The response of some materials to light may change substantially in a magnetic field. A particular case is presented for magneto-optical (MO) materials, in which the polarization of light is changed proportionally to the magnetization. This property is useful for applications in data storage, sensing and optical waveguides. The root of this MO effect is usually the material’s own magnetism. Yet, our research has uncovered that a surprisingly large MO effect can arise from an apparently non-magnetic source, namely, polarons. These physical entities have a particle-like character and they form when electrons interact so strongly with the surrounding ions that they deform the crystalline lattice around them (see Figure below). Indeed, this interaction may become so intense that the electrons can become self-trapped by the same distortion that they create in the solid.
Our work was focused on thin films of La2/3Ca1/3MnO3, a so-called manganite known for its “colossal” magnetoresistance. Briefly, around the ferromagnetic transition –relatively close to room temperature– the electric transport in this material is remarkably sensitive to magnetic fields, so that very large changes of resistance are induced by them. It is widely accepted that the interaction of polarons with magnetic fields plays an important role in the emergence of the colossal magnetoresistance and, therefore, La2/3Ca1/3MnO3is a very suitable system for the study of polaronic optical responses.
Bearing this in mind, the polarization of a light beam was compared before and after it had reflected from the film. The result was that, near the temperature where magnetoresistance is highest (265 K) and electrons self-trap forming polarons, a magnetic field induced a rotation of the beam’s polarization that was more than one order of magnitude higher than when electrons were free to move. The wavelength dependence of this rotation was measured as a function of the wavelength, and this analysis revealed features that were consistent with the excitation of polarons. According to the theoretical analysis, the magneto-optical response of polarons is particularly enhanced in La2/3Ca1/3MnO3 and possibly other manganites because of an interaction between the spin of the hopping polaron and its distortion of the lattice.
The observed phenomenon opens up interesting avenues. In particular, since straining a material can change the polaron concentration, the finding suggests a route to mechanical or electro-mechanical control of the MO effect that may find applications for sensing and optical communications.
Light incident with a given state of polarization is reflected off a surface of a CMR manganite. The incident beam interacts with a self-trapped polaron which changes the state of polarization of light.
B. Casals et al., Phys. Rev. Lett. 117, 026401 (2016). https://doi.org/10.1103/PhysRevLett.117.026401
Highlighted in APS Physics. Polarons Drive a Magneto-Optical Effect