Magnetic and Mössbauer studies of pure and Ti-doped YFeO 3 nanocrystalline particles prepared by mechanical milling and subsequent sintering

Single-phased nanocrystalline particles of pure and 10 % Ti ⁴⁺-doped perovskite-related YFeO ₃were prepared via mechanosynthesis at 450^°C. This temperature is ～150–350 ^°C lower than those at which the materials, in bulk form, are normally prepared. Rietveld refinements of the X-ray diffraction patterns reveal that the dopant Ti ⁴⁺ ions prefer interstitial octahedral sites in the orthorhombic crystal lattice rather than those originally occupied by the expelled Fe ³⁺ ions. Magnetic measurements show canted antiferromagnetism in both types of nanoparticles. Doping with Ti ⁴⁺ lowers the Néel temperature of the YFeO ₃ nanoparticles from ～ 586 K to ～ 521 K. The Ti ⁴⁺-doped YFeO ₃ nanoparticles exhibit enhanced magnetization and coercivity but less magnetic hyperfine fields relative to the un-doped nanoparticles. The ⁵⁷Fe Mössbauer spectra show ～ 15 % of the YFeO ₃ nanoparticles and ～22 of Ti ⁴⁺-doped YFeO ₃ ones to be superparamagnetic with blocking temperatures < 78 K. The broadened magnetic components in the ⁵⁷Fe Mössbauer spectra suggest size-dependent hyperfine magnetic fields at the ⁵⁷Fe nuclear sites and were associated with collective magnetic excitations. The ⁵⁷Fe Mössbauer spectra show the local environments of the Fe ³⁺ ions in the superparamagnetic nanoparticles to be more sensitive to the presence of the Ti ⁴⁺ ions relative to those in the larger magnetic nanoparticles.     

Mössbauer and magnetic studies of nanocrystalline zinc ferrites synthesized by microwave combustion method

Zinc ferrite nano-crystals were synthesized by a microwave assisted combustion route with varying the urea to metal nitrates (U/N) molar ratio The process takes only a few minutes to obtain Zinc ferrite powders. The Effect of U/N ratio on the obtained phases, particle size, magnetization and structural properties has been investigated. The specimens were characterized by XRD, Mössbauer and VSM techniques. The sample prepared with urea/metal nitrate ratio of 1/1 was a poorly crystalline phase with very small crystallite size. A second phase is also detected in the sample. The crystallite size increases while the second phase decrease with increasing the urea ratio. The saturation magnetization and coercivity of the as prepared nano-particles changed with the change of the U/N ratio. The powder with the highest U/N ratio showed the presence of an unusually high saturation magnetization of 16 emu/g at room temperature. The crystallinity of the as prepared powder was developed by annealing the samples at 700 ^°C and 900 ^°C. Both the saturation magnetization (Ms) and the remnant magnetization (Mr) were found to be highly dependent upon the annealing temperature. Mössbauer studies show magnetic ordering in the powder even at room temperature. The Mössbauer and the magnetic parameters of this fraction are different from the standard values for bulk zinc ferrite.     

Mössbauer and PAC studies of nanocrystalline Fe

High-purity Fe powder was mechanically milled under argon at ambient temperature using an SPEX 8000 mill. The local atomic and magnetic structure was studied using⁵⁷Co/Fe Mössbauer and¹¹¹In/Cd perturbed angular correlations (PAC) spectroscopies. After 32 hours of milling, X-ray diffraction revealed effective grain diameters of 18 nm and energy-dispersive X-ray analysis indicated a Cr impurity concentration of 5%, presumably introduced by mechanical attrition of steel ball bearings used for milling. In addition to a spectral component very similar to bulk iron metal, the Mössbauer spectra exhibited hyperfine field shifts attributed to the Cr impurities. PAC spectra on Fe milled for 5 h, with no contamination, exhibited two components: (1) A slightly broadened magnetic interaction attributed to interior, defect-free sites of In/Cd probes with a mean hyperfine field slightly greater than in macroscopic grains. The defect-free site fraction grew appreciably during milling, even though In is essentially insoluble in Fe. (2) An indistinct signal due to mixed magnetic and quadrupole interactions attributed to probes at surface or other defect sites.     

Mössbauer studies and magnetic properties of Ln2SrCu2O6 compounds

Mössbauer and magnetic susceptibility measurements were used to study the magnetic properties of Ln_1.9Sr_1.1Cu₂O₆ (Ln=Pr,Nd) and La₂SrCu₂O₆ materials. These compounds were prepared by solid-state reaction and crystallize in a tetragonal structure, space group I4/mmm with two formula units per unit cell. There is only one crystallographic site for Cu atoms, which form a double layer of CuO₅ pyramids. These compounds are not superconducting, but we show, using Mössbauer spectroscopy (MS) on iron doped samples and susceptibility measurements, that the Cu planes order antiferromagnetically. The hyperfine fields on iron nuclei at 4.2 K extend from 472 kOe for La₂SrCu₂O₆ to 501 kOe for Nd_1.9Sr_1.1Cu₂O₆. The ordering temperaturesT _N are: R20, 190, and 250 K for Ln=La, Pr and Nd, respectively.     

Mössbauer spectroscopy and magnetic studies of orientated textured Fe3O4 ferrofluid

Nano scale magnetite based ferrofluid is synthesized by chemical co pre cipitation technique and stabilized with oleic acid. Magnetization and viscosity measurements were used to optimize for texturing purpose. The freeze-textured ferrofluid in two configurations, namely, (1) field texture system (FTS) and (2) zero field texture system (ZTS) are investigated by magnetization measurements at 298 K and Mössbauer spectroscopy measurements at 77 and 298 K. These results are analysed on the basis of the contributions from collective superparamagnetic reversal and the strength of the inter particle interactions.     

Mechanosynthesis of nanocrystalline MgFe2O4—neutron diffraction and Mössbauer spectroscopy

The evolution of nanocrystalline n-MgFe₂O₄ by high-energy milling a mixture of MgO and α-Fe₂O₃ for periods of between 0 h and 12 h has been investigated by neutron diffraction in addition to previous Mössbauer, XRD and HRTEM measurements. Complete transformation of the milled products to n-MgFe₂O₄ only occurs on milling to ～8 h even though the average particle size decreases to <?～10 nm after milling for 2 h. The applied field Mössbauer spectra of n-MgFe2O4 can be well described by two subspectra representing core and shell regions with different cation distributions and spin canting angles. The neutron pattern of nanocrystalline MgFe₂O₄ is described well by two components comprising nanoparticles of core and shell dimensions ～7(1) nm and ～0.7(1) nm, respectively, in support of the Mössbauer core-shell model.     

Mechanosynthesis,magnetic and Mössbauer characterization of pure and Ti4+-doped cubic phase BiFeO3 nanocrystalline particles

The mechanosynthesis of cubic γ-phase pure BiFeO₃ and Ti⁴⁺-doped BiFeO₃ nanocrystalline particles and their preliminary characterization with magnetic measurements and Mössbauer spectroscopy are reported. The BiFeO₃ nanoparticles (5–40 nm) were prepared by heating a 48 h pre-milled 1:1 molar mixture of α-Bi₂O₃ and α-Fe₂O₃ at 400 °C for (1 h). Doping α-Fe₂O₃ in the initial mixture of reactants with Ti⁴⁺ was found to lead to the formation of Ti⁴⁺-doped BiFeO₃ nanoparticles by milling the reactants for 32 h. The magnetization of the BiFeO₃ nanoparticles is found to be tripled under a maximum external field of 1.35 T and their magnetic hardness increases by ～15 times relative to those of the corresponding bulk. The Ti⁴⁺-doped BiFeO₃ nanoparticles exhibit higher magnetization relative to the pure ones. These observations are related to the spiral modulated spin structure of the compound. The Mössbauer data show ～12 % of the BiFeO₃ nanocrystalline particles to be superparamagnetic having blocking temperatures of less than 78 K. The quadrupole shift values of the magnetic spectral component favor the cubic structural symmetry. These observations were mainly associated with possible collective magnetic excitations as well as transverse relaxation of canted surface spins. The Ti-doped BiFeO₃ nanoparticles gave statistically-poor Mössbauer spectra with no signs of a superparamagnetic behavior.     

Mössbauer studies and magnetic properties of spinel lead ferrite

In the present communication, we have reported the synthesis of nanocrystalline lead ferrite (PbFe₂O₄) by citrate mediated autocombustion method. X-ray diffraction pattern reveals the single phase formation in cubic (spinel) structure. The particle size and the surface morphology of the samples are characterized by TEM and SEM analysis. Magnetic studies are carried out using vibrating sample magnetometer (VSM) shows a very high coercive field for the material. Mössbauer studies were performed to investigate the local symmetry i.e. Fe is in octahedral/tetrahedral site and the charge states of Fe ions.     

Mössbauer spectroscopy of magnetic small particles with emphasis on barium ferrite

Ferromagnetic and ferrimagnetic particles have been of scientific and technological interest for several decades. The study of nanometer clusters or particles is currently a developing subject. Such materials may be in a non-equilibrium or quasi-equilibrium phase; different properties as compared to the bulk and indeed even new physical phenomena may be expected. Some ways to synthesize clusters and fine particles are described. Mössbauer spectroscopy is shown to be particularly useful for the study of nanometer particles; an outline of how it actually works is given. As an illustration, barium ferrite small particles, a material of topical interest, is considered in detail. In particular, methods of preparation, the crystal and magnetic structure, the magnetic characteristics, recently obtained Mössbauer results, and potential and realized applications are reviewed.     

Microwave absorption and Mössbauer studies of Fe3O4 nanoparticles

Materials consisting of nanometer-sized magnetic particles are currently the subject of intensive research activities. Especially, much attention has been paid to their promising features for microwave magnetic properties. Well dispersed Fe₃O₄ nanoparticles of 30 nm have been synthesized by oxidization method with NaNO₂, and the microwave magnetic properties of the composites have been studied. The real and imaginary part of relative permittivity remained low and nearly constant in the region of 0.1–18 GHz, respectively. As a result, the resin composites having a thickness of 2.0–3.2 mm, and containing 20 vol% Fe₃O₄ in the form of nanoparticles with an average diameter of 30 nm, exhibited excellent electromagnetic wave absorption properties in the frequency range of 4.5–12.0 GHz.     

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.     

Mössbauer and mineral magnetic studies on archaeological potteries from Adhichanallur, Tamilnadu, India

Megalithic potteries collected from Adhichanallur, Tamilnadu, India (Lat. 8°44′ N; Long. 77°42′ E) have been subjected to various spectroscopic and rock magnetic studies. The type of clay, their origin, level of structural deformation due to firing, firing temperature and atmospheric condition followed during making the potteries are analyzed. The potteries were subjected to Mössbauer and X-ray diffraction studies to analyze the iron phases in them. It is found that the samples were made of local clay (red clay), fired above 600°C under open atmospheric and/or reduced atmospheric conditions and air has been allowed during cooling. The Mössbauer spectra reveal the presence of Fe^3?+?, Fe^2?+? and iron oxides of hematite and magnetite. The firing temperature and firing conditions established from Mössbauer studies are similar to the observation made from FT-IR studies. The magnetic mineral types, the mass fractions and the domain states of the constituent magnetic grains were elucidated from a range of rock magnetic measurements including variation of susceptibility with low field, frequency and temperature, hysteresis parameters and isothermal remanence acquisition data. The magnetic mineralogy of most pottery samples was dominated by magnetite/(titano) magnetite, while magnetic grain size spectrum varies from very fine (super paramagnetic) to fine (stable single domain, pseudo single domain). The reversible thermo magnetic behavior reflects no mineralogical transformations during reheating and all the samples show same Curie temperature 580°C due to magnetite. From the above information it is demonstrated that these samples are suitable for determining the reliable ancient geomagnetic field intensity values existed during that period.     

Mössbauer and magnetic studies of nanocomposites containing iron oxides and humic acids

A sample of volcanic ashes emanated from the Osorno volcano, southern Chile, was characterized with X-ray fluorescence, X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy, in an attempt to identify the iron-bearing minerals of that geologically recent magmatic deposit. X-ray patterns indicated that the sample is mainly constituted of anorthite, Fe-diopside-type and Ca-magnetite. The crystallographic structures of these dominant iron minerals are proposed on basis of their chemical composition and corresponding Mössbauer data to support models refined by fitting powder X-ray diffraction data with the Rietveld algorithm.     

Mössbauer investigation of non-aggregated γ-Fe2O3 particles

Investigation by Mössbauer spectroscopy of non-aggregated nanometric -Fe₂O₃ particles dispersed in polymer is reported. Magnetic interactions between the particles were controlled by varying the particle concentration in the polymer. The results show that over the investigated range, the interactions make the relaxation time shorter. Infield experiments show spin canting which increases with decreasing particle size.     

Mössbauer and magnetic characterization of TbRhSn

The intermetallic compound TbRhSn was investigated in detail by X-ray, magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy. This compound undergoes a transition from a paramagnetic to an antiferromagnetic state at T _N = 20.8(2) K. The ¹¹⁹Sn Mössbauer spectrum recorded at 4.2 K can be well fitted as a composition of three subspectra with the same intensities, magnitudes of H _hf, ΔE _Q, and δ _is, in agreement with the model of triangular-like antiferromagnetic arrangements of equal magnetic Tb moments lying in the basal ab-plane deduced from neutron diffraction studies (Szytu?a et al., J Alloys Compd 244:94–98, 1996).     

M?ssbauer and magnetic studies of Mn0.1Sr0.2Co0.7Fe2O4 nanoferrite

The magnetic properties of Mn_0.1Sr_0.2Co_0.7Fe₂O₄ nanoferrite with particle size of about 8 nm were investigated using magnetization and Mössbauer spectroscopy measurements. The sample shows a large increase in coercive field from 0.045 kOe at room temperature to about 3.00 kOe at 4 K. Room temperature coercive fields increased with increase in the annealing temperature between 300°C and 800°C. Our results show evidence of transformation from single domain to multi-domain structure with thermal annealing.     

Mössbauer spectroscopy,X-ray diffraction and magnetic measurements of iron-nickel ultrafine particles

Iron-nickel ultrafine particles with a composition in the Invar region (38–50% Ni) were prepared by the gas-evaporation-coalescence technique. The chemical composition was checked by electronprobe microanalysis, while X-ray diffraction Rietveld refinement was used to characterize the structure as well as to estimate the particle size. The temperature and field dependence of the magnetizationM(B, T) was measured for 0B25 kOe in the temperature range 4.2 KT400 K. Transmission Mössbauer spectra were taken at room temperature and at liquid helium temperature. The results obtained show that the predominant phase is a disordered Ni-rich alloy.On leave from Physics Department, University of Khartoum, P.O. Box 321, Khartoum, Sudan.     

Mössbauer study on surface magnetic properties in magnetic fluids

The presence of m=0 lines of the Mössbauer spectra of small Fe₃O₄ particles coated with an organic surfactant in an applied field of 7 T at 5 K shows a non-collinear magnetic structure in the surface layer of these particles. From the temperature dependence of the hyperfine field, the anisotropy constantK was calculated.Projects supported by the Science Fund of the Chinese Academy of SciencesNow working at the Shanghai Institute of Education     

Mössbauer and X-ray studies for Ni0.2ZnxMg0.8−xFe2O4 ferrites

The spinel ferrites of Ni_0.2Zn_xMg_0.8−xFe₂O₄, 0⩽x⩾0.8, were studied at room temperature using X-ray diffraction and Mössbauer patterns. The analysis of the X-ray diffraction patterns proved that the samples have a single phase cubic spinel structure. The calculated values of the theoretical, true and average lattice constants, tetrahedral bond, tetrahedral edge and unshared octahedral edge were found to increase while the shared octahedral edge and octahedral bond decrease as the Zn²⁺ ion substitution increases. Mössbauer studies showed that the samples for x=0, 0.2 and 0.4 are magnetic and show rather broad lines, while for x=0.6 and 0.8 are paramagnetic. The hyperfine parameters of the tetrahedral and octahedral sites were determined as functions of composition x. The cation distributions were deduced and supported by X-ray studies. The B-site pattern was composite and has been fitted into multicomponents and the deduced hyperfine parameters have been discussed as a function of x.     

Mössbauer studies of baotite and bafertisite

The Mössbauer effect was used to study silicate minerals of baotite and bafertisite at 298 K and 95 K. Each spectrum of baotite at 298 K and 95 K consists of two doublets, and they are contributed from Fe²⁺ and Fe³⁺ in the octahedral Tisites, respectively. Each spectrum of bafertisite at 298 K and 95 K is composed of two doublets, and they are mainly caused by Fe²⁺ in the octahedral Fe(I) and Fe(II)sites, respectively. The average effective ionic radii of the Ti sites in baotite and the Fe(I) and Fe(II)sites in bafertisite were estimated based on the correlation of the isomer shifts with the average effective ionic radii in silicates, and they are 0.56 , 0.73 and 0.73 , respectively.