TEMPERATURE TRANSITION FROM 3D TO QUASI-1D ANTIFERROMAGNETISM IN CUO SINGLE-CRYSTALS

The susceptibility of CuO1-delta and Cu0.99Li0.01O single and polycrystals has been investigated in the temperature range 56-600 K in magnetic fields of up to 13 kOe. It is shown that defects produce a strong effect on the magnitude and behaviour of chi-(T). Anisotropy of susceptibility in CuO single crystals both below and above T(N) has been revealed. The analysis of susceptibility has shown that in the T < 230 K region CuO is a 3D collinear antiferromagnet with spin S = 1/2 directed along the b axis. A small chi-minimum was observed for the direction H perpendicular-to b in the vicinity of T = 212 K. The anomalous behaviour of susceptibility above T(N) is explained by the quasi-1D magnetic structure comprising Cu-O-Cu antiferromagnetic chains oriented along [101BAR]. The dependence chi-(T) for a polycrystal is well described by the Heisenberg antiferromagnetic chain model with S = 1/2, I/k = 430 K and g = 1.97. It has been found not possible to describe the dependences chi parallel-to (T) and chi perpendicular-to (T) in single crystals by means of Heisenberg and Ising models: this may be due to both g-factor and I/k anisotropies.