J.B.M. da Cunha

Time effects in the thermal annealing of Fe/V multilayers

We report a study on the structural and magnetic properties of iron–vanadium thin films grown in multilayer form and mixed by thermal treatment. The multilayer samples were annealed at 610°C for times ranging from 10 to 540 min. The samples were structurally characterized by means of x-ray diffraction (XRD) and by x-ray absorption spectroscopy (XAS). The magnetic characterization was carried out with a conventional alternating gradient magnetometer (AGM) and by conversion electron Mossbauer spectroscopy (CEMS). The XRD result for the as-deposited multilayer shows a high degree of crystallinity while the CEMS result suggests an abrupt interface, since no significant contribution from vanadium in iron is observed. After the thermal treatment, the results from XRD show a phase transformation of the disordered body-centred-cubic structure (α-phase) into a tetragonal structure (σ-phase) and a subsequent return to the α-phase. This α–σ–α oscillation is not reported in the literature available to the authors.

Synthesis and characterization of Fe-Al2O3 composites

Abstract We have synthesized composites of the type Fex(Al2O3)100−x by arc-melting compacted pellets of iron and alumina powders with starting compositions of x=0.4, 2, 3, 5, 10, 20, 40 and 60. Aluminas of two different nominal grades (99.7% and 99.99%) were used in the preparation of the composites. The samples were characterized by X-ray diffractometry, magnetization and Mossbauer spectroscopy. The results revealed the formation of the spinel FeAl2O3+z (hercynite phase) and of metallic iron partially nanostructured by this melting process. The Mossbauer subspectral areas of the formed phases were plotted against x, showing that increasing the starting iron content of the mixed powders favors the hercynite reaction. It was also observed that when the more pure alumina is used for cast, more hercynite is formed or, alternatively, less iron nanocrystallites precipitate, when compared to the less pure oxide. An arc-melted sample (x=2) was annealed at 1200°C in a hydrogen atmosphere, revealing hercynite reduction and phase separation of metallic iron.

Phase evolution and magnetic properties of a high-energy ball-milled hematite–alumina system

The system (α-Fe2O3)x(α-Al2O3)1−x was subjected to 24 h of high-energy ball-milling varying its nominal concentration, x. The milled samples were structurally and magnetically characterized at room temperature by x-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. Mossbauer studies were also performed in the temperature range 250–6 K. As a result of the earlier analyses, it was observed that the milling products were extremely dependent on the hematite starting concentration. In samples with low α-Fe2O3 initial concentration (i.e., x⩽0.12), the paramagnetic solid solution α-(FeΔYAl1−ΔY)2O3, the α-Fe and the FeAl2O4 phases were identified, along with alumina, which was always residual. The presence of spinel and metallic iron was attributed to the stainless-steel vial and balls abrasion. For x>0.12, the iron component was no longer present but another magnetic component, corresponding to an aluminum-substituted hematite phase, α-(Fe1−ΔWAlΔW)2O3, could be seen to increase with increasing x...

Structural, Magnetic, and Dielectric Investigations of the FeAlO3 Multiferroic Ceramics

The FeAlO3 phase presents an orthorhombic symmetry (Pna21 space group) with a double combination of hexagonal and cubic closed packing of oxygen ions. In this phase, there are four distorted different cationic sites, promoting piezoelectricity, ferrimagnetism and magnetoelectric effects at low temperatures. In this work, high-energy ball milling was employed to produce the FeAlO3 multiferroic compound. Its structural (Rietveld refinement), dielectric and magnetic properties were carefully investigated.

New investigation of the magnetic structure of CoNb2O6 columbite

The structural and magnetic properties of the CoNb2O6 compound have been investigated with a particular interest in their low-dimensional magnetic behavior which is characterized by the presence of weakly interacting ferromagnetic chains. We investigate this cobalt niobate by combining magnetic measurements; x-ray and neutron diffraction (ND). The ND was carried out on powder samples at different temperatures above (20 K) and below the ordering temperature (2.4, 1.8, and 1.4 K). The compound exhibits an orthorhombic crystal structure of Pbcn symmetry, typical of a columbite structure. Magnetic ordering at 2.5 K is found with the propagation vector (0, 0.4, 0), in agreement with earlier studies. However, at lower temperatures the present investigation shows the coexistence of two different magnetic phases: the propagation vector (0.5, 0.5, 0) is found to be necessary to refine the ND measurements at both 1.8 and 1.4 K, in addition to (0, 0.5, 0) which was the only one reported in earlier works.

Characterization of europium implanted LiNbO3

LiNbO 3 single crystals were implanted at room temperature with Eu + ions at 70 keV with fluence ranging from 0.5 to 5 × 10 16 ions · cm −2 . The damage in the implanted layer has been investigated by Channeling Rutherford Backscattering (RBS-C), and the oxidation states of the cations have been determined by x-ray photoelectron spectroscopy (XPS). Following implantation, a fully amorphized layer of 60 nm is generated, even for the lowest fluence employed. Subsequent annealing in air, in the range 800–1250 K, was applied to restore tentatively the crystallinity and promote the substitutional incorporation of Eu in the crystal. Only a partial recrystallization of the damaged layer was observed. For as-implanted samples, XPS spectra clearly reveal europium in Eu 2+ and Eu 3+ states, and the Nb 5+ ions are driven to lower charge states.

Structure refinement of mixed oxides FexCo1−xTa2O6

Abstract We report X-ray powder diffraction measurements performed on synthetic samples of the tapiolite-like Fe x Co 1− x Ta 2 O 6 solid solution. The crystal structure has been refined by the Rietveld method, by using the program fullprof . All the samples are tetragonal and were indexed to the space group P 4 2 / mnm . The end-members (FeTa 2 O 6 and CoTa 2 O 6 ) have unit-cell parameters quite similar to those previously published. The unit-cell parameters a and c for the mixed samples vary linearly with the cation substitution (Co→Fe) according to the Vegards law in all the range of substitution.

Crystal chemistry and structure of the orthorhombic (Fe,Mn)(Ta,Nb)2O6 family of compounds

In this paper we review the crystal chemistry and structure data from the literature and our results reported in the last decade for natural and synthetic samples of the tantalite-columbite series. Order-disorder transitions and oxidation reactions on the ixiolite-columbite-wodginite system are reported for one Brazilian sample. It is shown that reports on these subjects are sometimes dubious and incomplete. In addition to the reaction atmosphere (air or vacuum), order-disorder and oxidation are strongly dependent on the physical state of the samples (powdered or crystal). Structure and chemical composition have been investigated in our laboratory using X-ray diffraction (XRD), electron probe microanalysis (EPMA) and Mossbauer spectroscopy (MS).

Iron oxidation and order-disorder in the (Fe2+, Mn)(Ta, Nb)2O6(Fe2+, Mn)Fe3+(Ta, Nb)2O8 transition

Heat treatments in air and in vacuum have been performed on crystal and powder of the natural tantalite (Mn2+0.88Fe2+0.09)(Ta5+0.86Nb5+0.14)2O2-6, as well as on powder of synthetic Fe2+(Nb5+0.6Ta5+0.4)2O6. Crystal parameters and hyperfine interactions were obtained by use of x-ray diffraction and Mossbauer spectroscopy. It is shown that the partially ordered natural sample is completely ordered after heat treatment in vacuum. Conversely, heat treatment in air induced the tantalite [(Mn2+0.88Fe2+0.09)(Ta5+0.86Nb5+0.14)2O2-6]wodginite [(Mn2+,Fe2+)Fe3+(Ta,Nb)2O8] transformation on the powdered sample. When applied to the crystal sample, the heat treatment in air produced a mixture of two phases: the one in large amount is the ordered (Mn2+0.88Fe2+0.09)(Ta5+0.86Nb0.145+)2O62-, the other, in minor amount, is (Mn2+0.88,Fe2+0.09)Fe3+(Ta,Nb)2O8. The former arrives from cation ordering in the bulk portion of the sample, while the latter results from the near-surface oxidation. The Mn content as well as the oxidant atmosphere appears to play an important role in the transition mechanism. The same heat treatment applied to the synthetic columbite induces a different reaction: ferrocolumbite is transformed into the ixiolite Fe3+(Nb0.6Ta0.4)O4 with a minor amount of (Nb,Ta)2O5.

X-ray diffraction and magnetic susceptibility measurements for FexNi1−xTa2O6

The trirutile family of the tapiolite-like compound FexNi1−xTa2O6 was investigated by means of x-ray powder diffraction (XRD) and magnetic susceptibility measurements. From Rietveld refinement it is demonstrated that the family is a homogeneous solid solution obeying Vegards law. Magnetic susceptibility curves exhibit typical signatures of low-dimensional systems, with broadened maxima due to short range correlations immediately above the antiferromagnetic transition temperature TN. The T versus x phase diagram presents a minimum that suggests bicritical behaviour.