A living-dead magnetic layer in ferrimagnetic DyTiO₃ thin films and how to revive it

When ferromagnetic films become ultrathin, key properties such as the Curie temperature and the saturation magnetization are usually depressed. This effect has been thoroughly investigated in half-metallic manganites, but much less in ferrimagnetic insulating perovskites such as rare-earth titanates RTiO₃, despite the appeal of these materials to design correlated two-dimensional electron gases. Here, we report on the magnetic properties of epitaxial DyTiO₃ thin films. Above 50 nm the films show a bulk-like response, but surprisingly their saturation magnetization strongly increases at low thickness. We model this behavior using a classical model of “dead layer” but we assume that this layer is actually “living”, i.e. it responds to the magnetic field through a strong paramagnetic susceptibility. Through depth-dependent X-ray spectroscopy, we show that the “living-dead layer” corresponds to surface regions where magnetic (S=1/2) Ti³⁺ become non-magnetic Ti⁴⁺ ions, leaving Dy³⁺ ions magnetically uncoupled and thus unleashing their paramagnetic response. Accordingly, we find that capping the film with an epitaxial LaAlO₃ layer preserves the Ti³⁺ state and restores the pristine ferrimagnetic behavior.