On the birefringence of magnetoelectric BiFeO3

The birefringence dispersion of a ferroelastic single domain of magnetoelectric BiFeO3 crystal has been measured at room temperature (24 degrees C) in the rhombohedral phase. A thin (11 mu m) (110)(cubic) cut was used although it was not a principal cut. The optical axis (and spontaneous polarization) formed an angle of 35 degrees with the normal to the cut. Due to anomalous interference colors, the right order was deduced with the help of conoscopy. The birefringence of this cut shows normal dispersion and its value is very large (dn=0.13 at lambda=550 nm to 0.08 at lambda=850 nm). The principal birefringence (dn=0.34 at lambda=550 nm) has been evaluated with the help of the computed (Gladstone-Dale relationship) mean refractive index ((n) over bar=2.62). From dn'(T), (8K < T < 820K) the Neel temperature has been found for the first time optically, T-N=653K (380 degrees C), and a critical exponent has tentatively been derived near T-N.