The use of single molecule magnets in mainstream electronics requires their magnetic moment to be stable over long times. One can achieve such a goal by designing compounds with spin-reversal barriers exceeding room temperature, namely with large uniaxial anisotropies. Such strategy, however, has been defeated by several recent experiments demonstrating under-barrier relaxation at high temperature, a behaviour today unexplained. Here we propose spin-phonon coupling to be responsible for such anomaly. With a combination of electronic structure theory and master equations we show that, in the presence of phonon dissipation, the relevant energy scale for the spin relaxation is given by the lower-lying phonon modes interacting with the local spins. These open a channel for spin reversal at energies lower than that set by the magnetic anisotropy, producing fast under-barrier spin relaxation. Our findings rationalize a significant body of experimental work and suggest a possible strategy for engineering room temperature single molecule magnets.

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Interference and Intermediate-Level-Width Corrections to the Orbach Relaxation Rate

The role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets

This thesis demonstrates the applicability of far-infrared spectroscopy for the study of the crystal-field splitting of lanthanides in single-molecular magnetic materials. The far-infrared studies of three different kinds of single-molecular-magnetic materials, a single-ion magnet, a single-chain magnet and an exchange-coupled cluster, yielded a deeper understanding of the crystal-field splitting of the lanthanides in these materials. In addition, our results offered the opportunity to gain a deeper insight into the relaxation processes of these materials.

 
Diese Arbeit zeigt die Anwendbarkeit der Ferninfrarotspektroskopie fur die Untersuchung der Kristallfeldaufspaltung von Lanthanoiden in Einzelmolekulmagnetmaterialien. Die Ferninfrarotstudien an drei verschiedenartigen Einzelmolekulmagnetmaterialien, einem Einzelionenmagnet, einem Einzelkettenmagnet und einem austauschgekoppelten Cluster, fuhrten uns, zu einem tieferen Verstandnis der Kristallfeldaufspaltung der Lanthanoide in den untersuchten Materialien. Zudem gaben unsere Ergebnisse Einblicke in die jeweiligen Relaxationsprozesse.

background

Far-infrared spectroscopy of lanthanide-based molecular magnetic materials

While exponential and power law relaxation-time-temperature dependences are widely used to fit experimental data, these known relations in bulk materials have not been well-foundedly validated in the context of single-molecule magnets. Based on pattern of the phonon density of states and spin-phonon coupling strength, we derived a theory showing how these dependences arise and how transition among them occurs with temperature variation. The theory also resolves the puzzle of lower-than-expected Orbach barrier and multiple barriers, and elucidates the presumed Orbach process as Raman process in disguise. Except for very high temperature at which real Orbach processes are accessible, the results justify the ground state tunnelling as the dominant relaxation channel, and identify small tunnelling rate and enclosure of the magnetic atom as reasons for the slowness. Our findings issue a call for shifting our research efforts from achieving super large Orbach barriers to magnetic engineering that diminishes the transverse and high order anisotropies, and to dynamic engineering that lifts vibrational modes possessing strong coupling with the spin.

Interference and Intermediate-Level-Width Corrections to the Orbach Relaxation Rate

Citations

The role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets

Far-infrared spectroscopy of lanthanide-based molecular magnetic materials

Theory of spin-lattice relaxation in single-molecule magnets: Reasons for the slowness

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