Temperature Dependence of the Mössbauer Effect on Prussian Blue Nanowires

Highly ordered Prussian blue nanowires with diameter of about 50 nm and length up to 4 m have been fabricated by an electrodepositing technology with two-step anodizing anodic aluminum oxide films. The Mössbauer spectra taken between 15 and 300 K indicate that the hyperfine parameters decrease as the temperature increases. The temperature dependence of the quadrupole splitting, the isomer shift and the spectra area are discussed. A decrease of Debye temperature for Prussian blue nanowires was found with respect to that of Prussian blue bulk.     

Evaluation of the generalized Goodwin-Staton integral using binomial expansion theorem

A new analytical technique for evaluating the generalized Goodwin-Staton integral (GGS) is described. A closed-form evaluation is presented. The GGS integral are expressed in terms of linear combinations of binomial coefficients and incomplete gamma function. A further comparison of analytical results with numerical models demonstrates a high accuracy of the developed analytical solution. The convergence of the results is shown.     

On the Role of Aluminum in Finemet

The role of aluminum with respect of its influence on some intrinsic magnetic properties of the nanocrystalline Finemet, such as saturation magnetization, Curie temperature, spin-wave stiffness constant and hyperfine magnetic fields was investigated. The strengthening effect of Al on the exchange interaction was observed for the alloys with small Al content (up to 3 at.%). Higher concentration of Al (5-7 at.%) led to considerably suppressed exchange interaction.     

Concerning the cation distribution in MnFe2O4 synthesized through the thermal decomposition of oxalates

A single phase manganese ferrite powder have been synthesized through the thermal decomposition reaction of MnC₂O₄·2H₂O-FeC₂O₄·2H₂O (1:2 mole ratio) mixture in air. DTA-TG, XRD, Mössbauer spectroscopy, FT-IR and SEM techniques were used to investigate the effect of calcination temperature on the mixture. Firing of the mixture in the range 300-500 °C produce ultra-fine particles of α-Fe₂O₃ having paramagnetic properties. XRD, Mössbauer spectroscopy as well as SEM experiments showed the progressive increase in the particle size of α-Fe₂O₃ up to 500 °C. DTA study reveals an exothermic phase transition at 550 °C attributed to the formation of a Fe₂O₃-Mn₂O₃ solid solution which persists to appear up to 1000 °C. At 1100 °C, the single phase MnFe₂O₄ with a cubic structure predominated. The Mössbauer effect spectrum of the produced ferrite exhibits normal Zeeman split sextets due to Fe³⁺ions at tetrahedral (A) and octahedral (B) sites. The obtained cation distribution from Mössbauer spectroscopy is (Fe_0.92Mn_0.08)[Fe_1.08Mn_0.92]O₄.     

On the magnetic properties of mechanosynthesized Co-Fe-Ni ternary alloys

X-ray diffraction, Mössbauer spectroscopy and magnetization measurements were used as complementary methods to obtain structural data and to determine magnetic properties of the mechanically synthesized and subsequently thermally treated Co-Fe-Ni alloys. New, however approximate, phase diagrams were established on the basis of X-ray diffraction investigations. Mössbauer spectroscopy and magnetization measurements allowed to reveal practically linear correlation between the average values of the hyperfine magnetic field induction, 〈B_hf〉, and the effective magnetic moments, μ_eff, of the alloys. The decrease in 〈B_hf〉 with the number of electrons per atom, e/a, was observed. Moreover, the dependence of μ_eff on the valence 3d and 4s electrons per atom follows the Slater-Pauling curve. Thermal treatment of mechanosynthesized Co-Fe-Ni alloys led to some changes in the phase diagrams, increase in the grain size and decrease of the level of internal strains in alloys. Dependencies of lattice constants, average hyperfine magnetic fields, effective magnetic moments and Curie temperatures on the number of electrons per atom have the same trends for mechanically synthesized as well as for thermally treated alloys.     

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.     

Determination of the Recoilless Fraction in Iron Oxide Nanoparticles Using the Two-lattice Method

We propose a two-lattice method for direct determination of the recoilless fraction using a single room-temperature transmission Mössbauer measurement. The method is first demonstrated for the case of iron and metallic glass two-foil system and is next generalized for the case of physical mixtures of two powders. We further apply this method to determine the recoilless fraction of hematite and magnetite particles. Finally, we provide direct measurement of the recoilless fraction in nanohematite and nanomagnetite with an average particle size of 19nm. A list of values obtained for the recoilless fraction in various materials using the two-lattice method is given.     

On the origin of discontinuity of the hyperfine fields at Fe nuclei in bulk iron and aerosol Fe nanoparticles

Advancing the early work in which a discontinuity of hyperfine fields at ⁵⁷Fe nuclei in bulk iron and in aerosol Fe nanoparticles has been revealed by analyzing their Mössbauer spectra the present Letter evidences that the existence of several peaks in the hyperfine distribution (HFD) for bulk Fe is caused with the internal magnetic fields owing to its multidomain structure whereas aerosol Fe nanoparticles are single-domain and show only a unique peak in HFD. This argument has been corroborated by transformation of the HFD pattern for Fe foil after applying the external magnetic field of 0.03 T.     

A Mössbauer Study of Austenitic Steel Originating from the Steam Generator of a VVER-440 Type Nuclear Reactor,Paks, Hungary

Real specimens after more than ten years of operation in the steam generator of a VVER-440 type nuclear reactor, Paks, Hungary have been studied. Conversion electron Mössbauer spectroscopy was used for phase analysis of the primary side surfaces of austenitic stainless steel pipes. From the measurements, we concluded that the surface layer of the pipes consisted of a thin (<100 nm) and highly iron depleted oxidic film of nickel and chromium rich compounds. The ratio of the metallic constituents was found to be substantially different from that of the bulk steel to a depth of about 1–1.5 microns as revealed by independent SEM-EDAX studies. These findings may most likely be attributed to the irradiation enhanced diffusion and selective dissolution of iron.The bulk steel was investigated by transmission Mössbauer spectroscopy and X-ray diffraction. The samples originated from different sections of the collector head of the steam generator representing different contact times with steam and liquid water. Besides the paramagnetic peak of austenite, the Mössbauer spectra contained a magnetic sextet of ferrite in various amounts. This difference in phase composition may be attributed to the combination of irradiation effect and different thermal stress.     

An estimation of the magnetic disorder at Fe/Cr interfaces in coupled and non-coupled Fe/Cr multilayers

The antiferromagnetic coupling at the Fe/Cr interfaces, inferred from the orientation of the Cr magnetic moments, is used to estimate the magnetic disorder resulting from the interfacial roughness in Fe/Cr multilayers. A crossover from in-plane to out-of-plane orientation of Cr moments depends on the energy cost in either case: (i) to break the interfacial Fe–Cr antiferromagnetic coupling or (ii) having sites with frustrated Cr–Cr antiferromagnetic coupling in the Cr interlayers. A quantitative model of the magnetic frustration due to interfacial disorder in Fe/Cr multilayer systems is described. The step edge density, or terrace size, required to break the interfacial Fe–Cr coupling and destroy the Fe–Fe interlayer exchange coupling is estimated.     

The magnetic behaviour of the magnetically diluted spinel compound Fe0.6Mg1.6Ni0.1Ti0.7O4 studied by Mössbauer spectroscopy

From a new magnetically diluted spinel oxide with composition Fe_0.6Mg_1.6Ni_0.1Ti_0.7O₄ an as-prepared sample and one after reheating three times have been investigated with Mössbauer spectroscopy. The spectra of the as-prepared sample clearly show a typical superparamagnetic behaviour of magnetic clusters with diverging sizes. On the other hand, the reheated sample exhibits a sharp magnetic transition at 16 K. External-field Mössbauer measurements of the latter reveal spin canting to be exclusively present on the octahedral sites which disappears at the magnetic transition temperature. These results show that this spinel compound exhibits a transition to a spin-glass for which the random freezing only occurs on the octahedral sites.     

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.     

Beneficial effect of boron on the structural and magnetostrictive behavior of Fe77Ga23 alloy

Alloys of Fe₇₇Ga₂₃B_x (x=0, 0.025, 0.050, 0.075 and 0.1) were heat treated at 1000 °C/10 h and characterized for microstructural features and magnetostriction. The study indicates that the parent alloy consists of three phases viz. A2 as the major phase and L1₂ and DO₃ as minor phases. However, the volume fraction of L1₂ and DO₃ decreases with B addition and as a result magnetostriction improves. The decrease in volume fraction of these phases is attributed to B-segregation to the grain boundary, which seems to prevent the formation of these phases. The alloy with x=0.1, however, exhibits an additional Fe₂B phase and consequently its presence leads to the re-emergence of the DO₃ phase, affecting the magnetostriction.     

Effect of Fe substitution on the phase stability and magnetic properties of Mn-rich Ni-Mn-Ga ferromagnetic shape memory alloys

The influence of Fe additions on the martensitic transformation and magnetic properties of Mn-rich Ni-Mn-Ga alloys was investigated by substituting either 1 at% Fe for each atomic species or by substituting Ni with varying amounts of Fe. The magnetic structure of the alloys was studied using ⁵⁷Fe Mössbauer spectroscopy. Mössbauer spectra revealed typical paramagnetic features in Mn-rich Ni-Mn-Ga-Fe alloys owing to the preferential site occupancy of Fe atoms at Ni sites. The evolution of the magnetic properties and phase stability has been correlated with the chemical and atomic ordering in these alloys.     

NAA and Mössbauer Study on the Colorative Mechanism of Yaozhou Celadon in Ancient China

The contents of 30 colorative elements in the glaze of Yaozhou celadon have been measured by neutron activation analysis (NAA) with the conclusion that iron is the dominant colorative element. It is further detected by Mössbauer spectroscopy that the iron in the glaze exists in the chemical state of structural iron (Fe²⁺, Fe³⁺) and that there is quantitative relation between the dominant wavelength of various colored glazes and the relative content of structural iron. Thus the colorative mechanism of Yaozhou celadon has been revealed entirely.     

On the role of lattice distortion in the catalytic properties of substituted orthovanadates La1−xFexVO4

A series of mixed orthovanadates with nominal compositions La_1−xFe_xVO₄ were synthesized and characterized using powder X-ray diffraction, Mössbauer spectroscopy and temperature-programmed reduction techniques. The substitution resulted in the co-presence of two distinct mixed metal compositions having either monoclinic LaVO₄ or a triclinic FeVO₄ structure. Both these constituent phases were however, found to be of distorted nature, with no measurable change in respective crystal symmetry. Furthermore, the extent of this distortion depended upon the value of x and is attributed to the partial substitutions at A-site, i.e. with a part of La by Fe in LaVO₄ lattice and a part of Fe by La in the FeVO₄ phase. The substitution-induced lattice distortion is found to result in the lowering of the reduction temperature in case of both the above mentioned phases, and also in the synergistic enhancement in catalytic activity for a model CO oxidation reaction.     

On the Evaluation of Two-Center Overlap Integrals over Integer and Noninteger n-Slater-Type Orbitals

In this study, two-center overlap integrals over Slater-type orbitals (STOs) with integer and noninteger principal quantum numbers in unaligned coordinate systems have been calculated using formulas for overlap integrals in aligned coordinate systems obtained by the author's previous work (T. Özdogan and M. Orbay, Int. J. Quant. Chem. 87 (2002) 15). The obtained results for integer case have been found to be in excellent agreement with the prior literature. On the other hand, to the best of authors knowledge, because of the scarcity of the literatures on the use of noninteger n-STOs in unaligned coordinate systems, the results for noninteger case have been tested with the limit of integer case, and good agreement has been obtained too.     

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.     

On the Evaluation of Two-Center Overlap Integrals over Integer and Noninteger -Center Overlap Integrals over Integer and Noninteger n-Slater-Type Orbitals

In this study, two-center overlap integrals over Slater-type orbitals (STOs) with integer and noninteger principal quantum numbers in unaligned coordinate systems have been calculated using formulas for overlap integrals in aligned coordinate systems obtained by the author's previous work (T. Ozdogan and M. Orbay, Int. J. Quant. Chem. 87(2002) 15). The obtained results for integer case have been found to be in excellent agreement with the prior literature.On the other hand, to the best of authors knowledge, because of the scarcity of the literatures on the use of noninteger n-STOs in unaligned coordinate systems, the results for noninteger case have been tested with the limit of integer case,and good agreement has been obtained too.     

Mössbauer Investigation of the Itinerant Magnets U(Ga0.98Sn0.02)3 and CeFe2

The theoretical predictions that UGa₃ and CeFe₂ should be regarded as itinerant magnets stimulated new investigations. In this paper we focus on Mössbauer measurements aimed to characterize their magnetic properties. We show that the analysis of the transferred hyperfine interactions at the ¹¹⁹Sn nuclei in U(Ga_0.98Sn_0.02)₃, a type II antiferromagnet, provides direct information on the f-p hybridization and allows to determine the orientation of the U moments. The study of CeFe₂ demonstrates that the intrinsic ferromagnetism coexists with short range antiferromagnetic correlations. The instability of the ferromagnetic state is illustrated by doping CeFe₂ with a small amount of Co and by application of pressure on pure CeFe₂. Our results will be discussed in connection with neutron and synchrotron experiments.