  The magnetic, thermal and transport properties of Na0.75CoO2 single crystals
grown by the floating zone (FZ) method are reported. Magnetic susceptibility,
resistivity, magnetoresistance, and heat capacity data from these crystals
indicate a bulk phase transition at T1 = 22 K. These data are most consistent
with the formation of an antiferromagnet spin-density-wave (SDW) at 22 K with
the easy axis for magnetization nearly along the c axis. Weak and soft
ferromagnetism is observed for applied magnetic fields less than 0.5 T, which
suggests a slight canting of the SDW magnetization with respect to the c axis.
The jump in the heat capacity at the SDW transition is 0.45 J/K-mole-Co or
about 50% of the value expected from mean-field weak-coupling theory. The
reduced jump and the decrease in the resistivity below T1 are consistent with a
gap for only part of the Fermi surface. The magnetoresistance is small at the
SDW transition but increases in both directions reaching a value of 100% at 2 K
for applied fields of 8 Tesla. The magnetoresistance data imply that the
mobility of the remaining carriers is large and increases below T1. The
observation of a SDW transition in this material is found to be sensitive to
the preparation conditions and the degree of order in the Na layers. No SDW
transition is observed in our polycrystalline powder with the same nominal
composition (Na0.75CoO2) and lattice constants. Differential scanning
calorimetry data, however, show distinct differences between the powder and
crystal, suggesting a higher degree of order in the Na layers within the
crystal. The crystal exhibits a sharp phase transition at T2 = 340 K while for
the powder this transition is smeared over the temperature range from 250 to
310 K.
