Quantum tunneling of the magnetization in [MnIII6M]3+ (MÂ =Â CrIII, MnIII) SMMs: Impact of molecular and crystal symmetry

Single-molecule magnets (SMMs) are compounds that exhibit a hysteresis in the magnetization of pure molecular origin and that stay magnetized for a certain time without applied magnetic field. This behavior is associated with an energy barrier for magnetization reversal resulting in a slow relaxation of the magnetization at low temperature. The energy barrier can be overcome by a thermal pathway over the top of the barrier and by a quantum tunneling through the barrier. In order to slow down the magnetization reversal, the probability for both pathways must be minimized. We evaluate the influence of the molecular and crystal symmetry on the quantum tunneling for a family of heptanuclear SMMs [MnIII6MIII]3+ (MIII = CrIII, MnIII) synthesized with the triplesalen ligand (talent-Bu2)6− using different salts and solvates. [MnIII6CrIII]3+ SMMs have only a moderate height of the energy barrier of 15-19 cm−1. Single-crystal magnetization measurements of a [MnIII6CrIII]3+ SMM of low molecular and crystal symmetry exhibit strong zero-field tunneling due to transversal components by dipolar stray fields and rhombic ESt terms. Crystallization of [MnIII6CrIII]3+ in a high-symmetry trigonal space group using the rod-shaped anion lactate results in a high molecular and crystal symmetry. This almost completely suppresses zero-field tunneling due to vanishing transversal components. [MnIII6MnIII]3+ SMMs have a diamagnetic MnIII l.s. ion in the center due to efficient spin-orbit coupling. The high molecular and crystal symmetry of the [MnIII6MnIII]3+ lactate salt results in a blocking temperature TB ≈ 2 K despite of only a St,1 = 4 spin ground state with an energy barrier of 0.18 cm−1. This demonstrates the strong impact of a high molecular and crystal symmetry on the suppression of QTM. At fields above 2 T, a St,2 = 12 state becomes ground state, which even exhibits an open hysteresis up to ±10 T. We propose five requirements, which must be simultaneously fulfilled, to make 'better' SMMs with higher blocking temperatures: (1) a high magnetic ground state (either St, Lt, or Jt), (2) a strong magnetic anisotropy, (3) a control of the molecular topology with symmetry of at least C3 but less than cubic and collinearly aligned local anisotropy tensors, (4) a control of the crystal structure with collinear aligned molecular easy axes and symmetric environments, and (5) isolated ground state multipletts, i.e. strong exchange couplings in polynuclear SMMs.