The development of alkoxy-based sol-gel processing for magnetoresistive manganite thin films

Abstract

This dissertation presents, for the first time, the successful development of an all-alkoxy based, sol-gel process for integrating thin films of magnetoresistive doped-lanthanide manganites onto silicon-based substrates. Crystallization of the requisite perovskite phase at temperatures below 650°C resulted from the incorporation of all-alkoxide precursors, and in particular, Mn[OC(CH 3 ) 3 )] 2 . This latter precursor, when combined with the polyfunctional solvent, 2-methoxyethanol, exhibited high solubility and hydrolytic reactivity. This accomplishment represents a significant new contribution because low carbon-content manganese(II) alkoxides are stable, insoluble coordinate polymers.

Orange and pinkish-orange solutions, also synthesized for the first time, were free from products of aerobic oxidation, and hence, contained no brown discoloration. A partial hydrolysis of h = 0.25 produced a polymeric sol system, conferring both spinnable viscosities and excellent sol longevity. Post-coating hydrolysis via humidified air proved essential to yield transparent, dense, and defect-free amorphous coatings. Conversion into a fine-grain, polycrystalline microstructure occurred above 600°C on platinized-Si(100) and above 650°C on Si(100).

The cubic lattice parameters of the films (i.e., a = ∼ 3.90 Å) were in excellent agreement with values published in the literature for bulk, polycrystalline powders. Typical grain sizes started at 10-15 nm, increasing to 20-25 nm by 750°C. For films deposited on Si(100), magnetoresistance was observed in specimens heat treated at 700°C and 750°C, and for platinized-Si(100), 650°C, 700°C, and 750°C. Magnetoresistive response improved with heat-treatment temperature for the more refractory La0.67Ba 0.33 MnO 3 composition. The lead-doped counterpart offered the best property evolution, with T C = 320 K and T IM = 254 K by 750°C on platinized-Si(100). All corresponding transport curves were symmetric, demonstrating clear metal-insulator transitions (i.e., T IM ). High resistivities (i.e., ∼ 10 6 Ω-cm) were attributed to the fine-grain microstructure. Weak-field cycling between ±500 Oe yielded symmetrical loops with appreciable linear regions, a highly-desirable characteristic for magnetic sensing applications.