Interface effects in modulation-doped manganite superlattices

Epitaxial superlattices of antiferromagnetic La$_{1/3}$Ca$_{2/3}$MnO$_{3}$ and ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ were grown using a high-pressure sputtering technique. Structural analysis was performed by simulation of X-ray diffraction data using the SUPREX program and Transmission Electron Microscopy. Both techniques confirmed interfacial roughness in the range of one unit cell. Magnetization measurements showed that magnetization depth profile is incommensurate with the doping profile, such that the ferromagnetic order extends into the antiferromagnetically doped region beyond the chemical doping interface. Thermal demagnetization in the low temperature range obtained from the temperature dependence of the saturation magnetization shows an $\alpha $=5/2 power law dependence, independent of the ferromagnetic layer thickness. The $T^{5/2 }$term is associated with long-wave length spin waves, and corresponds to the anharmonic second-order expansion of the magnetization. This work was supported by COLCIENCIAS contract 043-2005, and U.S. DOE-BES under contract W-31-109-ENG-38.