Electric and magneto-transport properties of magnetic and superconductive iron pnictide and skutterudite compounds

Superconductivity has gained its popularity after the high Tc materials have been explored. Cuprates (copper-oxide systems) and Hg based 1223 systems still have research interest allowing to examine new chemical compounds for high Tc superconductors (HTS). Unconventional materials such as Fe-based pnictides (group V-B elements in 122 systems) are frontier compounds in which magnetism and superconductivity can be observed together in these materials, presenting them as an option for high Tc superconductors. Iron-arsenide (Fe-As) and copper-oxide (Cu-O) layers in the structure of a crystal are responsible for the superconductivity phenomenon observed in these systems. Fe-As system has similar properties with Cu-O like having d electrons, layered structures and antiferromagnetic (AFM) spin ordering and they are still earning interest. In addition to this, iron has a magnetic moment which plays a key role in superconductivity-magnetism colleration. Up to now, two physical problem in our study are the superconducting phase transition temperature Tc and magnetic behavior of superconducting materials. Lattice changes, symmetry breaking in the unit cell, applying pressure are some of reasons that shift Tc up or down. For the magnetic behavior of superconductive samples, establishment of a new magnetic ordering is key parameter under the Tc. Magnetic changes are said to be affect superconductivity. The exclusion of magnetic field lines by Meissner effect is a clear evidence for superconductive mechanism. Our study here is focused on iron pnictide compounds on the basis of these two physical problem. The most popular iron pnictide systems are Fe-As based ones. The material BaFe2As2 has a tetragonal structure and it goes a structural phase transition from tetragonal I4/mmm to orthorhombic Fmmm lattice. Same behaviors are found in AFe2As2 (A:Ba, Sr, Ca, etc.) systems at low temperatures like in cuprates. This kind of phase transition is named as spin density wave (SDW) transition and it is valid for both cuprates and iron-pnictides. A chemical change in composition with a dopant or applying pressure results magnetic transition in Fe-As compounds and thereby superconductivity emerges. By doping electron or hole to a AFe2As2 system like BaFe2-xPtxAs2 , superconductivity can be observed. Since the change in chemical composition may effect the Tc, Lanthanum is doped to the BaFe2-xPtxAs2 system. And a new serie of Ba1-xLaxFe1.9Pt0.1As2 polycrystalline samples are successfully synthesized by a solid-state reaction method. The synthesis of Pt and La doped compounds was physically remarkable since some of them are synthesized for the first time. Data obtained from those samples are investigated in detail by Rietveld refinement. Our target to produce "single phase" polycrystalline samples is achieved successfully. Unit cell parameters and symmetry group of samples are determined in agreement with the literature. Structural analyses of samples are made by X-Ray diffraction after the chemical synthesis part. Results are analyzed by FullProf software. Profile matching are done for whole samples and structural parameters are determined. Some samples are found single phase in which we published in the articles during our study. Resistivity measurements are performed with the liquid nitrogen followed by liquid helium by the help of a hand made glass cryostat. It is seen that the temperature dependence of the resistivity for the parent sample, BaFe2As2 exhibits the same behavior including a very close match to reported values given in the literature. The structural phase transitions from tetragonal I4/mmm to orthorhombic Fmmm lattice in BaFe2As2 and BaFe1.9Pt0.1As2 samples are observed. Temperature dependent transition is compatible with the corresponding phase changes observed at XRD measurement, which is known to be tetragonal at room temperature. Superconductive phase transition temperature, Tc for the samples in different Pt doping levels is investigated in the literature. It is found that Pt doping is effective on the change of Tc at lower doping rates like x = 0.2. x=0 sample is the parent sample (BaFe2As2) in which it is not superconductive but the BaFe1.9Pt0.1As2 sample has the highest value in Pt doped group for phase transition temperature, Tc. In addition to the literature, we revealed that superconductivity can be observed even in x=0.3 doping rates in such materials. Lanthanum doped samples are successfully synthesized in addition to the Pt doped ones. And resistivity measurements are showed that superconductivity still can be observed in such tetragonal I4/mmm systems. Resistivity of these samples are showed a clear evidence for superconductivity that our results approve. The magnetic measurements of materials are done by measuring zero field cooling (ZFC) and field cooling (FC) curves. Critical temperature, critical current density, coherence length (also known as Ginzburg-Landau parameter), volume magnetization and magnetic susceptibility values are obtained from the analysis. Magnetization behavior of samples are observed as a function of temperature and phase transition temperatures are determined. The critical current density, which is remarkable sign for how much charge a superconductor can carry, is estimated by models given in the literature. Coherence length is calculated from the M-H curves (from the Meissner effect observed in these samples). Results show that samples based on a tetragonal structure are convenient to be a host material for the superconductive behavior. This means the doping of rare-earth ions to those samples provide materials that have characteristic properties like showing zero resistivity and the exclusion of magnetic field lines. In our study, platinum and lanthanum are doped to our host material, BaFe2As2; and new sample sets are prepared. It is seen that ZFC-FC curves are separated in the phase transition temperature, Tc, showing superconductive behavior compatible with the resistivity measurements. Meissner effect is observed in both platinum and lanthanum doped samples. Critical current densities are calculated from the magnetic measurements and phase transition temperatures are determined. The samples are in superconducting regime where their temperatures are lower than the Tc. M versus H curves are measured in several temperatures in this regime below the phase transition temperature. Analysis of the results helped us to calculate coherence length z for the samples by obtaining Hc readings at different measurement temperatures. From the M-H curves of samples measured below the Tc, lines are extrapolated to T=0 K and coherence lengths are found in nm range. By transforming magnetization from emu/g to volume magnetization, susceptibility for the samples are obtained in terms of magnetization data. The termination of ZFC and FC curves in to close values to -1 in y axis, shows that samples are mostly diamagnetic with a high volume fraction. Samples are mostly presented superconductivity in general. Phase transition temperatures found from ZFC-FC curves are compatible with the resistivity data. La doped samples are showed a clear evidence for superconductivity that both resistivity and magnetic results approve. Thin films of BaFe2As2 sample is prepared and the amorphous structure is proved by the XRD measurement. Resistivity of sample is measured between 4.2-300K at liquid He. A phase transition at Tc = 6.7K is seen clearly with reproducible experiments which is very similar to a superconductive phase transition. We observe superconductivity for the first time in an amorphous thin film. This result is interesting because current macroscopic theory of superconductivity is based on Cooper pairing mechanism. The drop of resistivity down to zero obeys filamentary superconductivity that is seen in literature in principle. The difference in our study is that we create a disorder in material and it is amorphous. This process is realized in a simple flash evaporation system under vacuum. This is important because same kind of samples are produced in molecular beam epitaxy (MBE) or similar systems that we are not able to. Some studies are reported that both in bulk and thin film samples, showing that it is possible to apply pressure to the system and create disorder. This method is a way in order to trigger the superconductivity in such systems. For the first time, we observe the superconductivity in an amorphous thin film above the liquid He temperature, at ambient conditions. Magnetic characterization of this sample still continues and results will be presented in an appropriate publisher. In case of any failure in preparing superconducting samples, antimony doped compounds are prepared. BaFe4Sb12 sample is prepared first. This sample is known as skutterudite compound in literature and it is thermoelectrically efficient. Since the synthesis was successful, the thermoelectric and magnetic properties of Pt substituted BaFe4 xPtxSb12 (x = 0, 0.1, 0.2) compounds were investigated in this part of study. Thermoelectric (TE) materials are available to capture a part of wasted energy that is rejected to atmosphere and convert it into the useful electrical power. The efficiency of a TE generator is strongly depending on the dimensionless figure of merit ZT, defined as ZT = S2sT/k where S, s and k are the Seebeck coefficient, electrical conductivity, total thermal conductivity (k = ke + kL where ke is the electronic contribution and kL the lattice contribution) and T stands for the absolute temperature. Efficient thermoelectric materials are required to have high electrical conductivity, high Seebeck coefficient and lower thermal conductivity. Filled skutterudites fulfill these requirements. In the past decade years, experimental and theoretical tremendous effort have been conducted on skutterudites compounds in order to optimize their TE and magnetic properties for enhancing ZT value further and characterize low temperature behavior. The filled skutterudites with chemical formula RT4M12 where R = alkaline earth and rare earth metal, T = Fe, Ru, and Os and M = P, As, and Sb have been studied extensively due to their both high TE performance and exotic magnetic properties. Even though, there are many reports in literature on thermoelectric and magnetic properties of RT4M12 family skutterudite compounds. There is still lack of understanding about correlation between magnetic and thermoelectric phenomena for these compounds. In this work, we aim for contribute to understanding of relation between two phenomenon by characterization low temperature magnetic and thermoelectric properties BaFe4-xPtxSb12 (x = 0, 0.1, 0.2) compounds. X-Ray diffraction, resistivity, magnetic measurements, Seebeck coefficient and thermal conductivity of samples are measured. Temperature depending resistivity shows that all samples show semi metallic behavior in the temperature range of 4.2 K to 300 K. Increases in resistivity with Pt content is interpreted as existing of different scattering mechanisms such as spin fluctuation and Kondo impurity in these compounds or decreasing hole concentration. Kondo minimum are confirmed by temperature dependence of logarithmic resistivity analysis at 2-50 K which might be due to local deformation in crystal structure as a result of spin fluctuation mechanism. Local minimum and maximum in Seebeck coefficient observed might be due to paramagnon drag or spin fluctuation effect in materials. Magnetic susceptibility for the compounds x = 0.1 and x = 0.2 show that there are two successive transition, paramagnetic to magnetic state at Tc = 48 K and short range magnetic to long range magnetic ordering at lower temperatures, however there is only one transition from paramagnetic to long range ordering transition in Pt free compound. The lattice part of thermal conductivity versus temperature plot shows that the major part of total thermal conductivity is dominated by lattice contribution. Local maximum & minimum in Seebeck coefficient versus temperature plot, Kondo minimum observed in logarithmic resistivity show that there is a close relation between magnetic and thermoelectric properties due to spin fluctuation effect in Pt substituted BaFe4-xPtxSb12 (x = 0, 0.1, 0.2) compounds.