Mondal Sourav, Netz Julia, Hunger David, Suhr Simon, Sarkar Biprajit, van Slageren Joris, Köhn Andreas, Lunghi Alessandro
School of Physics, AMBER and CRANN Institute, Trinity College, Dublin 2, Ireland.
Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
ACS Cent Sci. 2025 Mar 13;11(4):550-559. doi: 10.1021/acscentsci.4c02139. eCollection 2025 Apr 23.
Magnetic relaxation in coordination compounds is largely dominated by the interaction of the spin with phonons. Although a comprehensive understanding of spin-phonon relaxation has been achieved for mononuclear complexes, only a qualitative picture is available for polynuclear compounds. Large zero-field splitting and exchange coupling values have been empirically found to strongly suppress spin relaxation and have been used as the main guideline for designing molecular compounds with long spin lifetime, also known as single-molecule magnets, but no microscopic rationale for these observations is available. Here we fill this critical knowledge gap by providing a full first-principles description of spin-phonon relaxation in an air-stable Co(II) dimer with both large single-ion anisotropy and exchange coupling. Simulations reproduce the experimental relaxation data with excellent accuracy and provide a microscopic understanding of Orbach and Raman relaxation pathways and their dependency on exchange coupling, zero-field splitting, and molecular vibrations. Theory and numerical simulations show that increasing cluster nuclearity to just four cobalt units would lead to a complete suppression of low-temperature Raman relaxation. These results hold a general validity for polynuclear single-molecule magnets, providing a deeper understanding of their relaxation and revised strategies for their improvement.
配位化合物中的磁弛豫在很大程度上由自旋与声子的相互作用主导。尽管对于单核配合物已经实现了对自旋 - 声子弛豫的全面理解，但对于多核化合物仅存在定性描述。通过经验发现，大的零场分裂和交换耦合值会强烈抑制自旋弛豫，并已被用作设计具有长自旋寿命的分子化合物（也称为单分子磁体）的主要指导原则，但对于这些观察结果尚无微观理论依据。在此，我们通过对具有大的单离子各向异性和交换耦合的空气稳定的Co(II)二聚体中的自旋 - 声子弛豫进行完整的第一性原理描述，填补了这一关键的知识空白。模拟以极高的精度重现了实验弛豫数据，并对奥巴赫和拉曼弛豫途径及其对交换耦合、零场分裂和分子振动的依赖性提供了微观理解。理论和数值模拟表明，将簇核数增加到仅四个钴单元将导致低温拉曼弛豫的完全抑制。这些结果对多核单分子磁体具有普遍有效性，为深入理解其弛豫特性和改进策略提供了依据。
