Magnetic ordering induced by interladder coupling in the spin- 12 Heisenberg two-leg ladder antiferromagnet C9 H18 N2 CuBr4

We present specific-heat and neutron-scattering results for the S=1/2 quantum antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)CuBr4. The material orders magnetically at TN=1.99(2) K, and magnetic excitations are accompanied by an energy gap of 0.30(2) meV due to spin anisotropy. The system is best described as coupled two-leg spin-1/2 ladders with the leg exchange Jleg=0.60(2) meV, rung exchange Jrung=0.64(9) meV, interladder exchange Jint=0.19(2) meV, and an interaction-anisotropy parameter Î=0.93(2), according to inelastic neutron-scattering measurements. In contrast to most spin ladders reported to date, the material is a rare example in which the interladder coupling is very near the critical value required to drive the system to a Néel-ordered phase without the assistance of a magnetic field.