Axion Electrodynamics Realized in a Topologically Trivial Antiferromagnet

Axion electrodynamics, originally proposed by Frank Wilczek in the context of high energy physics, can manifest in condensed matter systems as an isotropic linear magnetoelectric (ME) response. Yet, a continuous and purely monopolar ME response has until now remained experimentally inaccessible. We realize such a desired response by converting the topologically trivial antiferromagnet chromia into an isotropic ME medium. This conversion is demonstrated through low-frequency AC ME susceptometry and theoretically supported by Monte Carlo simulations. Field-cooling a chromia single crystal into an antiferromagnetic single-domain state, followed by powdering and solidification while keeping the grains in the antiferromagnetically ordered state, suppresses ME quadrupolar contributions and yields an isotropic ME tensor α_ij=θδ_ij. The parameter θ is temperature dependent and crosses zero at 168 K, consistent with the axion contribution in crystalline chromia and our theoretical findings. Above the Néel temperature, the pure axion response becomes irretrievable, revealing an intrinsic asymmetry between ME annealing before and after powdering. By realizing and controlling the monopolar ME state, we establish a condensed-matter platform for axion electrodynamics and open pathways toward ME Hall, magnetophotovoltaic, and monopolar-based devices.