¹H NMR study of dimensionality crossover in a two-dimensional antiferromagnet

We report ¹H NMR study in the two-dimensional (2D) Heisenberg antiferromagnet, C₁₆H₂₆Mn₂O₉ (MnDC-6). Over a wide temperature range from 300 to ∼ 8 K, we find that the linewidth of the ¹H NMR spectrum follows the temperature dependence of the uniform magnetic susceptibility χ, and the spin-lattice relaxation rate T1-1 is proportional to χT. These NMR data show that the static and dynamic magnetic properties of the system are governed by 2D antiferromagnetic (AFM) Heisenberg interactions. Remarkably, T1-1 is critically enhanced in a very narrow temperature range below ∼ 6 K undergoing a three-dimensional (3D) AFM transition at T_N= 4.7 K. Based on the fact that the interlayer coupling is extremely small, and the temperature dependence of the critical enhancement of T1-1 is incompatible with the 3D Heisenberg model, we argue that the rapid growth of the transverse correlation length ξ_⊥ due to a small planar anisotropy leads to the spin-dimensionality crossover from 2D Heisenberg to 2D XY-like at low temperatures close to T_N. Once ξ_⊥ becomes sufficiently long, even the tiny interlayer exchange coupling could trigger the 3D AFM ordering.