Magnetic susceptibility of YBa2Cu3O6+x: Effects of spin frustration and correlation.

Abstract

YBa/sub 2/Cu/sub 3/O/sub 6+//sub x/ is known to undergo a transition from a magnetic semiconductor to a metallic high-temperature superconductor as the oxygen content is increased from the range 0.0less than or equal toxless than or equal to0.5 to 0.5less than or equal toxless than or equal to1.0. We report here detailed temperature-dependent magnetic-susceptibility studies for x in the composition range 0.0less than or equal toxless than or equal to1.0. For 0.05less than or equal toxless than or equal to0.5, the effective moment ..mu../sub e//sub f//sub f/ decreases with decreasing temperature from 300--160 K, as expected for an antiferromagnet whose Neel temperature is above room temperature. Below 160 K, ..mu../sub e//sub f//sub f/ increases, reaching a maximum at /similar to/40 K then decreases again. The magnitude of this low-temperature peak in ..mu../sub e//sub f//sub f/ increases with x, reaches a maximum at x = 0.35, then decreases toward zero. A Monte Carlo simulation method has been used to model the three-dimensional antiferromagnetic ordering of this system as a function of oxygen composition. The calculation reveals the presence of spin frustration as x increases from 0 to 0.3 in accord with the increasing number of effective moments at low temperatures. Abovemore » x = 0.3 a new long-range order (corresponding to a doubling of the magnetic unit cell perpendicular to the planes) is predicted to occur in agreement with the observed magnetic susceptibility and recent neutron-diffraction experiments of Kadowaki et al. and Lynn et al.« less