Abstract
A microscopic model for the room-temperature multiferroic BiFeO3 that includes two
Dzyaloshinskii-Moriya interactions and single-ion anisotropy along the ferroelectric polarization predicts
both the zero-field spectroscopic modes as well as their splitting and evolution in a magnetic
field. Due to simultaneously broken time-reversal and spatial-inversion symmetries, the absorption
of light changes as the magnetic field or the direction of light propagation is reversed. We discuss
three physical mechanisms that may contribute to this absorption asymmetry known as directional
dichroism: the spin current, magnetostriction, and single-ion anisotropy. We conclude that the
directional dichroism in BiFeO3 is dominated by the spin-current polarization and is insensitive to
the magnetostriction and easy-axis anisotropy. With three independent spin-current parameters,
our model accurately describes the directional dichroism observed for magnetic field along [1,−1, 0].
Since some modes are almost transparent to light traveling in one direction but opaque for light
traveling in the opposite direction, BiFeO3 can be used as a room-temperature optical diode at certain
frequencies in the GHz to THz range. Our work demonstrates that an analysis of the directional
dichroism spectra based on an effective spin model supplemented by first-principles calculations can
produce a quantitative microscopic theory of the magnetoelectric couplings in multiferroic materials.
