Thermodynamic and magnetic properties as well as simulation of molecular dynamics of the Gd complex $\left[\mathrm{Gd}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{\mathrm{Cl}}_2\right]\mathrm{Cl}$ are reported. The investigation of static susceptibility, magnetization, and electron paramagnetic resonance enabled the identification of the studied complex as an $S=7/2$ magnet with easy-axis anisotropy $D/k_B=-0.12\mathrm{K}$, rhombic anisotropy $E/D=0.008$, and effective magnetic interaction of maximal size $|J/k_B|\approx 12\mathrm{mK}$ of predominantly dipolar origin. Simulation of molecular dynamics suggests the presence of local low-energy vibrational modes with energies below $200{\mathrm{cm}}^{-1}$. Their existence is also indirectly supported by the analysis of lattice specific heat. The study of magnetic field–induced slow relaxation revealed the existence of two relaxation channels. The former is represented by extremal slow relaxation reaching a timescale of seconds, which was attributed to the coexistence of Raman and Orbach-like processes in a multilevel system in which degeneracy of Gd(III) ion doublets is lifted by static magnetic field. The latter process is effectively described by $\tau \approx T^{-2.18}$, deviating from the temperature dependence $\tau \approx T^{-3}$, which, as recently theoretically proposed, arises from the coexistence of standard Raman relaxation for a multilevel system with $\tau \approx T^{-5}$ and the interaction of Gd(III) ions with local low-energy vibrational modes. In addition, the magnetic field dependence of the relaxation time reveals a pronounced acceleration of dynamical response with increasing magnetic field, suggesting resonant phonon trapping as the dominant relaxation mechanism. The present work suggests that the interaction of a magnetic ion with local low-energy vibrational modes proposed as the origin of anomalous Raman relaxation in single-molecule magnets, despite the presence of local vibrational modes in the system, may not occur as the main relaxation mechanism.

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• Magnetic susceptibility
• Phonons
• Spin relaxation
• Molecular magnets
• Single-molecule magnets