SUCCESSIVE ANTIFERROMAGNETIC TRANSITIONS AND MAGNETIC-SUSCEPTIBILITY OF YBA2CU3-XCOXOY

Magnetic susceptibility of YBa2Cu3-xCoxOy (x = 0.06 approximately 0.3 with tetragonal structure) has been studied, being focused on the successive antiferromagnetic transitions, T(N2) and T(N3), where the susceptibility (chi)-versus-temperature (T) curve has kinks. The T(N2), ranging between 18 K and 90 K, was nearly proportional to x, but little dependent on y. The T(N3), near 6 K, was almost independent both on x and y. The transitions were analyzed based on a molecular field approximation, as secondary antiferromagnetic transitions accompanying a magnetic order of Co ions in the chain site. The characteristics of these transitions have been well explained with parameters of J0 = 1400 K, \J1\ congruent-to 160 K, J2 + 8D = (180 +/- 60) K, and J4 = (80 +/- 40) K; where the parameters are exchange interactions between Cu2+ spins (J0, J2), between Cu2+ and Co3+ spins (J1), between Co3+ spins (J4) and a crystalline magnetic anisotropy (D). The good fitting of the model suggests that these magnetic orders are inherent in the YBa2Cu3Oy structure.