Figure 1

(a) Temperature evolution of the proton NMR spectra in ${\mathrm{Fe}}_8$ obtained by sweeping the field at constant frequency $\nu =197.9\mathrm{MHz}$. The huge broadening of the spectra and their structuring reflect the blocking of the molecular magnetization and can be approximately reproduced by considering the nuclear-electron dipolar interaction [9]. This broadening is not due to a trivial field-induced paramagnetic shift but to the local static fields associated with the blocked state of ${\mathrm{Fe}}_8$. Indeed, a similar degree of broadening occurs in zero external field [9]. (b) Temperature evolution of the proton NMR in ${\mathrm{Ni}}_{10}$ obtained by sweeping the frequency at constant field. To compare the two figures, one can use Larmor’s relation for ${}^1\mathrm{H}$: $\nu =42.57\mathrm{MHz}/\mathrm{T}$. Hence, the maximum broadening in (b) (95 kHz for $T=1.6\mathrm{K}$) corresponds to about 0.002 T, 3 orders of magnitudes smaller than the maximum broadening in (a). Note that in ${\mathrm{Ni}}_{10}$ we have investigated a broad frequency window (about 2.5 MHz), finding no additional signal. Hence, the whole absorption takes place in the narrow frequency range of panel (b). Inset: Magnetic core of [${\mathrm{Ni}}_{10}(\mathrm{O})(\mathrm{thme}{)}_4(\mathrm{dbm}{)}_4({\mathrm{O}}_2\mathrm{CPh}{)}_2(\mathrm{EtOH}{)}_6$] (red and gray: Ni; cyan: O). More than 20 variants having different ligands exist. Only the spin degrees of freedom of the four external Ni ions (red spheres) are relevant at low $T$.Reuse & Permissions

Figure 2

Comparison between the temperature dependence of $\Delta {\nu }_m/H$ [from Eq. (1)] and the macroscopic susceptibility obtained with SQUID. The latter has been rescaled by a common factor so as to match NMR data in the high temperature range. Open and closed circles at 0.2 T refer to ZFC and FC measurements, respectively. The large nonlinearity of $M$ and $\Delta {\nu }_m$ with respect to $H$ is a signature of nonequilibrium behavior.Reuse & Permissions

Figure 3

Temperature dependence of the nuclear spin-lattice relaxation rate in ${\mathrm{Ni}}_{10}$. Inset: The same quantity for ${\mathrm{Fe}}_4$, a prototypical SMM which blocks around 1 K, much below the temperature at which $1/T_1$ peaks [13].Reuse & Permissions