Magnetization study on Ferro-Anti\-ferromag\-netic based on Manganites of the type $La_{x}Ca_{1-x}MnO_{3}$

Magnetization measurements were done on a series of $La_{2/3}Ca_{1/3}MnO_{3}$ ferromagnetic (F) / $La_{1/3}Ca_{2/3}MnO_{3}$ antiferromagnetic (AF) superlattices, which were prepared with a constant thickness of 3.9 nm for the F layer and thickness of the AF layer was varied in the range $3.9 nm \leq t_{AF} \leq 15.6 nm$. We observe from magnetothermal zero field cooling (ZFC) and field cooling (FC) curves, a bifurcation temperature $T_{bif}$, around 180 K identical for all samples. However, the F Curie temperature $T_{C}$ changes with AF layer thickness. Hysteretic loop measurements after field cooling (FC), from room temperature to 5 K exhibit an exchange bias loop shift, $H_{ex}$, which persists up to temperatures around the Néel temperature $T_{N}$, (~150 K) of the AF layer. The temperature parameter $T_{0}$, derived from an exponential fit of $H_{ex}$, increases with $t_{AF}$ up to approximately 32 K, which is well below the blocking temperature $T_{B} \approx T_{N}$. This result can be associated with a continuous distribution of $T_{B}$ caused by inhomogeneities at the interfaces, and suggests that AF/F interface-effects are of critical importance for exchange- biasing in La-Ca-Mn-O based multilayers. This work was supported by COLCIENCIAS project 1106-05-11458 CT-046-2002 and US DOE-BES.