Magnetic and ferroelectric properties of oxygen octahedron/tetrahedron mixed ultrathin multiferroic layer by oxygen desorption

BiFeO₃ epitaxial films (t_BFO = 1–10 nm) were grown on a LaSrMnO₃ (t_LSMO = 70 nm) buffer layer and capped with a Ru or Ru/Al layer. The BiFeO₃ ultrathin film (t_BFO = 3 nm) was confirmed to be continuous and flat over a wide area. Piezoelectric response and switching of spontaneous polarization were observed in the ultrathin films by switching-spectroscopy piezoelectric force microscopy measurements. Using the lock-in method with a modulation frequency of 10 Hz in x-ray absorption spectroscopy (XAS), a difference was observed between the left- and right-circularly polarized XAS signals of Fe L₃ and L₂ owing to x-ray magnetic circular dichroism (XMCD). The number of Fe L₃ and L₂ XMCD peaks indicated that a mixture of oxygen octahedron and tetrahedron (divalent and trivalent) structures might be formed. The t_BFO dependence of XMCD revealed that approximately 1 nm thick ferro-like magnetic properties emerged on the BiFeO₃ side of the BiFeO₃/Ru interface. Ru may partially extract oxygen from BiFeO₃, forming an oxygen tetrahedron structure. The BiFeO₃/Ru bilayer exhibits both a piezoelectric response and ferro-like magnetism, which is promising as the basic structure of multiferroic tunnel junctions.