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 N,el 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 N,el temperature, where the antiferromagnetic long-range order disappears. The change in refractive index was obtained from the peak shift in the THz electric field. The temperature dependence of 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.