Investigations Into Macroscopic Quantum Phenomena: Magnetism and Superconductivity.

Quantum mechanics manifests itself macroscopically in both magnetism and superconductivity. To gain insight into the nature of these phenomena, their electromagnetic response has been investigated over a wide range of frequencies. Magnetic systems which were measured include amorphous Fe-Ru-B ribbons, RTi_2Ga_4 (R = Er, Ho, Dy), La_{2/3 }B a_{1/3}MnO_3, and PrBa_2Cu_3O_7/YBa _2Cu_3O_7 superlattices. Superconducting materials studied were sintered YBa_2Cu _3O_7 (YBCO) pellets; a-axis-oriented YBCO films; and c-axis-oriented films of pristine, Fe-doped, and irradiated YBCO. FMR, dc magnetization, and ac susceptibility studies showed that a-Fe_{33}Ru _{33}B_{33} is a reentrant and that a-Fe_{80 }Ru_{10}B_{10} is a spin glass. Low-field magnetization and coercive field studies showed that RTi_2Ga_4 (R = Er, Ho, Dy) is a ferromagnet and that a peak in the susceptibility was not due to antiferromagnetism but to the temperature dependence of the coercive field. FMR studies on La_{2/3 }Ba_{1/3}MnO_3 indicated that the system was rather inhomogeneous; however, under proper conditions, films could be made quite homogeneous as evidenced by the temperature and frequency dependence of the linewidth. Microwave absorption studies on PrBa_2Cu_3O_7/YBa_2Cu_3O _7 superlattices indicated that unlike in bulk, PrBa_2Cu_3O_7 in thin layers is superparamagnetic. ac susceptibility studies on sintered YBCO showed a distinct frequency dependence which depends upon the grain size. The magnitude of the susceptibility was modeled by including an intergranular H_{rm cl}. The temperature dependence of the saturation microwave absorption of a sintered YBCO pellet was found to follow the RSJ model. a-axis-oriented films of YBCO showed a large anisotropy in the microwave absorption which was in direct contrast as to what was expected from simple theory. Fe-doped YBCO films with poor Fe incorporation showed microwave transitions as expected from simple theory while those with good Fe incorporation showed an anomalous peak in the absorption near T_{rm c}. The angular dependence of the microwave magnetoabsorption of YBCO films followed a simple sintheta and could be described by weakly pinned, viscously damped fluxons and invoking BGL scaling. The magnetoabsorption of YBCO films with columnar defects, at low fluences, was less than that of its pristine counterpart, and the angular dependence qualitatively agreed with the prediction by Nelson and Vinokur.