XRD,magnetic and Mössbauer spectral studies of nano size aluminum substituted cobalt ferrites (CoAlxFe2−xO4)

Aluminum substituted cobalt ferrites having particle size ∼10 nm were prepared using aerosol route, which increases with annealing temperature. The unit cell parameter ‘a’ and saturation magnetization decreases linearly with the increase of aluminum concentration. This is due to the smaller ionic radius and the diamagnetic nature of the Al³⁺, respectively. Room temperature Mössbauer spectra of the samples annealed at 1200 °C show broad sextets, which were fitted with different sextets, indicating different local environment of both tetrahedrally and octahedrally coordinated iron cation. Further, hyperfine field of the samples decreases with the increase of Al³⁺ concentration due to the fact that the Al³⁺ diamagnetic species reduce the magnetic interactions. Cations distribution indicated a increase in Fe³⁺(oct.)/Fe³⁺(tet.) ratio on increasing the Al³⁺ concentration.     

Effects of annealing temperature on structure and magnetic properties of CoAl0.2Fe1.8O4/SiO2 nanocomposites

CoAl_0.2Fe_1.8O₄/SiO₂ nanocomposites were prepared by sol–gel method. The effects of annealing temperature on the structure and magnetic properties of the samples were studied by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer and Mössbauer spectroscopy. The results show that the CoAl_0.2Fe_1.8O₄ in the samples exhibits a spinel structure after being annealed. As annealing temperature increases from 800 to 1200 °C, the average grain size of CoAl_0.2Fe_1.8O₄ in the nanocomposites increases from 5 to 41 nm while the lattice constant decreases from 0.8397 to 0.8391 nm, the saturation magnetization increases from 21.96 to 41.53 emu/g. Coercivity reaches a maximum of 1082 Oe for the sample annealed at 1100 °C, and thereafter decreases with further increasing annealing temperature. Mössbauer spectra show that the isomer shift decreases, hyperfine field increases and the samples transfer from mixed state of superparamagnetic and magnetic order to the completely magnetic order with annealing temperature increasing from 800 to 1200 °C.     

Recoilless fraction, structural, magnetic and thermal properties of Fe73.5Cu1Nb3Si13.5B9 alloy

Differential scanning calorimetry, X-ray diffraction and room temperature Mössbauer spectrum measurements of Fe_73.5Cu₁Nb₃Si_13.5B₉ (Finemet) alloy have been carried out in order to study its structural and magnetic properties as a function of annealing temperature. The DSC profile of as-quenched Finemet showed two exothermic peaks at 530 and 702 °C, corresponding to two crystallization processes. The Finemet alloy remains amorphous at 450 °C with one broad peak in XRD pattern and one broad sextet in Mössbauer spectrum. When the Finemet alloy was annealed at 550 °C, only well indexed body-center-cubic phase was detected. After being annealed at 650 and 750 °C, the XRD patterns showed the coexistence of α-Fe(Si) and Fe-B intermetallic phases with the increase in XRD peak intensities, indicating the growth of crystallites and the decomposition of Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy at elevated temperatures. The Mössbauer spectra of annealed Finemet alloy could be fitted with 4 or 5 sextets and one doublet at higher annealing temperatures, revealing the appearance of different crystalline phases corresponding to the different Fe sites above the crystallization temperature. The appearance of the nanocrystalline phases at different annealing temperatures was further confirmed by the recoilless fraction measurements.     

XRD, HRTEM, magnetic and Mössbauer studies on chemically prepared Fe-doped nanoparticles of cerium oxide

Nanocrystalline samples of Fe-doped cerium oxide (Ce_0.90Fe_0.1O₂) are prepared by sol-gel method. The precursor materials used for the synthesis are ferric nitrate and cerium nitrate. The as-prepared samples is annealed at different temperatures to obtain the sample with different particle sizes. The crystallographic phases of the nanocrystalline materials have been confirmed by X-ray diffractograms (XRD). The sizes of the nanoparticles estimated from the peaks of the XRD patterns using Debye-Scherrer equation are in the range 6-58 nm. Results extracted from the high-resolution transmission electron microscopy (HRTEM) are in agreement with the findings obtained from XRD. The average magnetic susceptibilities of all the samples with different particle sizes are measured in the temperature range 300-14 K. The average susceptibilities of the samples annealed below ∼740 °C show paramagnetic behaviour. The susceptibilities of the samples annealed at and above ∼740 °C sharply decrease at ∼240 K and this sharp transition is quite likely due to the anti-parallel alignment of Fe³⁺ spins and is attributed to Morin transition of α-Fe₂O₃. Mössbauer spectra of the samples annealed at and above ∼740 °C give sextet patterns indicating the presence of exchange interaction among the Fe³⁺ ions of these samples and these sextets are also of typical nature of the α-Fe₂O₃ phase. The Mössbauer spectra of the samples annealed below ∼740 °C are doublets which may be attributed to either superparamagnetic and/or paramagnetic type nanoparticles.     

Effects of annealing temperature on structure and magnetic properties of amorphous Fe61Co27P12 nanowire arrays

Arrays of Fe₆₁Co₂₇P₁₂ nanowire with an aspect ratio about 70 were prepared in anodic aluminum oxide templates by electrodeposition. The influences of annealing temperature on structure and magnetic properties of Fe₆₁Co₂₇P₁₂ nanowires were studied. When the specimens were annealed below 400 °C, there are no obvious changes in structure except relaxation. With the annealing temperature increasing from 400 to 600 °C, the Fe-Co phase is detected by X-ray diffraction and Mössbauer spectra. The crystalline fraction and hyperfine field can be derived from Mössbauer spectra. The room temperature magnetic hysteresis loops show that the coercivity and squareness of the nanowire arrays in parallel to the wire axis increase with the increasing of annealing temperature, which mainly attributes to the strengthening of anisotropy.     

Influence of aluminum doping in Li–Co–Ti ferrite

The crystallographic and magnetic properties of low aluminum doped lithium cobalt titanium ferrites, Li_0.5Co_0.2Ti_0.2Al_xFe_2.1−xO₄(0.0≤x≤0.5), in the scope of spinel structure and ferrimagnetic property were investigated. Ferrites were doped with aluminum in the range of 0.0–0.5 and were synthesized by using the conventional ceramic methods. Using X-ray diffraction and Mössbauer spectroscopy, we confirmed the formation of crystallized particles. All of the samples showed a single phase with a spinel structure, and the lattice parameters linearly decreased as the doped aluminum content was increased. The particle size of the samples also decreased as the doped aluminum content increased. Until x=0.4 in Li_0.5Co_0.2Ti_0.2Al_xFe_2.1−xO₄, the Mössbauer spectra could be fitted with two Zeeman sextets, which is the typical spinel ferrite spectra of Fe³⁺ with A- and B-sites. However, for x=0.5, the Mössbauer spectrum could be fitted with two Zeeman sextets and one doublet. From the variation of the Mössbauer parameters and the absorption area ratio, the cation distributions were determined. The magnetic behavior of the samples showed that an increase in the aluminum contents led to a decrease in the saturation magnetization, whereas the coercivity decreased until x=0.4 and then increased. The minimum coercivity was 52.4 Oe at x=0.4 in Li_0.5Co_0.2Ti_0.2Al_xFe_2.1−xO₄.     

Mössbauer and X-Ray Diffraction Study of the Phase and Structure Transformations During Annealing of Mechanically Alloyed Al65Cu23Fe12

A powder mixture of Al₆₅Cu₂₃Fe₁₂ composition milled for four hours in a planetary ball mill was used as a starting material for quasicrystalline structure formation. Phase analysis of as-milled samples shows a mixture of intermetallic phases with preponderance of A2 and D0₃ structures. Also the presence of residual Al and Fe was detected. Annealing the milled samples gives rise to complex solid-state transformations. Mössbauer spectroscopy shows that the residual Fe amount decreases gradually with annealing temperature and that starting from the temperature of 440°C only an asymmetrical doublet is present. The formation of a structure almost totally icosahedral was observed in the XRD patterns of samples annealed at 600°C and above. The qualitative differences in the Mössbauer spectra observed for these samples may be associated with the presence of approximant phases to quasicrystalline structure.     

Mössbauer Study of the Thermal Behaviour of Garnets Used in High-Energy Water Jet Technologies

A high-energy water jet combined with silicate garnets as abrasives has been proven to be a powerful tool for disintegration of hard materials. Thermal heating of the garnets is one way for structure improvement of the abrasive material. Room-temperature Mössbauer spectra of initial powdered almandine samples are characterised by one doublet corresponding to Fe²⁺ in dodecahedral position 24c. Almandine garnet, industrial product Barton HP 80 as reference material in all experiments, has a second doublet corresponding to Fe³⁺ in octahedral position 16a. In room-temperature spectra of heated almandine garnet samples from locality Ktí and Mdnec (heated under temperatures 200–1000°C by 100 degrees for 1 hour in air) a new doublet originating from -Fe₂O₃ nanoparticles appeared. Under a heating temperature of higher than 800°C, the broad sextets of -Fe₂O₃ and -Fe₂O₃ in spectra were discovered. No additional doublets or sextets appeared in room-temperature Mössbauer spectra of reference material of almandine garnet Barton HP 80 heated under each temperature.     

Structural, microstructural and hyperfine properties of nanocrystalline iron particles

Nanocrystalline Fe particles were successfully prepared by the mechanical milling process using a high-energy planetary ball mill. The physical properties of the samples were investigated as a function of the milling time, t (in the 0-54 h range) by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and Mössbauer spectroscopy. After 54 h of milling, the lattice parameter increases from 0.28620 (3) nm for the starting Fe powder to 0.28667 (3) nm, the grain size decreases from 110 to 13 nm, while the strain increases from 0.09% to 0.7%. The powder particle morphology was observed by SEM at different stages of milling. For t less than 24 h, the Mössbauer spectra are characterized by one sextet corresponding to the crystalline bcc Fe phase, while for t greater than 24 h, the iron particles exhibit a two-component Mössbauer spectrum due to the presence of two phases: the crystallites bcc Fe phase and the grain-boundary region. The appearance and the increase in intensity of the second sextet with t may indicate that the interfacial region effect increases with milling time due to the grain size reduction and a probable disordered state of the grain boundaries.     

Synthesis, structural, magnetic and optical properties of nanocrystalline ZnFe2O4

Nanocrystalline zinc ferrite (ZnFe₂O₄) is synthesized by high-energy ball-milling after 12 h from a powders mixture of zinc oxide (ZnO) and hematite (α-Fe₂O₃) with balls to powders mass ratio of 20:1. X-ray diffraction, vibrating sample magnetometer (VSM), the Mössbauer spectrometry and photoluminescence (PL) are used to characterize the samples. Rietveld analysis and VSM measurements show that the powder has an average crystallites size of 10 nm and a ferrimagnetic behavior with a saturation magnetization of 30 emu/g. After annealing at 700 °C, the lattice parameter reduces from 8.448 to 8.427 Å and the sample transforms into a superparamagnetic behavior, which was confirmed as well by the room temperature Mössbauer spectrometry. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed. Finally, the broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.13 eV.     

Nanostructured Fe50Ni50 alloy formed by chemical reduction

A high purity Fe₅₀Ni₅₀ nanometric alloy was synthesized by ultra rapid autocatalytic chemical reduction of the corresponding transition metal ions in an aqueous solution. The ratio of metal concentration in solution is preserved in the precipitated powder alloy and no metal segregation has been detected. The alloy was characterized as a nanostructured chemically disordered taenite phase by X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). Transmission electron microscopy (TEM) showed that the as prepared alloy contained spherical particles with 96 nm mean diameter size. The particles are composed of crystallites (of ∼15 nm size) and a predominant disordered interfacial region. A thermal treatment of 673 K/2 h produced a structural relaxation with a significant narrowing in the XRD and Mössbauer lines with a exothermic flow in the DSC signal and an increase in the crystallite size to 30 nm.     

The γ-Fe formation in epitaxial Cu(0 0 1)/Fe(0 0 1) thin films by the solid-state synthesis: Structural and magnetic features

The γ-Fe formation in epitaxial Cu(0 0 1)/Fe(0 0 1) bilayers with the annealing temperature increasing has been studied. Using Mössbauer spectroscopy, X-ray diffraction, and magnetic analysis, structural and chemical characterization of the interface between cooper and iron layers has been performed. After annealing at 850 °C and subsequent cooling to room temperature, paramagnetic γ-Fe(0 0 1) precipitates with an average size of 30 nm coherent to a Cu(0 0 1) matrix form. Conditions of intermixing and the γ-Fe formation at the Cu/Fe interface are explained in terms of the solid-state synthesis in thin films. Specific features of the martensitic transition γ↔α are discussed.     

Magnetic Structure of FexCu100−x Magnetoresistive Alloys Produced by Mechanical Alloying

Fe _x Cu_100–x magnetoresistive alloys were produced by mechanical alloying. X-ray diffraction shows fcc structure. The room-temperature Mössbauer spectra evolves from an asymmetrical doublet below x=25%, to a broad magnetic hyperfine field distribution above this concentration. Quadrupole splitting of the doublet varies between 0.48 and 0.57 mm/s, and its isomer shift from 0.16 to 0.29 mm/s. Low-temperature Mössbauer spectroscopy displays a B _hf distribution. Magnetization measurements display different features depending on concentration, from mictomagnetism to ferromagnetism. Low-temperature magnetoresistance is measured. Samples with x20% exhibit larger magnetoresistivity ratios. Bulk and hyperfine magnetic properties are correlated in order to explain magnetoresistivity features of these samples.     

Thermoluminescent and stuctural properties of BaAl2O4:Eu,Nd,Gdphosphors prepared by combustion method

BaAl₂O₄:Eu²⁺,Nd³⁺,Gd³⁺ phosphors were prepared by a combustion method at different initiating temperatures (400–1200 °C), using urea as a comburent. The powders were annealed at different temperatures in the range of 400–1100 °C for 3 h. X-ray diffraction data show that the crystallinity of the BaAl₂O₄ structure greatly improved with increasing annealing temperature. Blue-green photoluminescence, with persistent/long afterglow, was observed at 498 nm. This emission was attributed to the 4f⁶5d¹–4f⁷ transitions of Eu²⁺ ions. The phosphorescence decay curves were obtained by irradiating the samples with a 365 nm UV light. The glow curves of the as-prepared and the annealed samples were investigated in this study. The thermoluminescent (TL) glow peaks of the samples prepared at 600 °C and 1200 °C were both stable at ∼72 °C suggesting that the traps responsible for the bands were fixed at this position irrespective of annealing temperature. These bands are at a similar position, which suggests that the traps responsible for these bands are similar. The rate of decay of the sample annealed at 600 °C was faster than that of the sample prepared at 1200 °C.     

Synthesis and characterization of the xZnO-(1−x)α-Fe2O3 nanoparticles system

The xZnO-(1−x)α-Fe₂O₃ nanoparticles system has been obtained by mechanochemical activation for x=0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 h. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy. The Rietveld structure of the XRD spectra yielded the dependence of the particle size and lattice constant on the amount x of Zn substitutions and as function of the ball milling time. The x=0.1 XRD spectra are consistent with line broadening as Zn substitutes Fe in the hematite structure and the appearance of the zinc ferrite phase at milling times longer than 4 h. Similar results were obtained for x=0.3, while for x=0.5 the zinc ferrite phase occurred at 2 h and entirely dominated the spectrum at 24 h milling time. The Mössbauer spectra corresponding to x=0.1 exhibit line broadening as the ball milling time increases, in agreement with the model of local atomic environment. Because of this reason, the Mössbauer spectrum for 12 h of milling had to be fitted with two sextets. For x=0.3 and 12 milling hours, the Mössbauer spectrum reveals the occurrence of a quadrupole-split doublet, with the hyperfine parameters characteristic to zinc ferrite, ZnFe₂O₄. This doublet clearly dominates the Mössbauer spectrum for x=0.5 and 24 h of milling, demonstrating that the entire system of nanoparticles consists finally of zinc ferrite. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.     

Observation of bulk like magnetic ordering below the blocking temperature in nanosized zinc ferrite

We investigated the magnetic behavior of nanosized zinc ferrite with the help of vibrating sample magnetometry and in-field Mössbauer spectroscopy. The nanoparticles of zinc ferrite with crystallite size ranging from 10 to 62 nm were synthesized by a nitrate method. The structure and phase were determined with the help of X-ray diffraction. Attributes of cation inversion were found with the calculated values of lattice parameter. The saturation magnetization decreases with the increase in crystallite size at room temperature, while these values are almost the same at 10 K for all the samples except the one with crystallite size of 10 nm. The thermal magnetization measurement shows a decrease in blocking temperature with increase in particle size for these samples. The synthesized samples exhibit the presence of antiferromagnetic ordering below the blocking temperature as investigated by in-field Mössbauer spectroscopy.     

High-Temperature Mössbauer Spectroscopy of Mechanically Milled NiFe2O4

Oxide spinels, in particular those containing iron, often exhibit technically important electrical- and magnetic-properties. We report here on X-ray powder diffraction and Mössbauer studies of nanostructured NiFe₂O₄ particles prepared by high-energy ball milling from bulk NiFe₂O₄, which is an inverse spinel. The Mössbauer spectra were recorded in situ at different temperatures in the range of 300–850 K. The Mössbauer spectra of the milled samples show a broad distribution of magnetic hyperfine fields together with a paramagnetic state at room temperature. Initially, at 700 K the spectrum is mainly paramagnetic, but during the process of annealing, magnetic sextets emerge. The treatment results in a significant change in the B/A area ratio of the ferrite. The Neél temperature of the samples is estimated from the B(T) relation to be in the range of 800–850 K.     

Evolution of structural and magnetic properties of FePt/C granular films with thermal annealing

We report the structural and magnetic properties of as-deposited and thermally annealed FePt/C granular multilayer films. The as-deposited system exhibits a disordered fcc FePt phase with an average grain size of 3 nm. Thermal annealing at 650 °C results in partial L1₀ ordering and an associated grain growth to 7 nm. Mössbauer measurements show that there is no non-magnetic component present, suggesting that carbon resides only in the grain boundary region. The ferromagnetic grains are magnetically decoupled.     

Evolution of magnetic and microstructural properties of thick sputtered NdFeB films with processing temperature

Ta (100 nm)/NdFeB (5 μm)/Ta (100 nm) films have been deposited onto Si substrates using triode sputtering (deposition rate ∼18 μm/h). A 2-step procedure was used: deposition at temperatures up to 400 °C followed by ex-situ annealing at higher temperatures. Post-deposition annealing temperatures above 650 °C are needed to develop high values of coercivity. The duration of the annealing time is more critical in anisotropic samples deposited onto heated substrates than in isotropic samples deposited at lower temperatures. For a given set of annealing conditions (750 °C/10′), high heating rates (?2000 °C/h) favour high coercivity in both isotropic and anisotropic films. The shape and size of Nd₂Fe₁₄B grains depend strongly on the heating rate.     

Effects of annealing temperature on YAG:Ce synthesized by spray-drying method

In this report, YAG:Ce phosphors were synthesized by spray-drying method. The effects of annealing temperature on crystal structure, morphology and photoluminescence property (PL) of as-prepared samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectrofluorometer, respectively. The XRD patterns showed all the samples are in consistence with a single garnet phase, and the location of strongest peak shifts to smaller angle with increasing the annealing temperature. The SEM micrographs revealed the sample annealed at 1200 °C appears to be a spherical polycrystalline aggregate; as the samples were annealed at 1300?1400 °C, spherical grains obviously grow up; but the sample annealed at over 1400 °C forms an irregular bulk. The emission spectra of samples indicated the PL of samples annealed at 1200?1400 °C improve with increasing the annealing temperature because of the diffusion of Si⁴⁺ ions; whereas the PL of sample annealed at the temperature over 1400 °C decreases likely resulting from inflection effects of multiangular shape of grains. Therefore, the samples annealed at 1400 °C are suitable for gaining phosphor with high brightness and good morphology.