Terahertz field induced spin excitations via anharmonicity in a spin spiral hexaferrite

Anharmonic coupling among elementary excitations such as phonons and magnons in coherent processes provides novel functionality for controlling of the states of matter. One intriguing phenomenon driven by the anharmonic coupling is the indirect creation of particular elementary excitations triggered by resonantly excited other elementary excitations. However, the coherent anharmonic processes for spin system have been rarely investigated. Multiferroics with spin-driven ferroelectricity exhibit versatile spin excitations including magnons and electromagnons, which possess the strong terahertz response. Furthermore, the presence of strong anharmonicity among these spin excitations has been suggested in multiferroics. Here we show the indirect creation of electromagnon and magnon by high-field terahertz pulses through the anharmonicity among spin excitations in spin-spiral helimagnet Ba₂Mg₂Fe₁₂O₂₂. We conducted terahertz pump-near infrared probe spectroscopy. The resonance excitation of electromagnon at 0.7 THz is found to cause the coherent oscillation of low frequency modes including the magnon and electromagnon.