Symmetry change at the Neel point in the high-Tc superconductor HoBa2Cu3O7

Landau's type theory has been applied to the description of the TN=190 mK magnetic phase transition in the high temperature superconductor HoBa2Cu3O7. It has been demonstrated that the magnetic order parameter is one-dimensional and that the magnetic ordering has an Ising character. All possible (symmetry allowable) low-temperature spin structures were subsequently determined. They are antiferromagnetic, with the magnetic moments directed along the basis vectors of the Bravais lattice and they exactly fulfill the non-linear differential equations which follow from the variational problem for the thermodynamic potential density. One of the obtained spin structures is identical to that observed by Roessli et al. in neutron diffraction experiments. The domain structure of the superconductor was subsequently determined on the grounds of symmetry considerations