The nonlinear effects in strongly driven magnetic and spintronic devices are gradually becoming key challenges in their practical applications and physical understanding. Here, an analytical theory of nonlinear ferromagnetic resonance is developed by solving the Landau-Lifshitz-Gilbert equation in the presence of a uniaxial anisotropy field under arbitrary microwave excitation. The solution shows that the magnetization undergoes stable conical precession, allowing the nonlinear resonance condition, resonance field, linewidth, and switching boundaries to be obtained analytically. The analysis reveals that the anisotropic-induced nonlinear restoring force is responsible for the asymmetric foldover resonance and the power-dependent resonance shift, in excellent agreement with spin-pumping measurements on ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{Pt}$ bilayers. By incorporating the microwave current-induced spin-transfer torque, we further show that the ac spin-transfer torque acts as a phase-sensitive perturbation that modifies the stability of nonlinear precession without changing the nonlinear resonance condition, thereby modulating the up-jump field. This work provides a unified analytical framework for understanding nonlinear foldover resonance and spin-transfer torque modulated magnetization dynamics.

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• Magnetic anisotropy
• Magnetization dynamics
• Spin dynamics
• Ferromagnets
• Magnetic thin films
• Ferromagnetic resonance
• Landau-Lifschitz-Gilbert equation