TShin film growth and magnetotransport study of (La, Sr)MnO_3

摘要

Thin-film smples of la_(1-x)Sr_xMnO_3(x chemical bouns 0.20-0.30) were grown by pulsed-laser deposition using various target compositions, substrate materials, growth temperatures, oxygen partial pressures, and laser-pulse repetition rates. The crystal structure and the transport and magnetic properties of these films were then examined. Of the growth conditions, the oxygen partial pressure (Po_2_ had the greatest influence on the electrical and magnetic properties. Films grown under a low Po_2 had a low ferromagnetic transition temperature (T_c) and a wide resistive transition width. None of the heat treatments doen after growth improved these films' quality. The films morphology was significantly affected by the substrate material. Our x-ray diffraction analysis and AFM measurements revealed that the films deposited on both MgO (100) and LaAlO_3 (100) were epitaxially grown but contained defect structures. In contrast, grain-free thin films were epitaxially grown npn the SrTiO_3 (100) substrates. The surface roughness of films grown on SrTiO_3 was less tan 0.3 nm, enven for films up to 150 nm thick. Under optimized growth conditions, as0deosited films for x=>0.2 showed a sharp transition in resistivity at T-C. magnetoresistance at far below T_c was as low as that reorted for single-crystal sample. Since large magnetoresistance was often observed in polycrystalline samples and believed to be a grain boundary effect, these results indicate the high quality of the films grown on the SrTiO_3 substrates.