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Coherent control of individual molecular spins in nano-devices is a pivotal prerequisite for fulfilling the potential promised by molecular spintronics. By applying electric field pulses during time-resolved electron spin resonance measurements, we measure the sensitivity of the spin in several antiferromagnetic molecular nanomagnets to external electric fields. We find a linear electric field dependence of the spin states in Cr$_7$Mn, an antiferromagnetic ring with a ground-state spin of $S=1$, and in a frustrated Cu$_3$ triangle, both with coefficients of about $2~\mathrm{rad}\, \mathrm{s}^{-1} / \mathrm{V} \mathrm{m}^{-1}$. Conversely, the antiferromagnetic ring Cr$_7$Ni, isomorphic with Cr$_7$Mn but with $S=1/2$, does not exhibit a detectable effect. We propose that the spin-electric field coupling may be used for selectively controlling individual molecules embedded in nanodevices., 6 pages, 3 figures

Muon spin rotation measurements were used to investigate the spin dynamics of heterometallic ${\mathrm{Cr}}_{7}\mathrm{Ni}$ and ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ molecular clusters. In ${\mathrm{Cr}}_{7}\mathrm{Ni}$ the magnetic ions are arranged in a quasiplanar ring and interact via an antiferromagnetic exchange coupling constant $J$, while ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ is composed of two ${\mathrm{Cr}}_{7}\mathrm{Ni}$ linked by a bridging moiety containing one Cu ion, that induces an inter-ring ferromagnetic interaction ${J}^{\ensuremath{'}}\ensuremath{\ll}J$. The longitudinal muon relaxation rate $\ensuremath{\lambda}$ collected at low magnetic fields ${\ensuremath{\mu}}_{0}Hl0.15$ Tesla, shows that the two systems present differences in spin dynamics vs temperature. While both samples exhibit a main peak in the muon relaxation rate vs temperature, at $T\ensuremath{\sim}10$ K for ${\mathrm{Cr}}_{7}\mathrm{Ni}$ and $T\ensuremath{\sim}8$ K for ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$, the two compounds have distinct additional features: ${\mathrm{Cr}}_{7}\mathrm{Ni}$ shows a shoulder in $\ensuremath{\lambda}(T)$ for $Tl8$ K, while ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ shows a flattening of $\ensuremath{\lambda}(T)$ for $Tl2$ K down to temperatures as low as $T=20$ mK. The main peak of both systems is explained by a Bloembergen-Purcell-Pound (BPP)-like heuristic fitting model that takes into account of a distribution of electronic spin characteristic times for $Tg5$ K, while the shoulder presented by ${\mathrm{Cr}}_{7}\mathrm{Ni}$ can be reproduced by a BPP function that incorporates a single electronic characteristic time theoretically predicted to dominate for $Tl5$ K. The flattening of $\ensuremath{\lambda}(T)$ in ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ occurring at very low temperature can be tentatively attributed to field-dependent quantum effects and/or to an inelastic term in the spectral density of the electronic spin fluctuations.

We present a refined model of the ${{\mathrm{Fe}}_{9}}$ tridiminished icosahedron magnetic molecule system. This molecule was originally modeled as being composed of two (${{\mathrm{Fe}}_{3}}$ and ${{\mathrm{Fe}}_{6}}$) clusters, with the ${\mathrm{Fe}}^{3+}$ ions within each cluster being coupled via exchange interactions, but with no coupling between the clusters. The present inelastic neutron scattering (INS) measurements were used to probe the low-lying energy spectrum of ${{\mathrm{Fe}}_{9}}$, and these results demonstrate that the previously published model of two uncoupled clusters is incomplete. To achieve agreement between the experiment and theory, we have augmented the model with relatively small exchange coupling between the clusters. A combination of Lanczos matrix diagonalization and quantum Monte Carlo simulations have been used to achieve good agreement between the experimental data and the improved model of the full ${{\mathrm{Fe}}_{9}}$ system despite the complexity of this model (with Hilbert space dimension $g{10}^{7}$).

(magnetization, susceptibility, and specific heat) allow us to determine the total spin of the ground state and to estimate the spin-Hamiltonian parameters related to magnetic anisotropy. X-ray spectroscopies (XAS and XMCD) are used to probe the local magnetization of the Cr and Cu ions, and provide results that are consistent with the bulk magnetization data. We finally investigate the relation between heterometallicity and entanglement in these prototypical spin systems. In particular, we focus on the spatial modulation of entanglement induced by the Cu defect spins and on the long-distance entanglement between them induced by the two Cr chains.

We have extensively studied the magnetic features of a heterometallic molecular trimer, ${\text{Cr}}_{2}\text{Cu}$. We have characterized molecular ${\text{Cr}}_{2}\text{Cu}$ by low-temperature ac susceptibility and specific-heat measurements and this allowed us to determine the microscopic parameters of the spin Hamiltonian. Then, we used x-ray magnetic circular dichroism to selectively probe local symmetries, electronic configuration, orbital, and spin magnetic moments of the magnetic ions. We found that at low temperatures only the ground state $S=3/2$ is occupied and the spin of Cu is found opposite to that of Cr in agreement with spin-Hamiltonian calculations.

We present magnetic susceptibility, $^{1}\text{H}$ nuclear magnetic resonance (NMR) spectra, spin-spin- and spin-lattice-relaxation rates data, collected in the temperature range $1.65lTl300\text{ }\text{K}$ at two applied magnetic fields $H=0.35$ and 1.5 T, on a single crystal of ${\text{Cr}}_{7}\text{Fe}$ heterometallic molecular nanomagnet. From a simple analysis of the magnetic susceptibility data, we deduce that the ring has a magnetic total spin ${S}_{T}=1/2$ ground state and Cr-Cr antiferromagnetic (AFM) exchange interactions comparable to similar ${\text{Cr}}_{7}\text{M}$ heterometallic rings. The proton NMR data indicate that the main coupling between nuclei and electronic moments is of dipolar origin. The spin-lattice-relaxation rate behavior is qualitatively explained by a model which assumes a single-correlation time as used previously for homometallic AFM rings. The direct comparison of the relaxation data in ${\text{Cr}}_{7}\text{Fe}$ and ${\text{Cr}}_{8}$ shows a substantial change in the spin dynamics as the result of Fe replacing one Cr ion. The difference is attributed to the change in the elastic properties of the sample with Fe substitution and/or to the multi-Lorentzian behavior of the spin-spin correlation function resulting from the presence of inequivalent ions in the heterometallic ring.

We characterize supramolecular magnetic structures, consisting of two weakly coupled antiferromagnetic rings, by low-temperature specific heat, susceptibility, magnetization and electron paramagnetic resonance measurements. Intra- and inter-ring interactions are modeled through a microscopic spin-Hamiltonian approach that reproduces all the experimental data quantitatively and legitimates the use of an effective two-qubit picture. Spin entanglement between the rings is experimentally demonstrated through magnetic susceptibility below 50 mK and theoretically quantified by the concurrence.

We present tunnel diode oscillator (TDO) measurements of dynamic magnetic susceptibility, inelastic neutron-scattering (INS) measurements, and theoretical predictions based on quantum Monte Carlo (QMC) calculations for a magnetic molecule system, ${\text{Cr}}_{12}{\text{Cu}}_{2}$. The TDO measurements show not only ground-state level crossings (as can also be observed in low-temperature dc magnetization measurements) but also clear evidence of crossings between certain excited energy levels. These TDO results are in excellent agreement with our theoretical predictions for a Heisenberg Hamiltonian and are further confirmed by our INS measurements. Our present findings demonstrate that the TDO technique is a valuable magnetic spectroscopic tool for studying magnetic molecules, and that the QMC method is a valuable tool for predicting properties of computationally demanding systems such as ${\text{Cr}}_{12}{\text{Cu}}_{2}$.

We present detailed magnetic measurements and the theoretical analysis of the recently synthesized magnetic molecule Cr10Cu2 . Due to the heterometallic nature of this molecule, there are three distinct intramolecular interactions, which we describe using an isotropic Heisenberg model with three distinct exchange constants. The magnetic properties of the model are calculated using the quantum Monte Carlo method, including the low-field magnetic susceptibility T and the magnetization M H ,T versus magnetic field H up to the saturation field 80 T for arbitrary temperature T. Of particular relevance to experiment, we have calculated the full set of ground-state level-crossing fields corresponding to peaks in M / H versus H for low T. A detailed search of the three-dimensional parameter space yields two well-separated sets of exchange constants, both of which give good agreement between the predictions of the model and the measured T . The present low-temperature tunnel-diode resonator measurements provide values of ground-state levelcrossing fields, as well as the level-crossing fields for certain low-lying excited states up to 16 T that are in good agreement with theory. The full set of theoretical crossing fields is very nearly equal for both sets of exchange constants. The theory also provides quantitative predictions for the site-dependent local magnetic moments of this molecule, which could perhaps be tested by future nuclear-magnetic-resonance measurements.

Magnetization measurements on a crystal of ${\mathrm{Cr}}_{7}\mathrm{Ni}$ antiferromagnetic rings are presented. Irradiation with microwaves at frequencies between 1 and $10\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ leads to observation of very narrow Gaussian resonant photon absorption lines which are mainly broadened by hyperfine interactions. The relaxation of magnetization after a nanosecond RF pulse is dominated by the phonon bottleneck effect. A two-pulse hole burning technique allowed us to estimate the characteristic energy diffusion time (spectral diffusion).

We present a parallel-mode EPR study of a very high-spin ground-state cluster complex [Cr 1 2 O 9 (OH) 3 (O 2 CCMe 3 ) 1 5 ], where Me indicates the methyl group. This high-symmetry (D 3 ) molecule has a well-isolated S=6 groundstate characterized by D=+0.088 cm - 1 , E=0, g z z =1.965, g x x =g y y =1.960. Low-temperature (5 K) single-crystal and powder parallel- and perpendicular-mode EPR spectra are presented. The forms of the spectra are discussed with respect to the composition of the wave functions as a function of the angle of the static magnetic field to the molecular Z axis, the selection rules of the two excitation modes, and the resultant transition probabilities. This is the largest spin state studied by parallel mode EPR spectroscopy to date, and the results demonstrate the applicability of parallel-mode EPR to high-spin ground-state molecular clusters such as single-molecule magnets.