Gabriele Spinac

The use of single molecule magnets (SMMs) as cornerstone elements in spintronics and quantum computing applications demands that magnetic bistability is retained when molecules are interfaced with solid conducting surfaces. Here, we employ synchrotron Mössbauer spectroscopy to investigate a monolayer of a tetrairon(III) (Fe₄) SMM chemically grafted on a gold substrate. At low temperature and zero magnetic field, we observe the magnetic pattern of the Fe₄ molecule, indicating slow spin fluctuations compared to the Mössbauer timescale. Significant structural deformations of the magnetic core, induced by the interaction with the substrate, as predicted by ab initio molecular dynamics, are also observed. However, the effects of the modifications occurring at the individual iron sites partially compensate each other, so that slow magnetic relaxation is retained on the surface. Interestingly, these deformations escaped detection by conventional synchrotron-based techniques, like X-ray magnetic circular dichroism, thus highlighting the power of synchrotron Mössbauer spectroscopy for the investigation of hybrid interfaces.

Doping of biocompatible nanomaterials with magnetic phases is currently one of the most promising strategies for the development of advanced magnetic biomaterials. However, especially in the case of iron-doped magnetic hydroxyapatites, it is not clear if the magnetic features come merely from the magnetic phases/ions used as dopants or from complex mechanisms involving interactions at the nanoscale. Here, we report an extensive chemical–physical and magnetic investigation of three hydroxyapatite nanocrystals doped with different iron species and containing small or no amounts of maghemite as a secondary phase. The association of several investigation techniques such as X-ray absorption spectroscopy, Mössbauer, magnetometry, and TEM allowed us to determine that the unusual magnetic properties of Fe^2+/3+-doped hydroxyapatites (FeHA) occur by a synergy of two different phenomena: i.e., (i) interacting superparamagnetism due to the interplay between iron-doped apatite and iron oxide nanoparticles as well as to the occurrence of dipolar interactions and (ii) interacting paramagnetism due to Fe³⁺ ions present in the superficial hydrated layer of the apatite nanophase and, to a lesser extent, paramagnetism due to isolated Fe³⁺ ions in the apatite lattice. We also show that a major player in the activation of the above phenomena is the oxidation of Fe²⁺ into Fe³⁺, as induced by the synthesis process, and their consequent specific positioning in the FeHA structure.

Improving cathode materials is mandatory for next-generation Li-ion batteries. Exploring polyanion compounds with high theoretical capacity such as the lithium metal orthosilicates, Li₂MSiO₄ is of great importance. In particular, mixed silicates represent an advancement with practical applications. Here we present results on a rapid solid state synthesis of mixed Li₂(FeMnCo)SiO₄ samples in a wide compositional range. The solid solution in the P2₁/n space group was found to be stable for high iron concentration or for a cobalt content up to about 0.3 atom per formula unit. Other compositions led to a mixture of polymorphs, namely Pmn2₁ and Pbn2₁. All the samples contained a variable amount of Fe³⁺ ions that was quantified by Mössbauer spectroscopy and confirmed by the T_N values of the paramagnetic to antiferromagnetic transition. Preliminary characterization by cyclic voltammetry revealed the effect of Fe³⁺ on the electrochemical response. Further work is required to determine the impact of these electrode materials on lithium batteries.

Structural and magnetic characterization of Pb₂FeMoO₆ reveals the central role played by nanometer-sized antiphase domains involved in the Fe/Mo cation ordering.

Based on 1-(cyclohexylmethyl)-1H-tetrazole (C₆mtz), a series of mononuclear iron(II) spin-crossover complexes with the general formula [Fe(C₆mtz)₆]X₂, where X is the non-coordinating anion (1), (2), or (3), have been synthesized and characterized. Temperature-dependent magnetic susceptibility measurements for 1, 2, and 3 show reversible one-step spin crossover (SCO) behavior between high-spin (HS, S = 2) and low-spin (LS, S = 0) states without hysteresis. The compound shows spin transition at T_1/2 = 213 K at a considerably higher temperature than the other compounds, (T_1/2 = 126 K) and (T_1/2 = 119 K). Temperature-dependent Far-IR and Mössbauer spectra of 2 and 3 were compared with the measured magnetic susceptibility and confirm the one-step SCO behavior of the compounds.

We report a comprehensive study of the spontaneous magnetization reversal (MRV) performed on the disordered polycrystalline perovskite BiFe _0.5 Mn _0.5 O ₃ , an intriguing compound synthesized in high pressure–high temperature conditions. In disordered systems, the origin of MRV is not completely clarified, yet. In BiFe _0.5 Mn _0.5 O ₃ , compositional disorder involves the ions on the B-site of the perovskite determining the presence of mesoscopic clusters, characterized by high concentrations of iron or manganese and thus by different resultant magnetization. This leads to the observation of two singular fields H ₁ and H ₂ dependent on the degree of inhomogeneity, unpredictably changing from sample to sample due to synthesis effects. These fields separate different magnetic responses of the system; for applied fields H < H ₁ , the Fe and Mn clusters weakly interact in a competitive way, giving rise to MRV, while for an intermediate field regime the energy of this weak interaction becomes comparable to the energy of the system under field application. As a consequence, the zero field cooled magnetization thermal evolution depends on the sample degree of inhomogeneity. In this field regime, applied field Mössbauer spectroscopy indicates that the iron rich clusters are highly polarized by the field, while the largest part of the material, consisting of AFM clusters characterized by axial anisotropy and uncompensated moments, shows soft or hard magnetism depending on T . Above the higher singular field, the M ( T ) curves show the trend expected for a classical antiferromagnetic material and the competitive character is suppressed. The MRV phenomenon results to be highly sensitive on both the thermal and magnetic measurement conditions; for this reason the present work proposes a characterization strategy that in principle has a large applicability in the study of disordered perovskites showing similar phenomenology.

Olivine-type LiFePO₄ is nowadays one of the most important cathode materials of choice for high-energy lithium ion batteries. Its intrinsic defectivity, and chiefly the so-called lithium ironanti-site, is one of the most critical issues when envisaging electrochemical applications. This paper reports a combined diffractometric (Synchrotron Radiation XRD with Rietveld and PDF analyses) and spectroscopic (Mössbauer) approach able to give a thorough characterization of the material defectivity. Such analytical procedure has been applied to a sample prepared following an innovative microwave-assisted hydrothermal synthesis route that, in a few minutes, allowed us to obtain a well crystallized material. PDF analysis, which is applied for the first time to this type of battery material, reveals the presence of disorder possibly due to Li/Fe exchange or to a local symmetry lowering. A 5% amount of iron on the lithium site has been detected both by PDF as well as by Mössbauer spectroscopy, which revealed a small percentage of Fe³⁺ on the regular sites.