Magnetic ordering and phase separation in high-temperature superconductors

The Monte–Carlo method has been used to investigate the magnetic properties of doped copper-oxide high-temperature superconductors and phase separation in La$_2$CuO$_{4+\delta}$. The calculations were based on a model in accordance with which a hole induced by the dopant or excess oxygen changes the antiferromagnetic interaction between neighboring copper spins to a ferromagnetic interaction in a group of Cu–Cu bonds forming a nest of defect bonds. For copper oxides with impurities in which the nests are distributed in accordance with a random law and frozen the concentration of impurities at which an antiferromagnet–spin-glass phase transition occurs has been calculated. Good agreement with the experimental value is found. A mechanism that explains the phenomenon, of phase separation in La$_2$CuO$_{4+\delta}$ is found. It is shown that the phase separation is due to magnetic effects. Mobile nests formed with excess oxygen atoms exhibit a clear tendency to clustering forming regions with high oxygen concentration. The clustering is energetically advantageous, since it leads to a decrease of the frustration energy.