L. Joly

Magnetic Properties of Gold Nanoparticles: A Room-Temperature Quantum Effect

Gold nanoparticles elicit a huge research activity in view of ntheir applications in diagnostics,[1, 2] therapy,[3] drug or gene delivery,[ n4] sensing[5, 6, 7] and imaging.[8] Gold nanoparticles also display ninteresting catalytic[9, 10] and optical[11, 12, 13, 14] properties. nThis Communication focuses on the least understood and so nfar unused property of gold: its becoming magnetic when prepared nin the form of nanoparticles. All these desirable properties, nbound together in one nanometric piece of matter, possibly nself-organized thanks to its ligands, make functionalized ngold nanoparticles a treasurable entity for nanosciences. The nex nihilo magnetic properties of functionalized gold (and other ndiamagnetic metals, such a Ag or Cu) nanoparticles, that is, ntheir ferromagnetic-like behavior, are well documented, nthough still poorly understood.[15] This unexpected property nopens new perspectives in materials science, in particular for nthe design of metamaterials. One may also envisage applications nin information storage and processing: nanometric magnetic nparticles with no obvious temperature limitation and possibly nself-organizing are currently much sought-after by the ncomputer industry and developing a room-temperature magnetic semiconductor is paramount for the realization of spintronics ntechnologies. nHerein, we wish to present the results of our own investigations ninto the magnetic properties of functionalized gold nanoparticles. nWe have made attempts at understanding this magnetic nbehavior using both traditional techniques (e.g. superconducting nquantum interference device, SQUID, magnetometry) nand other methods less common in this field, such as zerofield n197Au NMR (nuclear magnetic resonance) and SANS (smallangle nneutron scattering). We also directly probed the local nmagnetic field at the surface of gold nanoparticles using paramagnetic nTEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] radicals nand ESR (electron spin resonance) spectrometry. Surprisingly, nnone of these experiments provided a clearer picture in nfine. These “negative” results led us to pondering whether or nnot the explanation could be elsewhere. Our hypothesis is that nthe magnetism of gold (and possibly other metals) could very nwell originate in self-sustained persistent currents. We shall ndemonstrate hereafter that this hypothesis is indeed very plausible nand would actually reconcile all of the experimental data nreported to date.Striking results are often obtained when SQUID magnetometry nis performed on functionalized Au nanoparticles, such as ndodecanethiol-coated ones. Rather than being diamagnetic, as nexpected, the nanoparticles can be found to be para- or ferromagnetic nat room temperature and above. When hysteresis is nobserved, the magnetization curve looks like that of a soft ferromagnet nand exhibits a remnant magnetization MR and a coercive nfield HC, though both are rather weak. These parameters nhave been observed to have values that vary by orders of nmagnitude from sample to sample[15] (see Figure ESI-1 of the nSupporting Information). Very often, the magnetization does nnot saturate. Diamagnetic samples are more diamagnetic than nthe bulk metal. Also, the magnetic observables show little dependence non temperature between 2 and 400 K. The measurements nreported so far have been performed by totally independent ngroups, on systems that were synthesized using nknown chemical procedures. Figure 1 compares the magnetization nof bulk gold with that of two diamagnetic samples of ngold nanoparticles. It can be seen that nanoparticles have na much larger absolute diamagnetic susceptibility than massive ngold. nFigure 2 compares two samples of gold nanoparticles, exhibiting na paramagnetic behavior and a ferromagnetic-like one. nThere is a weak but clear hysteresis, and the magnetization ndoes not really saturate even at high field values.

Hysteresis and change of transition temperature in thin films of Fe{[Me2Pyrz]3BH}2, a new sublimable spin-crossover molecule.

Thin films of the spin-crossover (SCO) molecule Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) were sublimed on Si/SiO2 and quartz substrates, and their properties investigated by X-ray absorption and photoemission spectroscopies, optical absorption, atomic force microscopy, and superconducting quantum interference device. Contrary to the previously studied Fe(phen)2(NCS)2, the films are not smooth but granular. The thin films qualitatively retain the typical SCO properties of the powder sample (SCO, thermal hysteresis, soft X-ray induced excited spin-state trapping, and light induced excited spin-state trapping) but present intriguing variations even in micrometer-thick films: the transition temperature decreases when the thickness is decreased, and the hysteresis is affected. We explain this behavior in the light of recent studies focusing on the role of surface energy in the thermodynamics of the spin transition in nano-structures. In the high-spin state at room temperature, the films have a large optical gap (∼5 eV), decreasing at thickness below 50 nm, possibly due to film morphology.

First glimpse of the soft x-ray induced excited spin-state trapping effect dynamics on spin cross-over molecules

The dynamics of the soft x-ray induced excited spin state trapping (SOXIESST) effect of Fe(phen)2(NCS)2 (Fe-phen) powder have been investigated by x-ray absorption spectroscopy (XAS) using the total electron yield method, in a wide temperature range. The low-spin (LS) state is excited into the metastable high-spin (HS) state at a rate that depends on the intensity of the x-ray illumination it receives, and both the temperature and the intensity of the x-ray illumination will affect the maximum HS proportion that is reached. We find that the SOXIESST HS spin state transforms back to the LS state at a rate that is similar to that found for the light induced excited spin state trapping (LIESST) effect. We show that it is possible to use the SOXIESST effect in combination with the LIESST effect to investigate the influence of cooperative behavior on the dynamics of both effects. To investigate the impact of molecular cooperativity, we compare our results on Fe-phen with those obtained for Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) powder, which exhibits a similar thermal transition temperature but with a hysteresis. We find that, while the time constant of the dynamic is identical for both molecules, the SOXIESST effect is less efficient at exciting the HS state in Fe-pyrz than in Fe-phen.

Electronic structure and soft-X-ray-induced photoreduction studies of iron-based magnetic polyoxometalates of type {(M)M5}12FeIII30 (M = MoVI, WVI)

Giant Keplerate-type molecules with a {Mo72Fe30} core show a number of very interesting properties, making them particularly promising for various applications. So far, only limited data on the electronic structure of these molecules from X-ray spectra and electronic structure calculations have been available. Here we present a combined electronic and magnetic structure study of three Keplerate-type nanospheres--two with a {Mo72Fe30} core and one with a {W72Fe30} core by means of X-ray absorption spectroscopy, X-ray magnetic circular dichroism (XMCD), SQUID magnetometry, and complementary theoretical approaches. Furthermore, we present detailed studies of the Fe(3+)-to-Fe(2+) photoreduction process, which is induced under soft X-ray radiation in these molecules. We observe that the photoreduction rate greatly depends on the ligand structure surrounding the Fe ions, with negatively charged ligands leading to a dramatically reduced photoreduction rate. This opens the possibility of tailoring such polyoxometalates by X-ray spectroscopic studies and also for potential applications in the field of X-ray induced photochemistry.

Fast continuous energy scan with dynamic coupling of the monochromator and undulator at the DEIMOS beamline

In order to improve the efficiency of X-ray absorption data recording, a fast scan method, the Turboscan, has been developed on the DEIMOS beamline at Synchrotron SOLEIL, consisting of a software-synchronized continuous motion of the monochromator and undulator motors. This process suppresses the time loss when waiting for the motors to reach their target positions, as well as software dead-time, while preserving excellent beam characteristics.

Magnetism of CoPd self-organized alloy clusters on Au(111)

Magnetic properties of gold-encapsulated CoxPd1−x self-organized nano-clusters on Au(111) are analyzed by x-ray magnetic circular dichroism for xu2009=u20090.5, 0.7, and 1.0. The clusters are superparamagnetic with a blocking temperature decreasing with increasing Pd concentration, due to a reduction of the out-of-plane anisotropy strength. No magnetic moment is detected on Pd in these clusters, within the detection limit, contrary to thick CoPd films. Both reduction of anisotropy and vanishing Pd moment are attributed to strain.

Probing a Device's Active Atoms

Materials science and device studies have, when implemented jointly as operando studies, better revealed the causal link between the properties of the devices materials and its operation, with applications ranging from gas sensing to information and energy technologies. Here, as a further step that maximizes this causal link, the paper focuses on the electronic properties of those atoms that drive a devices operation by using it to read out the materials property. It is demonstrated how this method can reveal insight into the operation of a macroscale, industrial-grade microelectronic device on the atomic level. A magnetic tunnel junctions (MTJs) current, which involves charge transport across different atomic species and interfaces, is measured while these atoms absorb soft X-rays with synchrotron-grade brilliance. X-ray absorption is found to affect magnetotransport when the photon energy and linear polarization are tuned to excite Feuf8ffO bonds parallel to the MTJs interfaces. This explicit link between the devices spintronic performance and these Feuf8ffO bonds, although predicted, challenges conventional wisdom on their detrimental spintronic impact. The technique opens interdisciplinary possibilities to directly probe the role of different atomic species on device operation, and shall considerably simplify the materials science iterations within device research.

Versatile variable temperature insert at the DEIMOS beamline for in situ electrical transport measurements

The design and the first experiments are described of a versatile cryogenic insert used for its electrical transport capabilities. The insert is designed for the cryomagnet installed on the DEIMOS beamline at the SOLEIL synchrotron dedicated to magnetic characterizations through X-ray absorption spectroscopy (XAS) measurements. This development was spurred by the multifunctional properties of novel materials such as multiferroics, in which, for example, the magnetic and electrical orders are intertwined and may be probed using XAS. The insert thus enables XAS to in situ probe this interplay. The implementation of redundant wiring and careful shielding also enables studies on operating electronic devices. Measurements on magnetic tunnel junctions illustrate the potential of the equipment toward XAS studies of in operando electronic devices.

Linking Electronic Transport through a Spin Crossover Thin Film to the Molecular Spin State Using X-ray Absorption Spectroscopy Operando Techniques

One promising route toward encoding information is to utilize the two stable electronic states of a spin crossover molecule. Although this property is clearly manifested in transport across single molecule junctions, evidence linking charge transport across a solid-state device to the molecular films spin state has thus far remained indirect. To establish this link, we deploy materials-centric and device-centric operando experiments involving X-ray absorption spectroscopy. We find a correlation between the temperature dependencies of the junction resistance and the Fe spin state within the devices [Fe(H2B(pz)2)2(NH2-phen)] molecular film. We also factually observe that the Fe molecular site mediates charge transport. Our dual operando studies reveal that transport involves a subset of molecules within an electronically heterogeneous spin crossover film. Our work confers an insight that substantially improves the state-of-the-art regarding spin crossover-based devices, thanks to a methodology that can benefit device studies of other next-generation molecular compounds.

Simple and advanced ferromagnet/molecule spinterfaces

Spin-polarized charge transfer between a ferromagnet and a molecule can promote molecular ferromagnetism 1, 2 and hybridized interfacial states3, 4. Observations of high spin-polarization of Fermi level states at room temperature5 designate such interfaces as a very promising candidate toward achieving a highly spin-polarized, nanoscale current source at room temperature, when compared to other solutions such as half-metallic systems and solid-state tunnelling over the past decades. We will discuss three aspects of this research. 1) Does the ferromagnet/molecule interface, also called an organic spinterface, exhibit this high spin-polarization as a generic feature? Spin-polarized photoemission experiments reveal that a high spin-polarization of electronics states at the Fermi level also exist at the simple interface between ferromagnetic cobalt and amorphous carbon6. Furthermore, this effect is general to an array of ferromagnetic and molecular candidates7. 2) Integrating molecules with intrinsic properties (e.g. spin crossover molecules) into a spinterface toward enhanced functionality requires lowering the charge transfer onto the molecule8 while magnetizing it1,2. We propose to achieve this by utilizing interlayer exchange coupling within a more advanced organic spinterface architecture. We present results at room temperature across the fcc Co(001)/Cu/manganese phthalocyanine (MnPc) system9. 3) Finally, we discuss how the Co/MnPc spinterface’s ferromagnetism stabilizes antiferromagnetic ordering at room temperature onto subsequent molecules away from the spinterface, which in turn can exchange bias the Co layer at low temperature10. Consequences include tunnelling anisotropic magnetoresistance across a CoPc tunnel barrier11. This augurs new possibilities to transmit spin information across organic semiconductors using spin flip excitations12.