The Role of Multiferroics in the Negative Index of Refraction
  We explore the possibility of realizing intrinsic far infrared negative index materials (NIM) in multiferroic crystals (crystals simultaneously possessing a ferroelectric and ferromagnetic phase) possessing electric and magnetic dipole resonances with nearby resonance frequencies, or overlapping regions of negative permittivity and permeability. We demonstrate the functionality of such a material using finite difference time domain simulations. In order to motivate the connection between multiferroics and negative index materials, we discuss the negative index of refraction in the polariton picture. 
David W. Ward, Eric Statz, Kevin J. Webb, and Keith A. Nelson