  We report magnetotransport measurements on high purity sintered samples of
spintronic CrO2 in an unexplored crystallographic regime between 5-300 K. The
negative magnetoresistance (MR) as derived from RH isotherms is observed to be
unhysteretic up to temperatures as high as 200 K. Between 240-290 K, RH
isotherms exhibit some unusual features including a positive MR and strong
pinning effects. These feature disappear above 290 K and is apparently related
with the antiferromagnetic ordering of the insulating grain boundary.
Qualitatively similar features with significantly enhanced MR are also observed
when the GB density is increased. These results bring out the role played by
the magnetic and crystallographic microstructure on the magnitude, sign and
hysteresis of the magnetoresistance in this technologically important material.
