  Microwave (MW) transmission, absorption, and reflection loss spectra of the
ferrimagnetic U-type hexaferrite Sr$_4$CoZnFe$_{36}$O$_{60}$ ceramics were
studied from 100 MHz to 35 GHz at temperatures between 10 and 390 K. 9 MW
magnetic excitations with anomalous behavior near the ferrimagnetic phase
transitions were revealed. They also change under the application of weak bias
magnetic field (0 - 700 Oe) at room temperature. 6 pure magnetic modes are
assigned to dynamics of the magnetic domain walls and inhomogeneous magnetic
structure of the ceramics, to the natural ferromagnetic resonance (FMR) and to
the higher-frequency magnons. Three modes are considered as the
magnetodielectric ones with dominating influence of the magnetic properties on
their temperature and field dependences. Presence of the natural FMR in all
ferrimagnetic phases proves existence of the non-zero internal magnetization
and magnetocrystalline anisotropy. Splitting of the FMR into the two components
without magnetic bias was observed in the collinear phase and is attributed to
a change of the magnetocrystalline anisotropy during the phase transition. The
high-frequency FMR component critically slows down to the phase transition. At
room temperature, the FMR splitting and essential suppression of the
higher-frequency modes was revealed under the weak bias field (300 - 700 Oe).
The highly nonlinear MW response and the FMR splitting are caused by the
gradual evolution of the polydomain magnetic structure to a monodomain one. The
high number of magnetic excitations observed in the MW region confirms the
suitability of using hexaferrite Sr$_4$CoZnFe$_{36}$O$_{60}$ ceramics as MW
absorbers, shielding materials and highly tunable filters.
