  Hole induced changes in the antiferromagnetic structure of a lightly Ca doped
Gd:Y_{1-x}Ca_xBa_2Cu_3O_6 copper oxide single crystal with x~0.008 is
investigated by Gd3+ electron spin resonance. Holes do not localize to Ca2+
ions above 2.5 K since the charge distribution and spin susceptibility next to
the Ca2+ are independent of temperature. Both hole doped and pristine crystals
are magnetically twinned with an external magnetic field dependent
antiferromagnetic domain structure. Unlike the undoped crystal, where the easy
magnetic axis is along [100] at all temperatures, the easy direction in the
hole doped crystal is along the [110] diagonal at low temperatures and changes
gradually to the [100] direction between 10 K and 100 K. The transition is
tentatively attributed to a magnetic anisotropy introduced by hole ordering.
