Observation of magnon-polarons in a uniaxial antiferromagnetic insulator Cr2O3

Abstract

Magnons and phonons are two elementary excitations associated with spin waves and lattice vibrations in solids. In the presence of magneto-elastic interactions, the coupling of the spin and lattice degrees of freedom leads to the formation of a new type of hybridized excitation, i.e., magnon-polarons. Manifested as anomalies in the spin Seebeck effect (SSE) in a ferrimagnet YIG, magnon-polarons have profound consequences in modifying the magnon and phonon dispersion relations and affecting spin transport properties in ferrimagnetic insulators. In antiferromagnetic insulators, the magnon spectrum is distinctly different due to the existence of two equivalent, antiferromagnetically coupled spin sub-lattices. Here we report an observation of antiferromagnetic magnon-polarons in Cr2O3, a prototypical uniaxial antiferromagnet, featured as anomalies in the SSE signals of Cr2O3/heavy metal heterostructures. Despite the relatively high magnon energy in the absence of an external magnetic field, near the spin-flop transition of ~ 6 T, the left-handed magnon spectrum shifts downward to intersect those of acoustic phonons (both longitudinal and transverse) to cause hybridization. We have measured the temperature dependence of the SSE magnon-polaron anomalies in Cr2O3 and revealed the strength of the magneto-elastic coupling. Our study indicates that the hybridized excitations can be employed to manipulate spin transport in antiferromagnetic systems. This work was supported as part of the SHINES, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award No. SC0012670.