IMF2017

Materials with the Z-type hexaferrite structure belong to the rare cases of room-temperature magneto-electric multiferroics, featuring a high resistivity, low permittivity and a low loss tangent. They exhibit a transverse conically ordered magnetic state up to ca. 400 K. Their particular feature is the absence of ferroelectric polarization at zero magnetization. However, a polar state can be magnetically induced via the inverse Dzyaloshinski-Moriya interaction; it arises at very weak intensities of external magnetic field and persists up to ca. 1.5 T. The dynamical magnetic properties of Z-type hexaferrites are still largely unknown; they present a great interest both from the fundamental point of view and with respect to potential applications in memories, spintronics and magnonics. We have measured THz transmittance and far-infrared reflectivity of ceramics at temperatures from 5 to 900 K. Further, we carried out measurements of their magnetic susceptibility, magnetization and magneto-electric effect. For THz measurements, a magnetic field of up to 7 T was applied in the Faraday geometry. We observed a sharp resonance, whose position and damping depend strongly on temperature and applied magnetic field; we attribute it to an electromagnon, i.e. a magnon excited by electric field of the THz radiation. For field intensities above 2 T, another narrow resonance appears in the low-frequency part of the spectrum. At H = 7 T, its absorption peaks near 0.22 THz, a value which is temperature-independent within 5–250 K. This is probably a ferromagnetic resonance, whose frequency linearly increases with magnetic field.