Investigations of spin-excitations and intramolecular-exchange interaction of mixed 3d-4f molecular moments using scanning tunneling microscopy and X-ray magnetic circular dichroism
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Abstract
Molecular spintronics, a cutting-edge field at the intersection of molecular systems and spin physics, seeks to harness spin degrees of freedom for innovative technological applications. Within this context, diverse molecular systems have attracted considerable interest due to their multifunctionality. This thesis focuses on the magnetic properties of 3d − 4f mixed-metallic-centered molecules, investigated through scanning tunneling spectroscopy, particularly inelastic electron tunneling spectroscopy (IETS), and x-ray magnetic circular dichroism (XMCD).
In the first part, IETS measurements were performed on the 3d-4f molecule TbNi2. These experiments identified inelastic excitations on the in-plane Fe monolayer and additional excitations in out-of-plane magnetic regions. These variations in excitations suggest possible spin excitation phenomena, offering a foundation for future studies using advanced theoretical approaches to uncover the underlying mechanisms.
The second part explores polynuclear 3d − 4f 15–MC–5 and 12–MC–4 metallacrowns through XMCD experiments, providing element-specific insights into spin and orbital magnetic moments. The first XMCD measurement on RENi5[15–MC–5] molecules revealed that antiferromagnetically coupled Ni(II) ions contributed minimally to the overall molecular moment. Rare-earth ions, such as Tb(III) and Dy(III), exhibited spin and orbital moments lower than the expected ionic Hund’s rule values, attributed to finite magnetic anisotropy. To explain these discrepancies, a phenomenological model was proposed, enabling the determination of magnetic anisotropy energy barriers at the atomic level. This approach provides a nuanced alternative to conventional macroscopic magnetometry techniques.
Further XMCD studies on RENi8[12–MC–4] double-decker metallacrowns examined the influence of solvents on magnetic properties. A significant spin-state crossover from low-spin to high-spin was observed in Ni(II) ions when dissolved in methanol, attributed to changes in coordination geometry, while dichloromethane preserved the initial spin state. Charge-transfer multiplet calculations supported these findings, underscoring the critical role of ligand fields in shaping spin behavior.
Finally, angular-dependent XMCD experiments on single crystals of REMn4[12–MC–4], with RE=Dy(III) and Tb(III), revealed a fourth-order magnetic anisotropy. This behavior is linked to the anisotropic charge distribution of 4f orbitals within the tetragonal crystal field. Crystal-field multiplet simulations validated the experimental results after corrections, showing angular dependence in the magnetic moments of DyMn4, with maxima and minima along the easy and hard axes, respectively.