High Blocking Temperature of Magnetization and Giant Coercivity in the Azafullerene Tb2 @C79 N with a Single-Electron Terbium-Terbium Bond

Abstract

The azafullerene Tb₂ @C₇₉ N is found to be a single-molecule magnet with a high 100-s blocking temperature of magnetization of 24 K and large coercivity. Tb magnetic moments with an easy-axis single-ion magnetic anisotropy are strongly coupled by the unpaired spin of the single-electron Tb-Tb bond. Relaxation of magnetization in Tb₂ @C₇₉ N below 15 K proceeds via quantum tunneling of magnetization with the characteristic time τ_QTM =16 462±1230 s. At higher temperature, relaxation follows the Orbach mechanism with a barrier of 757±4 K, corresponding to the excited states, in which one of the Tb spins is flipped.

Keywords: endohedral fullerenes; exchange coupling; metal-metal bonds; single-molecule magnets; terbium.

Figure 1

Schematic depiction of Tb₂@C₈₀ with a single‐electron Tb−Tb bond and an unpaired spin on the fullerene cage, which can be stabilized by addition of an electron, substitution of one carbon atom by nitrogen to yield azafullerene Tb₂@C₇₉N, or by functionalization with a radical group, as realized in Tb₂@C₈₀(CH₂Ph). Also shown are spin density distributions in Ln₂@C₇₉N and Ln₂@C₈₀(CH₂Ph) (low isovalue for semitransparent isosurface, and high isovalue for solid isosurface). Three regions with high spin density correspond to 4f‐electrons of two Ln atoms and to the unpaired electron residing on the Ln−Ln bonding orbital.

Figure 2

a) Determination of the blocking temperature of magnetization, T _B, for Tb₂@C₇₉N (μ₀ H=0.2 T, temperature sweep rate 5 K min⁻¹); b) Magnetic hysteresis of Tb₂@C₇₉N measured between 1.8 and 26 K (sweep rate 2.9 mT s⁻¹).

Figure 3

Relaxation times of magnetization of Tb₂@C₇₉N measured in zero field (dots); lines are results of the fit with Equation (1) and contributions of different relaxation mechanisms. The inset shows the out‐of‐phase magnetic susceptibility χ′′ measured at different temperatures (dots) and fits with generalized Debye model (lines).

Figure 4

a) Experimental χT curve for Tb₂@C₇₉N measured in the field of 1 T and the simulations with different values of K ^eff (lines); note that below T _B, the experimental curve does not represent the thermodynamic behavior and cannot be reproduced by simulations. b) Experimental magnetization curves of Tb₂@C₇₉N measured at different temperatures above T _B and the simulations with K ^eff=45 cm⁻¹. Experimental data are in arbitrary units scaled to match simulated curves.

Figure 5

a) Alignment of individual spins in Tb₂@C₇₉N in the ground state (quantization axes of Tb ions are shown as green arrows, the red arrow represents the unpaired electron spin, whereas red isosurfaces represent the valence spin density distribution). b) Low‐energy part of the spectrum of the Hamiltonian (2) with K ^eff=45 cm⁻¹; dashed arrows denote QTM and Orbach relaxation mechanisms, numbers are transition probabilities (in μ_B ²), thickness of the red lines between the levels scales with transition probability. c) Dependence of the energy (left) and g_y component (right) of the lowest‐energy exchange‐excited states as a function of the tilting angle α. Green and red arrows schematically show alignment of the magnetic moments of Tb (green) and the unpaired electron (red).