## 2 Materials and Methods  
The experiments were performed in a laboratory-scale sputtering machine equipped with a circular Magneto 3 GABS magnetron (Pinch, LLC, Moscow, Russia) having a 76.2 mm diameter aluminum target (purity 99.999%, Girmet, LLC, Moscow, Russia). Experimental setup scheme is demonstrated in Figure 1.  
Figure 1: Setup of the magnetron sputtering system. The vacuum chamber was pumped down to a base pressure of 10\({}^{-4}\) Pa with a turbomolecular pump backed by a Roots pump. Argon and oxygen were introduced into the chamber by the automated mass-flow controllers El-Flow (Bronkhorst High-Tech B.V., Ruurlo, The Netherlands). The total operating pressure was fixed at 0.5 Pa. The cumulative flow rate of argon and oxygen in each mode was varied between 0 and 4.2 sccm in both directions to characterize the hysteresis behavior of the system. Correspondingly, since the total gas flow was fixed at 30 sccm, the oxygen flow fraction in the Ar/O\({}_{2}\) gas mixture was varied from 0 to 14%. To guarantee the stability of the magnetron operation mode, all measurements described below were taken after 5 min from the moment of changing the oxygen flow rate. Magnetron discharge was operated with a DC power supply APEL-M-5 PDC (Applied Electronics, LLC, Tomsk, Russia). The power rating was fixed at 100 W in all presented experiments. A magnetic sector mass analyzer (MA) was used to determine the mass-resolved ion fluxes in the typical substrate placement area. MA comprised an electrostatic extractor attached to the vacuum chamber pointed at the racetrack region of the magnetron. The extractor was essentially a three-electrode electrostatic lens realizing the functions of sampling ions from plasma, accelerating them and forming an ion beam. The entrance plasma-facing electrode was grounded, while the accelerating electrode, the magnetic sector, and all related equipment were under high voltage (\(U_{\mathrm{acc}}\)) relative to the ground potential. In order to improve the sensitivity of the instrument, a secondary electron multiplier (SEM) was used as a detector of mass-filtered ions. The further details of the mass spectrometer configuration are discussed in [44; 62]. The acquisition time for each ion mass spectrum was 1 min. In order to have an ability to measure the mass-resolved fluxes of both positive and negative ions, the detection system was arranged in two different configurations depending on the charge sign. The corresponding diagrams of electric potential distribution inside the MA are shown in Figure 2.  
A quartz crystal microbalance (QCM) Mikron-5 (Izovac Ltd., Minsk, Belarus) with crystal diameter of 14 mm and resonant frequency of 6 MHz was used to obtain the qualitative data on the deposition rate's dependence on the oxygen flow [52]. Mass deposition rate in arbitrary units was obtained by approximating the time dependence of the resonant oscillation frequency with a linear fit and calculating its slope. No assumptions of material density were made, and no coating thickness was calculated because the film's composition varies with changing the oxygen flow. Optical emission from plasma was characterized by a three-channel AvaSpec-ULS2048L (Avantes B.V., Apeldoorn, The Netherlands) spectrometer. It comprised three  
Figure 2: Schematic of electric potential distribution along the ion path in MA used for detection of (**a**) positive ions; (**b**) negative ions. measurement ranges of 200-365 nm (resolution 0.12 nm), 364-603 nm (resolution 0.18 nm), and 360-810 nm (resolution 0.15 nm). The fiber was mounted outside the vacuum chamber, and the emission was collected through a quartz viewport. No special protection was used to cover the viewport. Nevertheless, in our conditions, the measured spectra were not distorted because of the low deposition rate coupled with the high optical transparency of aluminum oxide coatings [1].