Effect of oxidation on the magnetism of Co and Cr clusters probed by Stern-Gerlach deflection Kobe De Knijf 1,* , Johan van der Tol 1 , Piero Ferrari 1 , Sandrien Scholiers 1 , Gao-Lei Hou 2 , Peter Lievens 1 and Ewald Janssens 1 1 Quantum Solid-State Physics, Department of Physics and Astronomy, Belgium, 2 MOE Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, School of Physics, Xi’an Jiaotong University, China * kobe.deknijf@kuleuven.be Transition metal oxides (TMOs) play an important role in a vast variety of applications [1,2]. In particular, Co oxide nanoparticles are candidates to inhibit cancer cell growth and have great potential as antimicrobial agents [3], whereas Cr oxide nanoparticles are employed in industrial applications, for example, as catalysts or pigments [4]. To date, mainly computational studies have been carried out to understand the evolution of the magnetic order in TMO clusters. The results are fascinating, showing a range of possible spin coupling configurations, and suggesting that the oxygen concentration can drastically change the magnetic order. However, there is a major lack of experimental data. Stern-Gerlach magnetic deflection experiments have been performed on neutral suboxide Co n O m and Cr n O m clusters (n+m ≤ 30). It was found that the magnitude of the magnetic moment increases with the number of Co atoms, suggesting a ferromagnetic coupling between the Co atoms. Interestingly, we also found that the magnetic moments in general seem to grow with oxygen concentration for a fixed value n, although absolute numbers fluctuate. Complementary density functional theory calculations suggest this may be due to charge transfer from cobalt to oxygen upon oxidation. Electron donation by the Co atoms is energetically more favourable to come first from the doubly occupied d -states. This has the effect of increasing the number of unpaired d -electrons and thus increasing the total spin of each Co atom. In addition, (preliminary) results of the Cr oxides clusters will be presented, complementing the data of the Co oxides. References 1. H. Osgood, S. V. Devaguptapu, H. Xu, J. Cho, and G. Wu, Transition metal (Fe, Co, Ni, and Mn) oxides for oxygen reduction and evolution bifunctional catalysts in alkaline media, Nano Today 11, 601-625 (2016). 2. N. T. Tung, N. M. Tam, M. T. Nguyen, P. Lievens, and E. Janssens, Influence of Cr doping on the stability and structure of small cobalt oxide clusters, J. Chem. Phys. 141, 044311 (2014). 3. N. Arsalan, E. H. Kashi, A. Hasan, M. E. Doost, B. Rasti, B. A. Paray, M. Z. Nakhjiri, S. Sari, M. Sharifi, K. Shahpasand, K. Akhtari, S. Haghighat, and M. Falahati, Exploring the interaction of cobalt oxide nanoparticles with albumin, leukemia cancer cells and pathogenic bacteria by multispectroscopic, docking, cellular and antibacterial approaches, Int. J. Nanomedicine 15, 4607-4623 (2020). 4. V. A. Senapati, A. K. Jain, G. S. Gupta, A. K. Pandey and A. Dhawan, Chromium oxide nanoparticle-induced genotoxicity and p53-dependent apoptosis in human lung alveolar cells, J. Appl. Toxicol. 35, 1179–1188 (2015).
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