Neutron-scattering study of the two-dimensional frustrated antiferromagnet Rb₂Cu_0.12Co_0.88F₄

Quasielastic neutron scattering is performed on a single crystal of the two-dimensional antiferromagnet Rb2Cu0.12Co0.88F4, whose magnetic structure is frustrated by the admixture of ferromagnetic bonds. From the critical scattering, the in-plane inverse correlation length is found to decrease linearly with the temperature, with a critical exponent nu = 1.02 +/- 0.04. At the onset of the sublattice magnetization, which occurs at T(N) = 88.1 +/- 0.1 K, however, the in-plane correlations have not yet become long ranged. At T(N), the planar domain size has grown to only 14 lattice spacings. Below T(N), the planar domains size increases further, to attain length scales beyond resolution at about 75 K. Conversely, the domains break up into smaller ones in this temperature range upon warming up. For the critical exponent of the susceptibility we find gamma = 1.5 +/- 0.2 after correction for the finite correlation remaining at T(N). The sublattice magnetization obeys the critical exponent beta = 0.16 +/- 0.04. The critical exponents found compare with the Ising values nu = 1, gamma = 7/4, and beta = 1/8. Another manifestation of the frustration is the failure to reach true equilibrium below T(N). This is reflected in a dependence of the c-axis correlations on the cooling rate, and in a small increment of the intralayer and in-layer correlation lengths over long times.