Temperature dependence of the effective interdimer exchange interaction in a weakly coupled antiferromagnetic dimer copper compound

Abstract

We report a variation with temperature T of the effective interdimeric interaction J´eff in the antiferromagnetic (AFM) copper dimeric organic compound Cu2[TzTs]4 (N-thiazol-2-yl-toluenesulfonamidate CuII). This T dependence was obtained from measurements of the effects in the electron paramagnetic resonance (EPR) spectra of the proposed quantum phase transition associated with the exchange-narrowing processes. Cu2[TzTs]4 contains exchange-coupled pairs of CuII spins SA and SB (S = 1/2), with intradimeric AFM exchange coupling J0 = (−115±1) cm−1 (Hex =−J0SA · SB). The variation of the EPR linewidth of single crystals with field orientation around a ?magic angle? where the transitions intersect and the integrated signal intensity of the so-called Upeak of the powder spectrum weremeasured as a function of T. Modeling these data using arguments of exchange narrowing in the adiabatic regime considering the angular variation of the single-crystal spectra and a geometric description, we find that the effective interdimeric coupling |J´eff| associated with the exchange frequency ωex is negligible for T<<|J0/kB| when the units are uncoupled and |J´eff|= (0.080±0.005) cm−1 (|J´eff/J0|= 7.0×10−4) at 298 K.Within this T interval, two ranges of |J´eff| with linear temperature variation but different slopes, with a kink at ∼80 K, are observed and discussed. This T dependence arises from the growing population of the triplet state, and its relevance to the properties of various arrays of dimeric units is discussed. Our experimental procedures and results are compared with those of previous works in ion radical salts and dimeric metal compounds. The relation between the effective coupling |J´eff | and the real interdimeric exchange coupling |J´| related to the chemical paths connecting neighbor units is discussed.