  A novel approach to tune the ferromagnetic resonance frequency of a soft
magnetic Ni$_{80}$Fe$_{20}$ (Permalloy = Py) film with in-plane magnetic
anisotropy (IMA) based on the controlled coupling to a hard magnetic
NdCo$_\text{x}$ film with perpendicular magnetic anisotropy (PMA) through a
non-magnetic Al spacer is studied. Using transverse magneto-optical Kerr effect
(TMOKE), alternating gradient magnetometry (AGM) as well as vector network
analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, the influence of both
Co concentration and Al spacer thickness on the static and dynamic magnetic
properties of the coupled IMA/PMA system is investigated. Compared to a single
Py film, two striking effects of the coupling between IMA and PMA layers can be
observed in their FMR spectra. First, there is a significant increase in the
zero-field resonance frequency from 1.3 GHz up to 6.6 GHz, and second, an
additional frequency hysteresis occurs at low magnetic fields applied along the
hard axis. The maximum frequency difference between the frequency branches for
increasing and decreasing magnetic field is as high as 1 GHz, corresponding to
a tunability of about 20% at external fields of typically less than $\pm$70 mT.
The origin of the observed features in the FMR spectra is discussed by means of
magnetization reversal curves.
