Speaker
Description
Static and dynamic magnetic properties, electron-spin resonance spectra together with calculation of low-energy vibrational modes for complex [Gd(H$_2$O)$_6$Cl$_2$]Cl are presented. The studied compound can be identified as $S = 7/2$ Heisenberg magnet with easy-axis anisotropy $D/k_B \approx - 50$ mK and dipolar magnetic coupling of nominal size $\left| J/k_B \right| \approx 12$ mK. The existence of low-energy local vibrational modes possessing energies lower than $200$ cm$^{-1}$ was revealed by numerical simulations. The analysis of low-temperature lattice specific heat indirectly supported their existence. The presence of two relaxation channels was revealed by the investigation of slow magnetic relaxation induced by static external magnetic field $0.4$ T. For the first process, extremal slow relaxation in the time scale of seconds was attributed to the coexisting Raman and Orbach-like processes involving various energy levels from a multi-level system created by Gd$^{3+}$ ion doublets split in the external magnetic field. Temperature dependence of the relaxation time for the second process could be described by $\tau \approx T^{-2.18}$, which represents substantial deviation from $\tau \approx T^{-3}$ being anticipated to arise from the coexistence of standard Raman relaxation for a multilevel system $( \tau \approx T^{-5} )$ and the interaction of a magnetic ion with low-energy local vibrational modes $(\tau \approx \exp(\Delta_{loc}/T) )$ proposed for various single molecule magnets [1]. However, magnetic field dependence of the relaxation time was found to be consistent with resonant phonon trapping $(\tau \approx T^{-2})$ and the assumptions for the onset of the trapping in [Gd(H$_2$O)$_6$Cl$_2$]Cl were verified. The obtained results suggest that the low-energy local vibrational modes, despite being present in a system, may not introduce the governing relaxation mechanism.
Acknowledgements
This work was supported by Slovak research and development agency under contract APVV-22-0172, and partially by the EU NextGenerationEU through the Recovery and Resilience Plan for Slovakia under the project No. 09I03-03-V04-00176.
References
[1] L. Gu and R. Wu, “Origin of the anomalously low Raman exponents in single molecule magnets,” Physical Review B, vol. 103, no. 1. American Physical Society (APS), Jan. 04, 2021. https://doi.org/10.1103/physrevb.103.014401
