Temperature dependence of the training effect in electrodeposited Co/CoO nanotubes

High aspect ratio Co/CoO nanotubes (NTs) were obtained by potentiostatic electrodeposition of Co inside nanoporous alumina templates followed by the natural oxidation of their inner walls. Magnetic measurements performed at low temperatures after field cooling the samples from above its blocking temperature (T B ∼ 220 K), evidenced the existence of exchange bias (EB) coupling between the Co ferromagnetic outer wall and the CoO antiferromagnetic inner wall of the NTs. A decrease in the magnitude of the EB field was measured at T < T B when cycling the Co/CoO NT arrays through consecutive hysteresis loops. This decrease is known as the training effect (TE) and is here studied in the 6 K ≤ T < T B temperature range. The TE was fitted using the recursive Binek formula, giving small values for the characteristic decay rate of the training behavior, and evidencing a decrease of EB with increasing antiferromagnetic layer thickness. A phenomenological theory for the temperature dependence of the TE in exchange biased systems was applied for the first time to core-shell nanotubular structures. The good agreement obtained between the experimental results and the theoretical data, provided a strong confirmation of the qualitative correctness of the spin configuration relaxation model used in these systems. © 2013 AIP Publishing LLC.M.P.P. and C.T.S. are thankful to FCT for post-doctoral grants SFRH/BPD/84948/2012 and SFRH/BPD/82010/2011, respectively. J.V. acknowledges financial support through FSE/POPH. M.V. thanks the Spanish Ministry of Economia y Competitividad, MEC, under project MAT2010-20798-C05-01. The authors acknowledge funding from FCT under project PTDC/FIS/098943/2008 and through the Associated Laboratory - IN.Peer Reviewe