Mechanism of giant exchange bias in a rare earth superlattice

After cooling a DyFe$_{2}$/YFe$_{2}$ superlattice [3 nm DyFe$_{2}$/12 nm YFe$_{2}$ repeated 22 times] to 12 K in a 1 T field, which aligns the Fe-spins parallel to the field, the magnetization vs. field curve of the superlattice was dramatically shifted along the \textit{magnetization and applied field} \textit{axes}. The exchange bias was -2 \textit{Tesla}. We developed a one dimensional spin-chain-model that completely explains the polarized neutron reflectometry, magnetometry and X-ray magnetic circular dichroism data. Two in-plane domain configurations were identified in the model. Both configurations contribute to the extraordinarily large exchange bias of the DyFe$_{2}$/YFe$_{2}$ superlattice. Until just recently, we lacked direct evidence for the existence of the domain configurations; however, SANS measurements of the thin film superlattice show compelling proof that magnetization reversal is accompanied by formation of small domains in the sample plane. The domain scattering exhibits a pronounced enhancement at the exchange bias field, and remarkably, is a minimum at the cooling field rather than at high (7 T) fields.