Detection of uncompensated magnetization at the interface of an epitaxial antiferromagnetic insulator

We have probed directly the temperature and magnetic field dependence of pinned uncompensated magnetization at the interface of antiferromagnetic ${\mathrm{FeF}}_{2}$ with Cu, using ${\mathrm{FeF}}_{2}\text{\ensuremath{-}}\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Co}$ spin valves. Electrons polarized by the Co layer are scattered by the pinned uncompensated moments at the ${\mathrm{FeF}}_{2}\text{\ensuremath{-}}\mathrm{Cu}$ interface giving rise to giant magnetoresistance. We determined the direction and magnitude of the pinned uncompensated magnetization at different magnetic fields and temperatures using the angular dependencies of resistance. The strong ${\mathrm{FeF}}_{2}$ anisotropy pins the uncompensated magnetization along the easy axis independent of the cooling field orientation. Most interestingly, magnetic fields as high as 90 kOe cannot break the pinning at the ${\mathrm{FeF}}_{2}\text{\ensuremath{-}}\mathrm{Cu}$ interface. This proves that the pinned interfacial magnetization is strongly coupled to the antiferromagnetic order inside the bulk ${\mathrm{FeF}}_{2}$ layer. Studies as a function of ${\mathrm{FeF}}_{2}$ crystalline orientation show that uncompensated spins are only detected in a spin valve with (110) crystal orientation, but not in valves containing ${\mathrm{FeF}}_{2}(100)$ and ${\mathrm{FeF}}_{2}(001)$. This observation is in agreement with symmetry-related considerations which predict the equilibrium boundary magnetization for the ${\mathrm{FeF}}_{2}(110)$ layer.