Magnetic Anisotropy and Magnetization Dynamics in Si/Ni Multilayers Probed by FMR and VSM Magnetometry

Abstract

The magnetic anisotropy and magnetization dynamics of sputtered [Si(40~Å)/Ni($t_{\mathrm{Ni}}$~Å)]$_{20}$ multilayers were systematically investigated at room temperature using SQUID--VSM magnetometry and angle-dependent ferromagnetic resonance (FMR). Static measurements reveal soft ferromagnetic behaviour in thicker Ni layers and mixed ferromagnetic and superparamagnetic contributions at reduced thicknesses. The saturation magnetization increases toward the bulk Ni value with increasing thickness, consistent with Ni nanocrystallite growth. Magnetic dead-layer analysis yields an interfacial dead-layer thickness of approximately $9$~Å per interface, accounting for the suppressed magnetization in thinner Ni layers. Angle-dependent FMR measurements reveal effective easy-plane magnetic anisotropy, while out-of-plane spectra exhibit additional non-uniform spin-wave modes. The effective magnetization remains lower than the static saturation magnetization, indicating a perpendicular anisotropy contribution that partially compensates the demagnetizing field. Quantitative anisotropy analysis yields an interfacial anisotropy constant of $K_S = +0.17~\mathrm{erg\,cm^{-2}}$, favouring perpendicular magnetization, and a volume anisotropy constant of $K_V = -1.23 \times 10^6~\mathrm{erg\,cm^{-3}}$, favouring in-plane alignment, with an extrapolated anisotropy crossover thickness of $t_c \approx 27$~Å. Increasing FMR line-shape asymmetry with decreasing Ni thickness is consistent with enhanced interfacial roughness. These findings highlight the competing interfacial and volume anisotropies and the importance of Ni thickness and interface quality in tailoring Si/Ni multilayers for silicon-compatible spintronic applications.