  Birefringence is an indicator of structural anisotropy of materials. We
measured the birefringence of Pb(II)-doped silica hydrogels prepared under a
high magnetic field of various strengths. Because the silica is diamagnetic,
one does not expect the structural anisotropy induced by a magnetic field. In
previous work [Mori A, Kaito T, Furukawa H 2008 Mater. Lett. 62 3459-3461], we
prepared samples in cylindrical cells made of borosilicate glass and obtained a
preliminary result indicating a negative birefringence for samples prepared at
5T with the direction of the magnetic field being the optic axis. We have
measured the birefringence of Pn(II)-doped silica hydrogels prepared in square
cross-sectional cells made of quartz and reverted the previous conclusion.
Interestingly, the magnetic-influenced silica hydrogels measured have been
classified into four classes: two positive birefringent ones, no birefringent
one, and negative birefringent one. Proportionality between birefringence and
the strength of magnetic field is seen for the former two.
