ИСТИНА

Recent experiments revealed the formation of novel 1D metal oxide wires on step edges of a vicinal metal surfaces [1,2]. These experimental evidences made these fascinating nanostructures very attractive for further investigations of their electronic and magnetic properties. Since 1D oxides, like many molecular systems, can exhibit unusual optical properties, they can be used for tailoring magnetic properties using electromagnetic radiation [3]. In this work we studied the magnetic and optical properties of cobalt nanowires (NWs) built up at the step edges of vicinal platinum surfaces (Pt(322) and Pt(332)) in the presence of oxygen impurities. Our main goal is to expose the role of oxygen on the magnetic behaviour and its relation to optical characteristics of Co NWs in low- (0.1 monolayer (ML) oxygen coverage) and high- (0.4 ML oxygen coverage) oxidized states. The magnetic and optical properties of Co oxide nanowires have been explored in two stages. At the first stage, the ground state magnetic behavior of Co oxides were studied in the framework of density functional theory (DFT) as implemented in VASP code [4], at the second stage, the optical behavior were exposed by means of the numerical scheme proposed by Gajdos et al. using the VASP code and LOPTICS method [5]. The DFT calculations have shown that Co oxides (CoO and CoO2) related to low- and high-oxidized states exhibit magnetic behaviour. However, their magnetic ground state depends on the step edge geometry. For instance, CoO NW at step edge of Pt(332) demonstrates ferromagnetic (FM) coupling between Co atomic spins with the large value of local magnetic moment (~ 1.5µB), while in CoO NW at Pt(322) surface these atomic spins reveal the antiferromagnetic (AFM) solution for spin coupling with the same value of local magnetic moment (~1.5µB,-1.5µB) per each Co spin. However, we found the significant decrease of the magnetic behavior in CoO2 NW at high-oxidized state. The local magnetic moment per Co atom within CoO2 NW decreased strongly up to 0.6µB and 0.2µB at the step edges of Pt(332) and Pt(322) surfaces, respectively. Besides that, CoO2 NW revealed FM and AFM solutions for spin coupling for these vicinal surfaces. Having obtained the magnetic spin configurations for Co oxides, we have carried out the calculations of their optical characteristics. For this purpose, the reflectance difference spectra (RDS) and also surface dielectric anisotropy (SDA) spectra has been calculated [6]. We observed the formation of a singular peak at energy 6eV for clean Pt surface in SDA spectra, the position and polarity of this peak are not specific and do not differ for two types of step edges. The formation of this peak we associated with the optical response of the step edges as heterogeneities of Pt surface. This peak was observed also for pure Co NW. However we found the peak displacement on Pt(332) surface on 0.1eV to the low-energy region relative to a clean Pt(332) surface (for Co NW-Pt(322) system, no peak shift was observed). Low-oxidized state led to small changes in RDS and SDA spectra. We observed a significant changes in the RD and SDA spectra at high-oxidized state of Co NW on Pt(322) surface. We found the reverse of the polarity of the SDA peak in this system and its position does not change in compare to low-oxidized state. We did not observe the significant changes in spectral characteristics for CoO2 NW on Pt(332) surface. The band structure analysis of Co oxide nanowires has shown that the change of polarity of singular peak in SDA spectra can be associated with magnetic transition in CoO2 NW on the Pt(322) surface. [1] L.-Y. Ma, et al., JPCC 117, 18464 (2013) [2] Netzer, F. P., Surface Science 604, 485 – 489 (2010). [3] López-Ortega A., et al., Light Sci Appl 9, 49 (2020) [4] J. Harl, et al., PRB 76, 035436 (2007) [5] M. Gajdoš, et al., PRB 73, 045112 (2006) [6] D.E. Aspnes, et al., JVSTA 6, 1327 (1988)