A hexa-tert-butyldysprosocenium complex, [Dy(Cp^ttt)₂][B(C₆F₅)₄], exhibits magnetic hysteresis up to 60 K under a magnetic field sweep rate of 22 Oe s⁻¹. This is the highest temperature at which magnetic hysteresis has been observed in a single-molecule magnet (SMM). The hysteresis is attributed to localized metal-ligand vibrational modes unique to dysprosocenium. Ab initio spin dynamics show that magnetic relaxation at high temperatures is due to local molecular vibrations, suggesting that magnetic data storage in single molecules at temperatures above liquid nitrogen may be possible with careful molecular design. The complex was synthesized by reacting [Dy(Cp^t tt)₂(Cl)] with a silylium reagent in benzene. Single crystal X-ray diffraction studies show no significant interactions between the anion and cation, due to steric demands of the six 'Bu groups. The bent dysprosocenium cation exhibits a Cp_centroid1...Dy...Cp_centroid2 angle of 152.56(7)°, with the two C₅ rings approximately eclipsed. The mean Dy...Cp_centroid distance is relatively short, as expected from electrostatic considerations for an isolated cation. Magnetic measurements reveal that the complex shows open hysteresis at and below 60 K using a sweep rate of 22 Oe s⁻¹, with 83% remanent magnetisation and a coercive field of 20–25 kOe at 2 K. The low temperature hysteresis data show a step at zero-field, likely due to enhanced relaxation by quantum tunnelling of the magnetisation (QTM). The magnetic hysteresis is of molecular origin and not due to long-range ordering, as confirmed by structurally analogous doped samples and frozen dichloromethane solutions. The computational and magnetic data for the complex are in excellent agreement with those reported by Guo et al. The origin of the discrepancy between the power-law relaxation exponents is not obvious, but the results suggest that the properties observed are intrinsic to the [Dy(Cp^ttt)₂]⁺ cation. Ab initio spin dynamics show that magnetic relaxation is moderated by localized molecular vibrations. The calculated relaxation rate follows an Arrhenius law, with U_eff = 1223 cm⁻¹ (1760 K) and τ₀ = 1.986 × 10⁻¹¹ s. The relaxation rate has a power-law dependence on temperature and is well-modelled by r⁻¹ = CTⁿ with C = 1.664 × 10⁻⁶ s⁻¹ K⁻ⁿA hexa-tert-butyldysprosocenium complex, [Dy(Cp^ttt)₂][B(C₆F₅)₄], exhibits magnetic hysteresis up to 60 K under a magnetic field sweep rate of 22 Oe s⁻¹. This is the highest temperature at which magnetic hysteresis has been observed in a single-molecule magnet (SMM). The hysteresis is attributed to localized metal-ligand vibrational modes unique to dysprosocenium. Ab initio spin dynamics show that magnetic relaxation at high temperatures is due to local molecular vibrations, suggesting that magnetic data storage in single molecules at temperatures above liquid nitrogen may be possible with careful molecular design. The complex was synthesized by reacting [Dy(Cp^t tt)₂(Cl)] with a silylium reagent in benzene. Single crystal X-ray diffraction studies show no significant interactions between the anion and cation, due to steric demands of the six 'Bu groups. The bent dysprosocenium cation exhibits a Cp_centroid1...Dy...Cp_centroid2 angle of 152.56(7)°, with the two C₅ rings approximately eclipsed. The mean Dy...Cp_centroid distance is relatively short, as expected from electrostatic considerations for an isolated cation. Magnetic measurements reveal that the complex shows open hysteresis at and below 60 K using a sweep rate of 22 Oe s⁻¹, with 83% remanent magnetisation and a coercive field of 20–25 kOe at 2 K. The low temperature hysteresis data show a step at zero-field, likely due to enhanced relaxation by quantum tunnelling of the magnetisation (QTM). The magnetic hysteresis is of molecular origin and not due to long-range ordering, as confirmed by structurally analogous doped samples and frozen dichloromethane solutions. The computational and magnetic data for the complex are in excellent agreement with those reported by Guo et al. The origin of the discrepancy between the power-law relaxation exponents is not obvious, but the results suggest that the properties observed are intrinsic to the [Dy(Cp^ttt)₂]⁺ cation. Ab initio spin dynamics show that magnetic relaxation is moderated by localized molecular vibrations. The calculated relaxation rate follows an Arrhenius law, with U_eff = 1223 cm⁻¹ (1760 K) and τ₀ = 1.986 × 10⁻¹¹ s. The relaxation rate has a power-law dependence on temperature and is well-modelled by r⁻¹ = CTⁿ with C = 1.664 × 10⁻⁶ s⁻¹ K⁻ⁿ