Enhanced Low Field Magnetoresistance in La$_{0.67}$Ca$_{0.33}$MnO$_{3}$/La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ superlattices

We have grown [1 nm/ 1nm]$_{n }$ superlattices of manganite (n = 20, 30,40) in which La$_{1-x}$Ca$_{x}$MnO$_{3}$(x=0.33) serves as the ferromagnetic layer while La$_{1-x}$Ca$_{x}$MnO$_{3}$(x=0.5) serves as the spacer layer on LaAlO$_{3}$ substrates by magnetron sputtering. The samples were characterized by XRD, Magneto-transport measurements, Rutherford backscattering spectroscopy, and atomic force microscopy. Enhanced longitudinal magnetoresistance (MR) under an applied field B, defined as MR (B) = $\rho $(B)/$\rho $(0) -- 1, was as much as -49{\%} at B=0.5 Tesla and T=90 K. The causes for this enhancement not seen at low field in other single-layered films of x=0.33 and the correlation of oxygen annealing with the MR effects for the superlattices will be discussed.