Antiferromagnetic domains in YBa2Cu3O6+x probed by Gd3+ ESR

The anisotropy of the static homogeneous magnetic spin susceptibility of th e antiferromagnetic CuO2 bilayers and the crystal-field parameters are meas ured in Gd-doped YBa2Cu3O6+X (small x) single crystals using Gd3+ ESR at 9, 75, 150, and 225 GHz. We show that the easy magnetization direction is alo ng [100] and that there is a magnetostriction leading to an orthorhombic la ttice distortion. We observe an antiferromagnetic domain structure correspo nding to the two equivalent orthorhombic distortions of the tetragonal latt ice which depends on magnetic fields of the order of 1 T. The domain struct ure is unchanged between 10 and 150 K and is independent of thermal and mag netic history. We discuss two models: (i) charged domain walls, (ii) magnet ization pinned to a small number of defective oxygen-rich Cu(1) layers. [S0 163-1829(99)01702-6].