the NbB2+x and TaB2+x samples are studied for existence of superconductivity. The non-stoichiometry induces an increase in c parameter with Boron excess in both borides. Magnetization (M-T) and Resistivity measurements (R-T) in case of niobium boride samples show the absence of superconductivity in stoichiometric NbB2 sample (c = 3.26 A) while a clear diamagnetic signal and a R = 0 transition for Boron excess NbB2+x samples. On the other hand, superconductivity is not achieved in TaB2+x case. The probable reason behind is the comparatively lesser or insufficient stretching of c-parameter.

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arXiv2008

Two-Fluid Behaviour at the Origin of the Resistivity Peak in Doped Manganites

We report a series of magnetic and transport measurements on high-quality single crystal samples of colossal magnetoresistive manganites, La_{0.7} Ca_{0.3} Mn O_3 and Pr_{0.7} Sr_{0.3} Mn O_3. 1 % Fe doping allows a Moessbauer spectroscopy study, which shows (i) unusual line broadening within the ferromagnetic phase and (ii) a coexistence of ferro- and paramagnetic contributions in a region, T_1 T_2. This shows that phase separation into metallic (ferromagnetic) and insulating (paramagnetic) phases cannot be generally responsible for the resistivity peak (and hence for the associated colossal magnetoresistance). Our results can be understood phenomenologically within the two-fluid approach, which also allows for a difference between T_C and T_{MI}. Our data indeed imply that while magnetic and transport properties of the manganites are closely interrelated, the two transitions at T_C and T_{MI} can be viewed as distinct phenomena.

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arXiv2008

Mossbauer Spectroscopy Determination of iron foreign phases in the Superconducting Systems; RAsFeO1-x, RAsFeO1-xFx and Sr1-xKxFe2As2

The recently discovered superconducting - spin density wave materials, containing Fe and As, have raised huge interest. However most materials prepared to date, suffer from a varying degree of content of foreign Fe-As phases, Fe2As, FeAs2 and FeAs, which can lead to wrong conclusions concerning the properties of these materials. We show here that Mossbauer Spectroscopy is able to determine quite easily the relative content of the foreign phases. This procedure is demonstrated by a study of seven samples of superconducting or spin density wave materials, prepared in three different laboratories.

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arXiv2007

Linear and nonlinear low frequency electrodynamics of the surface superconducting states in an yttrium hexaboride a single crystal

We report the low-frequency and tunneling studies of yttrium hexaboride single crystal. Ac susceptibility at frequencies 10 - 1500 Hz has been measured in parallel to the crystal surface DC felds, H0. We found that in the DC feld H0 > Hc2 DC magnetic moment completely disappears while the ac response exhibited the presence of superconductivity at the surface. Increasing of the DC field from Hc2 revealed the enlarging of losses with a maximum in the feld between Hc2 and Hc3. Losses at the maximum were considerably larger than in the mixed and in the normal states. The value of the DC field, where loss peak was observed, depends on the amplitude and frequency of the ac feld. Close to Tc this peak shifts below Hc2 which showed the coexistence of surface superconducting states and Abrikosov vortices. We observed a logarithmic frequency dependence of the in-phase component of the susceptibility. Such frequency dispersion of the inphase component resembles the response of spin-glass systems, but the out-of-phase component also exhibited frequency dispersion that is not a known feature of the classic spin-glass response. Analysis of the experimental data with Kramers-Kronig relations showed the possible existence of the loss peak at very low frequencies (< 5 Hz). We found that the amplitude of the third harmonic was not a cubic function of the ac amplitude even at considerably weak ac felds. This does not leave any room for treating the nonlinear effects on the basis of perturbation theory. We show that the conception of surface vortices or surface critical currents could not adequately describe the existing experimental data. Consideration of a model of slow relaxing nonequilibrium order parameter permits one to explain the partial shielding and losses of weak ac field for H0 > Hc2.

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arXiv2005

Transport, thermal and magnetic properties of RuSr_2(Gd_{1.5}Ce_{0.5})Cu_2O_{10-δ}, a magnetic superconductor

Resistivity, thermoelectric power, heat capacity and magnetization for samples of RuSr_2(Gd_{1.5}Ce_{0.5})Cu_{2}O_{10-δ} were investigated in the temperature range 1.8-300 K with a magnetic field up to 8 T. The resistive transitions to the superconducting state are found to be determined by the inhomogeneous (granular) structure, characterized by the intragranular, T_{c0}, and intergranular, T_{cg}, transition temperatures. Heat capacity, C(T), shows a jump at the superconducting transition temperature T_{c0}\approx 37.5 K. A Schottky-like anomaly is found in C(T) below 20 K. This low temperature anomaly can be attributed to splitting of the ground term $^{8}S_{7/2}$ of paramagnetic Gd^{3+} ions by internal and external magnetic fields.

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arXiv2005

The possible origin of the higher magnetic phase transition in RuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222)

Two magnetic transitions are observed in the magneto-superconducting RuEu2-xCexSr2Cu2O10 (Ru-1222), at TM 160 K and TM2~80 K. Below TM2 the Ru moments are weak-ferromagnetically ordered and wide ferromagnetic hysteresis loops are observed, they become narrow and disappear at ~ 60-70 K. Above TM2, (i) small antiferromagnetic-like hysteresis loops reappear with a peak in the coercive fields around 120 K. (ii) A small peak at ~120 K is also observed in the dc and ac susceptibility curves. The two phenomena are absent in the non-SC x=1 samples. For x<1, the decrease of the Ce4+ content, is compensated by non-homogeneous oxygen depletion, which may induce a reduction of Ru5+ ions to Ru4+. The higher ordering temperature, TM, which does not change with x, may result from Ru4+ rich clusters, in which the Ru4+-Ru4+ exchange interactions are stronger than the Ru5+-Ru5+ interactions. In the superconducting Ru1-xMoxSr2Eu1.5Ce0.5Cu2O10 (x=0-0.4) system, TM2 shifts to low temperature with x (14 K for x=0.4), whereas TM is not affected by the Mo content, indicating again that TM may not correspond to the main phase. Two scenarios are suggested to explain the magnetic phenomena at TM2<T< TM. (i) They are due to a small fraction of nano-size islands inside the crystal grains, in which the Ru4+ concentration is high and are magnetically ordered below TM. (ii) The presence of nanoparticles of a foreign minor extra Ru4+ magnetic phase of Sr-Cu-Ru-O3, which orders at TM, in which Cu is distributed inhomogeneously in both the Ru and Sr sites

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arXiv2005

Anisotropic magnetization of RuEu1.5Ce0.5Sr2Cu2O10 (Ru-1222) thin films

A dc magnetic study is reported on c-axis oriented epitaxial thin films of EuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222) on (100) SrTiO3 wafers. Magnetic measurements on 200 and 500 nm thick films were performed with the magnetic field either parallel or perpendicular to the wafer. We found that the films order magnetically at 180 K. The easy axis of the magnetization is in the basal plane and the Ru5+ ions are in their high-spin state, as deduced from the isothermal magnetization curves at low temperatures. The anisotropy ratio was found to be 7.5. The observed results are compared to data of the Ru-1222 ceramic material.

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arXiv2004

Micro-structure and Magnetization of the 80-K Superconductor, TbSr2Cu2.7Mo0.3O7+d

The results of micro-structural and detailed magnetization studies are reported here for our recently published TbSr2Cu2.7Mo0.3O7+d superconductor with a transition temperature (Tc) at as high as 80 K [1]. From XRD and EDX the sample was confirmed to be of single-phase and possess the nominal cation stoichiometry. The SEM images showed well-developed grains for this ceramic high-Tc superconductor. Magnetization measurements at various temperatures and fields revealed bulk superconductivity below 80 K. Magnetization versus applied field (M vs. H) loops at various temperatures below the Tc exhibited clear full penetration field HP. In normal state (> 100 K) the susceptibility follows the Curie-Weiss behavior with an effective paramagnetic moment of 9.83 mB.

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arXiv2004

Tunneling and Magnetic Characteristics of Superconducting ZrB$_{12}$ Single Crystals

Bulk and surface properties of high-quality single crystals of zirconium dodecaboride have been studied in the temperature range from 4.5 K up to the superconducting transition temperature which is found to be nearly 6.06 K. Scanning tunnelling spectroscopy data, together with dc and ac magnetization measurements, are consistent with the conventional s-wave pairing scenario, whereas they disagree in estimates of the electron-phonon coupling strength. We explain the divergence, supposing a great difference between the surface and bulk superconducting characteristics of the compound. This assertion is supported by our findings of a non-linear magnetic response to an amplitude-modulated alternating magnetic field, testifying to the presence of surface superconductivity in the ZrB$_{12}$ samples at dc fields exceeding the thermodynamic critical field.

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arXiv2003

Coexistence of Superconductivity and Magnetism in MoSr2RCu2O8 (R=rare-earth, Mo-1212)

The properties of the MoSr2RCu2O8 (R=rare earth) system are found to systematically change with the contraction of the R ions. For the light R ions (La-Nd) the samples are paramagnetic down to 5 K, whereas in the intermediate range (Sm-Tb), the Mo sublattice orders antiferromagnetically at TN, ranging from 11 to 24 K. For the heavy R ions, Ho-Tm and Y, superconductivity appears at TC in the range 19-27 K and antiferromagnetism sets in at TN < TC. This latter behavior resembles most of the magneto-superconductors, but is in sharp contrast to the iso-structural RuSr2RCu2O8 system where TN > TC.

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arXiv2003

Tuning the superconducting state in the magnetic MoSr2YCu2O8 materials

The properties of the MoSr2YCu2O8 materials are found to systematically change with the oxygen concentration determined by the sintering and annealing conditions. The as-prepared (asp) sample is anti-ferromagnetically (AFM) ordered at 16 K. The magnetic features are related to the Mo5+ sublattice. Annealing under an oxygen atmosphere induces superconductivity (SC) in the Cu-O planes, and the TC and the shielding fraction values obtained depend strongly on the oxygen concentration. Annealing the asp material at 1030 C under ambient oxygen atmosphere yields TC of 18 K, whereas further annealing at 650 C under 92 atm. of oxygen, shifts TC to 32 K. The AFM ordering, which coexists with SC state through effectively decoupled subsystems, is not affected by the presence or absence of the SC state. In all samples studied, TN < TC. This behavior resembles most of the inter-metallic magneto-superconductors, but is in sharp contrast to the iso-structural RuSr2GdCu2O8 system where TN > TC.

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arXiv2003

Magnetism and Superconductivity in RuM1.5Ce0.5Sr2Cu2O10 (M=Eu and Y)

RuY1.5Ce0.5Sr2Cu2O10, is not superconducting (SC) and orders magnetically at TM =152 K. The Ru moment at 5K is 0.75 mB. Ferromagnetic hysteresis loops are observed below Tirr= 100 K. At 100 K, the remanent magnetization (Mrem) and the coercive field (HC) become zero. Surprisingly, at Tirr <T< TM a reappearance of antiferromagnetic (AFM) hysteresis loops are observed with a peak at 120 K for both Mrem and HC. The paramagnetic constants are: Peff =2.06 mB (greater than the 1.73 mB expected for low spin Ru5+) and q= 136 K which agrees well with TM. Substitution of Zn (2.5% atm.) for Cu in RuEu1.5Ce0.5Sr2Cu2O10 (TC=38 K), suppresses TC but does not affect the magnetic features such as: TM and Tirr, Mrem and HC. This indicates that both SC and magnetic states are practically decoupled. We argue that the magnetic structure of this system becomes (i) AFM ordered at TM. (ii) Due to the tilting of the RuO6 octahedra away from the crystallographic c axis, at Tirr < TM, weak-ferromagnetic (W-FM) order is induced by the canting of the Ru moments. (iii) At TC (TM/TC~4), the specific materials (M=Eu, Gd) become SC. Below TC, both SC and W-FM states coexist.

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