2D THz Measurement of Magnon-Phonon Coupling in Multiferroic BiFeO₃

Quantum phonon, magnon, and electronic excitations are the building blocks of lattice, spin, and electronic degrees of freedom. Couplings between these modes can lead to new excitations, like electromagnons, and exotic macroscopic properties like colossal magnetoresistance, where a magnetic field dramatically alters the electrical conductivity. Terahertz (THz) frequency light is uniquely suited to resonantly probe collective electronic, phonon, magnon, and electromagnon modes. New developments in 2D THz spectroscopy are enabling the direct measurement of nonlinear excitation of phonon, magnon, and electronic modes, and preliminary measurements are directly showing couplings between them.<br/><br/>BiFeO₃ is one of the most studied multiferroic materials. It is a prototypical example of an ABO₃ type I magnetoelectric multiferroic, where the ferroelectric polarization (due to lone-pair-active structural distortions involving the “A” Bi atoms) arises independently from the magnetic ordering (that is due to the spin structure of electrons associated with the “B” Fe atoms). BiFeO₃ is ferroelectric below the Curie temperature of ~1100 K and it becomes antiferromagnetic below the Néel temperature of ~650 K, making it multiferroic at room temperature. Because it is a type I multiferroic where spin and ferroelectric ordering arise independently of each other, coupling between magnon, electronic, and phonon subsystems is not expected to be strong. Nevertheless, the 2D THz measurements of BiFeO₃ at room temperature reveal that magnon-phonon coupling occurs.<br/><br/>Direct phonon-phonon coupling between two phonon modes at 4 and 2.3 THz are observed. We additionally see direct magnon-phonon coupling between the phonon at 2.3 THz and a magnon at 0.6 THz. This direct measurement of magnon-phonon coupling not only enables us to prove that it occurs even in a weakly coupled multiferroic like BiFeO₃, but now we can further consider how to control the fundamental material degrees of freedom by using these mode couplings.