Epitaxial thin (110) films of the weak ferromagnet ${\text{NiF}}_2$ were deposited on single-crystal ${\text{MgF}}_2$ (110) substrates via molecular-beam epitaxy. Subsequently polycrystalline Co was grown on the ${\text{NiF}}_2$ film. The antiferromagnetic ordering of the ${\text{NiF}}_2$ was monitored as a function of temperature via neutron diffraction and the exchange bias $H_E$ of the Co layers was measured via standard magnetometry measurements. Because in ${\text{NiF}}_2$ the spins lie in the (001) plane, the maximum $H_E$ is observed after cooling the sample with a cooling field in the film plane perpendicular to the [001] direction of the ${\text{NiF}}_2$. In $60\text{nm}$, $49\text{nm}$, and $38\text{nm}$ thick ${\text{NiF}}_2$ samples, the Néel temperature is $T_N\approx 81\mathrm{K}$, which is significantly larger than the bulk value of $T_N=73.2\mathrm{K}$. This enhancement also occurs in films without Co overlayers and thus is not due to a proximity effect. For the $38\text{nm}$ sample with a Co overlayer cooled in a $50\text{kOe}$ field, $H_E>0$ and vanishes at a blocking temperature $\left(T_B\right)$ which coincides with the $T_N$ of the films. When the sample is cooled in $2.0\text{kOe}$, $H_E<0$, disappearing at $T=55\mathrm{K}$, reappearing at $T=65\mathrm{K}$, and finally disappearing once again at $T=81\mathrm{K}$. For the $12\text{nm}$ thick ${\text{NiF}}_2$ sample, $T_B⪡T_N$. Strain-induced enhancement of ferromagnetic exchange interactions between the nearest-neighbor ${\text{Ni}}^{2+}$ ions along the $c$ axis may be responsible for the $T_N$ enhancement. These results also demonstrate that in general, a diminished $T_B$ is not necessarily due to a lower $T_N$.

• Received 29 December 2003

DOI:https://doi.org/10.1103/PhysRevB.69.214416