Anomalous behaviour of the weak exchange coupling between water bibridged dimers in crystals. Temperature and pressure EPR studies

Abstract

Angular, temperature (4.2-300 K) and pressure (0-400 MPa) variations of the EPR spectra of single crystals show that magnetic dimers of bibridged by water molecules exist with a triplet state produced by dipolar coupling between ions. This is consistent with preliminary structural x-ray diffraction data. Using a decoupling computer procedure the superexchange coupling parameter between dimers was determined as at room temperature. The parameter depends strongly on temperature and pressure with an anomalous minimum in J(T) at about 200 K. Such behaviour is discussed in terms of a phenomenological model of competing contributions to the effective superexchange coupling based on existing superexchange theories. We have found that the interdimer exchange coupling is antiferromagnetic, and leads to a magnetic ordering at very low temperatures as observed by the huge EPR line broadening.

Molecular One‐ and Two‐Qubit Systems with Very Long Coherence Times

“…[ 74,76 ] Finally, at high temperatures the T 2 temperature dependence is found again. [ 74 ] Using the real phonon spectrum rather than a fictitious Debye phonon distribution has been reported to yield somewhat but not dramatically better agreement between experiment and simulation. [ 77 ] From the foregoing, the local mode‐assisted relaxation scenario in the intermediate temperature regime would appear most reasonable.…”

“…A large number of models exist that aim to take into account the contribution of local modes to the spin–lattice relaxation. [ 74 ] Such local modes can be molecular vibrations, but also phonons that are localized on a defect in the crystal lattice and do not propagate through the lattice. Local mode models typically predict an exponential temperature dependence of the spin–lattice relaxation rate at lowest temperature.…”

“…Local mode models typically predict an exponential temperature dependence of the spin–lattice relaxation rate at lowest temperature. [ 74,75 ] At intermediate temperatures, for resonance mode frequencies much higher than the Debye frequency, a T 3 temperature dependence has been proposed. [ 74,76 ] Finally, at high temperatures the T 2 temperature dependence is found again.…”

“…[ 74,75 ] At intermediate temperatures, for resonance mode frequencies much higher than the Debye frequency, a T 3 temperature dependence has been proposed. [ 74,76 ] Finally, at high temperatures the T 2 temperature dependence is found again. [ 74 ] Using the real phonon spectrum rather than a fictitious Debye phonon distribution has been reported to yield somewhat but not dramatically better agreement between experiment and simulation.…”