Antiferromagnetic Order in Epitaxial FeSe Films on ${\mathrm{SrTiO}}_3$

Single monolayer FeSe film grown on a Nb-doped ${\mathrm{SrTiO}}_3(001)$ substrate shows the highest superconducting transition temperature ($T_C\sim 100\mathrm{K}$) among the iron-based superconductors (iron pnictides), while the $T_C$ value of bulk FeSe is only $\sim 8\mathrm{K}$. 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 $\sim 140\mathrm{K}$ for a single monolayer film. The antiferromagnetic order disappears after electron doping.

Figure 1

The MEBE in an ${\mathrm{Fe}}_{21}{\mathrm{Ni}}_{79}/\mathrm{FeSe}/\mathrm{STO}$ film. (a), (b) The schematic MHLs of the MEBE after the positive and negative FC. (c) Layout of the film. (d) MHLs of the sample in (c) measured at 5 K after FC from room temperature to 5 K. (e) The corresponding enlarged plots near the zero field.

Figure 2

Control experiments. (a) Layout of control sample 1. (b) MHLs and (c) the corresponding enlarged plots of the sample in (a). (d) Layout of control sample 2. (e) MHLs and (f) the corresponding enlarged plots of the sample in (d). (g) Layout of control sample 3. (h) MHLs and (i) the corresponding enlarged plots of the sample in (g). The loops did not shift in all of the control experiments.

Figure 3

Temperature dependence of the MEBE. (a)–(f) Enlarged curves of original and inverted loops of $\mathrm{Au}(10\mathrm{nm})/{\mathrm{Fe}}_{21}{\mathrm{Ni}}_{79}(0.7\mathrm{nm})/\mathrm{FeSe}(100\mathrm{ML})/\mathrm{STO}$ film at different temperatures after FC. (g) $H_{\mathrm{EB}}$ as a function of temperature. The solid line is a guide for the eye.

Figure 4

The MEBE in 1-ML and 2-ML FeSe/STO films. (a) Layout of the films. (b) MHLs of $\mathrm{Au}(10\mathrm{nm})/{\mathrm{Fe}}_{21}{\mathrm{Ni}}_{79}(0.7\mathrm{nm})/\mathrm{as}\text{−}\mathrm{grown}$ $\mathrm{FeSe}(1\mathrm{ML})/\mathrm{STO}$ film and (c) the corresponding enlarged plots. (d) MHLs of $\mathrm{Au}(10\mathrm{nm})/{\mathrm{Fe}}_{21}{\mathrm{Ni}}_{79}(0.7\mathrm{nm})/\mathrm{annealed}$ $\mathrm{FeSe}(1\mathrm{ML})/\mathrm{STO}$ film and (e) the corresponding enlarged plots measured at 5 K. (f)–(h) Enlarged curves of the original and inverted loops of the $\mathrm{Au}(10\mathrm{nm})/{\mathrm{Fe}}_{21}{\mathrm{Ni}}_{79}(0.7\mathrm{nm})/\mathrm{as}\text{−}\mathrm{grown}$ $\mathrm{FeSe}(1\mathrm{ML})/\mathrm{STO}$ film at three temperatures. (g) $H_{\mathrm{EB}}$ of the $\mathrm{Au}(10\mathrm{nm})/{\mathrm{Fe}}_{21}{\mathrm{Ni}}_{79}(0.7\mathrm{nm})/\mathrm{as}\text{−}\mathrm{grown}$ (1 and 2 ML) and annealed 1-ML FeSe/STO films as a function of temperature.