Mössbauer spectroscopy of Fe-based nanomaterials

X-ray diffraction, magnetic measurements and Mössbauer spectroscopy were used to study magnetic properties and hyperfine interaction parameters of nanocrystalline (< 10 nm) and bulk bcc Fe, Fe₉₀Ge₁₀, and Fe₇₇Al₂₃ alloys. It has been established that nanocrystalline state does not influence the formation of specific saturation magnetization, Curie temperature, isomer shift and hyperfine magnetic field. No additional sextets in Mö ssbauer spectra as well as special features in temperature dependences of a.c. magnetic susceptibility have been found. A slight increase (～ 20%) of the width of the nanocrystalline Fe Mössbauer spectral lines has been observed.     

Some magnetic properties of nanoperm alloy after irradiation

Transmission⁵⁷Fe Mössbauer spectrometry was used to study the microstructural and magnetic changes induced by neutron irradiation of nanocrystalline Fe-Zr-B alloy. Measurements were performed in the temperature range from 77 K up to 500 K. The evolutions of both the hyperfine field distribution, the average hyperfine field value, the mean orientation of the ferromagnetic domains and the value of the Curie temperature are consistent with short-range atomic order changes resulting from the neutron irradiation damages. In addition, the neutron irradiation affects significantly the proportions of amorphous, crystalline and interfacial phases. The results of Mössbauer spectrometry were correlated with those obtained by X-ray diffraction.     

Mössbauer study of a ferromagnetic metallic glass: Fe74Co10B16

Amorphous Fe₇₄Co₁₀B₁₆ (METGLAS 2605CO) has been studied in the temperature range of 77 K – 700 K by Mössbauer spectroscopy. Its crystallization temperature is found to be 665 ± 5 K and Curie temperature is estimated to be 760±10 K. The observed rapid decrease in reduced hyperfine fields can be explained well by Handrich's model for amorphous ferromagnets if one assumes a temperature dependent δ, a measure of fluctuations in the exchange interactions in such solids.     

Mössbauer study of Fe−Zr amorphous alloys hydrogenated at different cathodic potentials

Mössbauer spectroscopy was used for studying the effect of hydrogenation on amorphous Fe₉₀Zr₁₀ and Fe₈₉Zr₁₁ alloys electrolytically charged to high hydrogen content. The differences observed between the room temperature Mossbauer spectra of uncharged and hydrogenated amorphous alloys can be attributed to a change in the Curie temperature. Up to a certain hydrogen content the Curie temperature increases with the hydrogen concentration while the Curie temperature goes thorough a maximum as the hydrogen content increases.     

Magnetic and Mössbauer spectroscopic studies of NiZn ferrite nanoparticles synthesized by a combustion method

The properties of nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ synthesized by an auto-combustion method have been investigated by magnetic measurements and Mössbauer spectroscopy. The as-synthesized single phase nanosized ferrite powder is annealed at different temperatures in the range 673–1,273 K to obtain nanoparticles of different sizes. The powders are characterized by powder X-ray diffraction, vibrating sample magnetometer, transmission electron microscopy and Mössbauer spectroscopy. The as-synthesized powder with average particle size of ~9 nm is superparamagnetic. Magnetic transition temperature increases up to 665 K for the nanosized powder as compared to the transition temperature of 548 K for the bulk ferrite. This has been confirmed as due to the abnormal cation distribution, as evidenced from room temperature Mössbauer spectroscopic studies.     

Nanocrystalline iron-based alloys investigated by Mössbauer spectrometry

Nanocrystalline alloys exhibit great fundamental and technological interests because of their microstructural properties, and their excellent soft magnetic properties. ⁵⁷Fe Mössbauer spectrometry is a well suitable technique to investigate Fe-based nanocrystalline alloys: its local probe behaviour permits to elucidate the nature of hyperfine interactions at different resonating iron nuclei and to distinguish their immediate atomic surroundings. We review on the recent Mössbauer developments performed on first FeCuMBSi and then FeCuBSi nanocrystalline alloys. From Mössbauer studies, one can estimate the crystalline (i.e., amorphous) fraction, the Si-content in Fe--Si nanocrystalline grains emerging from amorphous alloys of the first series, the temperature dependence of magnetic behaviours of both crystalline and amorphous phases; finally, we present a novel fitting procedure applied to FeCuBSi nanocrystalline alloys which result from bcc-Fe crystalline grains embedded in an amorphous matrix. In this case, the hyperfine structure is able to model the intergranular phase.     

Nanogranular Phase Formation During Devitrification of Fe(NiCo)ZrB Amorphous Alloys

A comprehensive review of our recent experimental and theoretical developments in the processing of nanocrystalline soft magnetic materials made by crystallization of amorphous precursors and containing new nanocrystalline phases is given. The relationship between the structures of the metastable and equilibrium phases and their transformations are discussed. Nickel-rich amorphous precursors with stoichiometry Ni₆₄Fe₁₆Zr₇B₁₂Au₁ were produced by melt-spinning technique and then heat-treated at temperatures ranged from 420 °C to 600 °C for one hour to form nanostructured alloy. The transformation from the amorphous state into the nanocrystalline state was investigated by the differential scanning calorimetry (DSC), the x-ray diffraction (XRD), the vibrating sample magnetometry (VSM) and Mössbauer techniques. The annealing favours the emergence of cubic Fe_xNi_23-xB₆ crystalline grains (10-25 nm in diameter). Magnetic measurements made at 4.2-1100 K reveal rather high value of saturation magnetization (nearly 60 and 40 Am²/kg at 4.2 K and room temperature, respectively) in amorphous as well as in nanocrystalline states. These facts are consistent with 300 K ⁵⁷Fe Mössbauer results which are well supported by the calculations of Ni and Fe magnetic moments in Ni₂₃B₆ and Fe₂₃B₆ phases, using the spin polarized tight binding linear muffin-tin orbital (TB-LMTO) method. However, anomalous high magnetic moments of Fe and Co atoms were found in some inequivalent positions in the crystal structure.     

Surface and bulk Mössbauer effect studies of annealed NANOPERM-type alloys

Structure, hyperfine interactions, and magnetic behaviour of Fe₈₀M₇Cu₁B₁₂ (M=Mo, Nb, Ti) nanocrystalline alloys are studied by Mössbauer spectrometry. As-quenched and heat-treated specimens are investigated. Transmission and Conversion Electron Mössbauer effect techniques are used to compare surface and bulk crystallization as a function of annealing temperature with the aim to unveil the crystallization onset. In addition, magnetic structure comprising distributions of hyperfine fields is discussed as a function of composition and annealing temperature. Hyperfine field distributions are obtained separately for the amorphous residual phase and for interface regions. Crystalline phases are represented by discrete components.     

Effect of aluminum on the hyperfine field and crystallization behaviour of NANOPERM alloy

The effect of Al on the hyperfine fields and crystallization of NANOPERM alloy has been studied by introducing 2% Al for Fe in Fe₈₈Zr₇B₄Cu₁. Al is found to enhance the nucleation rate in these alloys. Mössbauer spectroscopic technique has been used to trace the effect of Al on the hyperfine interactions in the amorphous and nanocrystalline state of these alloys. The hyperfine field clearly shows the presence of Al both in the amorphous and the nanocrystalline phase. Magnetic measurements using VSM and also XRD, TEM and DSC studies have been used to support the conclusions derived using Mössbauer spectroscopy.     

Magnetic and thermal Mössbauer effect scans: a new approach

Mössbauer transmission recorded at fixed photon energies as a function of a given physical parameter such as temperature, external field, etc. (Mössbauer scan), is being developed as a useful quantitative tool, complementary of Mössbauer spectroscopy. Scans are performed at selected energies, suitable for the observation of a given physical property or process. It is shown that one of main advantages of this approach is the higher speed at which the external physical parameter can be swept, which allows the recording of quasi-continuous experimental response functions as well as the study of processes which occur too fast to be followed by Mössbauer spectroscopy. The applications presented here are the determination of the temperature dependence of the ⁵⁷Fe hyperfine field in FeSn₂, the thermal evolution and nanocrystallization kinetics of amorphous Fe_73.5Si_13.5Cu₁Nb₃B₉ and the measurement of the dynamic response of Fe magnetic moments in nanocrystalline Fe₉₀Zr₇B₃ to an external ac field.     

Temperature dependence of the Mössbauer spectra of amorphous and nanocrystallized Fe86Zr7Cu1B6

Mössbauer measurements have been performed on amorphous and nanocrystalline alloy ribbons of nominal composition Fe₈₆Zr₇Cu₁B₆. The nanocrystalline samples were obtained by annealing the as-quenched alloy at different temperatures in the range between 650 and 870 K. Mössbauer spectra of the as-quenched amorphous sample have been recorded at 77 K, room temperature and above the Curie temperature (330 K) at 360 K. We have also performed Mössbauer measurements at room temperature in the nanocrystalline alloys to characterize the phases that appear after the annealing and their relative concentration. The as-quenched sample spectra reveal the existence of two inequivalent sites for Fe. Such a feature is also observed in the remaining amorphous phase of the annealed samples. In the first steps of crystallization, -Fe precipitates and its concentration increases with the annealing temperature. The experimental results suggest that the composition of the whole amorphous phase does not suffer large changes during crystallization.     

An amorphous magnetically ordered ultra-thin film of Fe2O3 on graphite: Mössbauer spectroscopy

An amorphous ultra-thin film of Fe₂O₃ was prepared on partially oriented graphite (Papyex) by surface oxidation of an adsorbed monolayer of Fe(CO)₅. Mössbauer spectra exhibit super-paramagnetism below 79 K. The Mössbauer fraction parallel to the film exceeds that perpendicular by 16% at room temperature.     

Nanocrystallization process and ferromagnetic properties of amorphous (Fe0.99Mo0.01)78Si9B13 ribbons

The nanocrystallization process of soft ferromagnetic (Fe_0.99Mo_0.01)₇₈Si₉B₁₃ ribbons has been studied in detail. Microstructural and ferromagnetic properties are examined by transmission electron microscopy (TEM), X-ray diffraction (XRD), Mössbauer spectroscopy (MS), differential scanning calorimetry (DSC) and magnetization measurements. The Curie and crystallization temperatures are determined to be T_C=665 K and T_x=750 K, respectively. The T_x value is in well agreement with DSC measurement results. XRD patterns had shown two metastable phases (Fe₂₃B₆, Fe₃B) which were formed under in situ nanocrystallization process. These metastable phases embedded in the amorphous matrix have a significant effect on magnetic ordering. The ultimate nanocrystalline (NC) phases of α-Fe(Mo, Si) and Fe₂B at optimum annealing temperature had been observed respectively. It is notable that the magnetization of the amorphous phase decreases more rapidly with increasing temperature than those of NC ferromagnetism, which suggest the presence of the distribution of exchange interaction in the amorphous phase or high metalloid contents.     

Mössbauer study of the martensitic transformation in a Ni–Fe–Ga shape memory alloy

We present the results of an extensive Mössbauer study of the magnetic and martensitic transformation at room temperature of a polycrystalline alloy with a Ni₅₅Fe₁₉Ga₂₆ nominal composition. From calorimetric measurements, we have determined the martensitic transformation temperature of T _M ≈ 240 K, in good agreement with the one obtained by magnetic characterization. This sample has a Curie temperature of T _C ≈ 287 K. Additional Curie temperatures, belonging to a γ phase, have been also detected. Mössbauer spectroscopy performed at different temperatures monitored all these transformations and the fitting of the obtained spectrum at the highest temperature allow us to give percentages of the different phases in the sample.     

Nanostructured Y-Fe Alloys Produced by Ball Milling

Ball milling was used to produce nanostructured Y-Fe alloys. Depending on preparation conditions, nanocrystalline and amorphous components are formed to coexist. The transmission Mössbauer spectra exhibit YFe₂ and amorphous components. The influence of superparamagnetic YFe₂ particles was separated from the amorphous part by measuring at 77 K. The thermal stability of the samples and the growth of equilibrium phases was studied by annealing.     

Nanostructures, disordered ferromagnetism and spin glasses

Magnetic systems with a considerable amount of irregular interfaces were investigated by ⁵⁷Fe Mössbauer spectroscopy. Chemically homogeneous ferromagnets around the percolation threshold composition of disappearing magnetism and chemically heterogeneous alloys prepared by nanocrystallization of amorphous alloys belong to this class of materials. Low temperature and high field measurements were performed on nanocrystalline FeZrBCu alloys, on ball‐milled Fe with nano‐size grains and on melt‐quenched amorphous Fe–Zr and Fe–Y alloys in order to clarify the origin of large high‐field susceptibility and to investigate the common features of the approach to magnetic saturation. Curie point determination of the residual amorphous phase in the nanocrystalline FeZrBCu alloys, results on the structure of the nanocrystalline b.c.c. phase and of the interfacial region will be reported.     

Mössbauer studies on Al−Co ferrites

Mössbauer spectroscopy studies have been performed on the spinel CoAl_xFe_2?xO₄ (2<-x<-1.7) in the temperature range 77–750 K using either a liquid nitrogen bath cryostat or a furnace. The samples are magnetic at 77 K giving spectra that have magnetic sextets coexisting with a central line which increases in population with the Al-content indicating relaxation effects. The data shows that Al possesses no preference to either tetrahedral or octahedral sites of the ferrite over the whole range of concentration. The Mössbauer hyperfine interaction parameters and magnetic transition temperatures were determined. As expected the hyperfine field and Curie temperature decrease when the Al-content increases.     

High temperature stability of maghemite in partially oxidized basalt lava

Mössbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (J_S-T) curves with Curie temperatures of about 580°C, has revealed considerable amounts of maghemite (γ-Fe₂O₃) in many samples. In view of the expected instability, of maghemite at temperatures above 350°C, this reversibility is rather surprising. Here we report Mössbauer studies on heated lava samples, showing high content of maghemite. The samples were kept at 600°C in oxidizing, reducing, and inactive atmospheres, respectively, for different lengths of time, and then analyzed with Mössbauer spectroscopy at room temperature. The Mössbauer spectra showed that maghemite is stable in the oxidizing atmosphere for at least several hours. In the inactive atmosphere a considerable amount of maghemite still exists after two hours heating. In the reducing atmosphere maghemite had transformed to magnetite after only 30 minutes.     

Ferrimagnetic to Paramagnetic Transition in Magnetite: Mössbauer versus Monte Carlo

Powder magnetite was analyzed in situ via Mössbauer with temperatures ranging from 170 K up to 900 K. Hyperfine fields of the tetrahedral and octahedral sites of magnetite as well as the corresponding average field were followed as a function of temperature in order to elucidate the critical behavior of magnetite at around the Curie temperature. Results evidence a progressive collapse of the Mössbauer spectra onto a singlet-type line at a critical temperature of around 870 K characterized by a critical exponent β = 0.28(2) for the hyperfine field. In order to describe such temperature dependence of the hyperfine field, a Monte Carlo-Metropolis simulation based on a stoichiometric magnetite and an Ising model with nearest magnetic neighbor interactions was also carried out. In the model, we have taken into account antiferromagnetic and ferromagnetic interactions depending on the involved ions. A discussion about the critical behavior of magnetite and a comparison between the hyperfine field obtained via Mössbauer and the magnetization obtained via Monte Carlo is finally presented.     

Temperature dependence of the hyperfine fields in NiFe2O4 and CuFe2O4 oxides

We report the temperature dependence of the magnetic properties of (Ni, Cu)Fe₂O₄ spinel oxides. Mössbauer spectra for NiFe₂O₄ at various temperatures (79 ≤?T?≤ 900 K) are well fitted by two sextets associated with ⁵⁷Fe nuclei at tetrahedral (A) and octahedral (B) sites. The Curie point T _C was deduced by zero velocity Mössbauer technique to be 873 ± 3 K. The hyperfine fields are observed to vary with temperature according to the equation $B_{\rm hf} (T)=B_{\rm hf} (0)[{1-(T/T_{\rm C})^n}]^{\beta_n}$ where n?=?1 (based on the Landau–Ginzburg theory) and n?=?2 (based on the Stoner theory). A systematic decrease of the Mössbauer spectrum shift with increasing temperature is observed.