Perovskite-type cobalt oxide at the multiferroic Co/Pb Zr_0.2Ti_0.8O_3 interface

Magnetic Tunnel Junctions whose basic element consists of two ferromagnetic electrodes separated by an insulating non-magnetic barrier have become intensely studied and used in non-volatile spintronic devices. Since ballistic tunnel of spin-polarized electrons sensitively depends on the chemical composition and the atomic geometry of the lead/barrier interfaces their proper design is a key issue for achieving the required functionality of the devices such as e.g. a high tunnel magneto resistance. An important leap in the development of novel spintronic devices is to replace the insulating barrier by a ferroelectric which adds new additional functionality induced by the polarization direction in the barrier giving rise to the tunnel electro resistance (TER). The multiferroic tunnel junction Co/PbZr_0.2Ti_0.8O_3/La_2/3Sr_1/3MnO_3 (Co/PZT/LSMO) represents an archetype system for which - despite intense studies - no consensus exists for the interface geometry and their effect on transport properties. Here we provide the first analysis of the Co/PZT interface at the atomic scale using complementary techniques, namely x-ray diffraction and extended x-ray absorption fine structure in combination with x-ray magnetic circular dichroism and ab-initio calculations. The Co/PZT interface consists of one perovskite-type cobalt oxide unit cell [CoO_2/CoO/Ti(Zr)O_2] on which a locally ordered cobalt film grows. Magnetic moments (m) of cobalt lie in the range between m=2.3 and m=2.7μ_B, while for the interfacial titanium atoms they are small (m=+0.005 μ_B) and parallel to cobalt which is attributed to the presence of the cobalt-oxide interface layers. These insights into the atomistic relation between interface and magnetic properties is expected to pave the way for future high TER devices.