Magnetic Anisotropy Measurements of CoO Single Crystal

Torque and susceptibility measurements with well annealed and chemically polished crystals of CoO were made down to 90°K. When cooled through the Néel point of 20°C with a temperature gradient along one of the cubic principal axes, the crystals yielded torque and susceptibility curves that are attributable to nearly untwinned tetragonal crystals. The torque amplitude for rotation about the axis of the temperature gradient is very small, while those for rotations about the other two axis are large; the torque does not always vanish in the directions of the principal axes, which means that the antiferromagnetic spin axis is parallel to none of them, as theory predicts. The susceptibility measured along the axis of the temperature gradient decreased with decreasing temperature, while those for the other two axes were nearly constant and equal. These susceptibilities, when extrapolated linearly to absolute zero, agreed with the theoretical values for the single-domain crystal predicted by Tachiki.