Magnetic ordering induced by interladder coupling in the spin-12Heisenberg two-leg ladder antiferromagnetC9H18N2CuBr4

Abstract

We present specific-heat and neutron-scattering results for the $S=1/2$ quantum antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)${\mathrm{CuBr}}_{4}$. The material orders magnetically at ${\mathit{T}}_{N}=1.99(2)$ 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 ${J}_{\mathrm{leg}}=0.60(2)$ meV, rung exchange ${J}_{\mathrm{rung}}=0.64(9)$ meV, interladder exchange ${J}_{\mathrm{int}}=0.19(2)$ meV, and an interaction-anisotropy parameter $\ensuremath{\lambda}=0.93(2)$, 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 N\'eel-ordered phase without the assistance of a magnetic field.