Evolution of structure, microstructure and hyperfine properties of nanocrystalline Fe50Co50 powders prepared by mechanical alloying

Nanostructured Fe₅₀Co₅₀ powders were prepared by mechanical alloying of Fe and Co elements in a vario-planetary high-energy ball mill. The structural properties, morphology changes and local iron environment variations were investigated as a function of milling time (in the 0-200 h range) by means of X-ray diffraction, scanning electron microscopy (SEM), energy dispersive X-ray analysis and ⁵⁷Fe Mössbauer spectroscopy. The complete formation of bcc Fe₅₀Co₅₀ solid solution is observed after 100 h milling. As the milling time increases from 0 to 200 h, the lattice parameter decreases from 0.28655 nm for pure Fe to 0.28523 nm, the grain size decreases from 150 to 14 nm, while the meal level of strain increases from 0.0069% to 1.36%. The powder particle morphology at different stages of formation was observed by SEM. The parameters derived from the Mössbauer spectra confirm the beginning of the formation of Fe₅₀Co₅₀ phase at 43 h of milling. After 200 h of milling the average hyperfine magnetic field of 35 T suggests that a disordered bcc Fe-Co solid solution is formed.     

Characterization of nanocrystalline FeSiCr powders prepared by ball milling

FeSi₁₀Cr₁₀ powder was mechanically alloyed by high energy planetary ball milling, starting from elemental powders. The microstructural and magnetic properties of the milled powders were characterized by scanning electron microscopy, X-ray diffraction, ⁵⁷Fe Mössbauer spectrometry and a vibratory sample magnetometer.After 3 h of milling, the formation of two bcc solid solutions α-Fe₁ (Si, Cr) and α-Fe₂ (Si, Cr) is observed. Their grain sizes decrease with increase in milling time attaining, at 15 h of milling, 23 and 11 nm, respectively. Mössbauer spectra of the milled powder show the presence of two components. One is a ferromagnetic type with a broad sextuplet. Its distribution of hyperfine field is characterized by high and low hyperfine field’s peaks and a mean value of 26.5 T. The other is a single paramagnetic peak. Its low concentration increases to ∼4% at 15 h of milling. These results can be explained by different atomic environments affected by Si or/and Cr elements, as well as the increased disordered grain boundaries.Magnetic measurements of the milled FeSi₁₀Cr₁₀ alloy powder exhibit a soft ferromagnetic character with a decrease of both magnetization at saturation (Ms) and coercive force (Hc) with milling time attaining values of Ms=151 emu/g and Hc=2500 A/m at 30 h of milling time.     

X-ray diffraction,microstructure, Mössbauer and magnetization studies of nanostructured Fe50Ni50 alloy prepared by mechanical alloying

Nanocrystalline Fe₅₀Ni₅₀ alloy samples were prepared by the mechanical alloying process using planetary high-energy ball mill. The alloy formation and different physical properties were investigated as a function of milling time, t, (in the 0–50 h range) by means of the X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), energy dispersive X-ray (EDAX), Mössbauer spectroscopy and the vibrating sample magnetometer (VSM). The complete formation of γ-FeNi is observed after 24 h milling. When milling time increases from 0 to 50 h, the lattice parameter increases towards the Fe₅₀Ni₅₀ bulk value, the grain size decreases from 67 to 13 nm, while the strain increases from 0.09% to 0.41%. Grain morphologies at different formation stages were observed by SEM. Saturation magnetization and coercive fields derived from the hysteresis curves are discussed as a function of milling time.     

Study of magnetic phases in mechanically alloyed Fe50Zn50 powder

The mechanosynthesis of Fe₅₀Zn₅₀ alloy resulted in the formation of the bcc Fe(Zn) solid solution after 20 h of milling. Structural transformations induced by mechanical alloying and heating, and magnetic properties of the powders were studied by Mössbauer spectroscopy, X-ray diffraction, Faraday balance and vibrating sample magnetometry techniques. All alloys studied exhibit strong magnetic ordering with Curie temperatures close to 900 K. Room temperature Mössbauer measurements revealed distinguished magnetic environments in the samples. The decrease of coercivity with prolonged milling time was attributed to the reduction or averaging of local magnetic anisotropies.     

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.     

Nanocrystalline FeAl intermetallics obtained in mechanically alloyed Fe50Al40Ni10 powder

B2-Fe₄₇Al₅₃ intermetallics has been produced by mechanical alloying in a planetary ball mill, using elemental Fe, Al and Ni powder mixture. The microstructural and magnetic properties of the mechanically alloyed Fe₅₀Al₄₀Ni₁₀ powdered samples were investigated by X-ray diffraction and ⁵⁷Fe Mössbauer spectrometry at 300 and 77 K. As resulted from the X-ray diffraction studies, the ordered B2 structure was formed in the Fe₅₀Al₄₀Ni₁₀ powder, together with the bcc α_i-Fe(Al, Ni) (i = 1, 2) solid solutions. Further milling led to a partial disordering of B2-Fe₄₇Al₅₃; it has undergone an order–disorder transition which is characterized by an expansion of the volume Δa₀ (lattice disorder) and a magnetic transition from the paramagnetic to ferromagnetic state which is characterized by strong ferromagnetic interactions in the alloy. The nanocrystalline bcc α_i-Fe(Al, Ni) solid solution was ferromagnetic with a mean crystallite size of 6 nm.     

Fe-contacts on InAs(100) and InP(100) characterised by conversion electron Mössbauer spectroscopy

We have grown 4 nm thin films of ⁵⁷Fe on InAs(100) and InP(100) surfaces by use of MBE and studied the samples by ⁵⁷Fe conversion electron Mössbauer spectroscopy. In the case of InAs, the Mössbauer spectrum showed a sextet due to α-Fe and a further magnetically split component with slightly smaller hyperfine field, which is attributed to interface components. This result indicates that there is a relatively sharp interface between Fe and InAs. On the contrary, the spectrum of the InP sample showed a sextet with very broad lines and a smaller average hyperfine field. This suggests a strong chemical reaction between iron and the substrate, which results in the formation of a poorly crystalline phase.     

High-Energy Ball Milling of NiAl(Fe) System

High-energy ball milling was used to promote the solubilization of iron into NiAl powder for an iron concentration range of 10–30 wt.%. The microstructural evolution induced by the intense mechanical deformations, under different milling conditions, was followed by X-ray diffraction and Mössbauer spectroscopy. The Mössbauer spectra are dominated by a magnetic sextet of about 33 T. Increasing the time and the speed of milling gives rise to a non-resolved doublet, having parameters typical of a NiAl compound with Fe atoms in solution. At the same time a reduction of lattice parameter occurs, which can be correlated to composition variations and partial disordering of the NiAl structure. Subsequent annealing modifies the Mössbauer spectra noticeably. In particular, the non-magnetic component becomes a broad singlet. Both diffraction analysis and Mössbauer spectroscopy indicate that a fcc Ni(Al,Fe) solid solution is forming in samples milled in agate. It is observed that the grain size of the milled products remains in the nanometric range even after thermal treatment, which adds interest to possible applications.     

Mössbauer effect study of fine atomic structure of Fe50Al40Ni10 powders

In the present paper, we discuss the local atomic environment of Fe atoms in the mechanically alloyed Fe₅₀Al₄₀Ni₁₀ powders on the basis of hyperfine data estimated from ⁵⁷Fe Mössbauer spectra. B_hf decreases with increasing milling time due to the diffusion of Al and/or Ni into Fe grains. Nickel atoms did not diffuse inside the first coordination sphere of Fe and if the diffusion takes place the number is not more than one atom. Analyses of P(B_hf), indicate that the high hyperfine field values ranging from 30 to 33 T have to be partially attributed to Fe crystalline nanograins and the presence of the defects in them, the hyperfine field values ranging from 15 to 30 T can be associated to the nanocrystalline bcc Fe(Al, Ni) solid solution while the low hyperfine field values (<15 T) result from Fe atoms located in the disordered grain boundaries.     

Properties of Fe–Ga based powders prepared by mechanical alloying

Alloys of Fe–Ga with starting compositions of 17, 19, 21, 23, and 25 at% Ga and Fe₈₁Ga₁₇Z₂ (Z=Si, Sn) have been prepared by mechanical alloying. Samples were milled in a SPEX Model 8000 mill with a ball to sample weight ratio of about 4:1. Phase formation as a function of milling time has been investigated for the 19 at% Ga sample and suggests that milling times of 12 h produce fully alloyed samples. Alloys have been studied by electron microprobe, X-ray diffraction, vibrating sample magnetometery and ⁵⁷Fe Mössbauer effect spectroscopy. Fully milled powders have measured compositions of Fe_100−xGa_x with x=15.7, 17.0, 19.0, 22.4, and 24.0 and Fe_83.1Ga_15.2Z_1.7 (for both Z=Si and Sn). X-ray diffraction showed the presence of a disordered bcc phase with no indication of an ordered D0₃ phase. However, the latter is difficult to observe with X-ray diffraction because of the low intensity of the fcc superlattice peaks. A bimodal Fe hyperfine field distribution as obtained from Mössbauer effect spectra indicated the presence of two discrete Fe environments. The results suggested a lower degree of Ga clustering than has been previously observed in Fe–Ga alloys, of similar composition, prepared by melt spinning. The microstructure is similar to that of Fe–Ga thin films prepared by combinatorial sputtering. Some samples have also been studied after annealing at 800 °C for 8 h. No changes were observed in X-ray diffraction patterns after annealing. However, Mössbauer effect studies show the formation of D0₃ and L1₂ order in annealed samples analogous to the phases observed in melt spun ribbons of similar composition.     

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 Spectroscopy Study of Fe–Si Solid Solution Prepared by Mechanical Milling

A specimen of Fe–Si solid solution is prepared by ball milling of proper amounts of the pure elements (3Fe:Si) for different milling times. X-ray diffraction and Mössbauer spectroscopy have been used to characterize the solid solution. The Mössbauer spectra show four different sites corresponding to Fe atoms in a bcc structure having 0, 1, 2 and 3 Si atoms in the 1st nearest-neighbor (nn) shell. The hyperfine magnetic field decreases by 31 kOe for each Si atom in the 1st nn shell. A magnetic component with hyperfine field around (180 kOe) characterized by a broadened sextet was observed which could be due to iron sites having more than 3 Si atoms in the 1st nn. A theoretical model based on the binomial distribution was adopted to analyze the data. Good agreement between the experimental and the theoretical hyperfine field distribution in the high hyperfine field region was found, and the silicon content in the disordered A2 phase is deduced from the parameters which give the best agreement.     

Evolution of magnetic order in mechanically alloyed Al–1 at%Fe

The evolution of ferromagnetic order in high-energy ball-milled Al–1 at% Fe before the onset of a considerable Fe–Al solid solution phase has been investigated using ⁵⁷Fe Mössbauer and bulk magnetization studies. The unmilled sample does not exhibit bulk magnetic properties and an onset of bulk magnetization is observed only after 30 min of milling, when the grain size becomes comparable to the ferromagnetic exchange length. The Curie temperatures of all the samples are less than that of pure iron. The reduction in grain size is accompanied by an increase in coercivity and reduced remanence and a decrease in T_C. The effective magnetic moment per iron atom decreases with the development of a non-magnetic, Al-rich Fe–Al solution on longer milling. The clustering of Fe at grain boundaries is responsible for the observed bulk magnetic ordering. The systematic variation of the magnetic properties has been qualitatively correlated with the evolution of microstructure, reduction in grain size and enhanced inter-granular exchange coupling.     

Partial Reduction of MgFe2O4 with Al and Mg via Ball Milling

Nanocomposites consisting of metallic Fe particles and a nonmagnetic oxide were prepared by reducing MgFe₂O₄ with Al or Mg in a ball mill. The reaction takes place as a fast self-propagating process sometime between 0.5 and 1 h of milling. Combining XRD and Mössbauer spectroscopy reveals that the bcc Fe phase contains a few percents of dissolved Al, it has a high defect concentration, and the surface tension of the ultrafine (about 15 nm) grains results in local compressive strains. The magnetization is 25% less than expected for pure ferromagnetic Fe.     

Mössbauer spectroscopy and magnetic properties in thin films of FexNi100−x electroplated on silicon (1 0 0)

Fe_xNi_100−x thin films were produced by galvanostatic electrodeposition on Si (1 0 0), nominal thickness 2800 nm, and x ranging 7-20. The crystalline structure of the sample was determined by X-ray diffraction (XRD). The magnetic properties were investigated by vibration sample magnetometry (VSM) and room temperature ⁵⁷Fe Mössbauer spectroscopy. Conversion Electron Mössbauer spectroscopy (CEMS) in both film surfaces for the thick self-supported films showed that the magnetic moment direction is in the plane and conventional transmission (MS) that the directions are out of the plane films. The results were interpreted assuming a three-layer model where the external layer has in-plane magnetization and the internal one, out of plane magnetization.     

Magnetic properties of mechanochemically prepared iron-wüstite (Fe-FeyO) nanocomposites

In this work, iron-wüstite (Fe-Fe_yO) nanocomposites have been prepared via high-energy ball milling (HEBM), using high-purity hematite (α-Fe₂O₃) and iron (Fe) powders as the raw materials with different Fe/Fe₂O₃ mole ratios (MR)=0.6, 0.9, 1.0, 2.3, 4.9 and 13.6. X-ray diffraction studies of the as-milled powders show that a single-phase wüstite was formed for the lowest mole ratio (MR=0.6) and mixtures with MRs higher than 0.6 result in iron-wüstite nanocomposites, except for MR=13.6 that is dominantly a pure iron phase. The mean crystallite sizes of the iron and wüstite in the nanocomposites have been calculated by Scherrer's formula, which were 9±1 and 7±1 nm, respectively. Using the formula a=3.856+0.478y, for Fe_yO, where “a” is the lattice parameter of wüstite, it is possible to estimate the value of “y” for different nanocomposites and a composition of Fe_0.93O was estimated for the wüstite single phase (MR=0.6). In addition, a gradual decrease in “y” from 0.87 to 0.83 was obtained by increasing MR values from 0.9 to 4.9, respectively. The room-temperature Mössbauer spectrum of the single-phase wüstite shows considerable asymmetry due to two overlapping quadrupole doublets. For higher MRs, room-temperature Mössbauer spectra exhibit sextets, which confirm the existence of iron in the samples. The Mössbauer spectrum of the sample with the highest mole ratio (MR=13.6) shows only a sextet related to α-Fe without any detection of wüstite, which is in agreement with the XRD results. The nanosized prepared wüstite shows ferrimagnetic like behavior, which was interpreted according to spinel-like defect clusters. The M_s values obtained from VSM measurements and those calculated based on the Mössbauer data and chemical reaction are in good agreement. By increasing MR from 0.6 to 2.3, the coercivity (H_c) increases sharply to its maximum value at about MR=2.3, for which the value of Fe content is 45% and then drops off. This behavior is discussed based on α-Fe contents in the nanocomposites and percolation threshold.     

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.     

Exhibition of High- and Low-spin States of the High-temperature Fcc Phase in Nanoparticles of Fe,Fe-rich and Co-rich Alloys

The results of combined X-ray and Mössbauer studies of structure and local magnetic ordering in massive substances Fe, Fe–Ni, Fe–Mn, Fe–Ni–Mn, Fe–Pt, Fe–Co and aerosol nanoparticles produced by their evaporation in rare Ar atmosphere are discussed. This technique provides a stochiometric composition of alloys in nanoparticles. The smallest (5–8 nm) particles for all alloys containing Fe 60–65% are shown to have a bcc structure whereas with doubling a size the particles acquire a fcc structure. This is explained by the fact that by cooling the particles in the course of preparation they quickly reach a state close to the equilibrium and, according to the constitution diagram, must decompose into two phases. Such decomposition in massive alloys was never observed at temperatures below 300°C because of diffusive difficulties. It is found that as-fresh aerosol particles are covered with an X-ray amorphous oxide shell, which is displayed in the room temperature Mössbauer spectra as a superparamagnetic doublet and is transformed into sextet at lower temperatures. An availability of the oxide shell has no practical influence on the particles structure. The obtained Mössbauer spectra are considered with the model suggested by R.J. Weiss in 1963, on existence of two-spin states in the high-temperature fcc modification of Fe and its alloys. Both states coexist, moreover, in the Mössbauer spectra the ferromagnetic state dominates at high temperature and anti-ferromagnetic one at low temperature. The ferromagnetic state manifests itself as a remnant of the frozen magnetic ordering of the high-temperature fcc modification in the resulting bcc structure, whereas the anti-ferromagnetic state is related to some fcc fraction retained under the particles quenching.     

Eu and Fe Mössbauer study of mechanically alloyed EuFeO3

EuFeO₃ was prepared by mechanical alloying starting from europium and iron oxides. After 20 h of milling the resulting compound is pure EuFeO₃. Samples were studied as a function of milling period using XRD, Mössbauer, SEM, and magnetic measurements. Mössbauer spectroscopy was used to probe both the transition metal and the rare-earth sites. Results are compared with previous works on EuFeO₃ prepared by different methods.     

Swift ion irradiations of Fe/Fe/Si trilayers

We report here on changes in magnetism and microstructure when implanting, at 92 or 300 K, up to 5 × 10¹⁵ Au²⁶⁺-ions cm⁻² of 350 MeV into ^natFe(45 nm)/⁵⁷Fe(20 nm)/Si trilayers. This choice of ions and energy allowed to test the irradiation effects in the regime of pure electronic stopping. The samples were analysed before and after irradiation by Rutherford back-scattering spectroscopy, X-ray diffraction, conversion electron Mössbauer spectroscopy, and magneto-optical Kerr effect. Up to 1 × 10¹⁵ ions cm⁻², there was interface broadening at a mixing rate of Δσ²/Φ = 55(5) nm⁴, followed by full Fe-Si inter-diffusion. The Mössbauer spectra revealed fractions of α-Fe and amorphous ferromagnetic and paramagnetic iron silicides, but no crystalline Fe-Si phase. The magnetic remanence in the as-deposited Fe-layer showed small components of uniaxial and four-fold magnetization. For increasing ion fluence, the component with four-fold symmetry grew at the expense of the uniaxial component. For the highest fluences, an isotropic magnetization was found.