With the use of the Hubbard model bound hole states in YBa_2Cu_3O_(6+x) are studied. For the parameters of this crystal the exchange interaction between the spin-carrying chain ion O^- and Cu-O plane sites is shown to ensure the formation of a large ferromagnetically ordered clusters around holes in the plane.
@article{arxiv.cond-mat/9809129,
title = {Ferron-like states in YBa_2Cu_3O_(6+x)},
author = {P. Rubin and A. Sherman},
journal= {arXiv preprint arXiv:cond-mat/9809129},
year = {2009}
}