Parallel Versus Antiparallel Interfacial Coupling In Exchange-biased Co/FeF$_2$

The nature of exchange bias in FeF$_2$ (110) remains elusive due to it's nominally compensated surface. Other interesting phenomena include positive exchange bias and an enhancement of the the coercivity near $T_N$. In order to address these issues, soft x-ray dichroism absorption spectroscopy was used to investigate the direction of interfacial exchange coupling in a antiferromagnetic/ferromagnetic exchange-coupled Co (2.5 nm) /FeF$_2$ (68 nm) bilayer. The FeF$_2$ was epitaxially grown on MgF$_2$ (110) and the Co layer was polycrystalline. The sample was capped with a 2.0 nm layer of Pd to protect it from oxidation. For comparison, a nominally identical sample without Co was also grown. A small portion of interfacial Fe spins couples antiparallel to the ferromagnet, causing the positive exchange bias for cooling fields. A larger potion of interfacial spins, coupled more strongly and parallel to the ferromagnet, increases the degree of antiferromagnetic order and plays an important role in the observed coercivity increase at high temperatures.\\ \\ Work supported by DOE at SSRL and by NSF grant DMR-0400578 at WVU.