A Living-Dead Magnetic Layer at the Surface of Ferrimagnetic DyTiO₃ Thin Films

When ferromagnetic films become ultrathin, key properties such as the Curie temperature and the saturation magnetization are usually depressed. This effect is thoroughly investigated in magnetic oxides such as half-metallic manganites, but much less in ferrimagnetic insulating perovskites such as rare-earth titanates RTiO₃ , despite their appeal to design correlated 2D electron gases. Here, the magnetic properties of epitaxial DyTiO₃ thin films are reported. While films thicker than about 50 nm show a bulk-like response, at low thickness a surprising increase of the saturation magnetization is observed. This behavior is described using a classical model of a "dead layer" but assuming that this layer is actually "living," that is, it responds to the magnetic field with a strong paramagnetic susceptibility. Through depth-dependent X-ray absorption and photoemission spectroscopy, it is shown that the "living-dead layer" corresponds to surface regions where magnetic (S = 1/2) Ti³⁺ ions are replaced by nonmagnetic Ti⁴⁺ ions. Hysteresis cycles at the Dy M ₅ and Ti L ₃ edges indicate that the surface Ti⁴⁺ ions decouple the Dy³⁺ ions, thus unleashing their strong paramagnetic response. Finally, it is shown how capping the DyTiO₃ film can help increase the Ti³⁺ content near the surface and thus recover a better ferrimagnetic behavior.