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.     

Fe-doped TiO2 prepared by mechanical alloying

The effect of different milling conditions on the formation of Fe-doped TiO₂ powders by mechanical alloying was investigated by Mössbauer spectrometry. The milling conditions investigated were ball to powder weight ratio, milling time, rotation velocity of supporting disc, and the type of starting reactive iron and its concentration. X-ray diffraction shows that high energy mechanical milling of undoped anatase TiO₂ induce the anatase to rutile phase transformation via high pressure srilankite. Mössbauer spectra for the majority of the doped samples were decomposed into one sextet and one or two doublets. The sextets was attributed to the presence of α-Fe or hematite impurities. The doublets were assigned to Fe^3?+? incorporated in the TiO₂ structure, and to the Fe^2?+? located either at the surface or the interstitial sites of TiO₂. A greater incorporation of Fe in the TiO₂ structure was observed when samples were prepared from hematite instead of α-Fe.     

Visible light activated photo-catalytic effect and local structure of iron silicate glass prepared by sol-gel method

A relationship between methylene blue (MB) decomposition ability under visible light and local structure of xFe₂O₃·(100-x)SiO₂ glass abbreviated as xFS prepared by sol-gel method was investigated by ⁵⁷Fe-Mössbauer spectroscopy, X-ray diffractometry (XRD) and ultraviolet-visible light absorption spectroscopy (UV-Vis). Mössbauer spectra of xFS glass with x of 10, 30 and 50 annealed at 1000 °C for 3 h were mainly composed of a paramagnetic doublet due to fayalite (Fe₂SiO₄), and magnetic sextets due to magnetite (Fe₃O₄) or hematite (α-Fe₂O₃). The absorption area (A) of α-Fe₂O₃ gradually increased from 0.0 to 10.3 and 100 % with the increasing Fe₂O₃ content (x) of annealed xFS glass. A leaching test performed by 20 mL of MB aqueous solution and 40 mg of annealed 50FS glass showed that MB concentration decreased from 16.2 to 4.7 μmol L^?1 after 2 h with the first order rate constant of 1.8 × 10^?4 s^?1. These results prove that annealed iron silicate glass containing α-Fe₂O₃ can decompose MB effectively under visible light irradiation.     

Coatings and weathering rinds at Gusev crater, Mars, investigated by depth selective Mössbauer spectroscopy

Powders of Al substituted Ni-ferrites (NiAl _x Fe_2???x O₄ x?=?0, 0.5, 1.0 and 1.5) were prepared using the calcination method. Mössbauer spectroscopy results of all samples show two well defined magnetic sextets corresponding to Fe^3?+? ions in the A (tetrahedral) and B (octahedral) sites of the spinel structure. In addition, a quadrupole doublet with quadrupole splitting of (~0.53 mm/s) starts evolving upon Al substitution. This doublet is associated with a new Fe–Al-oxide phase. The X-ray diffraction patterns show, in addition to the original peaks, new peaks evolve and increase in intensity as the Al concentration increases, confirming the evolution of this new phase.     

Decrease of superparamagnetic fraction at room temperature in ultrafine CoFe2O4 particles by Ag doping

Co_1???x Ag _x Fe₂O₄ nanoparticles have been prepared by the combustion route. The average crystallite sizes for compositions with x = 0 and 0.2 are found to be 36 and 33 nm respectively from the XRD line broadening. Compared to the pure CoFe₂O₄, Ag-doping reduces the intrinsic magnetization values (M, M _r), but enhances coercivity (H _c). Mössbauer spectra show two sextets, indicating occupancies of tetrahedral and octahedral sites by Fe^3?+?. Hyperfine fields of 505 and 477 kOe in pure CoFe₂O₄ have been found for octahedral and tetrahedral sites respectively at liquid nitrogen temperature. The hyperfine field decreases with Ag-doping which also corroborates the magnetization studies. EPR study confirms the room temperature ferromagnetic behavior for Co_1???x Ag _x Fe₂O₄ (x = 0.2). The room temperature Mössbauer studies on x?=?0.0 and 0.2 show the ferromagnetic sextets (95%) along with superparamagnetic doublet (5%). However, x = 0.6 sample shows the ferromagnetic sextets only at room temperature. Highly Ag doped samples could be useful for the fabrication of the high-density magnetic materials as well as magnetic drug delivery.     

Mössbauer characteristics of glauconitisation

Using 50 MeV Li^3?+? ion irradiation, the change induced in polycrystalline ferrites Li_0.5(1?+?x)Ti _x Al_0.1Fe_2.4???1.5x O₄ (x = 0.0 to 0.3, step–0.1)[LTAF] and Li_0.5(1?+?x)Ti _x Cr_0.1Fe_2.4???1.5x O₄ (x = 0.0 to 0.3, step–0.1; LTCF) in the electronic stopping power regime is studied. Both the systems were irradiated with the same fluence of 5 × 10¹³ ions/cm². The modifications of the structural and magnetic properties are studied by means of X-ray diffraction (XRD), magnetization, ⁵⁷Fe Mössbauer spectroscopy and low field a. c. susceptibility. The contrast in the role of Ti^4?+? in the presence of Al^3?+? and Cr^3?+? causing the formation of paramagnetic centres through Swift Heavy Ion Irradiation (SHII) induced cation rearrangement has been revealed through the comparative Mössbauer signatures of both the systems. The hyperfine interaction parameters deduced through Mössbauer spectra are also discussed before and after irradiation. The observed reduction in the saturation magnetic moment and Curie temperature after irradiation supports the partial formation of paramagnetic centres and rearrangement of cations in the lattice.     

Mössbauer study of spin structure transformation from an incommensurate to a commensurate state

We present crystallographic and magnetic properties of NiCr_1.98 ⁵⁷Fe_0.02O₄ by using X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), and Mössbauer spectroscopy. The lattice constants a₀ were determined to be 8.318 Å. The ferrimagnetic Neel temperature (T _N) for NiCr_1.98 ⁵⁷Fe_0.02O₄ is determined to be 90 K. The Mössbauer absorption spectra for all chromites at 4.2 K show two well developed sextets superposed with small difference of hyperfine fields (H _hf) caused by Cr^3?+? ions in two different magnetic sites. The values of the isomer shifts show that the charge states of Fe are Fe^3?+? for all temperature range. Ni-chromites Mössbauer spectra below T _N present aline broadening due to a Jahn–Teller distortion and show that spin structure behavior of Cr ions change from an incommensurate to a commensurate state.     

α-Fe2O3-In2O3 mixed oxide nanoparticles synthesized under hydrothermal supercritical conditions

α-Fe₂O₃-In₂O₃ mixed oxide nanoparticles system has been synthesized by hydrothermal supercritical and postannealing route, starting with (1−x)Fe(NO₃)₃·9H₂O·xIn(NO₃)₃·5H₂O aqueous solution (x=0-1). X-ray diffraction and Mössbauer spectroscopy have been used to study the phase structure and substitutions in the nanosized samples. The concentration regions for the existence of the solid solutions in the α-Fe₂O₃-In₂O₃ nanoparticle system together with the solubility limits of In³⁺ ions in the hematite lattice and of Fe³⁺ ions in the cubic In₂O₃ structure have been evidenced. In general, the substitution level is considerably lower than the nominal concentration x. A justification of the processes leading to the formation of iron and indium phases in the investigated supercritical hydrothermal system has been given.     

Hyperfine interactions and dynamics characteristics of 119Sn in xSnO2-(1-x)α-Fe2O3 nanoparticle system

The hyperfine interactions and dynamics aspects in nanoparticles system of the xSnO₂-(1-x)α-Fe₂O₃ (x = 0.0–1.0) have been investigated by ¹¹⁹Sn-Mössbauer and XRD techniques. The change revealed by the XRD (XRDSs) and Mössbauer spectra (MSs) vs. x was discussed in terms of the phase and lattice-dynamics changes. The evidences of the triple hyperfine magnetic fields’ distributions (MHFDs) and their dependencies x (<0.175) are discussed taking into account the super-transferred hyperfine interactions (STHI), the local disorder Sn^4?+?/Fe^3?+? around the tin isotope probe and the reduced dimension effect.     

Synthesis and characterization of magnetite-maghemite nanoparticles obtained by the high-energy ball milling method

We present the process of synthesis and characterization of magnetite-maghemite nanoparticles by the ball milling method. The particles were synthesized in a planetary ball mill equipped with vials and balls of tempered steel, employing dry and wet conditions. For dry milling, we employed microstructured analytical-grade hematite (α-Fe₂O₃), while for wet milling, we mixed hematite and deionized water. Milling products were characterized by X-ray diffraction, transmission electron microscopy, room temperature Mössbauer spectroscopy, vibrating sample magnetometry, and atomic absorption spectroscopy. The Mössbauer spectrum of the dry milling product was well fitted with two sextets of hematite, while the spectrum of the wet milling product was well fitted with three sextets of spinel phase. X-ray measurements confirmed the phases identified by Mössbauer spectroscopy in both milling conditions and a reduction in the crystallinity of the dry milling product. TEM measurements showed that the products of dry milling for 100 h and wet milling for 24 h consist of aggregates of nanoparticles distributed in size, with mean particle size of 10 and 15 nm, respectively. Magnetization measurements of the wet milling product showed little coercivity and a saturation magnetization around 69 emu g^?1, characteristic of a nano-spinel system. Atomic absorption measurements showed that the chromium contamination in the wet milling product is approximately two orders of magnitude greater than that found in the dry milling product for 24 h, indicating that the material of the milling bodies, liberated more widely in wet conditions, plays an important role in the conversion hematite-spinel phase.     

Monitoring by Mössbauer spectroscopy the thermal reduction of hematite into magnetite: the surface effect and charge disproportionality in iron oxide nanoparticles

Nanoparticles of magnetite Fe₃O₄ were synthesized by thermal reduction of hematite α-Fe₂O₃ powder in the presence of high boiling point solvent. The structural transformations and magnetic properties of the obtained nanoparticles were investigated by the ⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at each step of the chemical and thermal treatment of the product. An increase of saturation magnetization with the reaction time correlates with an increase of concentration of magnetite in the samples. The electron hoping between Fe^2?+? and Fe^3?+? ions in the octahedral sites of the magnetite nanoparticles and Verwey phase transition were investigated. It was established that not all iron ions in the octahedral sites participated in electron hoping Fe^2?+????Fe^3?+? above the Verwey temperature T _V, and the charge distribution could be expressed as $\big( {{\rm Fe}^{3+}}\big)_{{\rm tet}} \big[ {{\rm Fe}_{1.85}^{2.5+} {\rm Fe}_{0.15}^{3+} }\big]_{{\rm oct}} {\rm O}_4$ .     

Magnetic studies of Bi x Y3-x Fe5O12 fabricated using conventional method

A series of Bi substituted yttrium iron garnet (Bi-YIG) nanoparticles with nominal formula of Bi _x Y_3???x Fe₅O₁₂ in which x varied in steps of 0.0, 0.25 and 0.5 are prepared by conventional method. Vibration sample magnetometer (VSM) at Room temperature (RT) shows saturation magnetization decreases from 27.4 to 25.2 (emu/g) as x value increases from 0.0 to 0.5. Room temperature ⁵⁷Fe Mössbauer spectra are recorded for these series. The hyperfine field value for octahedral and tetrahedral of samples increases from 484 and 390 kOe to 491 and 397 kOe respectability, as Bi replaces Y in (Bi _x Y_3???x Fe₅O₁₂) atom with increasing x value. The effect of Bi^3?+? substitution for Y^3?+? on lattice constants, morphology and magnetic properties of pure YIG has been investigated.     

Magnetic and Mössbauer studies on nanocrystalline Co1−x Li x Fe2O4 (x = 0, 0.2)

Ultrafine particles of Co_1???x Li _x Fe₂O₄ (x?= 0, 0.2) samples are prepared by glycine–nitrate combustion route. X-ray diffraction and transmission electron microscopy studies show that the samples have cubic spinel structure and average crystallite sizes of x?= 0 and 0.2 are 36 and 44 nm respectively. Vibrating sample magnetometer studies revealed the ferromagnetic nature of the samples. Li-doped CoFe₂O₄ sample showed higher values of coercive field, remanent magnetization and saturation magnetization compared to pure CoF₂O₄ indicating the enhancement of magnetic interactions. Mössbauer spectra at 77 K exhibited two broad sextets indicating that Fe^3?+? ions occupy both tetrahedral and octahedral sites. From these studies, it is concluded that Co_1???x Li _x Fe₂O₄ (x?= 0, 0.2) samples exhibit an inverse spinel structure. At room temperature, two sextets are superimposed on a very broad non-Lorentzian background indicating the presence of superparamgnetic fraction in agreement with the microscopic observations.     

Effect of Co 2 + content on the magnetic properties of Co x Fe 3 − x O 4 /SiO 2 nanocomposites

Non-stoichiometric Co_xFe_3???xO₄/SiO₂ (x = 0.8, 0.9, 1.0, 1.1) nanocomposites have been prepared by sol-gel method. The structure, morphology and magnetic properties of the obtained samples were characterized by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer and Mössbauer spectroscopy at room temperature. As the Co^2?+? content increases, the average particle size of the spherical Co_xFe_3???xO₄ in the samples decreases and the lattice constants increases. The hyperfine fields for both A- and B-site decrease, while the fraction of Co^2?+? occupying the A-site increases. Magnetization measurements show the saturation magnetization and coercivity of Co_xFe_3???xO₄/SiO₂ decrease with increasing Co^2?+? content. The decrease in magnetization results from the weakened A-B interactions between Fe^3?+?, and the change in coercivity can be related to the variation of Co^2?+? at B-site and the decreasing particle size.     

Effect of Ti4+ substitution on the hyperfine properties of Li–Sb–Ti ferrites using Mössbauer spectroscopy

Mössbauer investigations were carried out at room temperature on the ferrite system Li_0.6?+?0.5tFe_2.3???1.5tTi_tSb_0.1O₄ (0.0 ≤ t ≤ 1.0 in steps of 0.2). The effect of Ti^4?+? concentration on the various hyperfine interactions like Isomer shift, quadrupole splitting and internal magnetic field have been studied. The spectra exhibited well-defined Zeeman sextets at low substitution level, corresponding to the A and B sites. The sample with t = 1.0 showed paramagnetic behaviour. The results obtained have been discussed.     

Effect of indium addition on the structure and magnetic properties of YIG

In this work we report the structure and magnetic properties of a series of single-phase indium-substituted yttrium iron garnet (In-YIG) nanoparticles with nominal composition of Y₃In_xFe_5−xO₁₂ (x=0.1, 0.2, 0.3 and 0.4) prepared by conventional mixed oxide route. Based on XRD results, the lattice parameters of the samples increased with increase in In³⁺ content due to its larger ionic radius. Mössbauer results confirmed the substitution of In³⁺ for Fe³⁺ in [a] site of YIG structure. Further, the magnitudes of the magnetic hyperfine field (MHF) were seen to reduce due to indium substitution. Moreover, a rising trend was observed for saturation magnetization (M_S) of the samples with x>0.2 owing to the substitution of non-magnetic In³⁺ for Fe³⁺. However, the observed initial drop of M_S for the sample with x=0.2 compared to that with x=0.1 is possibly attributed to the dominance of spin canting over the net magnetization rise caused by In³⁺ in [a] sites.     

Magnetic properties of the mixed spinel Co1+xSnxFe2−2xO4

The structural and magnetic properties of the mixed spinel Co_1+xSn_xFe_2?2xO₄ system for 0.1≤x≤0.5 have been studied by means of X‐ray diffraction, magnetization, a.c. susceptibility and Mössbauer effect measurements. X‐ray intensity calculations indicate that Sn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Co²⁺ ions substitute for A‐site Fe³⁺ ions. The lattice constants are determined and the applicability of Vegard's law has been tested. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at tetrahedral (A) and octahedral (B) sites for x≤0.4. The Mössbauer intensity data show that Sn possesses a preference for the B‐site of the spinel. As expected, the hyperfine field and Curie temperature determined from a.c. susceptibility decreases with increasing Sn content. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with Sn content is satisfactorily explained on the basis of Néel's collinear spin ordering model for x=0.1–0.4.     

Mössbauer study of Al and Cr co-substituted Yttrium iron garnets

The Mössbauer spectra of Y₃Al _x Cr _X Fe_5???2x O₁₂ (x?=?0.0 to 0.6) measured at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at the octahedral (a) and the tetrahedral (d) sites for x?≤ 0.6. The isomer shifts (δ) and quadrupole splitting (ΔE_Q) indicate the presence of high spin Fe³⁺ ions in the tetrahedral (d) and octahedral (a) sites, typical of yttrium of yttrium iron garnet structure. Mössbauer results have shown that Al³⁺, enters a-sites only but Cr³⁺ enters both a-and d-sites.     

Weathering of Martian and Earth surface studied by Mössbauer spectroscopy

Martian regolith and Earth’s basaltoid samples have been investigated by means of Mössbauer spectroscopy. The identification of the same minerals: olivine, pyroxene, magnetite, hematite and confrontation of the Fe^3?+?/Fe^2?+?, Fe^3?+?/Fe_tot, Fe^2?+?/Fe_tot ratios are presented. Co-existence of olivine and hematite in Martian regolith, absent in presented by authors terrestrial samples has been tentatively explained.     

57Co-emission Mössbauer study on diluted magnetic semiconductor TiO2 films

Rutile-type Co _x Ti_1???x O_2???d laser deposited on alumina substrate was studied after ⁵⁷Co doping, using emission Mössbauer spectroscopy. It was found that although under certain conditions (i.e., which result in high conductivity) the thin rutile layers showed magnetooptical effect, the Mössbauer probe did not reveal any magnetic splitting, but showed paramagnetic Co^3?+? (Fe^3?+?) in the films of low electric resistance. This supports that these samples have carrier induced magnetism which is also connected to the defect structure of the rutile films.