High exchange-bias blocking temperature in an ultrathin amorphous antiferromagnet system

We experimentally observed an ultrahigh blocking temperature, above 600 K, in an exchange-biased Fe/FeO bilayer after field annealing at 773 K. This is far above the 198 K Néel temperature of bulk FeO. By comparing with an Fe/FeO/Fe trilayer and studying the Fe thickness dependence, we find that the magnetic proximity effect, due to coupling between the Fe layer and the ultrathin amorphous FeO layer, is responsible for the high blocking temperature. The realization of high-temperature exchange bias using an ultrathin antiferromagnet paves the way to further miniaturize magnetic devices for high-information-density applications.

Figure 1

(a) The field annealing temperature dependence of the magnitude of the exchange-bias field $|H_{\text{E}}|$ and the coercivity $H_{\text{C}}$ for an Fe(18 nm)/FeO(2 nm) bilayer at room temperature. The inset shows a high-resolution TEM cross section of the Fe/FeO bilayer after field annealing at 773 K. (b) Hysteresis loop of an Fe(18 nm)/FeO(2 nm) bilayer at room temperature (300 K). The inset shows $|H_{\text{E}}|$ vs the angle $\theta$ between the in-plane applied magnetic field and the cooling field $H_{\text{cool}}$. Blue filled squares represent the measured $|H_{\text{E}}|$ values, and the red curve depicts the $\left|\cos \theta \right|$ dependence of $|H_{\text{E}}|$.

Figure 2

Temperature dependence of the (a) exchange-bias field $H_{\text{E}}$ and (b) coercivity $H_{\text{C}}$ of the Fe(18 nm)/FeO(2 nm) bilayer and the Fe(18 nm)/FeO(2 nm)/Fe(1.6 nm) trilayer.

Figure 3

Temperature dependence of (a) $H_{\text{E}}$ and (b) $H_{\text{C}}$ in Fe/FeO bilayers with Fe thickness $t_{\text{Fe}}$ ranging from 2 to 14 nm. The inset in (a) shows $H_{\text{E}}$ as a function of $t_{\text{Fe}}$ in the Fe/FeO bilayer at room temperature (300 K). The inset in (b) shows the hysteresis loop of the Fe(2 nm)/FeO(2 nm) bilayer at room temperature (300 K).

Figure 4

(a) Temperature dependence of the magnetization $M\left(T\right)$ in Fe/FeO bilayers, for $t_{\text{Fe}}$ from 2 to 18 nm. (b) $t_{\text{Fe}}$ evolution of $T_{\text{C}}$ and $T_{\text{B}}$ in the Fe/FeO bilayers. The solid line is plotted according to Eq. (1).

Figure 5

Comparison of blocking temperature $T_{\text{B}}$ and AFM thickness in the Fe/FeO bilayer with previously reported data for high-$T_{\text{B}}$ exchange-biased systems [40, 41, 42, 43, 44, 45, 46, 47, 48].