High Density Nanostructured Soft Ferrites Prepared by High Pressure Field Assisted Sintering Technique.

The synthesis of highly compacted, nanostructured soft magnets is highly desirable due to their promising properties for the development of electronic devices working at frequency higher than 2 MHz. In this work we investigated the potentiality of High Pressure Field Assisted Sintering Technique (HP-FAST). To this aim, we first synthesized soft Mn-Zn ferrite magnetic nanoparticles (MNPs) through an easy-scalable, eco-friendly strategy based on aqueous co-precipitation in basic media, starting from transition metal chlorides. Powder X-ray diffraction (PXRD) and Transmission Electron Microscopy (TEM) analyses evidenced the formation of crystalline nanoparticles with the cubic spinel structure and average crystal size of 7.5 nm. Standard magnetometric measurements showed a saturation magnetization value of ca. 56 emu/g and no magnetic irreversibility at room temperature. The MNPs were then compacted applying an uniaxial pressure over a toroidal shaped die. In order to obtain a material with a density close to the bulk one, the as-prepared green toroids underwent either a classic sintering treatment, obtaining a microstructured system, or to High Pressure Field Assisted Sintering Technique (HP-FAST), which allowed for preserving the nanostructure. The relative permeability and core losses of the toroidal samples were evaluated in the frequency range 1-2 MHz using an in-house built setup. The comparison of the behavior of samples obtained by the two different sintering approaches showed the nanostructured samples had a much smaller relative magnetic permeability (ten times lower than the microstructured sample) and, consequently, higher core losses. However, when samples with similar µr were compared, a significant decrease of core losses at the larger frequencies was observed. This result suggests HP-FAST is a very promising approach to prepare high density nanostructured soft magnetic materials.

Ecofriendly nanotechnologies and nanomaterials for environmental applications: Key issue and consensus recommendations for sustainable and ecosafe nanoremediation.

The use of engineered nanomaterials (ENMs) for environmental remediation, known as nanoremediation, represents a challenging and innovative solution, ensuring a quick and efficient removal of pollutants from contaminated sites. Although the growing interest in nanotechnological solutions for pollution remediation, with significant economic investment worldwide, environmental and human risk assessment associated with the use of ENMs is still a matter of debate and nanoremediation is seen yet as an emerging technology. Innovative nanotechnologies applied to water and soil remediation suffer for a proper environmental impact scenario which is limiting the development of specific regulatory measures and the exploitation at European level. The present paper summarizes the findings from the workshop: "Ecofriendly Nanotechnology: state of the art, future perspectives and ecotoxicological evaluation of nanoremediation applied to contaminated sediments and soils" convened during the Biannual ECOtoxicology Meeting 2016 (BECOME) held in Livorno (Italy). Several topics have been discussed and, starting from current state of the art of nanoremediation, which represents a breakthrough in pollution control, the following recommendations have been proposed: (i) ecosafety has to be a priority feature of ENMs intended for nanoremediation; ii) predictive safety assessment of ENMs for environmental remediation is mandatory; (iii) greener, sustainable and innovative nano-structured materials should be further supported; (iii) those ENMs that meet the highest standards of environmental safety will support industrial competitiveness, innovation and sustainability. The workshop aims to favour environmental safety and industrial competitiveness by providing tools and modus operandi for the valorization of public and private investments.

Temperature and pH sensors based on graphenic materials.

Point-of-care applications and patients' real-time monitoring outside a clinical setting would require disposable and durable sensors to provide better therapies and quality of life for patients. This paper describes the fabrication and performances of a temperature and a pH sensor on a biocompatible and wearable board for healthcare applications. The temperature sensor was based on a reduced graphene oxide (rGO) layer that changed its electrical resistivity with the temperature. When tested in a human serum sample between 25 and 43°C, the sensor had a sensitivity of 110±10Ω/°C and an error of 0.4±0.1°C compared with the reference value set in a thermostatic bath. The pH sensor, based on a graphene oxide (GO) sensitive layer, had a sensitivity of 40±4mV/pH in the pH range between 4 and 10. Five sensor prototypes were tested in a human serum sample over one week and the maximum deviation of the average response from reference values obtained by a glass electrode was 0.2pH units. For biological applications, the temperature and pH sensors were successfully tested for in vitro cytotoxicity with human fibroblast cells (MRC-5) over 24h.

Analysis of the electrostatics in Dy(III) single-molecule magnets: the case study of Dy(Murex)3.

A Dy(III)-based single-molecule magnet is reported. Ab initio calculations highlight that molecular symmetry plays a predominant role over site symmetry in determining the shape and orientation of Dy(III) magnetic anisotropy. Moreover the dipolar component of the electrostatic potential created by the surrounding ligands is shown to be the driving force of its magnetic behaviour.

Substrate-induced effects in thin films of a potential magnet composed of metal-free organic radicals deposited on Si(111).

We deposit a paramagnetic pyrene derivative of the nitronyl nitroxide radical on Si(111). The molecules experience a strong chemical interaction with the substrate that influences the film growth. We also study the time evolution of the nitronyl nitroxide radical under a micro-focused soft X-ray beam, observing a stable radical as a product. This result hints at the possibility of using this class of materials in dosimeters and sensors.

Coupling between magnetic and optical properties of stable Au-Fe solid solution nanoparticles.

Au-Fe nanoparticles constitute one of the simplest prototypes of a multifunctional nanomaterial that can exhibit both magnetic and optical (plasmonic) properties. This solid solution, not feasible in the bulk phase diagram in thermal equilibrium, can be formed as a nanostructure by out-of-equilibrium processes. Here, the novel magnetic, optical and magneto-optical properties of ion-implanted Au-Fe solid solution nanoparticles dispersed in a SiO(2) matrix are investigated and correlated. The surface plasmon resonance of the Au-Fe nanoparticles with almost equicomposition is strongly damped when compared to pure Au and to Au-rich Au-Fe nanoparticles. In all cases, the Au atoms are magnetically polarized, as measured by x-ray magnetic circular dichroism, and ferromagnetically coupled with Fe atoms. Although the chemical stability of Au-Fe nanoparticles is larger than that of Fe nanoparticles, both the magnetic moment per Fe atom and the order temperature are smaller. These results suggest that electronic and magnetic properties are more influenced by the hybridization of the electronic bands in the Au-Fe solid solution than by size effects. On the other hand, the magneto-optical transitions allowed in the vis-nIR spectral regions are very similar. In addition, we also observe, after studying the properties of thermally treated samples, that the Au-Fe alloy is stabilized, not by surface effects, but by the combination of the out-of-equilibrium nature of the ion implantation technique and by changes in the properties due to size effects.

Two-step magnetic ordering in quasi-one-dimensional helimagnets: possible experimental validation of villain's conjecture about a chiral spin liquid phase.

Low-temperature specific heat, magnetic susceptibility, and zero-field muon spin resonance (microSR) measurements have been performed in the quasi-one-dimensional molecular helimagnetic compound Gd(hfac)3NITEt. The specific heat presents two anomalies at T(0)=2.19+/-0.02 K and T(N)=1.88+/-0.02 K, which both disappear upon the application of a weak magnetic field. Conversely, magnetic susceptibility and muSR data show the divergence of two-spin correlation functions only at T(N)=1.88+/-0.02 K. These results suggest an experimental validation of Villain's conjecture of a two-step magnetic ordering in quasi-one-dimensional XY helimagnets; i.e., the paramagnetic phase and the helical spin solid phase are separated by a chiral spin liquid phase, where translational invariance is broken without violation of rotational invariance.

Polynuclear lanthanide hydroxo complexes: new chemical precursors for coordination polymers.

The synthesis of hexanuclear lanthanide hydroxo complexes by controlled hydrolysis led to polymorphic compounds. The hexanuclear entities crystallize in four different ways that depend on the extent of their hydration. The four structures can be described as hexanuclear lanthanide entities with formula [Ln(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(6)(H(2)O)(12)](2+). Two additional NO(3)(-) ions intercalate between the hexanuclear entities in order to ensure the electroneutrality of the crystal structure. Some crystallization water molecules fill the intermolecular space. The three first families of compounds (1-3) exhibit crystal structures that have previously been reported. The fourth family of compounds (4) is described here for the first time. Its chemical formula is [Ln(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(6)(H(2)O)(12)](NO(3))(2).2H(2)O (Ln = Gd, Er, and Y). In this paper, the chemical and thermal stabilities of the hexanuclear lanthanide compounds are reported together with the magnetic properties of the Gd(III)-containing species. To use these entities as precursors for new materials, the substitution of the nitrato groups by chloride ions has been studied. Two byproduct compounds have so been obtained: The first (compound 5) is a nitrato/chloride hexanuclear compound of chemical formula [Er(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(6)(H(2)O)(12)](NO(3))Cl.2H(2)O. The second one (compound 6) is a polymeric compound in which the hexanuclear entities are linked by an unexpected and original N(2)O(4) bridge. Its chemical formula is [Er(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(4)(H(2)O)(11)(OH)(ONONO(2))]Cl(3).2H(2)O. Its crystal structure can be described as the juxtaposition of chainlike molecular motifs. To the best of our knowledge, this is the first example of a coordination polymer synthesized from an isolated polylanthanide hydroxo complex.

Combined charge and spin density experimental study of the yttrium(III) semiquinonato complex Y(HBPz3)2(DTBSQ) and DFT calculations.

High-resolution X-ray diffraction and polarized neutron diffraction experiments have been performed on the Y-semiquinonate complex, Y(HBPz3)2(DTBSQ), in order to determine the charge and spin densities in the paramagnetic ground state, S = (1/2). The aim of these combined studies is to bring new insights to the antiferromagnetic coupling mechanism between the semiquinonate radical and the rare earth ion in the isomorphous Gd(HBPz3)2(DTBSQ) complex. The experimental charge density at 106 K yields detailed information about the bonding between the Y3+ ion and the semiquinonate ligand; the topological charge of the yttrium atom indicates a transfer of about 1.5 electrons from the radical toward the Y3+ ion in the complex, in agreement with DFT calculations. The electron density deformation map reveals well-resolved oxygen lone pairs with one lobe polarized toward the yttrium atom. The determination of the induced spin density at 1.9 K under an applied magnetic field of 9.5 T permits the visualization of the delocalized magnetic orbital of the radical throughout the entire molecule. The spin is mainly distributed on the oxygen atoms [O1 (0.12(1) mu B), O2(0.11(1) mu B)] and the carbon atoms [C21 (0.24(1) mu B), C22(0.20(1) mu B), C24(0.16(1) mu B), C25(0.12(1) mu B)] of the carbonyl ring. A significant spin delocalization on the yttrium site of 0.08(2) mu B is observed, proving that a direct overlap with the radical magnetic orbital can occur at the rare earth site and lead to antiferromagnetic coupling. The DFT calculations are in good quantitative agreement with the experimental charge density results, but they underestimate the spin delocalization of the oxygen toward the yttrium and the carbon atoms of the carbonyl ring.

Finite-size effects in single chain magnets: an experimental and theoretical study.

The problem of finite-size effects in s=1/2 Ising systems showing slow dynamics of the magnetization is investigated introducing diamagnetic impurities in a Co2+-radical chain. The static magnetic properties have been measured and analyzed considering the peculiarities induced by the ferrimagnetic character of the compound. The dynamic susceptibility shows that an Arrhenius law is observed with the same energy barrier for the pure and the doped compounds while the prefactor decreases, as theoretically predicted. Multiple spin reversal has also been investigated.

Ising-type magnetic anisotropy in a cobalt(II) nitronyl nitroxide compound: a key to understanding the formation of molecular magnetic nanowires.

The compound [Co(hfac)2-(NITPhOMe)2] (2) (hfac = hexafluoroacetylacetonate, NITPhOMe = 4'-methoxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) crystallizes in the triclinic P1 space group, a= 10.870(5), b = 11.520(5), c = 19.749(5) A, alpha = 78.05(5), beta = 84.20(5), gamma = 64.51(5) degrees, Z = 2. It can be considered a model system for studying the nature of the magnetic anisotropy of [Co(hfac)2(NITPhOMe)] (1), which was recently reported to behave as a molecular magnetic wire. The magnetic anisotropy of 2 was investigated by EPR spectroscopy and SQUID magnetometry both in the polycrystalline powder and in a single crystal. The experimental magnetic anisotropy was related to the anisotropy of the central ion and to the exchange interaction between the cobalt(II) ion and the radicals.

Hydroxo-bridged Cubane-type tetrairon(II) clusters supported by sterically-hindered carboxylate ligands.

A series of hydroxo-bridged cubane-type tetrairon(II) clusters, [Fe(4)(mu-OH)(4)(mu-O(2)CAr(4)(-)t(BuPh))(2)(mu-OTf)(2)L(4)] (L = C(5)H(5)N (1), 4-(t)BuC(5)H(4)N (2), 3-FC(5)H(4)N (3)), were synthesized by using a sterically hindered carboxylate ligand, 2,6-di(4-tert-butylphenyl)benzoate (Ar(4)(-)t(BuPh)CO(2)(-)). Three different bridging units that mediate weak antiferromagnetic coupling interactions between the metal centers support the unprecedented cubane-type [Fe(4)(mu-OH)(4)](4+) cores in 1-3. The solution structures of 1 and 3 probed by FT-IR and (19)F NMR spectroscopy are consistent with the solid-state geometry determined by X-ray crystallography. Zero-field Mössbauer spectra of 1-3 at 4.2 K are characteristic of high-spin iron(II) centers in nearly identical coordination environments. Compound 1 undergoes two irreversible oxidation processes at ca -10 and +880 mV (vs Fc/Fc(+)), the former approaching quasi-reversible behavior with increased scan rates and a narrow potential sweep range. Comparisons are made with analogous known [Fe(4)X(4)](n)()(+) (X = O, S) units, and the structural integrity of tetrairon fragments upon a change in oxidation state is discussed together with some possible biological implications.

Pressure- and temperature-induced valence tautomeric interconversion in a o-dioxolene adduct of a cobalt-tetraazamacrocycle complex.

An electronic switch at the molecular level has been realized by using a class of ionic compounds of the formula [Co(L)(diox)]Y (L = tetraazamacrocyclic ligand, Y = mononegative anion). Such compounds undergo temperature- and pressure-induced intramolecular one-electron transfer equilibria. The transition temperature of interconversion varies with the nature of the counterions Y (Y = PF6, BPh4, I). Surprisingly the effect of the anion on the transition temperature is not only governed by its volume but also by its coulombic interaction.

A bis-bidentate dioxolene ligand induces thermal hysteresis in valence tautomerism interconversion processes.

The use of a bis-bidentate ligand in a solid cobalt dioxolene complex affords the necessary cooperative properties that lead to thermal hysteresis in a valence tautomeric interconversion equilibrium.

Tuning the magnetic properties of the high-spin molecular cluster Fe8.

The synthesis, crystal structure, and magnetic characterization of a high-spin cluster comprising eight iron ions are presented in this contribution. The cluster has formula [(tacn)6Fe8O2(OH)12Br4.3(ClO4)3.7]·6H2O (Fe8pcl), where tacn is the organic ligand 1,4,7-triazacyclononane. It can be considered a derivative of Fe8 Br8 , a cluster whose low-temperature magnetization dynamics has been extensively investigated, in which four of the bromide ions have been replaced by perchlorate anions. The structure of the central core of the two molecules, [Fe8O(OH)12(tacn)6](8+), is essentially the same, but Fe8pcl has a higher symmetry (the bromide derivative crystallizes in the acentric P1 space group while Fe8pcl crystallizes in the monoclinic P2(1)/c space group). The magnetic properties of Fe8pcl suggest it is very similar to Fe8Br8 having a S=10 ground state as well. The zero-field splitting parameters were accurately determined by high field-high frequency EPR (HF-EPR) measurements. The two clusters have similar axial anisotropy D but Fe8pcl has a larger transverse anisotropy E: The value of E/D is 0.21 for the perchlorate derivative but 0.19 for Fe8Br8. AC susceptibility measurements revealed the cluster behaves like a superparamagnetic particle. However, due to the occurrence of large terms in the transverse anisotropy, the temperature dependence of the relaxation time cannot be reproduced by a simple Arrhenius law model. As observed in the bromide derivative, below 350 mK the relaxation time becomes temperature independent and indicating that a pure tunneling regime is attained. The comparison of the tunneling rate in the two clusters shows that in the perchlorate derivative the relaxation process is 35 times faster. The observed ratio of the tunneling rates is in reasonable agreement with that calculated from the tunneling splitting, namely the energy difference between the two almost-degenerate lowest levels Ms =±10, in the two clusters.

Phonon-assisted tunneling in the quantum regime of Mn12 acetate

Longitudinal or transverse magnetic fields applied on a crystal of Mn12 acetate allows one to observe independent tunnel transitions between m = -S+p and m = S-n-p ( n = 6-10, p = 0-2 in longitudinal field and n = p = 0 in transverse field). We observe a smooth transition (in longitudinal) from coherent ground-state to thermally activated tunneling. Furthermore, two ground-state relaxation regimes show a crossover between quantum spin relaxation far from equilibrium and near equilibrium, when the environment destroys multimolecule correlations. Finally, we stress that the complete Hamiltonian of Mn12 should contain odd spin operators of low order.

Effects of nuclear spins on the quantum relaxation of the magnetization for the molecular nanomagnet Fe8

The strong influence of nuclear spins on resonant quantum tunneling in the molecular cluster Fe8 is demonstrated for the first time by comparing the relaxation rate of the standard Fe8 sample with two isotopic modified samples: (i) 56Fe is replaced by 57Fe, and (ii) a fraction of 1H is replaced by 2H. By using a recently developed "hole digging" method, we measured an intrinsic broadening which is driven by the hyperfine fields. Our measurements are in good agreement with numerical hyperfine calculations. For T>1.5 K, the influence of nuclear spins on the relaxation rate is less important, suggesting that spin-phonon coupling dominates the relaxation rate.

Anions derived from squaric acid form interionic pi-stack and layered, hydrogen-bonded superstructures with organometallic sandwich cations: the magnetic behaviour of crystalline

Depending on the stoichiometric ratio, squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione, H2SQA) reacts with [(eta6-C6H6)2Cr] in THF to form the crystalline material [(eta6-C6H6)2Cr][HSQA] (1) and in water to yield [[(eta6-C6H6)2Cr]2][SQA] x 6H2O (3); it also reacts with [(eta5-C5H5)2Co][OH] in water to form [[(eta5-C5H5)2Co]2][SQA] x 6H2O (4). Compound 1 is almost isostructural with the previously reported salt [(eta5-C5H5)2Co][HSQA] (2); its structure is based on pi-pi stacks between the benzene ligands and the hydrogen squarate anionic chains (pi-pi distance 3.375 A). Compounds 3 and 4 are isomorphous and have a structure in which layers of organometallic cations intercalate with layers of water molecules hydrogen bonded to squarate dianions. All crystals contain charge-assisted C-Hdelta+...Odelta- hydrogen bonds between the organometallic and the organic components, while negative O-H(-)...O(-) and O-H...O(2-) interactions are present in the pairs 1/3 and 2/4, respectively. In constrast to most organic salts of [(eta6-C6H6)2Cr]+ and [(eta5-C5H5)2Co]+ which are yellow, crystals of compounds 1-4 are orange. Reflectance spectra measured on the crystalline material 1 show the presence of an intense tail that can be assigned to a charge-transfer transition through the [(eta6-C6H6)2Cr]+/[HSQA]- pi-stacking interactions, while the pi stacking in 2 causes only a broadening of the band. The magnetic behaviour of 1 and 3 has been investigated by SQUID magnetometry. Both compounds are characterised by a weak antiferromagnetic interaction between the S=1/2 Cr centres of the [(eta6-C6H6)2Cr]+ cations, which is significantly stronger in 1 due to the pi-stacking with the HSQA- anions.