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 NaC1 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 I .tm and lateral periodicity = 100 A. Magnetic properties of the Co layers were investigated by magnetooptical Kerr effect from 10 K to room temperature. They are dominated by a strong shapeinduced 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.