Magnetoelectric coupling in multiferroic Z-type hexaferrite revealed by electric-field-modulated magnetic resonance studies

Temperature dependence of ferromagnetic resonance (FMR) frequency f_FMR was revealed in Z-type hexaferrite (Ba_xSr_1−x)₃Co₂Fe₂₄O₄₁ using microwave spectroscopy in zero external magnetic field. A linear decrease in f_FMR toward magnetic phase transition temperature T_c2 = 500 K was observed, implicating magnetic anisotropy decrease. Ferromagnetic resonance studies performed near 9 GHz with the magnetic field up to 10 kOe also confirmed FMR anomaly near 500 K, where the magnetic structure changes from conical to collinear. FMR spectra are tunable by external dc and ac (100 kHz) electric fields which allowed us to determine the value of magnetoelectric coefficient α_E = 390 ps/m at 170 K. α_E is at about 0.5 kOe one order of magnitude lower than the reported value at 10 K, because in our case the electric field was applied along c-axis, while the previously reported value was obtained with the field in hexagonal plane. Nevertheless, our α_E is still one or even four orders of magnitude higher than in other "high-temperature" multiferroics.