FisMat2017 - Submission - View

Abstract's title: Cobalt-oxide reduction upon annealing: thermally-induced film purification after intercalation under Gr
Submitting author: Ilaria Carlomagno
Affiliation: Dipartimento di Scienze - Università Roma Tre, CNR-SPIN Roma
Affiliation Address: via della Vasca Navale 84, 00146, Roma (RM), via del Fosso del Cavaliere, 100, 00133 Roma (RM)
Country: Italy
Oral presentation/Poster (Author's request): Oral presentation
Other authors and affiliations: J. Drnec (ESRF, Grenoble, France) , A.M. Scaparro (Università Roma Tre, Roma, Italy) , S. Cicia (Università Roma Tre, Roma, Italy), R. Felici (SPIN-CNR Roma, Italy) , C. Meneghini (Università Roma Tre, Roma, Italy)
Abstract

Oxidation is a major issue in the development of magnetic memories based on ultra-thin Cobalt (Co) films as the Co oxidation could spoil the device’s magnetic response. In order to prevent the oxidation occurring under ambient pressure, Co films can be intercalated between a stabilizing Graphene (Gr) layer and an Ir(111) substrate: intercalated films not only are supposed to be oxidation resistant but they also exhibit an enhanced magnetic response [1].

This peculiar magnetic behaviour has originated a strong research interest mainly focussed on the films structure and magnetism. These properties were mostly investigated in-situ, i.e. in controlled environments, far from the working conditions of memory devices. Aiming to probe the chemical stability offered by Gr, our study deals with Gr-intercalated Co films exposed to air for several minutes. X-ray Absorption Spectroscopy at the Co K-edge provides information on the Co local structure and on the chemical environment of Co atoms; comparing spectra collected with different polarization geometries, we also investigate the local anisotropy of the films both in and out of the surface plane.

The near edge (XANES) region of the spectra suggests that after the 300°C annealing used to trigger the intercalation, some of the Co stays on the Gr surface and then oxidises upon ambient pressure exposure. The concentration of oxide decreases upon further annealing at 500°C while the amount of Co does not vary, ruling out the hypothesis of a simple desorption of the Co-oxide. Comparing the data collected after annealing at 500°C with those recorded in-situ on bare Co films on Ir [2], we demonstrate the restoration of the film metallic nature which is maintained even after air exposure, supporting a complete intercalation.

Details on the local anisotropy arise from the extended region (EXAFS) of the spectra collected after the 500°C annealing: a thickness-dependent in-plane stretching is found for the investigated films. This strain originates from the lattice mismatch between Co and Ir which seems to be enhanced by the intercalation under Gr. Our observations confirm the colander-like model we proposed in our previous investigation [3] to describe the role of the Gr. The strain also results into a magnetoelastic contribution to the overall magnetic anisotropy which could explain the enhanced magnetic response of Gr-intercalated Co films.

In conclusion, we report on thermal treatments focussing on oxide removal, intercalation effectiveness and chemical stability of intercalated films providing valuable information for the fabrication of magnetic memory devices. The thermally-induced structural modifications also affect the overall magnetic response of the systems.

 

[1] N. Rougemaille et al., Appl. Phys. Lett. 101, 142403 (2012)
[2] I. Carlomagno et al., J. Appl. Phys. 120, 195302 (2016)
[3] J. Drnec, S. Vlaic, I. Carlomagno et al., Carbon 94, 554 (2015)