M(?)ssbauer And Magnetometry Study Of Iron-Based High-Tc Superconductors

The recent discovery of superconductivity at Tc's up to 55 K in iron-arsenide systems has triggered enormous interest in this class of superconducting ma-terials. Initial characterization has revealed that superconductivity emerges upon doping carriers into an antiferromagnetic (AFM) parent compound, while the structural and AFM phase transitions are suppressed. This sug-gests an interesting interplay between magnetism and superconductivity. It is widely believed that the dynamic spin fluctuations associated with the gradual suppression of the AFM order are crucial for the mechanism of superconductivity. However, the origin of the magnetism and the driving force that responsible for both the structural and AFM phase transitions is still highly under debate. Detailed studies on the magnetic properties of the superconductor and the phase transitions of the parent compound may con-tribute to address this problem and thus shed lights on the understanding of the pairing mechanism in these materials.In this thesis two different kinds of Iron-based superconductors, namely the Polycrystalline 1111-type CeFeAsO0.84F0.16 superconductor and the single crystal 122-type CaFe2As2 non-superconducting parent compound, have been prepared.1. CeFeAsO0.84F0.16:The micro-and macroscopic magnetic properties of the CeFeAsO0.84F0.16 superconductor have been studied. Temper-ature dependent of DC and AC magnetic measurements and Moss-bauer measurement both in normal and superconducting state have been carried out. Our results show that the superconducting tran-sition temperature is Tc= 36 K, and the values of the critical field Hc1 at various temperatures are also deduced from M(H) curves. The M(T) and M(H) plots show that AFM coexist with superconductivity under temperatures less than TN= 3 K because of the Ce 4f local mo-ments. Furthermore, the 57Fe Mossbauer spectra consist of a singlet with isomer shift values that indicate the iron in CeFeAsO0.84F0.16 compound is most likely in a low spin (S=0) ferrous (Fe2+) state, and no magnetic order was detected in the temperature range of 15 K to 295 K. The Debye temperature is calculated to be 336 K.2. CaFe2As2: Detailed Mossbauer spectroscopy studies of structural and magnetic phase transitions in the undoped parent compound CaFe2As2 single crystal of the recently discovered Fe-based superconductors Ca1-xKxFe2As2 have been conducted. Above the transition tempera-ture, the Mossbauer spectra were fitted with an asymmetric doublet. The ratio of the two line intensities was found to be I3/2/I1/2= 2.04, revealing a positive Vzz. Analysis on the temperature dependence of Vzz reveals a first-order structural transition at Ts-165K, which is consistent with the isomer shift result and the magnetic measurement result. Theoretical fit of the hyperfine magnetic field data with the Bean-Rodbell model givesη= 1.35, indicating a first-order magnetic phase transition in the CaFe2As2 compound, and the compressibility, K, for the CaFe2As2 compound is calculated to be 1.67×10-2 GPa-1. The results provides clear evidence of strong magnetic-lattice coupling and suggests that the magnetic fluctuations may be the driving force for the structural-AFM phase transition.