Epitaxial NiO-Co exchange-biased bilayers grown on MgO single crystals Influence of the substrate orientation on the fil
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Epitaxial NiO-Co exchange-biased bilayers grown on MgO single crystals Influence of the substrate orientation on the film morphology, the Co structure and the magnetic behavior B.Warot1, E.Snoeck1, J.C.Ousset1, M.J.Casanove1, S.Dubourg2, A.R.Fert2, J.F.Bobo2 1 CEMES-CNRS, BP4347, 31055 Toulouse, France 2 LPMC-INSA, 31077 Toulouse, France ABSTRACT Co/NiO bilayers have been grown on MgO(001), MgO(110) and MgO(111) substrates in an ultra high vacuum sputtering chamber. Growth mode and surface morphology are investigated by X-ray diffraction, Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM) and High Resolution Transmission Electron Microscopy (HRTEM). NiO layers grow epitaxially whatever the substrate orientation. Flat surfaces are observed on NiO/MgO(001) whereas on MgO(110) the NiO surface exhibits a roof-like morphology consisting in (100) and (010) facets elongated along the [001] direction. On MgO(111), the NiO surface presents pyramids with {100} facets. A temperature dependence of the cobalt layer structure is observed: on NiO(001) at room temperature it grows in its high temperature face-centered cubic structure (fcc), whereas it has the hexagonal close-packed structure (hcp) when deposited at slightly higher temperatures. INTRODUCTION Magnetic exchange coupling between ferromagnetic (F) and antiferromagnetic(AF) layers induces active research mainly motivated by the development of spin valves. This magnetic coupling strongly depends on the spin arrangement in the AF layer at the AF/F interface which is determined by the structure and morphology of this interface (orientation, width, roughness, chemical composition…) and therefore by the growth characteristics of the AF layer. Many studies outline the influence of the microstructure on the exchange coupling [13]. In this study, we focussed on the structural and morphological characteristics of 33nm thick NiO films epitaxially grown on MgO(001), MgO(110) and MgO(111) substrates and of Co films deposited on these NiO layers. NiO is widely studied as an antiferromagnet since it is weakly affected by corrosion and easy to produce. It is a chemically stable ionic insulator with a good thermal stability and a high exchange field. The magnetic moments of the Ni atoms lie along a [110] direction and adopt a ferromagnetic order in a given (111) plane, successive (111) planes being antiferromagnetically ordered in the [111] direction. Thus, the NiO{001} and {110} planes are compensated surfaces with no resulting moment nor electrical charge while NiO{111} is a polar surface with a magnetic moment. Since MgO and NiO present the same rock-salt structure with very close lattice parameters (the mismatch is less than 1%) and nearly the same thermal coefficients, the epitaxial growth is then achievable. The NiO layer should then grow under a tensile elastic stress.
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EXPERIMENTAL DETAILS The NiO layers were grown by radio frequency (RF) sputtering in an ultra high vacuum chamber (10-9 mbar) using two sintered NiO facing targets. The pla
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