Antiferromagnetic Order in Epitaxial FeSe Films on SrTiO 3

Single monolayer FeSe film grown on a Nb-doped ${\mathrm{SrTiO}}_{3}(001)$ substrate shows the highest superconducting transition temperature (${T}_{C}\ensuremath{\sim}100\text{ }\text{ }\mathrm{K}$) among the iron-based superconductors (iron pnictides), while the ${T}_{C}$ value of bulk FeSe is only $\ensuremath{\sim}8\text{ }\text{ }\mathrm{K}$. Although bulk FeSe does not show antiferromagnetic order, calculations suggest that the parent $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_{3}$ films are antiferromagnetic. Experimentally, because of a lack of a direct probe, the magnetic state of $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_{3}$ films remains mysterious. Here, we report direct evidence of antiferromagnetic order in the parent $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_{3}$ films by the magnetic exchange bias effect measurements. The magnetic blocking temperature is $\ensuremath{\sim}140\text{ }\text{ }\mathrm{K}$ for a single monolayer film. The antiferromagnetic order disappears after electron doping.