Co nanotubes (NTs) were electrodeposited inside nanoporous alumina templates by a potentiostatic method and then left in air for the natural oxidation of their inner walls. Temperature-dependent magnetic measurements evidenced the existence of exchange-bias (EB) coupling between the Co ferromagnetic (FM) outer wall and the CoO antiferromagnetic (AFM) inner wall of the NTs. The values of the EB field measured at 6 K were found in the same order of magnitude as those already reported for Co/CoO bilayer thin films and core-shell nanoparticles. The effect of temperature and cooling field on the EB effect between the Co/CoO walls of the NTs is here reported. The training effect of the EB in the Co/CoO NT arrays, measured at 6 K, suggests the existence of a stronger coupling between the AFM and the FM layers that are shaped into a core-shell nanotubular structure, when compared with flat interfaces. A bimodal distribution of blocking temperatures is also found when applying the magnetic field parallel and perpendicular to the NTs’ axis.

• Received 20 November 2012

DOI:https://doi.org/10.1103/PhysRevB.87.134404