We investigate the effect of interfacial exchange coupling on the broadband ferromagnetic resonance (FMR) response of thin-film ${\mathrm{IrMn}}_3/\mathrm{NiFe}$ bilayers, focusing on the evolution of effective magnetization $\left(M_{\mathrm{eff}}\right)$ and rotatable anisotropy field $\left(H_{\mathrm{ra}}\right)$ as a function of ${\mathrm{IrMn}}_3$ layer thickness $\left(t_{\mathrm{AF}}\right)$ from 0 to 4.5 nm, encompassing the paramagnetic to antiferromagnetic (AF) phase transition of ${\mathrm{IrMn}}_3$. Our results reveal a $\sim 4\%$ enhancement in $M_{\mathrm{eff}}$ upon formation of AF ordering in ${\mathrm{IrMn}}_3$, without a corresponding change in the volumetric magnetization of NiFe. Theoretical modeling indicates that this effect arises from the hybridization between the standard FMR mode of NiFe and the fundamental mode of the ${\mathrm{IrMn}}_3$ layer, which is activated by interfacial exchange coupling. Because the fundamental mode frequency of ${\mathrm{IrMn}}_3$ lies far above the GHz FMR range, this hybrid mode penetrates the ${\mathrm{IrMn}}_3$ layer as an evanescent excitation, making $M_{\mathrm{eff}}$ predominantly sensitive to the interfacial exchange energy rather than to the bulk AF parameters. A control calculation in which the AF moments are held fixed produces only a resonance-field shift and no enhancement of $M_{\mathrm{eff}}$, demonstrating within the model that the observed increase requires a dynamical AF response. Our results establish $M_{\mathrm{eff}}$ as a valuable tool for probing AF spin excitations, given that it is a more phenomenon-specific indicator in comparison to other FMR parameters that produce homogeneous resonance field shifts.

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• Magnetization dynamics
• Spin dynamics
• Landau-Lifshitz model