Magnetic sublattice disorder mediated exchange bias at the interfaces of Ho/Er multilayers in their conical phases

An increase in spin imbalance induced by magnetic sublattice disorder within rare-earth (RE) metals can in turn increase the exchange bias coupling at the interfaces of RE--RE multilayers below the temperature of conical phases. In this work we have regulated the sublattice disorder by using different monolayers (MLs) of Ho within two highly textured multilayers Ho/Er comprising ${\mathrm{ML}}_{\mathrm{Ho}}=7$ and 29 while keeping ${\mathrm{ML}}_{\mathrm{Er}}=21$. For ${\mathrm{ML}}_{\mathrm{Ho}}=29$, the spin spiral constituting a fraction of the whole stack is expected to decrease the surface area--to--volume ratio and help in the profoundness of the conical phase, thereby contributing to the consequent increase in sublattice disorder. For ${\mathrm{ML}}_{\mathrm{Ho}}=7$, due to its limited extension, the spin spiral is expected to remain truncated and allow a relatively lower disorder. The sample with ${\mathrm{ML}}_{\mathrm{Ho}}=29$ exhibits two temperature-dependent phase transitions of the spin configurations (conical and helical), whereas only a single phase (conical) is identified for the ${\mathrm{ML}}_{\mathrm{Ho}}=7$ sample. Furthermore, a significant shift in temperature for the characteristic peak related to the conical-to-helical phase is observed as compared to an insignificant shift that is related to the helical-to-paramagnetic phase as we decrease the number of Ho MLs from 29 to 7, which eventually reduces the accumulated magnetic repeat distance (10 MLs) by three times. Interestingly, a meager exchange bias field of up to --0.007 $\mathrm{kOe}\ifmmode\pm\else\textpm\fi{}0.001$ kOe for ${\mathrm{ML}}_{\mathrm{Ho}}=7$ is found to increase by threefold to --0.02 $\mathrm{kOe}\ifmmode\pm\else\textpm\fi{}0.005$ kOe for ${\mathrm{ML}}_{\mathrm{Ho}}=29$, accounting for the increased sublattice disorder in its conical phase. For ${\mathrm{ML}}_{\mathrm{Ho}}=29$, as we replace 21 MLs of Er with 30 MLs of a ferromagnet (CoFe), we find the usual double hysteresis loops and the coupling strength also increases further to 0.07 $\mathrm{kOe}\ifmmode\pm\else\textpm\fi{}0.01$ kOe. Thus, the increased exchange bias phenomenon below 20 K in Ho/Er multilayers for the ${\mathrm{ML}}_{\mathrm{Ho}}=29$ sample reflects a higher spin imbalance induced by the magnetic sublattice disorder within Ho.