Investigations of Magnetic Thin Films and High T(c) Superconductivity.

Magnetic behavior of low dimensional systems has been investigated through the study of two kind of materials: (1) Fe/Ag single crystal thin films; (2) Fe doped high T_{c} superconductors. The techniques involved in the investigations are Mossbauer spectroscopy, SQUID magnetometry, X-ray diffraction, etc. Samples of Fe(110)/Ag(111) thin films were fabricated by molecular beam epitaxy. By studying the spin wave excitations at the interfaces of Fe/Ag/Fe sandwiche, the existence of magnetic interaction between the two Fe films across the Ag layer was proved. The relation between the spin wave excitations and the coupling strength of the two ferromagnets was calculated by a Green's function method, which shows that as the coupling strength increases from zero to the exchange interaction of the ferromagnetic material, the B factor at the interface decreases monotonically from 2B_{b} to B_ {b}. An island structure in quasi-two dimensional thin films and its influence to the magnetic properties were also studied. It was found that under certain circumstances, the island structure can result in a lower superparamagnetic blocking temperature than ferromagnetic Curie temperature, and can yield a linear temperature dependence of the magnetization. A simple theoretical model was proposed to describe these phenomena and was supported by experiments. In the study of Fe doped high T_ {c} superconductors of YBa_2(Cu _{1-x}Fe_{x})_3O _{7-delta} with 0 <=q x <=q 0.15, the superconducting critical temperature T_{c} was found to be suppressed by the Fe doping and drops to zero at x ~ 0.17. An orthorhombic -to-tetragonal structural transition occurs at x ~ 0.03. The Fe dopants were found to occupy primarily at the Cu(1) sites (~ 90%) and the Fe-Fe interactions can establish a magnetic ordering at T = 4.2K for x > 0.03, which coexists with the superconductivity. The antiferronmagnetism in the Cu(2)-O _2 planes of oxygen deficient samples was confirmed by means of Mossbauer spectroscopy. In addition, after annealing in Ar at 750^circ C and reannealing in O_2 at 210^circC, 30% of Fe in the samples can be caused to migrate from Cu(1) sites to Cu(2) sites. For this series of samples, the orthorhombic -to-tetragonal transition occurs at x ~ 0.09 and T_{c} drops to zero at x ~ 0.13. Comparison of this series samples with the previous one was made.