Anisotropic spin freezing in the S=1/2 zigzag chain compound SrCuO₂

We characterize an unusual magnetic phase in orthorhombic SrCuO₂ using neutron scattering. SrCuO₂ contains zigzag spin chains formed by pairs of linear S=1/2 chains (J=180 meV) coupled through a frustrated interaction (|J’| ≲ 0.1J). For T < T_c1=5.0(4) K, an elastic peak develops in a gapless spectrum indicating spin freezing on a time scale δ t > 210^-10 s. While the frozen state has long range order along chains (ξ_c > 200_c) and a substantial correlation length, ξ_a = 60(25)a perpendicular to zigzag planes, the correlation length is only ξ_b = 2.2(3)b in the direction of frustrated interactions. We argue that slow dynamics of stripelike cooperative magnetic defects yield this anisotropic frozen state.