B. Warot

Growth and stress relaxation of Co/NiO bilayers on MgO(001)

Co/NiO bilayers have been grown on a MgO(001) surface in an ultrahigh vacuum sputtering chamber under various deposition conditions. Investigations by x-ray diffraction, atomic force microscopy, and high resolution transmission electron microscopy give evidence for a close relationship between structure and deposition conditions for the two different layers. The role of the deposition rate on the oxide layer surface roughness is particularly emphasized. At high deposition rates, surface roughness increases with temperature, whereas the surface remains flat for low deposition rates. Whatever the growth conditions, the NiO layer grows epitaxially on the MgO substrate with the orientation relationship NiO(001)[100]//MgO(001)[100]. The misfit strain (about 1%), elastically accommodated in the thinner layers (3 nm), is fully relaxed in 50-nm-thick layers. A temperature dependence of the cobalt layer structure is observed: at room temperature it grows in its high temperature face-centered cubic structure, where...

Correlations between coercivity and exchange bias in epitaxial NiO–Co(110) bilayers

We have sputtered epitaxial NiO–Co samples on MgO (110) substrates. NiO epitaxially grows on the isostructural fcc MgO substrate but, due to surface energy minimization, its surface morphology is saw-tooth-like with terraces aligned along [001] direction and either (100) or (010) termination planes. The obtained nanostructures are 80–200 A wide facets with micron-size length. Subsequently deposited Co layers adopt a fcc structure conformal with the NiO nanofacets. It consists thus in a set of connected nanostripes as evidenced by complementary structural characterizations. Shape anisotropy induces a strong easy axis along the stripe edges [001] and a hard axis along the [-110] MgO direction. Magnetization loops recorded along [001] have a total squareness. Thermal treatments were done in zero field for investigating the NiO/Co exchange thermal stability and activation. We observed thermally assisted exchange bias field (HE) variations on 1000 Oe field treated samples for various temperatures between 300 K...

Exchange-Biased Nio-Co Nanofaceted Bilayers Grown on MgO (110)

Abstract : We have sputter-grown self-organised faceted NiO-Co epitaxial bilayers on MgO(110). Due to very close lattice parameters, NiO adopts the same NaCl crystallographic structure as the substrate but it minimises its surface energy growing in a stripe-shaped morphology elongated along 001 MgO direction. The Co layers then deposited on NiO adopt a fcc structure. They consist of a set of connected nanowires whose height is about 50 A, length is near to 1 micrometer and lateral periodicity 100 A. Magnetic properties of the Co layers were investigated by magneto-optical Kerr effect from 10 K to room temperature. They are dominated by a strong shape-induced uniaxial anisotropy and exchange coupling with the antiferromagnetic underlayer. Magnetisation loops recorded along the easy axis exhibit a perfect squareness and switch in a field range smaller than 10 Oe. Transverse measurements indicate that switching occurs by domain nucleation and/or domain wall propagation. On the contrary, close to the 110 hard axis, magnetic switching occurs by coherent rotation. The bi-stable Co magnetisation state along its easy axis has been used for ordering the NiO spins configuration from room temperature to 10 K. Sign and value of exchange bias induced by such a thermal treatment can be modulated thanks to a wide magnetocrystalline or local exchange path energies distributions.

Oscillatory Angular Dependence of Exchange Bias for Epitaxial NiO-Co (001) Bilayers

Abstract : We have sputter-deposited NiO-Co bilayers on MgO (001) substrates. NiO grows epitaxially on MgO at 900 deg C and subsequently the room deposited 80 A thick Co films have a fcc crystal structure in epitaxy with the oxide underlayer. These samples were warmed up to 300 deg C and then zero-field or field cooled through the NiO Neel temperature (a 300 Oe magnetic field was applied along the 100 or the 110 MgO axis). Magnetic hysteresis loops were obtained by magneto-optical Kerr effect, the magnetic field being oriented in the plane of the substrate for various angles alpha with respect to the 100 direction. The usually expected behavior for such experiments is a smooth angular a dependence of the exchange bias H(sub E) close to a cosine with only one sign change over 180 deg. The high crystallographic coherence of our NiO/Co bilayers induces a very unusual oscillatory HE (alpha) dependence with several sign changes according to the NiO axis field application. Despite of the Co magnetization switching mechanism which is not a pure coherent rotation. we propose a Stoner-Wohlfhart model including four fold anisotropy and unidirectional exchange anisotropy giving a realistic description of these typical magnetic properties.