Revealing a Living-Dead Magnetic Layer

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. 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 M₅ and Ti L₃ edges indicate that the surface Ti⁴⁺ ions decouple the Dy³⁺ ions, thus unleashing their strong paramagnetic response. The depth dependent X-ray absorption measurements were obtained by using three different types of detection as shown in the figure. The measurements were performed at the X-Treme beamline of the Swiss Light Source.