Suppression of Magnetic Quantum Tunneling in a Chiral Single Molecule Magnet by Ferromagnetic Interactions

摘要

Single-molecule magnets (SMMs) retain a magnetization without applied magnetic field for a decent time due to an energy barrier U for spin-reversal. Despite the success to increase U, the difficult to control magnetic quantum tunneling often leads to a decreased effective barrier U-eff and a fast relaxation. Here, we demonstrate the influence of the exchange coupling on the tunneling probability in two heptanuclear SMMs hosting the same spin-system with the same high spin ground state S-t = 21/2. A chirality-induced symmetry reduction leads to a switch of the Mn-III-Mn-III exchange from antiferromagnetic in the achiral SMM [(Mn6CrIII)-Cr-III](3+) to ferromagnetic in the new chiral SMM (RR)[(Mn6CrIII)-Cr-III](3+). Multispin Hamiltonian analysis by full-matrix i diagonalization demonstrates that the ferromagnetic interactions in (RR)[(Mn6CrIII)-Cr-III](3+) enforce a well-defined S, = 21/2 ground state with substantially less mixing of Ms substates in contrast to [(Mn6CrIII)-Cr-III](3+)and no tunneling pathways below the top of the energy barrier. This is experimentally verified as U-eff is smaller than the calculated energy barrier U in [114.n1116Cr1193 due to tunneling pathways, whereas U-eff equals U in (RR)[(Mn6CrIII)-Cr-III](3+) demonstrating the absence of quantum tunneling.