  The anisotropic triangular lattice of the crednerite system Cu(Mn1-xCux)O2 is
used as a basic model for studying the influence of spin disorder on the ground
state properties of a two-dimensional frustrated antiferromagnet. Neutron
diffraction measurements show that the undoped phase (x=0) undergoes a
transition to antiferromagnetic long-range order that is stabilized by a
frustration-relieving structural distortion. Small deviation from the
stoichiometric composition alters the magnetoelastic characteristics and
reduces the effective dimensionality of the magnetic lattice. Upon increasing
the doping level, the interlayer coupling changes from antiferromagnetic to
ferromagnetic. As the structural distortion is suppressed, the long-range
magnetic order is gradually transformed into a two-dimensional order.
