SPIN-STATE AND VARIATION OF THE SPIN ORIENTATION OF CO(III) IN THE 112-TYPE PHASE YBA(CO2-XCUX)O-5

The copper and cobalt valence states in the 112-phase YBa (CO2-xCUx)O-5 have been determined from a XANES analysis: copper is mainly in the Cu(II) state whereas cobalt appears in a mixed valent state Co(II)/Co(III) in agreement with the oxygen stoichiometry. The magnetic behavior of cobalt and copper in the structure of these compounds has been determined by magnetic susceptibility measurements and by powder neutron diffraction. The evolution of inverse molar susceptibility versus temperature shows that these phases are antiferromagnetic with T-N ranging from 395 to 515 K. At low temperature (T < T-N), one observes a second magnetic transition for 0.75 < x < 1.0 compositions. The temperature T2 corresponding to this transition increases when x decreases. The nuclear and magnetic structures of three compositions, x = 0.4, 0.8, and 0.9, have been investigated by neutron diffraction at 1.4 K. At this temperature, the nuclear structure of these phases has the space group P4/mmm and lattice parameters a(0) = a(p) and c(0) = 2a(p). The magnetic cell is related to the nuclear cell by the relationships: a = a(0) root 2, c = co for x = 0.4 and a a(0) root 2, c = 2c(0) for x = 0.8 and 0.9. For the x = 0.4 compound, the magnetic moments lie in the ab plane; on the other hand, the spins exhibit another component along c for x = 0.8 and 0.9 phases. The study of the magnetic structure of the x = 0.8 oxide versus temperature shows that their spins switch along the c axis as the temperature approaches T2. The values of the magnetic moment determined in this study for these phases agree with the t(2g)(5)e(g)(1) (C) 1994 Academic Press, Inc.