A neutron diffraction and Mössbauer study of NdFe9.25Si1.75Cx compounds

The crystallographic structures, magnetic properties and hyperfine interactions of NdFe_9.25Si_1.75C_x have been studied using neutron diffraction, magnetic and ⁵⁷Fe Mössbauer spectra measurements. The refinement of the powder neutron diffraction demonstrates that NdFe_9.25Si_1.75C_1.5 crystallizes in the BaCd₁₁-type structure with space group I4₁/amd and four formula units per unit cell. Nd atoms occupy 4a sites, and Fe atoms distribute on 4b, 8d and 32i sites. Si and C atoms occupy 8d and 8c sites, respectively. The saturation magnetization, the isomer shift (IS) and the hyperfine field reach their highest values at the carbon content x=1.0. With a set of phenomenological formulas, the contribution of the magneto-volume effect and the chemical bonding effect to the hyperfine parameters are successfully separated. The pure magneto-volume effect and the pure chemical bonding effect influence the hyperfine field and the IS differently. The former one increases both of them and the latter one decreases them. This is consistent with the rather strong bond between Fe(32i) and C(8c).     

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.     

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.     

Magnetic ordering in DyNi2Si2 andDyNi2Ge2 intermetallics studied by161Dy Mössbauer spectroscopy

The¹⁶¹Dy Mössbauer spectra of DyNi₂Si₂ andDyNi₂Ge₂ have been measured at temperatures between 4.2 and22 K. Temperature dependences of spectrum and-ray transmission rate at zero velocity as well as magnetic susceptibility measurements indicate clearly that anti-ferromagnetic orderings occur atT _N=7 K in DyNi₂Si₂ and9 K in DyNi₂Ge₂. Though the spectrum of DyNi₂Si₂ is interpreted approximately by a single set of hyperfine parameters, that of DyNi₂Ge₂ exhibits broadening of absorption lines which originates from distributed hyperfine parameters. The small values of hyperfine field, 707 MHz at 2.9 K in DyNi₂Si₂ and 765 MHz at 3.0 K in DyNi₂Ge₂ are attributable to the crystalline electric field interaction.     

Study of archaeological iron objects by PGAA,Mössbauer spectroscopy and X-ray diffraction

Archaeological iron objects often corrode rapidly after their excavation, even though they have survived long times of burial in the ground. Chlorine that accumulates during burial is thought to play a major role in this destructive post-excavation corrosion. It is therefore important for the conservation of such objects to determine the chlorine content in a non-destructive manner and, if necessary, to remove the chlorine from the artefacts by appropriate methods. Such methods are leaching in alkaline solutions or heating in a reducing atmosphere at temperatures up to 800 ^°C. We have studied the efficiency of the heating method using prompt gamma activation analysis (PGAA) for monitoring the Cl content and Mössbauer spectroscopy at room temperature (RT) and 4.2 K as well as X-ray diffraction to study the mineralogical transformations of the rust layers. The heat treatments were performed a N₂/H₂ (90/10) mixture at temperatures up to 750 ^°C. As test specimens sections of iron rods from the Celtic oppidum of Manching (Bavaria) were used. The initial Cl contents of the pieces varied in the range of several hundred ppm, referring to the iron mass. Annealing for 24 h at 350, 550 and 750 ^°C was found to reduce the Cl contents of the specimens, to about 70, 30 and 15 % of the original values, respectively. The rust consists mainly of goethite with admixtures of magnetite, lepidocrocite and akaganeite, which is thought to be a major carrier of chlorine, probably together with iron chlorides. Much of the goethite is so fine-grained that it does not split magnetically at RT. Annealing converts the rust mainly to maghemite at 350 ^°C, to magnetite at 550 ^°C and to wüstite plus magnetite and metallic iron at 750 ^°C. Pure akaganeite behaves in nearly the same manner.     

Iron-metal multilayers studied by Mössbauer spectroscopy,RBS and X-ray diffraction

Fe/M (M = Ag, Zn and Sn) multilayers prepared by a vacuum evaporation method are studied by Mössbauer spectroscopy (MS), Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). For the case of an M = Ag multilayer, MS reveals that Fe in the multilayer remains as an-phase down to the layer thickness of 10 nm. This result is in agreement with the RBS result that Fe and Ag form a completely discrete layer structure without any mutual mixing. For the case of M = Zn and Sn, RBS reveals that a considerable mixing has taken place between Fe and Sn during the specimen preparation. MS on Fe/Sn specimens with different layer thickness shows that an alloy phase of about 5 nm thickness is formed at the interface. Structural as well as magnetic properties of the alloy phase are discussed based on MS at different temperatures and on reported results of the intermetallic compound FeSn.     

Magnetic behaviour of γ-Fe2O3 nanoparticles by mössbauer spectroscopy and magnetic measurements

-Fe₂O₃ nanoparticles with varying state of dispersion in a polymer have been investigated by Mössbauer spectroscopy, static magnetic measurements at low applied field, and alternative susceptibility measurements over a large range of frequencies (2×10^–2–10⁴ Hz). The dynamical behaviour was characterized through the variation of the blocking temperature with the characteristic time of the measurement. The Mössbauer blocking temperature was determined according to a procedure described. For quasi-isolated particles an Arrhenius law is demonstrated. Effects of interparticle interactions in concentrated and aggregated systems are satisfactorily explained by the previous model. Dependence of the superparamagnetic susceptibility on the experimental conditions interpreted using the Lorentz or Onsager fields is mentioned.     

Characterization of Maghsail meteorite from Oman by Mössbauer spectroscopy, X-ray diffraction and petrographic microscopy

The meteorite found at Maghsail (16 55 70 N–53 46 69 E) west of Salalah Oman, has been studied by ⁵⁷Fe Mössbauer spectroscopy, X-diffractometry and petrographic microscopy. In the polished section the meteorite exhibits a porphyritic texture consisting of pyroxene and olivine phenocrysts in a fine to medium grained ground mass in addition to minor phases possibly skeletal chromite, troilite and minute amount of iron oxides. X-ray diffraction supports the existence of these compounds. The Mössbauer spectra of powdered material from the core of the rock at 298 K and 78 K exhibit a mixture of magnetic and paramagnetic components. The paramagnetic components are assigned to the silicate minerals olivine and pyroxene. On the other hand, the magnetic spectra reveal the presence of troilite and iron oxides. The petrographic analyses indicate that the iron oxides are terrestrial alteration products.     

Magnetic composites from minerals: study of the iron phases in clay and diatomite using Mössbauer spectroscopy, magnetic measurements and XRD

Magnetic particles as matrix for enzyme immobilization have been used and due to the enzymatic derivative can be easily removed from the reaction mixture by a magnetic field. This work presents a study about the synthesis and characterization of iron phases into magnetic montmorillonite clay (mMMT) and magnetic diatomaceous earth (mDE) by ⁵⁷Fe Mössbauer spectroscopy (MS), magnetic measurements and X-ray diffraction (XRD). Also these magnetic materials were assessed as matrices for the immobilization of invertase via covalent binding. Mössbauer spectra of the magnetic composites performed at 4.2 K showed a mixture of magnetite and maghemite about equal proportion in the mMMT, and a pure magnetite phase in the sample mDE. These results were verified using XRD. The residual specific activity of the immobilized invertase on mMMT and mDE were 83 % and 92.5 %, respectively. Thus, both magnetic composites showed to be promising matrices for covalent immobilization of invertase.     

Mössbauer spectroscopy study of interfaces for spintronics

The submonolayer sensitivity and element-specificity of conversion electron Mössbauer spectroscopy, combined with the use of ⁵⁷Fe enriched tracer layers, enable to carefully investigate thin films and interfaces at the atomic-scale. This paper reports on the main achievements we obtained so far in the study of structural, chemical, and magnetic properties of a variety of interfaces between oxides and Fe-based films having potential interest in the field of spintronics.     

Nanocrystalline oxide zinc materials studied by the Mössbauer effect, μSR and neutron diffraction

ZnO and ZnFe₂O₄ were investigated in nanocrystalline form using various hyperfine techniques in conjunction with neutron diffraction and bulk magnetic measurements. For the zinc oxide, the main interest was the influence of the nanocrystalline state on lattice dynamics and on the local symmetry of the Zn atom as reflected in its quadrupolar coupling, employing the high-resolution Mössbauer resonance of ⁶⁷Zn. Measurements were carried out on nanocrystalline materials with particle sizes between 4 and 25 nm. It was found that the asymmetry parameter increases drastically from η=0 in ZnO single crystals to η≈0.5 in nanostructured material, indicating an off-axis displacement of either Zn or O atoms in finite size grains. The dependences of the Lamb-Mössbauer factor on grain size and temperature are well described by a two-component model in which small crystallites are surrounded by a network of grain boundaries. For the zinc ferrite the question of changes in the magnetic properties stood in the foreground. Using the combination of techniques mentioned, the magnetic behavior of nanocrystalline ZnFe₂O₄, which had an average particle size of about 9 nm was compared to differently prepared crystalline samples. It was seen that the role of cation-site occupation, which changes substantially with nanocrystallinity, plays a decisive role for the magnetic properties.     

A Mössbauer and X-ray diffraction study of nonstoichiometry in magnetite

The paper deals with the applicability of Mössbauer spectroscopy and X-ray diffraction methods for the determination of the deviation of magnetite from stoichiometry. The results show that among the data obtainable by both methods, the ratio of intensities of two partial spectra composing the Mössbauer spectrum of magnetite enables to evaluate the deviation of magnetite from stoichiometry quantitatively.The authors express their gratitude to Prof. Dr. Ing. J.Cirák who enabled them to perform all measurements of Mössbauer spectra at the Department of Nuclear Physics and Technics, Slovak Technical University, Bratislava. The authors are also indebted to Ing. P.Holba (Institute of Solid State Physics, Czechoslovak Academy of Sciences) and to Ing. Z.Drbálek (Research Institute of Sound and Picture) for the preparation of magnetite samples, and to Mr. P.Chaloupek (Faculty of Mathematics and Physics, Charles University, Praha) for computer calculation of lattice constants. The aid provided by members of the G. V. Akimov State Research Institute for the Protection of Materials, Dipl. Chem. K.Jendelová who carried out chemical analysis of the samples and Ing. K.Turecká who took part in X-ray diffraction measurements, is gratefully acknowledged.     

Magnetic susceptibility and 119Sn Mössbauer spectroscopy studies of RAuSn compounds (R=La, Ce, Pr)

In this paper the results of bulk magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy measurements of LaAuSn, CeAuSn and PrAuSn intermetallic compounds are presented. New phase transitions in CeAuSn and PrAuSn compounds have been found by both methods employed. Structural and electronic properties are discussed.     

Investigations of the lanthanum-europium-copper-oxygen system by X-ray powder diffraction,thermal analysis and europium-151 Mössbauer spectroscopy

Lanthanum-europium-copper oxides of composition La_2?x Eu _x CuO₄ with structures related to those of the high temperature superconducting oxides have been prepared by solid state reactions between the component oxides in air. The X-ray powder diffraction data demonstrate that an orthorhombic to tetragonal structural transformation occurs at compositions betweenx=0.5 andx=0.8. The¹⁵¹Eu Mössbauer spectra show that europium is present in all phases as Eu³⁺. Thermal analysis studies in hydrogen show that a two-step reduction process occurs in the lanthanum-europium-copper oxides with the orthorhombic type structure.¹⁵¹Eu Mössbauer spectroscopy shows that the process does not involve the reduction of the lanthanide ion.