Observation of Antiferromagnetic Magnons and Magnetostriction in Manganese Oxide Using Terahertz Time-Domain Spectroscopy.

We observed antiferromagnetic magnons in manganese oxide (MnO) using terahertz time-domain spectroscopy (THz-TDS). The antiferromagnetic resonance (AFMR) frequency shows the behavior of a first-order phase transition near the Néel temperature (116 K). The temperature dependence of the AFMR frequency is explained well by a molecular field approximation including biquadratic exchange. It was found that the relaxation rate of the AFMR signal shows an abrupt increase near the Néel temperature, where the antiferromagnetic long-range order disappears. The change $\Delta n$ in refractive index was obtained from the peak shift in the THz electric field. The temperature dependence of $\Delta n$ can be explained by the lattice and magnetostrictive contributions to this refractive index change. The measurement of refractive index by THz-TDS is very useful for optically opaque materials such as MnO. [ABSTRACT FROM AUTHOR]