Interface phenomena in ferromagnet/ TaOx -based systems: Damping, perpendicular magnetic anisotropy, and Dzyaloshinskii-Moriya interaction

Abstract

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.

Interfacial Dzyaloshinskii–Moriya Interaction, Perpendicular Magnetic Anisotropy, and Damping in CoFeB-/Oxide-Based Systems

“…Ms and td have been deduced from the CoFeB thickness dependence of the saturation magnetic moment per unit area measured by VSM, shown in figure 1 for CoFeB/TaOx, Ta/CoFeB/TaOx and CoFeB/PtOx. The data indicate that Ms values are 1470±50 emu/cm 3 , 1536±60 emu/cm 3 [29] and 1520±50 emu/cm 3 for CoFeB/TaOx(0.8nm), Ta/CoFeB/TaOx(0.8nm) and CoFeB/PtOx(1nm), respectively. These high values of Ms are most probably due the CoFeB composition which is rich in Fe known to have high values of Ms.…”

“…These high values of Ms are most probably due the CoFeB composition which is rich in Fe known to have high values of Ms. The corresponding td are 0.71±0.16 nm, 0.7±0.1 nm [29] and 1.02±0.22 nm, respectively. This magnetic dead layer is most probably attributed to intermixing of the top Ta or Pt metals and the CoFeB layer when these top metals are not completely oxidized and to the interpenetrating oxygen atoms in CoFeB during oxidation of Ta or Pt, owing to the similar magnetic dead layer thickness of the unbuffered and Tabuffered CoFeB/TaOx samples.…”

“…In the following, all parameters will be discussed as a function of this effective thickness defined as teff=tCFB-td, unless if explicitly mentioned. For the gyromagnetic ratio  and consequently the g-factor, MS-FMR under perpendicular applied magnetic field has been used to measure the applied magnetic field dependence of the uniform precession mode frequency for unbuffered and Tabuffered CoFeB/TaOx samples [29]. The obtained values of /2 are 30.27±0.12 GHz/T (g=2.16±0.1) and 28.86±0.14 GHz/T (g=2.07±0.01), respectively.…”

“…The perpendicular anisotropy constant (K) is then determined. To have 4Meff, MS-FMR and BLS data, giving the in-plane applied magnetic field dependence of the uniform precession mode frequency (MS-FMR) and mean frequency (FS+FaS)/2, have been fitted using equation (2) reported in [29] and [31], respectively. Note that MS-FMR measurements revealed the existence of small uniaxial in-plane anisotropy, note exceeding 60 Oe.…”

“…For both systems,  varies linearly with 1/teff due the spin pumping, as shown in figure 2b. From the linear fit of these experimental data, it is then possible to determine values of CoFeB Gilbert damping αCFB and effective spin mixing conductance ↑↓ from the vertical axis intercept and the slope, respectively [29]. The obtained values of αCFB and ↑↓ are (2.1±0.4)10 -3 , (3.1±0.19)10 -3 , (16.4±2.1) and (14.5±1.2) nm -2 for CoFeB/TaOx and Ta/CoFeB/TaOx, respectively.…”