d0 Ferromagnetic Interface between Nonmagnetic Perovskites

Conventional ferroelectricity and magnetism are chemically contraindicated in ABO3-type perovskite-structure materials. The presence of unoccupied B d orbitals and the B d – O 2p orbital hybridization are considered as causing the polar ferroelectric distortion (e.g. Ti 3d – O 2p in BaTiO₃) in many perovskite ferroelectrics, while the partially filled 3d or 5f orbitals are a prerequisite for magnetism. Moreover, ferromagnetic coupling is favored if the density of states at the Fermi level is high enough. Here we demonstrate that d⁰ ferromagnetism can be obtained at a charge-imbalanced interface between non-magnetic perovskite-structure ferroelectrics.

Epitaxial films of KTaO₃,KNbO₃, and NaNbO₃are grown on Ti-terminated single-crystal SrTiO₃ substrates. First-principles calculations show that the charge imbalance at the interfaces between these films and SrTiO₃can introduce holes in SrTiO₃ and induce a d0 ferromagnetic half-metallic 2D hole gas at the interfacial oxygen 2p orbitals. Presence of such an interfacial layer is experimentally detected from spectroscopic ellipsometry measurements. Magnetic investigations evidence ferromagnetic behavior at room temperature. The phenomenon makes it possible to obtain materials combining polar and magnetic ordering without any magnetic elements.

SQUID measured total magnetization as a function of external magnetic field determined at room temperature in as-deposited 10x5 mm² films of KTaO₃/SrTiO₃ (red), KNbO₃/SrTiO₃ (blue), and NaNbO₃/SrTiO₃ (black).

¹COMP Centre of Excellence, Department of Applied Physics, Aalto University, P.O. Box 11100, 00076 Aalto, Helsinki, Finland
²Microelectronics and Materials Physics Laboratories, University of Oulu, P.O. Box 4500, 90014 Oulunyliopisto, Finland
³Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
⁴NanoSpin, Department of Applied Physics, Aalto University, P.O. Box 15100, 00076 Aalto, Helsinki, Finland