Pulsed laser deposition growth of heteroepitaxial YBa_2Cu_3O_7/La_(0.67)Ca_(0.33)MnO_3 superlattices on NdGaO_3 and Sr_(0.7)La_(0.3)Al_(0.65)Ta_(0.35)O_3 substrates

摘要

Heteroepitaxial superlattices of [YBa_2Cu_3O_7(n)/La_(0.67)Ca_(0.33)MnO_3(m)].t (YBCO/LCMO), where n and m are the number of YBCO and LCMO monolayers and x the number of bilayer repetitions, have been grown with pulsed laser deposition on NdGaO_3 (110) and Sr_(0.7)La_(0.3)Al_(0.65)Ta_(0.35)O_3 (001). These substrates are well lattice matched with YBCO and LCMO and, unlike the commonly used SrTiO_3, they do not give rise to complex and uncontrolled strain effects at low temperature. The growth dynamics and the structure have been studied in situ with reflection high-energy electron diffraction and ex situ with scanning transmission electron microscopy, x-ray diffraction, and neutron reflectometry. The individual layers are found to be flat and continuous over long lateral distances with sharp and coherent interfaces and with a well-defined thickness of the individual layer. The only visible defects are antiphase boundaries in the YBCO layers that originate from perovskite unit-cell height steps at the interfaces with the LCMO layers. We also find that the first YBCO monolayer at the interface with LCMO has an unusual growth dynamics and is lacking the CuO chain layer, while the subsequent YBCO layers have the regular Y-123 structure. Accordingly, the CuO_2 bilayers at both the LCMO/YBCO and the YBCO/LCMO interfaces are lacking one of their neighboring CuO chain layers and, thus, half of their hole-doping reservoir. Nevertheless, from electric transport measurements on a superlattice with n=2we obtain evidence that the interfacial CuO_2 bilayers remain conducting and even exhibit the onset of a superconducting transition at very low temperature. Finally, we show from dc magnetization and neutron refiectometry measurements that the LCMO layers are strongly ferromagnetic.