Magnetic-order crossover in coupled spin ladders

We report a novel crossover behavior in the long-range-ordered phase of a prototypical spin-1/2 Heisenberg antiferromagnetic ladder compound (C₇H₁₀N)₂CuBr₄. The staggered order was previously evidenced from a continuous and symmetric splitting of ¹⁴N NMR spectral lines on lowering the temperature below T_c ≃330mK, with a saturation towards ≃150 mK. Unexpectedly, the split lines begin to further separate away below T∼100mK, while the linewidth and the line shape remain completely invariable. This crossover behavior is further corroborated by the NMR relaxation rate T⁻₁¹ measurements. A very strong suppression reflecting the ordering, T⁻₁¹ ∼T^5.5, observed above T, is replaced by T⁻₁¹ ∼T below T. These original NMR features are indicative of the unconventional nature of the crossover, which may arise from a unique arrangement of the ladders into a spatially anisotropic and frustrated coupling network.