  Hexaferrite materials are highly demanded to develop and manufacture
electronic devices operating at radio- and microwave frequencies. In the light
of the prospects for their use in the forthcoming terahertz electronics, here,
we present our results on the terahertz and infrared dielectric response of a
typical representative of hexaferrites family, lead-substituted M-type barium
hexaferrite doped with aluminum, Ba0.2Pb0.8AlxFe12-xO19, x(Al)=0.0, 3.0, and
3.3. We studied uniquely large and high-quality single crystals of the.
Systematic and detailed investigations of the dependences of terahertz-infrared
(frequencies 8 - 8000 cm-1) spectra of complex dielectric permittivity on the
temperature, 4 - 300 K, and on the chemical composition, x(Al)=0.0, 1.2, 3.0,
3.3, were performed for polarizations of the electric field E-vector of the
probing radiation normal and parallel to the crystallographic c-axis. A number
of resonance absorption bands are discovered at infrared-terahertz frequencies
and assigned to polar phonons and transitions between energy levels of the
fine-structured ground state of Fe2+ (5E) ions. In contrast to undoped
BaFe12O19, no softening of the lowest frequency A2u phonon is observed,
indicating suppression of a displacive phase transition in substituted
compounds. Basing on dielectric data and detailed X-ray experiments, we find
that for all concentrations of Al3+ ions, x(Al)=0.0, 1.2, 3.0, and 3.3, they
mainly occupy the 2a and 12k octahedral site positions and that the degree of
substitution of iron in tetrahedral positions is not substantial. Along with
fundamental findings, the obtained data on broad-band dielectric properties of
Ba0.2Pb0.8AlxFe12-xO19 crystals provides the information that can be used for
development and manufacture of electronic devices with operating frequencies
lying in the terahertz spectral band.
