Seminars Archive

Exotic magnetism in ZnO based systems

Abstract
We present a study of the local structure of ZnO capped nanoparticles and ZnO/ZnS multilayers, performed by using X-ray absorption spectroscopy (XAS), and X-ray Magnetic Circular Dichroism (XMCD) at the Zn, O and S K-edge. The experimental XAS spectra have been compared to ab-initio calculations. ZnO nanoparticles (NPs) have been capped with three different organic molecules: tryoctylphosphine (TOPO), dodecylamine (AMINE), and dodecanethiol (THIOL), which bond to the particle surface through an O, N, and S atom respectively. Thin films were grown by RF sequential sputtering from ZnO and ZnS target. A combined element-specific XAS and XMCD study performed at the Zn K-edge demonstrates the occurrence of intrinsic ferromagnetic-like behaviour up to room temperature [1]. In addition, the analysis of XAS spectra of ZnO NPs capped with dodecanethiol points out the formation of a ZnO-ZnS core-shell structure and a ZnO-ZnS interface. Zn K-edge XMCD measurements reveal the coexistence of two different magnetic contributions: a paramagnetic response from the core-shell components of the NP, and a ferromagnetic contribution stemming from the interface. Our results indicate that ferromagnetism is favored in those regions of the interface where Zn-O bonds rather than Zn-S ones are involved [2,3]. We have extended these studies to the O K-edge, for which we have obtained a neat XMCD signal, and the S K-edge. The analysis of the XAS spectra fully support the conclusions derived from the analysis at the Zinc K-edge. Particularly, the comparison of the experimental spectra and the ab-initio calculations demonstrates the formation of the aforesaid ZnO-ZnS interface at both NPs capped with dodecanethiol and ZnO/ZnS multilayers. Accordingly, the ferromagnetic-like contribution is related to the structural details at this interface, where structural distortions play a fundamental role. Finally, we have performed a study at both Mn and Zn K edges in Zn(0.95)Mn(0.05)O thin films prepared with different sputtering gases (pure Ar, Ar/N2 and Ar/O2). In those systems, while little attention has been paid to the role of the structural effects on the magnetic properties, the origin of the observed exotic magnetism has been associated, from both theoretical and experimental sides, to the occurrence of vacancies. Based on similar experimental XAS spectra, different authors report opposing conclusions regarding the nature of the defects involved in the magnetism (such as oxygen and/or zinc vacancies). However, contrary to previous claims, our results indicate that by assuming a reliable defect concentration randomly distributed the presence of both oxygen and zinc vacancies is not detectable in the XANES spectra. On the contrary the theoretical computations presented here point out that Mn adapts its local environment by increasing the Mn-O interatomic distance with the nearest-neighbour oxygen atoms [4]. [1] J. Chaboy et al, Phys. Rev. B 82, 064411 (2010). [2] C. Guglieri and J. Chaboy , J. Phys. Chem. C 114, 19629–19634 (2010) [3] C. Guglieri, M. A. Laguna-Marco, M. A. García, N. Carmona, E. Céspedes, M. García- Hernández, A. Espinosa, J. Chaboy, J. Phys. Chem. C 116, 6608−6614 (2012) [4] C. Guglieri, E. Céspedes, C Prieto, J. Chaboy, J. Phys.: Condens. Matter 23 (2011) 206006