We measured the magnetoinfrared response of the quasi-one-dimensional quantum Heisenberg antiferromagnet $\text{Cu}\left(\text{pyz}\right){\left({\text{NO}}_3\right)}_2$ to investigate local lattice distortions through the field-driven transition to the fully polarized state. This magnetic quantum critical transition involves changes in the out-of-plane N and C-H bending modes of pyrazine with field that directly track the magnetization. We discuss our findings in terms of calculated spin densities, scaling laws, and extracted spin-phonon coupling constants, the latter of which are large due to the softness of the superexchange ligand.

• Received 22 January 2010

DOI:https://doi.org/10.1103/PhysRevB.81.104307