Magnetic birefringence of natural and synthetic ferritin

► Magnetic birefringence of ferritins were studied and compared with nanoscale magnetite. ► Magnetoferritin shows non-Langevin behaviour of magnetic birefringence in contrast to magnetite. ► Cotton-Mouton constant of synthetic ferritin is four orders higher than that of the natural one. ► Magnetic birefringence can be useful for identification of magnetic core of biogenic ferritins. ► The described magnetooptical method can be of interest in biomedicine.