Stabilization of Antiferromagnetic Order in FeO Nanolayers

We have studied the evolution of the magnetic state of a nanometer thick antiferromagnetic (AFM) FeO layer during its formation using nuclear resonant scattering of synchrotron radiation. In contact to ferromagnetic Fe, the FeO layer does not show magnetic order at room temperature (RT). Once embedded between two Fe layers, magnetic coupling to the adjacent ferromagnets leads to a drastic increase of the Néel temperature far above RT, while the blocking temperature remains below 30 K. The presented results evidence the role that the ferromagnetic surrounding plays in modifying the magnetic state of ultrathin AFM layers.

Figure 1

Nuclear time spectra (left panels) and corresponding hyperfine field distributions (right panels) recorded during the growth of an $\mathrm{Fe}/\mathrm{\text{native-oxide}}/\mathrm{Fe}$ trilayer at room temperature. The solid lines on the time spectra are fits. The 1.6 nm buried FeO layer appears to be magnetically ordered only when the top Fe layer is thick enough to develop long-range magnetic order.

Figure 2

Upper panels: Nuclear time spectra recorded on a ${}^{56}\mathrm{Fe}/{}^{57}\mathrm{FeO}(1.6\mathrm{nm})/{}^{56}\mathrm{Fe}$ trilayer at 300 K and at 10 K. The solid lines are fits. Lower panel: Hyperfine field distributions extracted from the fits of the time spectra.

Figure 3

Temperature dependence of the mean value of the large $B_{\mathrm{hf}}$ component displayed in Fig. 2. The solid line is a fit using a $(T_N-T{)}^{1/3}$ law, which yield a Néel temperature of $800\pm 30\mathrm{K}$.

Figure 4

(a) Selected hysteresis curves recorded by VSM on a $\mathrm{Fe}/\mathrm{FeO}(1.6\mathrm{nm})/\mathrm{Fe}$ trilayer. (b) Temperature dependence of the coercive field $H_c$ and exchange bias field $H_e$ extracted from the magnetization curves.