Superexchange Coupling and Slow Magnetic Relaxation in a Transuranium Polymetallic Complex

$\{{\mathrm{Np}}^{\mathrm{VI}}{\mathrm{O}}_2{\mathrm{Cl}}_2\}\{{\mathrm{Np}}^{\mathrm{V}}{\mathrm{O}}_2\mathrm{Cl}(\mathrm{thf}{)}_3{\}}_2$ is the first studied example of a polymetallic transuranic complex displaying both slow relaxation of the magnetization and effective superexchange interactions between $5f$ centers. The coupling constant for ${\mathrm{Np}}^{\mathrm{V}}\mathrm{\text{−}}{\mathrm{Np}}^{\mathrm{VI}}$ pairs is 10.8 K, more than 1 order of magnitude larger than the common values found for rare-earth ions in similar environments. The dynamic magnetic behavior displays slow relaxation of magnetization of molecular origin with an energy barrier of 140 K, which is nearly twice the size of the highest barrier found in polymetallic clusters of the $d$ block. Our observations also suggest that future actinide-based molecular magnets will have very different behavior to lanthanide-based clusters.

Figure 1

Left, structure of the ${\mathrm{Np}}_3$ cluster studied in the present work (Np blue [dark gray], Cl yellow [light gray], O red [medium-dark gray], C green [medium gray], H not shown for clarity). Right, schematic model of the superexchange interactions.

Figure 2

Plots of $\chi T$ vs $T$ for ${\mathrm{Np}}_3$. Data have been collected with applied field values of 0.5, 1, 3, and 7 T as reported in the legend. Solid lines are the results of calculations assuming ligand-field and superexchange interactions between pairs of Np centers as indicated in Fig. 1. Inset: Calculated energy spectra of the ${\mathrm{Np}}_3$ cluster as a function of the magnetic field. Dashed red lines are calculated with the magnetic field applied along the direction perpendicular to the plane of the ${\mathrm{Np}}_3$ triangle, while full blue lines are obtained when the magnetic field is applied within the plane and forms the same angle with the quantization axis of the two ${\mathrm{Np}}^{\mathrm{V}}$ centers. For the latter configurations, the eigenstates can be labeled by the $J_z$ component of each individual Np moment as indicated in the figure (note that changing the sign of all three quantum numbers results in states which are degenerate in absence of an applied magnetic field).

Figure 3

Left panel: ${\mathrm{Np}}^{\mathrm{V}}$ ($5f^2$) axial ligand-field parameters regions leading to a $J_z=\pm 3$ ground-state doublet for positive (light-blue zone, containing most of the first quadrant) and negative (light-red zone, containing most of the third quadrant) $B_2^0$ values. Right panel: ${\mathrm{Np}}^{\mathrm{VI}}$ ($5f^1$) axial ligand-field parameters region leading to a $J_z=\pm 1/2$ ground-state doublet.

Figure 4

Real (top panel) and imaginary (bottom panel) part components of the ac magnetic susceptibility as a function of temperature, measured at different frequencies $f$ of the driving field. The natural logarithm of the relaxation time $\tau$ is plotted in the inset as function of $1/T$.