University of Wollongong. Institute for Superconducting & Electronic Materials
Manshadi, Mahboobeh Shahbazi, Study of supercoducting and magneto transport properties of REFeAsO1-xFx (RE=La and Ce), Master thesis, University of Wollongong. Institute for Superconducting & Electronic Materials, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3254
Iron-based superconductors are the most recently discovered superconductors which could be suitable for a variety of applications. The high upper critical fields and relatively high critical current density in this group are good evidence that these compounds can be competitive with MgB2 and even high critical temperature (Tc) cuprates. Moreover, the first high temperature superconductors, cuprates, have been studied intensively for more than 20 years, but scientists still don’t know exactly how these materials work. Finding the first non-cuprate high temperature superconductors can help to unveil the mystery of superconductivity. It is possible that the clues to how these materials work could lead to the design of room temperature superconductors.
The first iron-based superconductor compound, LaFeAsO1-xFx, shows superconductivity at 26 K. However, the transition temperature is increased by replacing La by other rare earth elements with smaller atomic radii, such as Ce, Sm, Nd, Pr, and Gd, resulting in an increase of up to 56 K for GdFeAsO1-xFx compound. The parent compounds show antiferromagnetic spin density wave order, and superconductivity appears by introducing charge carriers, either through electron or hole doping.
Six families of iron based superconductors have been discovered so far. The first family has the formula REOFeAs, in which RE stands for rare earth element. These compounds have a tetragonal layered ZrCuSiAs structure with the P4/nmm space group. The second family has the formula AFe2As2, in which A is alkali- rear elements. They have the ThCr2Si2 structure with space group I4/mmm. The next family is LiFeAs, which has an infinite layered structure and crystallizes into the CuPb-type tetragonal structure. The next category, FeSe, presents a tetragonal structure, the simplest crystal structure among the iron-based superconductors. The latest families discovered so far are (Ca,Sr) FFeAsand Sr4Sc2Fe2As2O6. (Ca,Sr)FFeAs has the ZrCuSiAs type structure with P4/nmm space group. Sr4Sc2Fe2As2O6 has a layered structure with the space group I4/mmm.
For practical application of the Fe-based superconductors, two of the most important parameters are the upper critical field, Hc2, and the critical current density, Jc. The upper critical field is an intrinsic property, which has been approximated to be higher than 55 or 64 T in LaFeAs O0.9F0.1, 70 T in PrFeAsO0.85F0.15, over 100 T in SmFeAsO0.85F0.15,and 230 T in high-pressure (HP) fabricated NdFeAsO0.82F0.18. The Jc is sample dependent and controlled by the flux pinning behaviour. However, the available data for critical current density and pinning force in LaFeAsO1-xFx compound are very limited so far. As La is non-magnetic, LaFeAsO1-xFx was selected for study in this thesis, as it should be an ideal sample to study the flux pinning related properties. This is because all the other RE compounds contain a magnetic RE. Compared to LaFeAsO1-xFx, where Fe is the only possible ion carrying a significant magnetic moment, a rare earth oxypncictide with a paramagnetic ion such as Ce+3 in CeFeAsO1-xFx also offered a unique opportunity to study the interplay between the rare-earth element and the Fe magnetic ions.
Our results show that the critical current density for both compounds, LaFeAsO1-xFx and CeFeAsO1-xFx, depends on the level of fluorine doping. For LaFeAsO1-xFx compound, with increasing fluorine doping from x = 0.05 to x = 0.15, Jc is increased. After that, with further increases in x, the Jc is reduced. For CeFeAsO1-xFx, Jc is decreased with increasing fluorine doping. Both compounds show a superior Jc field dependence at low temperature. A peak effect was observed in the CeFeAsO1-xFx samples with x = 0.1 at T = 20 K. The peak effect was also detected at 5 K, 10 K and 15 K for LaFeAsO0.85F0.15 compound. Jc shows weak magnetic field dependence at T < 20 K for both compounds. By using the Ginzburg-Landau equation and the Werthamer-Helfand-Hohenberg (WHH) theory, we estimate Hc2ab(0) = 122 T and 185 T for LaFeAsO0.85F0.15 and CeFeAsO0.9F0.1, respectively.
The pinning potential scales as Uo/kB B-n, where Uo is the pinning potential energy, kB= Boltzmann’s constant, and n = 0.2 for B < 3 T and n = 0.7 for B > 3 T for CeFeAsO0.9F0.1, and n = 0.13 for B < 1 T and n = 0.68 for B > 1 for LaFeAsO0.85F0.15.So, it is expected that single-vortex pinning may coexist with collective creep in lowfield. The value of Uo for the CeFeAsO0.9F0.1 doped sample is two times higher than for the LaFeAsO0.95F0.05.
The Hc2 values of these compounds have the potential to be increased even more, through proper chemical doping and physical approaches, due to the two-gap superconductivity in the new iron-based superconductors. As the Jc values are still considerably lower than those of individual grains, the challenge is to produce these materials with more texture and connectivity, in order to allow these new superconductors to carry a high critical current density in low and high magnetic fields.