Electron Tunneling Studies of Spin-Flip Scattering with Native and Artificial Rare-Earth Barriers.
Abstract
Both artificial and native rare earth (RE) tunnel barriers have been successfully fabricated. Artificial barriers are of the form M-REOx-M (M = Al, Pb, PbBi; RE = Ho, Dy, Gd, Lu), and native barriers are of the form RE-REOx-s ((S = Pb, PbBi; RE = Dy,Ho). Artificial barriers are formed by oxidizing a thin (5-20 A) film of evaporated RE. Current-Voltage characteristics (I-V) produced by Al-REOx-Al junctions show substantial gap depression and gap smearing for magnetic rare earths (Ho, Dy, Gd). Using the Abrikosov-Gorkov (AG) theory of spin-flip scattering, we have successfully fit gap-depressed I-V's. Since the actual experimental temperature (T = 0.93^ circK) corresponds to a high reduced temperature (T/T_{rm c}) for Al, the AG predicted temperature dependence of the pair potential was necessary to obtain good fits. Our tunneling experiments utilize a variety of fabrication geometries. Our results do not support the view that contact between Al and the REOx barrier is responsible for gap depression. Instead, they indicate that gap depression is caused by inter-diffusion of Al and RE at the metal-metal interface. WKB fits to dynamic resistance (dV/dI) spectra of artificial RE barriers yield asymmetric ("tilted") barriers similar to our AlOx results. IETS spectra show two features characteristic of the artificial HoOx barriers: (1) a progressive down-shift in energy of the "Al-O" peak with increasing RE thickness, and (2) the growth of a peak at 12 mV with increased RE thickness. Native Dy barriers have been fabricated "in-situ" using a wet-oxygen ambient. The presence of water was found to be crucial for the formation of low leakage (0.6%) barriers. PbBi counter-electrodes show no sign of gap depression, despite clean contact with the DyOx barriers. Barrier characteristics for native RE barriers are quite different from those of artificial RE barriers. Native RE dV/dI's are quite symmetric corresponding to rectangular ("flat") WKB barriers. The average height and width are 1.02 eV and 27.8 A, respectively. IETS has been performed for both HoOx and DyOx barriers. We report new quantitative results for barrier phonons in both HoOx and DyOx.
- Publication:
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Ph.D. Thesis
- Pub Date:
- September 1987
- Bibcode:
- 1987PhDT........83G
- Keywords:
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- Physics: Elementary Particles and High Energy