Magneto-electric optics in non-centrosymmetric ferromagnets

Non-centrosymmetric ferromagnetic materials can show optical and x-ray responses unique to magneto-electrics. For example, the refractive index and absorption coefficient may be dependent on the directions of light (x-ray) propagation k and magnetization M , and these dependences can be termed directional birefringence and directional dichroism, respectively. Such a kind of magneto-electric (ME) optics has recently been observed in GaFeO 3 and CuB 2 O 4 . The ME optics can be explained from the microscopic point of view by the interference of an electric dipole (E1) process with a magnetic dipole (M1) or electric quadrupole (E2) process at non-centrosymmetric magnetic sites. Even in magnetic crystals with global inversion symmetry, the E1–E2 interference can occur at magnetic sites with local inversion breaking, causing unique x-ray diffraction. Such ME-effect-related x-ray diffraction has been found in GaFeO 3 and Fe 3 O 4 . Artificial breaking of space inversion by means of currently used techniques such as lithography can be applied to the exploration of multiferroics, as a way of investigating ME optics. Relatively large ME optical signals at room temperature have been successfully observed in diffraction from such tailor-made multiferroics.