Ultra-thin Perovskite Manganite films on SrTiO$_{3}$ and SrTiO$_{3}$/Si heterostructures

We report on the growth and characterization of high-quality ultra-thin La$_{0.7}$Ba$_{0.3}$MnO$_{3}$ and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ epitaxial films on SrTiO$_{3}$ and SrTiO$_{3}$-buffered Si (100) and Si (111) substrates by pulsed-laser deposition. The films demonstrate remarkable magnetic and electrical properties associated with the colossal magnetoresistance behavior at and above room temperature. The enhanced transition temperature of manganite films on buffered Si substrates is discussed in terms of the strain relaxation at the interface between the manganite film and the SrTiO$_{3}$ buffer layer which is caused by the smaller grain size. We also report the doping of Ru (x=0.3 to 0.4) into Mn sites in LSMO films grown both on STO and STO buffered Si. The films display remarkable hardening of $H_{c}$ due to the charge-transfer-enhanced exchange coupling. We have optimized doping for the compensation of hole doping by the valence effect of Ru,. This effect has been explained in terms of the charge transfer between the Mn$^{4+}$ and Ru$^{4+}$ species and ferromagnetic interaction between the resultant Mn$^{3+}$ and Ru$^{5+/4+}$. The electrons in Ru$^{4+}$ partially occupy the degenerated $t_{2g}$ orbitals due to the fact that Ru is a heavy metal, we expect a single-ion anisotropy of Ru spins through a spin-orbit channel. This structure is highly applicable for fabrication of the magnetic tunnel junctions.