Year

2014

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Discovered in 2008, iron pnictides are the latest high temperature superconductors which have aroused enormous attention in the scientific community. The discovery of iron based superconductors (FBSs) marked the foundation of a new era in the field of superconductivity by replacing the Copper Age by the Iron Age. This discovery has given scientists the chance to study the superconducting and magnetic properties in a different family of high temperature superconductors, as understanding the nature of superconductivity in unconventional superconductor is crucial for designing new materials with higher critical temperature (Tc). These materials would be good candidates for use in electricity generators, cheaper medical imaging scanners, and extremely fast levitating trains because superconducting materials with higher Tc would not require expensive coolants to reach the superconducting transition temperature. Therefore, the discovery of FBSs was a significant achievement in the condensed matter community.

The main focus and novelty of this work is twofold: firstly, the pinning potential, thermally activated flux flow behaviour and superconducting properties of iron based superconductors, mostly hole doped BaFe2As2 pnictides and arsenic free FeSe1-xTex chalcogenides was investigated in details. Secondly, the magnetic and transport properties of parent compound BaFe2As2 and non superconducting Ba(Fe1-xCrx)2As2 was studied using magnetic, magnetoresistance and neutron diffraction measurements.

Understanding the vortex pinning mechanism in FBSs is crucial for practical applications and fundamental study due to the relatively high critical temperature, high upper critical field (Bc2), high critical current density, very high intrinsic pinning potential, and nearly isotropic superconductivity of these compounds, and also due to the possibilities for the fabrication of superconducting wire. In order to understand the pinning mechanisms in these systems, scaling analysis of the normalized pinning force as a function of reduced field was performed. Analysis using the Dew-Hughes model has suggested that point pins alone cannot explain the observed field variation of the pinning force density. According to the collective flux pinning model, the field dependence of the magnetization shows that the flux pinning in Ba(Fe1-xNix)2As2 is dominated by the spatial variation in the charge carrier mean free path.

Irradiation has been employed in order to increase the pinning potential, and as a result, the critical current density in BaFe1.9Ni0.1As2 single crystal. The C4+irradiation cause little change in the superconducting critical temperature, but it can enhance infield critical current density (Jc) by a factor of up to 1.5, with enhanced flux jumping at 2 K. Also, the magnetic optical imaging results confirm the enhancement of Jc in the irradiated samples. These results suggest that light C4+ ion irradiation is an effective method for the enhancement of Jc in FBSs compared to heavy ion irradiation and neutron irradiation.

In addition, The angular dependence of the upper critical field and the pinning potential of underdoped BaFe1.9Co0.1As2 single crystals have been investigated by measuring magneto-transport at different magnetic fields and angles. Furthermore, by scaling the angular dependence of the resistance, based on the anisotropic Ginzburg-Landau (GL) theory, an anisotropy value of less than 2.1 was determined for different temperatures below the superconducting transition temperatures. Based on these results, the pinning potential is strongly angle dependent for θ ≤ 45° and almost angle independent for θ ≥ 45°, while Bc2 increase monotonically with increasing angle.

Also, the thermally activated flux flow (TAFF) behaviour of arsenic free Fe1.06Te0.6Se0.4 single crystals were analysed using the conventional Arrhenius relation and modified TAFF model. It was found that the Arrhenius curve slopes are directly related to, but not equal to, the activation energies of Fe1.06Te0.6Se0.4 single crystals. Therefore, the use of a modified TAFF model, ρ(T, B) = ρ0f exp(-U/T), is suggested, where the temperature dependence of the prefactor ρ0f = 2ρcU/T and the nonlinear relation of the thermal activation energy are considered.

Furthermore, a detailed investigation was carried out to understand the magnetic and magnetoresistance behaviour of non-superconducting Ba(Fe1-xCrx)2As2. It should be noted that understanding the antiferromagnetic order of iron ions itself is also important for both fundamental study and practical application. It is very interesting to design new magnetic device based on spin dependent transport properties of pnictide materials. Ba(Fe1-xCrx)2As2 is the first doped compound with no superconductivity phase and magnetic phases is the only competitor as Cr concentration increases. Transport and magnetic measurements show an interesting two fold symmetry in non superconducting Ba(Fe1-xCrx)2As2 (x = 0.303) compound which depend on temperature and magnetic field. In order to understand the temperature and magnetic field response of iron pnictide at atomic level, neutron diffraction studies were performed for Ba(Fe1-xCrx)2As2 (x=0.303).

FoR codes (2008)

0912 MATERIALS ENGINEERING

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.