[Pt(1.5 nm)/Co(tCo)/W(1.5 nm)]N multilayers of different Co thicknesses (tCo) and number of repeats (N) have been grown by sputtering on Si substrates, and their magnetic properties have been studied. The x-ray reflectivity has been used to measure thicknesses of each layer as well as their roughness. The dependence of the magnetic moment on tCo and N (as determined by vibrating sample magnetometry) indicates the existence of a magnetic dead layer, which increases with N and reaches its maximum values for N ≥ 3. A similar N dependence of the magnetization at saturation is found. Ferromagnetic resonance and Brillouin light scattering have been used to investigate perpendicular magnetic anisotropy, damping, and interfacial Dzyaloshinskii-Moriya interaction (iDMI) vs Co thickness and the number of Pt/Co/W sequence repeats. We show that these parameters result from interface contributions that vary in a similar way with N, confirming that the first two Pt/Co/W trilayers are of lower quality. We thus conclude that for these systems, the increase of N improves the quality of interfaces and the volume of the stack, leading to the enhancement of the magnetic properties. Moreover, the measured weak iDMI constant, even for the higher N values, suggests that most probably, this iDMI results mainly from the Pt/Co interfaces.

The perpendicular magnetic anisotropy (PMA) and the interfacial Dzyaloshinskii-Moriya interaction (iDMI) are investigated in as grown and 300 • C annealed Co-based ultrathin systems. For this, Co films of various thicknesses (0.8 nm t Co 5.7 nm) were deposited by magnetron sputtering on thermally oxidized Si substrates using Pt, W, Ir, Ti, Ru and MgO buffer or/and capping layers. X-ray diffraction was used to investigate their structural properties and vibrating sample magnetometry (VSM) was used to determine the magnetic dead layer thickness and the magnetization at saturation (M s ). VSM revealed that the M s for the Pt and the Ir buffered and capped films is the largest. Microstrip line ferromagnetic resonance (MS-FMR), used to extract the gyromagnetic ratio of the thicker Co films, revealed the existence of a second order PMA term, which is thickness dependent. Brillouin light scattering (BLS) in the Damon-Eshbach configuration was used to investigate the thickness dependence of the iDMI effective constant from the spin wave vector dependence of the frequency difference between Stokes and anti-Stokes lines. BLS and MS-FMR techniques were combined to measure the spin wave frequency variation as a function of the in-plane applied magnetic field (where the second order PMA contribution vanishes). The thickness dependence of the effective magnetization was then deduced and used to investigate PMA. For all the systems, PMA results from interface and volume contributions that we determined. The largest interface PMA constants were obtained for Pt-and Ir-based systems due to the electron hybridization of Co with these heavy metals having high spin orbit coupling. Annealing at 300 • C increases both the interface PMA and iDMI for the Pt/Co/MgO most probably due to de-mixing of interpenetrating oxygen atoms from the Co layer and the formation of a sharp Co/O interface.

Microstrip line ferromagnetic resonance (MS-FMR) and Brillouin light scattering (BLS) in the Damon-Eshbach geometry were used to investigate the perpendicular magnetic anisotropy (PMA), the damping and the interfacial Dzyaloshinskii-Moriya (iDMI) interaction in ferromagnetic (FM)/TaO x -based systems as a function of the ferromagnetic (FM = Co or Co 8 Fe 72 B 20 ) and the TaO x thicknesses (oxidation level). The analysis of the experimental FMR and BLS data has shown that the effective magnetization, the Gilbert damping parameter α and the iDMI are inversely proportional to the CoFeB and the Co films thickness. The BLS investigation of the iDMI variation versus the TaO x thickness and oxidation level reveals a contribution of FM/TaO x mediated by the presence of a Rashba field at this interface. Finally, we evidenced a correlation between iDMI and PMA by varying the Cu spacer thickness in the Pt/Cu/Co/TaO x system and we showed that both PMA and iDMI are localized at the first atomic monolayers of the Pt/Co interface. The observed non-linear dependence of PMA versus iDMI constant is attributed to similar interface orbital hybridizations involved in both quantities.

Correlation between interfacial Dzyaloshinskii–Moriya interaction (iDMI), perpendicular magnetic anisotropy (PMA) and spin pumping-induced damping was investigated in CoFeB-based systems grown by sputtering on Si substrates, using Pt, Ta, Cu, W and MgO capping layers. Vibrating sample magnetometer, Brillouin light scattering (BLS) and broadband ferromagnetic resonance techniques were combined for this aim. The CoFeB thickness dependence of iDMI and PMA constants, in CoFeB/X (where X = Pt, Cu/Pt, Ta/Pt or W/Al), revealed that only the CoFeB/Pt system presents a measurable iDMI and that the interfacial PMA is mostly similar except for the Ta/CoFeB/Ta/Pt system. Therefore, no clear correlation between the above-mentioned interfacially-driven and spin-orbit coupling related quantities was observed due to their different origins in our systems. An efficient sample design involving various spacer layers of variable thicknesses in Ta/CoFeB(1.5 nm)/Y/Pt (where Y = Cu, Ta, MgO) allowed evidence of a linear correlation between iDMI, PMA constants and the effective spin mixing conductance. The linear dependence, which could result from the narrow variation range of PMA and/or iDMI, is attributed to the similar interface orbital hybridizations involved in PMA, iDMI and spin pumping-induced damping.

This review article aims to provide a comprehensive overview of recent FMR studies on magnetic oxide nanoparticles and their potential applications. The use of the FMR technique is a powerful tool to study the magnetic properties of magnetic nanoparticles and can provide valuable information on their behavior. For this, we will start by discussing the purpose of these magnetic nanoparticles and their application in various fields, including biomedical applications, energy storage, and environmental remediation. We will then discuss the methods used to prepare magnetic nanoparticles and the theory behind FMR including the superparamagnetic effect. Additionally, we will present the most recent studies on FMR for magnetic oxide nanoparticles by highlighting the effect of temperature and doping on the magnetic properties of these nanoparticles.

Perpendicular magnetic anisotropy (PMA), spin pumping induced damping and interfacial Dzyaloshinskii-Moriya interaction (iDMI), which are spin-orbit coupling-related phenomena of utmost importance for applications, were experimentally investigated in as grown and 225°C annealed CoFeB/PtOx, CoFeB/TaOx and Ta/CoFeB/TaOx systems by means of vibrating sample magnetometry, microstrip ferromagnetic resonance and Brillouin light scattering techniques. By varying Co8Fe72B20 (CoFeB) thickness in the range 0.8-10 nm, the effect of Ta buffer layer on anisotropy and damping was first studied, where a large surface magnetic anisotropy (Ks=2.1±0.16 erg/cm 2 ) was measured in the unbuffered CoFeB/TaOx(0.8nm) system most likely due to their higher roughness induced by the substrate. Ks degrades significantly for CoFeB film thickness below 2 nm where spontaneous perpendicular magnetization was found to be impossible without Ta buffer layer. PMA, iDMI and damping of as-deposited and 225°C annealed CoFeB(1.5 nm)/PtOx systems were measured as a function of PtOx thickness in the range 0.7-1.6 nm. Their strong dependence versus the PtOx thickness was attributed to the decrease of the magnetic dead layer as PtOx thickness increases. Linear dependence of damping versus PMA constant were obtained confirming their relation with the spin orbit coupling. Moreover, annealing increases PMA and the effective mixing conductance probably due to the enhancement of the CoFeB crystal structure and interfaces.

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