Isotropic and anisotropic spin-spin interactions and a quantum phase transition in a dinuclear Cu(II) compound

Abstract

We report electron-paramagnetic resonance ͑EPR͒ studies at ϳ9.5 GHz ͑X band͒ and ϳ34 GHz ͑Q band͒ of powder and single-crystal samples of the compound Cu 2 ͓TzTs͔ 4 ͓N-thiazol-2-yl-toluenesulfonamidatecopper͑II͔͒, C 40 H 36 Cu 2 N 8 O 8 S 8 , having copper͑II͒ ions in dinuclear units. Our data allow determining an antiferromagnetic interaction J 0 = ͑−113Ϯ 1͒ cm −1 ͑H ex =−J 0 S 1 • S 2 ͒ between Cu͑II͒ ions in the dinuclear unit and the anisotropic contributions to the spin-spin coupling matrix D ͑H ani = S 1 • D • S 2 ͒, a traceless symmetric matrix with principal values D / 4=͑0.198Ϯ 0.003͒ cm −1 and E / 4=͑0.001Ϯ 0.003͒ cm −1 arising from magnetic dipole-dipole and anisotropic exchange couplings within the units. In addition, the singlecrystal EPR measurements allow detecting and estimating very weak exchange couplings between neighbor dinuclear units, with an estimated magnitude ͉JЈ͉ = ͑0.060Ϯ 0.015͒ 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 width͒, which is described with general ͑but otherwise simple͒ 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 cm −1 and 0.060 cm −1 ͒.

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

“…37,38,43 We collected EPR spectra of single crystals between 120 and 298 K at ∼34.3 GHz with a Varian E110 EPR spectrometer equipped with a nitrogen gas-flux T controller, and in powder samples between 4 and 298 K, using a Bruker 500 spectrometer working at ∼33.9 GHz with a helium gas T -controller. Matlab 38 Besides, the rhombic contribution |E| is much smaller than the axial contribution |D|, and may be discarded, and the g-matrices g A and g B are considered equal, with g x and g y differing within the experimental uncertainties.…”

“…37,39 Our method replaces the study of the quantum transition occurring when the lowest Zeeman component of the triplet state crosses the singlet state as a function of the magnitude of the magnetic field, used by researchers studying Bose-Einstein condensation, [14][15][16][17]20,21,40 by one where the two allowed EPR transitions intersect as a function of the orientation of B 0 . This replaces the method based in the Bloch equations used by other authors, 23 38,43 (|J ′ /J 0 | < 10 −3 ) and EPR is best suited for our purpose. |J ′ eff | is evaluated as a function of T with equal results from two independent sets of EPR data: line width as a function of field orientation in single crystals, and intensity of the "U-peak" in powder samples as a function of T .…”

EPR and magnetic studies of a carboxylate-bridged dinuclear copper(II) compound: [cu2(flu)4(dmf)2]

“…The exchange parameter J 0 was obtained from the variation with T of the EPR signal intensity of the dinuclear unit (Figures 6a and 6b). 24 The ratio R between the areas of the peak of the single crystal spectra and the peak corresponding to the marker ( Figure 6) is displayed in Figure 7 as a function of T. For antiferromagnetically coupled units, R(T) follows: 24 (8) and we obtained J 0 = (−283 ± 5) cm −1 from a least-squares fit of equation 8 to the data included as a solid line in Figure 7. Measuring the variation with T of a single crystal EPR signal (instead of a peak of the powder spectra) avoids errors arising from contribution of the mononuclear impurities whose resonance may partially overlap the dinuclear peak.…”

“…data in powder and single crystals as described in recent publications. 6,8,21 When the couplings between Cu II ions in neighbor units are negligible, the EPR spectra of a unit is described by the spin hamiltonian:…”

“…Interesting quantum phase transitions produced by randomly distributed interactions between one unit and the "bath" of other units in the lattice have been recently observed by EPR. 6,8 We report here magnetic measurements and singlecrystal and powder EPR spectra of tetrakis( (Figure 1c).…”