  We have measured the magnetoresistance of stretched sheets of carbon
nanotubes in temperatures ranging from 2 K to 300 K and in magnetic fields up
to 9 T, oriented either perpendicular or parallel to the plane of the sheets.
The samples have been partially aligned by post-fabrication stretching, such
that the direction of stretching was either parallel or perpendicular to the
direction of applied electric current. We have observed large differences
between the magnetoresistance measured under the two field orientations, most
pronounced at the lowest temperatures, highest fields, and for the
laterally-aligned sample. Treatment of the sheets with nitric acid affects this
anisotropy. We analyzed the results within the theoretical framework of weak
and strong localization and concluded that the anisotropy bears the mark of a
more unusual phenomenon, possibly magnetically-induced mechanical strain.
