Abstract
To investigate the magnetic properties and the spin entanglement of dinuclear arrays, we prepared compounds [{Cu(pAB)(phen)H2O}2·NO3·pABH·2H2O], 1, and [Cu2(pAB)2(phen)2pz]n, 2 (pABH = p-aminobenzoic acid, phen = 1,10-phenanthroline and pz = pyrazine). The structure of 1 is known and we report here that of 2. They contain similar dinuclear units of CuII ions with 1/2-spins S1 and S2 bridged by pairs of pAB molecules, with similar intradinuclear exchange and fine interactions , but different 3D crystal arrays with weak interdinuclear exchange J', stronger in 2 than in 1. To investigate the magnetic properties and the spin entanglement produced by J', we collected the powder spectra of 1 and 2 at 9.4 GHz and T between 5 and 298 K, and at 34.4 GHz and T = 298 K and single-crystal spectra at room T and 34.4 GHz as a function of magnetic field (B0) orientation in three crystal planes, calculating intradinuclear magnetic parameters J(1)0 = (-75 ± 1) cm-1, J(2)0 = (-78 ± 2) cm-1, |D(1)| = (0.142 ± 0.006) cm-1, |D(2)| = (0.141 ± 0.006) cm-1 and E(1) ∼ E(2) ∼ 0. Single crystal data indicate a quantum entangled phase in 2 around the crossing between two fine structure EPR absorption peaks within the spin triplet. This phase also shows up in powder samples of 2 as a U-peak collecting the signals of the entangled microcrystals, a feature that allows estimating |J'|. Transitions between the two quantum phases are observed in single crystals of 2 changing the orientation of B0. We estimate interdinuclear exchange couplings |J'(1)| < 0.003 cm-1 and |J'(2)| = (0.013 ± 0.005) cm-1, in 1 and 2, respectively. Our analysis indicates that the standard approximation of a spin Hamiltonian with S = 1 for the dinuclear spectra is valid only when the interdinuclear coupling is large enough, as for compound 2 (|J'(2)/J(2)0| ∼ 1.7 × 10-4). When J' is negligible as in 1, the real spin Hamiltonian with two spins 1/2 has to be used. Broken-symmetry DFT predicts correctly the nature and magnitude of the antiferromagnetic exchange coupling in 1 and 2 and ferromagnetic interdinuclear coupling for compound 2.
Highlights
Great advances in dinuclear units (DUs) have been achieved since the pioneering electron paramagnetic resonance (EPR) study of Bleaney and Bowers[1] on dinuclear copper acetate hydrate
We study here antiferromagnetic dinuclear compounds, where two atoms with unpaired spin 1/2 are coupled by isotropic and anisotropic spin–spin interactions, plus Zeeman coupling with by integrating dχ′′/dB0 (B0) (eqn (2))
This system should be compared with that considered by Tachiki and Yamada in 1970.70,71 They studied the effect of the magnetic response of a “spin pair” or dinuclear compound of the crossing of one level of the triplet state with the singlet state, produced as a function of the magnitude of the applied magnetic field
Summary
Great advances in dinuclear units (DUs) have been achieved since the pioneering electron paramagnetic resonance (EPR) study of Bleaney and Bowers[1] on dinuclear copper acetate hydrate. S1 = S2 = 1/2 for which eqn (1) predicts a ground singlet state and an excited triplet state with a gap ΔE = |J0| are the simplest non-trivial quantum and magnetic spin systems. They gave rise to important research lines in molecular magnetism,[2,4,5,6,7] inorganic, organic and biological chemistry The periodic structures give rise to travelling spin excitation (triplons)[25,26,27] transforming the finite quantum dinuclear units into infinite collective systems with quantum many-body effects modifying the magnetic properties and showing the fascinating properties of molecular magnets, including quantum phase transitions,[28,29] Bose–Einstein condensation,[27,29] quantum spin ladders[30] and more.[31,32]
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