We report electron-paramagnetic resonance (EPR) studies at $\sim 9.5\text{GHz}$ ($X$ band) and $\sim 34\text{GHz}$ ($Q$ band) of powder and single-crystal samples of the compound ${\text{Cu}}_2{\left[\text{TzTs}\right]}_4$ [$N$-thiazol-2-yl-toluenesulfonamidatecopper(II)], ${\text{C}}_{40}{\text{H}}_{36}{\text{Cu}}_2{\text{N}}_8{\text{O}}_8{\text{S}}_8$, having copper(II) ions in dinuclear units. Our data allow determining an antiferromagnetic interaction $J_0=\left(-113\pm 1\right){\text{cm}}^{-1}$ $\left({\mathcal{H}}_{\text{ex}}=-J_0{\mathbf{S}}_{\mathbf{1}}\cdot {\mathbf{S}}_{\mathbf{2}}\right)$ between Cu(II) ions in the dinuclear unit and the anisotropic contributions to the spin-spin coupling matrix $\mathcal{D}\left({\mathcal{H}}_{\text{ani}}={\mathbf{S}}_1\cdot \mathcal{D}\cdot {\mathbf{S}}_2\right)$, a traceless symmetric matrix with principal values $\mathcal{D}/4=\left(0.198\pm 0.003\right){\text{cm}}^{-1}$ and $\mathcal{E}/4=\left(0.001\pm 0.003\right){\text{cm}}^{-1}$ arising from magnetic dipole-dipole and anisotropic exchange couplings within the units. In addition, the single-crystal EPR measurements allow detecting and estimating very weak exchange couplings between neighbor dinuclear units, with an estimated magnitude $\left|J^{\prime }\right|=\left(0.060\pm 0.015\right){\text{cm}}^{-1}$. The interactions between a dinuclear unit and the “environment” of similar units in the structure of the compound produce a spin dynamics that averages out the intradinuclear dipolar interactions. This coupling with the environment leads to decoherence, a quantum phase transition that collapses the dipolar interaction when the isotropic exchange coupling with neighbor dinuclear units equals the magnitude of the intradinuclear dipolar coupling. Our EPR experiments provide a new procedure to follow the classical exchange-narrowing process as a shift and collapse of the line structure (not only as a change of the resonance width), which is described with general (but otherwise simple) theories of magnetic resonance. Using complementary procedures, our EPR measurements in powder and single-crystal samples allow measuring simultaneously three types of interactions differing by more than three orders of magnitude (between $113{\text{cm}}^{-1}$ and $0.060{\text{cm}}^{-1}$).

• Received 25 January 2008

DOI:https://doi.org/10.1103/PhysRevB.77.214423