  Ferromagnetic resonance properties of F$_1$/f/F$_2$/AF multilayers, where
weakly ferromagnetic spacer f is sandwiched between strongly ferromagnetic
layers F$_1$ and F$_2$, with F$_1$ being magnetically soft and F$_2$ -
magnetically hard due to exchange pinning to antiferromagnetic layer AF, are
investigated. Spacer-mediated exchange coupling is shown to strongly affect the
resonance fields of both F$_1$ and F$_2$ layers. Our theoretical calculations
as well as measurements show that the key magnetic parameters of the spacer,
which govern the ferromagnetic resonance in F$_1$/f/F$_2$/AF, are the magnetic
exchange length ($\Lambda$), effective saturation magnetization at $T=0$
$(m_0)$, and effective Curie temperature ($T_{\text{C}}^{\text{eff}}$). The
values of these key parameters are deduced from the experimental data for
multilayers with f = Ni$_x$Cu$_{100-x}$, for the key ranges in Ni-concentration
($x=54\div70$ at. %) and spacer thickness ($d=3\div 6$ nm). The results
obtained provide a deeper insight into thermally-controlled spin precession and
switching in magnetic nanostructures, with potential applications in spin-based
oscillators and memory devices.
