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 MLHo=7 and 29 while keeping MLEr=21. For MLHo=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 MLHo=7, due to its limited extension, the spin spiral is expected to remain truncated and allow a relatively lower disorder. The sample with MLHo=29 exhibits two temperature-dependent phase transitions of the spin configurations (conical and helical), whereas only a single phase (conical) is identified for the MLHo=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 kOe±0.001 kOe for MLHo=7 is found to increase by threefold to --0.02 kOe±0.005 kOe for MLHo=29, accounting for the increased sublattice disorder in its conical phase. For MLHo=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 kOe±0.01 kOe. Thus, the increased exchange bias phenomenon below 20 K in Ho/Er multilayers for the MLHo=29 sample reflects a higher spin imbalance induced by the magnetic sublattice disorder within Ho.