Unusual exchange bias in Sr₂FeIrO₆/La_0.67Sr_0.33MnO₃ multilayer.

Here, we study an interface induced magnetic properties in 3$d$-5$d$ based superlattice made of La$_{0.67}$Sr$_{0.33}$MnO$_3$ and double perovskite Sr$_2$FeIrO$_6$, respectively. Bulk LSMO is metallic and shows ferromagnetic (FM) ordering above room temperature. In contrast, bulk SFIO is an antiferromagnet (AFM) with N$\acute{e}$el temperature around 45 K ($T_N$) and exhibits an insulating behavior. Two set of superlattices have been grown on SrTiO$_3$ (100) crystal with varying thickness of FM LSMO layer. Superlattice with equal thickness of La$_{0.67}$Sr$_{0.33}$MnO$_3$ and Sr$_2$FeIrO$_6$ ($\sim$ 10 nm) shows exchange bias (EB) effect both in conventionally field cooled (FC) as well as in zero field cooled (ZFC) magnetic hysteresis measurements which is rather unusual. The ZFC EB effect is weakened both with increasing maximum field during initial magnetization process at low temperature and with increasing temperature. Interestingly, superlattice with reduced thickness of La$_{0.67}$Sr$_{0.33}$MnO$_3$ ($\sim$ 5 nm) does not exhibit ZFC EB phenomenon, however, the FC EB effect is strengthened showing much higher value. We believe that an AFM type exchange coupling at interface and its evolution during initial application of magnetic field cause this unusual EB in present superlattices.