Optimization of Single‐Molecule Magnets by Suppression of Quantum Tunneling of the Magnetization

Abstract

The ligand system triplesalen was rationally designed following requirements for polynuclear 3d single‐molecule magnets (SMMs). The essential central part is the C3 symmetric, meta‐phenylene bridging unit phloroglucinol for ferromagnetic interactions via the spin‐polarization mechanism. The triplesalen‐based [MnIII6CrIII]3+ SMMs strongly suppress the quantum tunneling of the magnetization (QTM) but exhibit blocking temperatures not exceeding 2 K. We have analyzed the reason for this behavior and found that the triplesalen ligands are not in the anticipated aromatic phloroglucinol form but in a non‐aromatic heteroradialene form. Here we present our strategies to optimize the triplesalen ligand system to suppress the heteroradialene formation and to enforce ferromagnetic interactions. This allowed us to study in detail the influence of exchange coupling on the QTM and relaxation properties of SMMs and provides valuable insights for further rational improvements of our triplesalen ligand system and of SMMs in general.

Optical Phenomena in Molecule-Based Magnetic Materials

“…The energy positions of the m J levels within the ground multiplet can be extracted spectroscopically which provides the energy gaps that can be further compared with the best-fit parameters obtained in the analysis of slow magnetic relaxation processes, e.g., an energy barrier related to the Orbach relaxation. 744,745,830,811 with the support of theoretical calculations, 61,827,838,839 lanthanide molecular nanomagnets, due to their relative simplicity, are excellent objects for such magneto-optical correlations. 237,840 Below, we describe a few examples presenting the general idea of the application of highresolution optical spectra, mainly emission ones, in the elucidation of magnetic phenomena of lanthanide-based SMMs.…”

“…The energy positions of the m J levels within the ground multiplet can be extracted spectroscopically which provides the energy gaps that can be further compared with the best-fit parameters obtained in the analysis of slow magnetic relaxation processes, e.g., an energy barrier related to the Orbach relaxation. ,,, In this context, far-infrared (FIR) absorption spectra as well as emission characteristics were employed to gain information on the lowest-lying energy levels of the Ln SMMs (refs , , , , , , , , − , , , , , , , and − ). When compared with SMMs constructed of high-nuclearity clusters based on various metal ions whose mechanisms of slow magnetic relaxation processes are often difficult to be explained using optical methods even with the support of theoretical calculations, ,,, lanthanide molecular nanomagnets, due to their relative simplicity, are excellent objects for such magneto-optical correlations. , Below, we describe a few examples presenting the general idea of the application of high-resolution optical spectra, mainly emission ones, in the elucidation of magnetic phenomena of lanthanide-based SMMs. ,,,,,− …”

“…For future applications of SMMs in high-density information storage and spintronics, it is essential to understand the mechanisms staying behind their magnetic hysteresis effect which is related to the energies and features of sublevels of the ground electronic multiplets. 59,72,746,827,828 In this context, direct-current (dc) magnetic measurements, including temperature dependence of magnetic susceptibility and field dependences of magnetization, can be used to get insight into the energy splitting of the ground multiplet; however, it becomes a difficult task when the molecular symmetry of a metal complex is low. 794,829 The theoretical approaches, such as ab initio calculations, are the solution as they can provide the energy splitting of the ground multiplet but the experimental support is crucial to validate the performed calculations.…”

“…For future applications of SMMs in high-density information storage and spintronics, it is essential to understand the mechanisms staying behind their magnetic hysteresis effect which is related to the energies and features of sublevels of the ground electronic multiplets. ,,,, In this context, direct-current ( dc ) magnetic measurements, including temperature dependence of magnetic susceptibility and field dependences of magnetization, can be used to get insight into the energy splitting of the ground multiplet; however, it becomes a difficult task when the molecular symmetry of a metal complex is low. , The theoretical approaches, such as ab initio calculations, are the solution as they can provide the energy splitting of the ground multiplet but the experimental support is crucial to validate the performed calculations. ,,, Moreover, some important features could not be fully taken into account, such as deviation from the ideal geometry which is often not easily reflected in the structural model, e.g., due to the significant structural disorder, non-negligible magnetic interactions that cannot be treated easily using the ab initio approach; in addition, the effect of the magnetic field on the energy scheme of the ground electronic multiplet should be examined experimentally to fully understand the scope of opto-magnetic interactions in an investigated SMM system (see section . ). ,, …”