  Using magnetic neutron scattering we characterize an unusual low temperature
phase in orthorhombic SrCuO2. The material contains zigzag spin ladders formed
by pairs of S=1/2 chains (J=180 meV) coupled through a weak frustrated
interaction |J'|<0.1J. At T<Tc1=5.0(4)K an elastic peak develops in a gapless
magnetic excitation spectrum indicating spin freezing on a time scale larger
than 200 picoseconds. While the frozen state has long range commensurate
antiferromagnetic order along the chains with the correlation length exceeding
200 lattice periods along the c-axis and a substantial correlation length of
60(25) spacings along the a-axis perpendicular to the zigzag plane, only 2
lattice units are correlated along the b-axis which is the direction of the
frustrated interactions. The frozen magnetic moment of each Cu ion is very
small, 0.033(7) Bohr magneton even at T=0.35K, and has unusual temperature
dependence with a cusp at Tc2=1.5K reminiscent of a phase transition. We argue
that slow dynamics of stripe-like cooperative magnetic defects in tetragonal
a-c planes yield this anisotropic frozen state.
