Exchange coupling in the conical phase of Ho within Ho/Tb multilayers due to Ho magnetic sublattice disorder

We explore the exchange bias coupling at the RE-RE interfaces (RE denotes rare earth) due to a plausible increase in the spin-imbalance induced by magnetic sublattice disorder around the temperature range where one of the REs exists in its conical phase while the other remains ferromagnetic (FM). In this regard, two highly textured multilayers with 29 monolayers (ML) of Ho are grown, which interface with 7 and 21 ML of Tb. For ${\mathrm{ML}}_{\mathrm{Tb}}=7$, the noncollinear spin configuration constituting a fraction of the whole stack regulates the effect of the surface-area-to-volume ratio as the helix usually remains truncated, while for ${\mathrm{ML}}_{\mathrm{Tb}}=21$, the helix can be sufficiently profound. Both samples exhibit at least two temperature-dependent phases of the spin configuration of Ho: conical and helical. For the ${\mathrm{ML}}_{\mathrm{Tb}}=7$ sample, significant exchange bias fields of up to $\ensuremath{-}0.57\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{4pt}{0ex}}\mathrm{kOe}$ are observed along with double hysteresis loops (DHLs) below 20 K. For ${\mathrm{ML}}_{\mathrm{Tb}}=21$, the coupling strength diminishes to $\ensuremath{-}0.055\ifmmode\pm\else\textpm\fi{}0.01\phantom{\rule{4pt}{0ex}}\mathrm{kOe}$ as we also find shifts in the temperature behavior regimes related to the conical-to-helical and helical-to-paramagnetic phase fractions within Ho. This emblemizes the effect of helical phase formation in Tb on the Ho spin configuration. Instead of nanoclustering, regular FM behavior is seen around the temperature range of conical-to-helical phase evolution. As we replace 21 ML of Tb with 10 ML of a ferromagnet (CoFe), we find the usual DHLs and an exchange bias field up to $\ensuremath{-}0.02\ifmmode\pm\else\textpm\fi{}0.01\phantom{\rule{4pt}{0ex}}\mathrm{kOe}$. The exchange bias phenomenon in Ho/Tb multilayer below 20 K is attributed to the small spin imbalance in the magnetic sublattice disorders in the conical phase of Ho.