Axion electrodynamics with magnetoelectric Chromia*

Axion electrodynamics with magnetoelectric Chromia

Syed Q. A. Shah¹, Ather Mahmood¹, Allan. H. MacDonald² and Christian Binek¹

syedshah@huskers.unl.edu¹, ather.mahmood@unl.edu¹, macdpc@physics.utexas.edu², cbinek@unl.edu<sup>1

1</sup>Department of Physics & Astronomy and the Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, NE 68588-0299, USA.

²Department of Physics, University of Texas at Austin, Austin, TX 78712

Abstract:

The magnetoelectric (ME) susceptibility, α , of ME materials such as Cr₂O₃ (Chromia) can be decomposed into a trace free tensor component and a pseudoscalar or axion piece component with isotropic ME response. Powder samples have been suggested as a pathway to fabricate isotropic ME materials which effectively only have a pseudoscalar ME response. However, activating a non-vanishing axion piece requires a ME field cooling protocol which tends to induce preferred axes. In our work we noticed shortcomings in the literature on ME powders where effects of the magnitude of the ME annealing field product on the ME response have been ignored. We investigate the evolution of ME susceptibility in powder chromia samples for various ME field cooling protocols. A strong dependence of the functional form of α vs. T of Chromia powders on the ME field cooling protocol is observed. It provides a pathway to realize the elusive isotropic ME response. In addition, we invest the macroscopic magnetic monopole response of an isotropic ME material when excited by the electric field of an electric point charge.