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The design of nanoscale information storage devices based on molecular magnets requires the preparation of materials with well-designed spin-phonon interactions such that magnetic memory persists at ambient temperatures. Understanding the fine details of spin-phonon coupling is thus crucial in designing new materials for this task. Of crucial importance for molecular spin dynamics is the phonon density of states, which directly impacts the timescale of magnetic memory, and a core part of this is the knowledge of phonon linewidths, which are inversely proportional to their lifetimes. In the past, we have assumed a fixed empirical linewidth of ca. 10 cm-1 [1], but others have suggested a strong temperature and energy dependence on the phonon linewidth [2]. However, the phonon linewidths are, in principle, accessible from first principles calculations; albeit at an extraordinary computational cost. By judiciously choosing a compact, high-symmetry molecular magnet viz [Dy(bbpen)Br] [3], we have directly obtained the ab initio linewidths. At 300 K, the linewidths vary on the order of 0.1 to 40 cm-1, as a function of both energy and wavevector and have a marked temperature dependence. Subsequent ab initio calculations of the spin-phonon coupling and magnetic relaxation rates using these calculated linewidths are in good agreement with the experiment, however, there is no significant difference between rates obtained with fixed, mode-dependent or ab initio-calculated linewidths. Rather, the most important factor is the density of the q-point mesh of reciprocal space.
References:
[1] D. Reta, J. G. C. Kragskow and N. F. Chilton, J. Am. Chem. Soc., 2021, 143, 5943.
[2] A. Lunghi, F. Totti, R. Sessoli and S. Sanvito, Nature Commun., 2017, 8, 14620.
[3] J. Liu, Y.-C. Chen, J.-L. Liu, V. Vieru, L. Ungur, J.-H. Jia, L. F. Chibotaru, Y. Lan, W. Wernsdorfer, S. Gao, X.-M. Chen and M.-L. Tong, J. Am. Chem. Soc., 2016, 138, 5441.
