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The iron(II1) "picket-fence" porphyrin complex [ Fe1ii(TPpivP)(OS0,CF3)(H20)] (1) was synthesized and characterized by its UV-visible, 'H NMR, EPR, magnetic, and Mossbauer properti'es. The X-ray structure of 1 was determined at -100 OC. Crystal data: [Fe(TPpivP)(OS0,CF3)(HzO)] (C65H6sN80sF3sFe); monoclinic; a = 13.161 (3), b = 19.196 (6), c = 26.212 (6) A; 0 = 103.34 (2)"; Z = 4; d,,, = 1.270 g cm-); space group P2i/c. The six-coordinate iron atom is bonded to the four porphyrinato nitrogens (Fe-N, = 2.021 (16) A), to an oxygen atom of the triflate ion (Fe-O(triflate) = 2.188 (5) A), placed inside the molecular cavity of the picket-fence porphyrin, and to a water molecule (Fe-O(water) = 2.133 (5) A). The susceptibility measurementsin an external field of 1.5 T show that the effective magnetic moment varies from 4.0 to 5.7 pB in the temperature range 2-300 K. The EPR data yield g, = 5.7, which corresponds to a mixture of 85% spin sextet $Ai and 15% spin quartet 'A2 in the lowest Kramers doublet. This mixture is somewhat stronger than in typical high-spin iron porphyrins. Mossbauer spectra were recorded at temperatures varying from 4.2 to 300 K in fields of 0-6.21 T. They exhibit temperature-independent quadrupole splitting, AEQ = 2.2 mm s-l, which lies between the AEQ values characteristic for high-spin (S = 5/2) and intermediate-spin (S = 3/z) porphyrins. The magnetic hyperfine patterns of the measured Mossbauer spectra, in the slow and fast relaxation limit, are successfully simulated within the IO-state model for the spin mixture between 6A, and 4A2 by using parameters that have been derived from susceptibility and EPR data within the same model. The applicability of the usual spin (S = 5/2) Hamiltonian analysis and its relation to the IGstate model are discussed. Spinspin and spin-lattice relaxation effects are explicitly accounted for in the intermediate relaxation regime within the framework of the (S = 5/z) spin Hamiltonian. The degree of spin mixture together with the axial and porphyrinato-nitrogen coordination of iron in 1 and related complexes is discussed on the basis of a putative spin-state/stereochemical relationship.

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The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL(1)) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL(2)) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe(II)(HL)(2)](ClO(4))(2), [Fe(II)L(2)] and [Fe(III)L(2)]ClO(4) (HL = HL(1) or HL(2)) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe(III)(L(1))(2)]ClO(4) is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe(III)(L(1))(2)]ClO(4) and [Fe(III)(L(2))(2)]ClO(4) are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the (6)A(1)↔(2)T(2) transition curve is sigmoidal with T(½)∼280 K and the SCO is virtually complete. As regards [Fe(III)(L(1))(2)]ClO(4), Mössbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state.

Nanocrystalline EuCrO3 particles (∼25 nm) have been prepared by pre-milling a 1 : 1 molar mixture of Eu2O3 and Cr2O3 for 60 h followed by sintering at 700 °C (12 h). This temperature is ∼500–600 °C lower than those at which the material, in bulk form, is conventionally prepared. Rietveld analysis of the x-ray powder diffraction pattern of the EuCrO3 nanoparticles favours a structural model involving a slight degree of cationic exchange where ∼11% of the Eu3+ and Cr3+ ions exchange their normal dodecahedral A- and octahedral B-sites, respectively, in the perovskite-related structure. This cationic site exchange, which is unusual in a perovskite structure, has been well supported by the corresponding room-temperature 151Eu Mössbauer spectrum of the nanoparticles that in addition to displaying a distribution in the principal component of the EFG tensor (V zz ) at the usual A-sites of the 151Eu nuclei, also revealed the presence of a subcomponent with ∼11% area fraction and a considerably increased |V zz | value that was associated with Eu3+ ions at octahedral B-sites. X-ray photoelectron and Auger electron spectroscopic techniques reveal a complex surface structure where extremely thin layers of un-reacted Eu2O3 and Cr2O3 cover most of the EuCrO3 nanoparticles' surfaces together with some traces of elemental Cr. The binding energies associated with Eu3+ 3d5/2, Eu3+ 4d3/2, Cr3+ 2p3/2 and O2− 1s core-level electrons in EuCrO3 are estimated from the x-ray photoelectron data for the first time.

We have carried out extensive measurements on novel Fe3O4–γ-Fe2O3 core–shell nanoparticles of nearly similar core diameter (8 nm) and of various shell thicknesses of 1 nm (sample S1), 3 nm (sample S2), and 5 nm (sample S3). The structure and morphology of the samples were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The direct current (DC) magnetic measurements were carried out using a superconducting quantum interference device (SQUID). Exchange bias and coercivity were investigated at several temperatures where the applied field was varied between 3 and −3 T. Several key results are obtained, such as: (a) the complete absence of exchange bias effect in sample S3; (b) the occurrence of nonconventional exchange bias effect in samples S2 and S1; (c) the sign-change of exchange bias field in sample S2; (d) the monotonic increase of coercivity with temperature above 100 K in all samples; (e) the existence of a critical temperature (100 K) at which the coercivity is minimum; (f) the surprising suppression of coercivity upon field-cooling; and (g) the observation of coercivity at all temperatures, even at 300 K. The results are discussed and attributed to the existence of spin glass clusters at the core–shell interface.

Nickel -chromium ferrites NiCr x Fe 2-x O 4 (0 ≤ x ≤ 1.4) were studied using X-ray diffraction, magnetization measurements and Mössbauer spectroscopy. X-ray diffraction patterns show that all samples have cubic spinel structure. The temperature dependent magnetic measurements revealed that a magnetic compensation point appears at chromium concentration x = 0.9, 1.2 and 1.4. Moreover, it was observed that the Curie temperature T C decreases and approaches the compensation temperature T K as Cr 3+ substitution for Fe 3+ increases. The magnetization data at all concentrations are discussed in the light of Néel's molecular field model taking into account the cation distributions obtained using the analysis of Mössbauer spectra.