We report ac susceptibility and continuous wave and pulsed EPR experiments performed on GdW10 and GdW30 polyoxometalate clusters, in which a Gd3+ ion is coordinated to different polyoxometalate moieties. Despite the isotropic character of gadolinium as a free ion, these molecules show slow magnetic relaxation at very low temperatures, characteristic of single molecule magnets. For T≲200  mK, the spin-lattice relaxation becomes dominated by pure quantum tunneling events, with rates that agree quantitatively with those predicted by the Prokof'ev and Stamp model [Phys. Rev. Lett. 80, 5794 (1998)]. The sign of the magnetic anisotropy, the energy level splittings, and the tunneling rates strongly depend on the molecular structure. We argue that GdW30 molecules are also promising spin qubits with a coherence figure of merit Q(M)≳50.

A robust, stable and processable family of mononuclear lanthanoid complexes based on polyoxometalates (POMs) that exhibit single-molecule magnetic behavior is described here. Preyssler polyanions of general formula [LnP(5)W(30)O(110)](12-) (Ln(3+) = Tb, Dy, Ho, Er, Tm, and Yb) have been characterized with static and dynamic magnetic measurements and heat capacity experiments. For the Dy and Ho derivatives, slow relaxation of the magnetization has been found. A simple interpretation of these properties is achieved by using crystal field theory.

In this work we study theoretically the coupling of single-molecule magnets (SMMs) to a variety of quantum circuits, including microwave resonators with and without constrictions and flux qubits. The main result of this study is that it is possible to achieve strong and ultrastrong coupling regimes between SMM crystals and the superconducting circuit, with strong hints that such a coupling could also be reached for individual molecules close to constrictions. Building on the resulting coupling strengths and the typical coherence times of these molecules (∼ µs), we conclude that SMMs can be used for coherent storage and manipulation of quantum information, either in the context of quantum computing or in quantum simulations. Throughout the work we also discuss in detail the family of molecules that are most suitable for such operations, based not only on the coupling strength, but also on the typical energy 6

We investigate the mechanism of spin-lattice relaxation of Er ions encapsulated in polyoxometalate clusters, which below 4 K can only reverse its spin via quantum tunneling processes. The temperature-independent rate −1 is, at zero field, ten orders of magnitude larger than the rates predicted for direct phonon-induced processes. In addition, we observe that −1 is suppressed by external magnetic bias and hyperfine interactions but enhanced by increasing the concentration of Er ions. The observed relaxation agrees with predictions for pure quantum tunneling, showing that this phenomenon drives the thermalization of electronic spins. A possible link between these two phenomena is discussed, involving the collective emission of phonons from particular spin configurations attained via quantum tunneling. The fundamental equations of magnetism, including Curie's law, rely on the ability of magnetic moments to attain thermal equilibrium with the solid lattice. In spite of the progress achieved in studying and manipulating individual spins in solids, 1 the spin-lattice relaxation ͑SLR͒ mechanisms are not well understood yet. An intriguing situation arises, near zero field, for strongly anisotropic spins, e.g., magnetic molecular clusters or rare-earth ions. When thermally activated tunneling processes 2 die out, at sufficiently low temperatures ͑typically T Շ 1 K͒, spins can only flip by pure quantum tunneling ͑QT͒ across the anisotropy energy barrier. Theoretical descriptions 3-5 of QT in the presence of hyperfine couplings and dipolar spin-spin interactions account well for experiments that measure the time-dependent magnetization under such conditions. 6-8 Concerning SLR, a major difficulty arises. It stems from the fact that QT modifies the magnetization but conserves the energy of the ensemble of nuclear and electronic spins. Therefore, equilibrium states might well be reached long after the characteristic time scales of QT. However, a few experiments suggest otherwise. Specificheat studies 10 indicate that Mn 4 and Fe 8 single-molecule magnets ͑SMM͒ attain thermal equilibrium at rates comparable to those found in magnetization relaxation experiments. In addition, NMR experiments on Mn 12 clusters 11 show that the nuclear spin and bath temperatures remain the same down to the neighborhood of absolute zero.In order to elucidate the nature of the SLR mechanism and its relationship with QT, direct measurements of the SLR rates as a function of temperature, magnetic field, concentration of spins, etc., are clearly desirable. Studying the SLR of molecular nanomagnets at very low temperatures and under weak magnetic fields is, however, a demanding experimental task because tunneling time scales are on the order of days even for clusters made of a few atoms. In order to overcome this difficulty, simpler molecules need to be studied. In the present work, we report the SLR rates of polyoxometalate ͑POM͒ clusters containing individual lanthanide ions. The results evidence that the thermalization of electronic spins is dictat...

The polyoxometalate clusters with formula [Gd(W(5) O(18) )(2) ](9-) and [Gd(P(5) W(30) O(110) )](12-) each carry a single magnetic ion of gadolinium, which is the most widespread element among magnetic refrigerant materials. In an adiabatic demagnetization, the lowest attainable temperature is limited by the presence of magnetic interactions that bring about magnetic order below a critical temperature. We demonstrate that this limitation can be overcome by chemically engineering the molecules in such a way to effectively screen all magnetic interactions, suggesting their use as ultra-low-temperature coolers.

Magnetic properties of Au nanoparticles deposited on an archaeal S layer are reported. X-ray magnetic circular dichroism and superconducting quantum interference device magnetometries demonstrate that the particles are strongly paramagnetic, without any indication of magnetic blocking down to 16 mK. The average magnetic moment per particle is M part ¼ 2:36ð7Þ B . This contribution originates at the particle's Au 5d band, in which an increased number of holes with respect to the bulk value is observed. The magnetic moment per Au atom is 25 times larger than any measured in other Au nanoparticles or any other configurations up to date.

We report a detailed experimental study of maghemite nanoparticles, with sizes ranging from 1.6 to 6 nm, synthesized inside a biological mould of apoferritin. The structural characterization of the inorganic cores, using TEM and x-ray diffraction, reveals a low degree of crystalline order, possibly arising from the nucleation and growth of multiple domains inside each molecule. We have also investigated the molecular structure by means of atomic force microscopy in liquid. We find that the synthesis of nanoparticles inside apoferritin leads to a small, but measurable, decrease in the external diameter of the protein, probably associated with conformational changes. The magnetic response of the maghemite cores has been studied by a combination of techniques, including ac susceptibility, dc magnetization and Mössbauer spectroscopy. From the equilibrium magnetic response, we have determined the distribution of magnetic moments per molecule. The results show highly reduced magnetic moments. This effect cannot be ascribed solely to the canting of spins located at the particle surface but, instead, it suggests that magnetoferritin cores have a highly disordered magnetic structure in which the contributions of different domains compensate each other. Finally, we have also determined, for each sample, the distribution of the activation energies required for the magnetization reversal and, from this, the size-dependent magnetic anisotropy constant K. We find that K is enormously enhanced with respect to the maghemite bulk value and that it increases with decreasing size. The Mössbauer spectra suggest that low-symmetry atomic sites, probably located at the particle surface and at the interfaces between different crystalline domains, are the likely source of the enhanced magnetic anisotropy.

We report the experimental results that show the operation of superconducting quantum interference device ͑SQUID͒ microsusceptometers immersed in the 3 He mixture inside the mixing chamber of a dilution refrigerator at high frequency ͑1 MHz͒ and down to very low temperatures ͑13 mK͒. The devices are based on highly sensitive and easy-to-use commercial SQUID sensors. The integrated susceptometers are fabricated by rerouting some connections of the SQUID's input circuit. Examples of measurements on molecular magnets Mn 12 and HoW 10 are shown. © 2010 American Institute of Physics. ͓doi:10.1063/1.3280169͔The research on the properties of magnetic materials at very low temperatures has been stimulated in the last fifteen years by the observation of fascinating quantum phenomena, such as quantum tunneling of spins, 1 quantum coherence, [2][3][4][5] and quantum entanglement. 6,7 Besides providing a direct hindsight on the not yet fully understood quantum-toclassical transition, these phenomena might also find application in the development of solid-state quantum information technologies.8 Their study has fuelled the development of new magnetic sensors with improved performances that approach the experimental limits in terms of sensitivity and operation conditions. Experiments need to be performed at very low temperatures, when zero-point quantum fluctuations become dominant over classical thermal fluctuations. A very high spin sensitivity is also desirable to enable detecting the magnetic response of single layers of nanomagnets and eventually of individual ones. In addition, by magnetically diluting the samples, the decoherence caused by spin-spin interactions can be minimized. Another crucial point is the access to broader frequency bandwidths than those offered by commercial magnetometers, since the time scales of quantum dynamics can vary over many decades depending on the spin value, the magnetic anisotropy, and the couplings to the environment. The many recent advances in this field include the development of miniature magnetometers, based on the use of either micro-Hall bars or micro-and nanosuperconducting quantum interference device ͑SQUID͒ sensors.9 Another application with similar stringent performance is found in the use of metallic magnetic microcalorimeters as radiation detectors. 10Our purpose has been to develop an ac magnetic susceptometer that combines high sensitivity and broad frequency bandwidth with operation down to very low temperatures and user friendliness. Commercial SQUID technology and readout electronics ͑e.g., from Magnicon GbR, Germany͒ are now available with sensitivities near the quantum limit at mK temperatures and room temperature electronics adapted for operation with broad bandwidths. 11 We have taken advantage of such optimized devices as the starting point for the fabrication of our susceptometers. It turns out that, via a simple modification of the chip's input wiring, a fully integrated thin-film SQUID susceptometer can be obtained. This modification is carried out by focused...