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THz spectra of Z-type hexaferrite (Ba0.2Sr0.8)3Co2Fe24O41 reveal an electrically active spin wave (electromagnon) at temperatures below 250 K. This excitation is activated due to dynamic magnetoelectric coupling (exchange striction). Its intensity is highly sensitive to an external magnetic field: its strength drastically drops for magnetic field values above 0.5 Tesla and it completely disappears above 2 Tesla, when the spin structure changes from transverse conical to collinear.
Description
Magnetic field dependence of index of refraction n and extinction coefficient k in the THz region measured at 50 K. The electromagnon resonance seen at 35 cm-1 completely disappears for magnetic field above 2 Tesla. A ferromagnetic resonance appears at 4 Tesla and its frequency increases linearly with magnetic field (see the features below 10 cm-1).