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

Temperature dependence of ferromagnetic resonance (FMR) frequency fFMR was revealed in Z-type hexaferrite (BaxSr1−x)3Co2Fe24O41 using microwave spectroscopy in zero external magnetic field. A linear decrease in fFMR toward magnetic phase transition temperature Tc2 = 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 αE = 390 ps/m at 170 K. αE 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 αE is still one or even four orders of magnitude higher than in other “high-temperature” multiferroics.