Low Hyperfine Field Research Articles

Cationic migration effect on the dielectric and magnetic properties of CoFe2O4/ZnO composites

The present paper focusses on the effect of cationic migration in the sintered CoFe2O4/ZnO composite. The increase in lattice parameter of the CoFe2O4 after sintering indicates substitution of Zn. Mӧssbauer analysis of the samples indicates the transformation of composites to paramagnet in the sample sintered at 600 and 750 °C. At 900 °C, a low hyperfine field sextet was evolved complimented by MH loop which is due to redistribution of cations and formation of ferromagnetic phase. Maxwell-Wagner dispersion was identified by the frequency-dependent fluctuation of dielectric constants (ε׳) and dielectric loss (tan δ). Dielectric constant values of 8.66, 7.90, 9.31 and dielectric loss value of 0.11, 0.03, 0.05 were found for CZ6, CZ7 and CZ9 respectively at 1 MHz frequencies. The composite are suitable for high frequency applications due to its low loss, high resistance, low sintering temperature. The mechanisms behind these observations are discussed in this paper.

Unveiling the Hidden Entropy in ZnFe2O4.

The antiferromagnetic (AFM) transition of the normal ZnFe2O4 has been intensively investigated with results showing a lack of long-range order, spin frustrations, and a “hidden” entropy in the calorimetric properties for inversion degrees δ ≈ 0 or δ = 0. As δ drastically impacts the magnetic properties, it is logical to question how a δ value slightly different from zero can affect the magnetic properties. In this work, (Zn1-δFeδ)[ZnδFe2-δ]O4 with δ = 0.05 and δ = 0.27 have been investigated with calorimetry at different applied fields. It is shown that a δ value as small as 0.05 may affect 40% of the unit cells, which become locally ferrimagnetic (FiM) and coexists with AFM and spin disordered regions. The spin disorder disappears under an applied field of 1 T. Mossbauer spectroscopy confirms the presence of a volume fraction with a low hyperfine field that can be ascribed to these spin disordered regions. The volume fractions of the three magnetic phases estimated from entropy and hyperfine measurements are roughly coincident and correspond to approximately 1/3 for each of them. The “hidden” entropy is the zero point entropy different from 0. Consequently, the so-called “hidden” entropy can be ascribed to the frustrations of the spins at the interphase between the AFM-FiM phases due to having δ ≈ 0 instead of ideal δ = 0.

Magnetic and 57Fe hyperfine structural features of nitrided austenitic stainless steel

Samples of an austenitic stainless steel were plasma-nitrided at 673 K, for 4 h, in 80% H2−20% N2 gas mixture, at different pressures: 4; 6 and 10 Torr. The samples were characterized by four different techniques: X-ray diffraction, Mössbauer spectroscopy, magnetic force microscopy and energy dispersive X-ray. The nitrogen concentration and thickness of the nitrided layer was found to be 7 at.% and 2 μm for the 4 Torr-sample; for those at 6 and 10 Torr, the concentration was 16 and 24 at.%, with nearly the same thickness, namely 7 μm. These values are well correlated with the framework of an expanded austenite, as pointed by the X-ray diffraction data. The magnetic force microscopy data are well correlated with the observed nitrogen concentration, evidencing the absence of any magnetic domain for the 4 Torr-sample; the magnetic patterns for the 6 Torr and 10 Torr-samples are quite similar, despite of the great difference in their nitrogen concentrations. The 57Fe hyperfine parameters at two different sample depths were assessed by Mössbauer spectroscopy, by collecting data with two distinct backscattering setup geometries. Fitting the corresponding spectra with model-independent hyperfine field distributions, starting from very low hyperfine field values, allowed correlating the magnetic force microscopy data to find out that the transition from paramagnetic to magnetic for the expanded austenite occurs at smaller hyperfine magnetic fields than it is usually reported in the scientific literature. Such correlations point to the formation of a submicrometer layer occurring on the outermost surface of the sample. Despite the complexity and difficulties in differentiating the nitrides and the martensitic structure on the surface of the samples obtained at 6 Torr and 10 Torr pressures, the results obtained in this work strongly indicate the existence of them in the surface layer.

Effect of low Zn doping on the Verwey transition in magnetite single crystals: Mössbauer spectroscopy and x-ray diffraction

To observe, by microscopic probe, how low Zn doping in $\mathrm{F}{\mathrm{e}}_{3\ensuremath{-}x}\mathrm{Z}{\mathrm{n}}_{x}{\mathrm{O}}_{4}$ ($x$ is below 1%) changes the Verwey transition, we have performed M\"ossbauer spectroscopy measurements on three single crystalline samples with various Zn doping. In spectra analysis we used the recently published model of M\"ossbauer data treatment formulated as a result of ab initio calculations for a low-temperature monoclinic structure (of Cc symmetry) of magnetite. It was suggested there that the hyperfine parameters for all 24 Fe distinct positions in the lattice can be grouped into four major components with very similar hyperfine parameters within each set. Using these parameters as starting values, very good fits were obtained for magnetite with low doping level, while for higher doping, $x=0.03$, where the Verwey transition changes its character, one component is significantly different. In particular, low hyperfine field ${B}_{\mathrm{eff}}=36\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, considered as a characteristic feature of the Cc phase spectrum, is absent here. Also, in this case, the high-temperature spectra are different from those for lower doped magnetite showing more pronounced continuous alteration with temperature. This might be due to crystal structure of lower than Fd-3m symmetry, a fact suggested by our x-ray synchrotron studies. All this triggered a discussion about an experimental fingerprint for the difference between these two classes of magnetite, frequently referred to as magnetite of first- and second-order Verwey transition, and about the electronic structure of both kinds of systems.

Thermal oxidation behaviors and structures of Fe-9Cr-1Mo alloy

Isothermal oxidation behaviors of Fe-9Cr-1Mo alloy were investigated at 600, 700, 750 and 850 ∘C for 72 h in air atmosphere. The oxidation rates were measured using a thermogravimetric analyzer (TGA). The structure and composition of the oxide scale were characterized by X-ray diffraction (XRD), energy dispersive spectrometry (EDS) and conversion electron Mossbauer spectrometry (CEMS). In this study it was found that the oxide layers form duplex structures consisting of Fe and Cr oxides. CEMS spectra are composed of one doublet due to dispersed Fe 3+ in Cr2O3 oxidation layers produced at high temperatures and two magnetic components due to Fe-Cr-Mo alloy substrate and hematite (Fe, Cr)2O3 with lower hyperfine field than pure hematite (52 T). Fe rich oxides are formed at the surface by oxidation at relatively low temperatures of 600 ∘C and 700 ∘C, while Cr rich (Fe, Cr)-oxides are formed in the top surface layers by oxidation at higher than 750 ∘C.

Local structure and magnetic properties of cubic iron oxide nanoparticles formed in zeolite as deduced using Mössbauer spectroscopy

Local structure and magnetic properties of cubic iron oxide nanoparticles as prepared in zeolite matrix have been studied in a detailed manner using Mössbauer spectroscopy. Superparamagnetic particles of cubic iron oxide are observed to be strongly bound in zeolite matrix. Annealing around 870 K is understood to result in detrapping of the bound nanoparticles. This leads to the onset of coarsening of the nanoparticles thus resulting in an appearence of a broad sextet characterised by a low hyperfine field. On the otherhand, the occurrence of the sextet, due to the onset of coarsening of the bare cubic iron oxide superparamagnetic nanoparticles, has been observed subsequent to annealing around 520 K. Linewidth and quadrupole splitting corresponding to the fraction of 57Fe absorbers associated with superparamagnetic particles are shown to be sensitively dependent upon the annealing temperature with respect to detrapping and aggregation of nanoparticles formed in zeolite.

Mössbauer and X-ray study of the Fe 65 Ni 35 invar alloy obtained by mechanical alloying

Fe65Ni35 samples were prepared by mechanical alloying (MA) with milling times of 5, 6, 7, 10 and 11 h, using a ball mass to powder mass ratio of 20:1 and at 280 rpm. The samples were characterized by X-ray diffraction (XRD) and transmission 57Fe Mossbauer spectrometry. The X-ray diffraction pattern showed the coexistence of one body centered cubic (BCC) and two face centered cubic (FCC1 and FCC2) structural phases. The lattice parameters of these phases did not change significantly with the milling time (2.866 A, 3.597 A and 3.538 A, respectively). After 10 h of milling, the X-ray diffraction pattern showed clearly the coexistence of these three phases. Hence, Mossbauer spectrometry measurements at low temperatures from 20 to 300 K of this sample were also carried out. The Mossbauer spectra were fitted using a model with three components: the first one is a hyperfine magnetic field distributions at high fields, related to the BCC phase; the second one is a hyperfine magnetic field distribution involving low hyperfine fields related to a FCC phase rich in Ni, and the third one is a singlet related to a FCC phase rich in Fe, with paramagnetic behavior. As proposed by some authors, the last phase is related with the antitaenite phase.

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. Bhf 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(Bhf), indicate that the high hyperfine field values ranging from 30 to 33T 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 30T can be associated to the nanocrystalline bcc Fe(Al, Ni) solid solution while the low hyperfine field values (<15T) result from Fe atoms located in the disordered grain boundaries.

Characterization of nanocrystalline FeSiCr powders prepared by ball milling

FeSi 10Cr 10 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-Fe 1 (Si, Cr) and α-Fe 2 (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 FeSi 10Cr 10 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.

Mechanical amorphization of Fe 75Nb 10B 15 powder: Microstructural and magnetic characterization

The evolution of the amorphous fraction developed during the mechanical alloying of a mixture of pure 75 at.% Fe, 10 at.% Nb and 15 at.% B, X Am, has been followed by different techniques: X-ray diffraction, Mössbauer spectroscopy and magnetic permeability measurements; in order to compare their sensitivity in the detection of small fractions. The values obtained for the amorphous fraction from the three techniques show a roughly linear correlation above ∼20%. The most sensitive technique was Mössbauer spectroscopy ( X Am obtained from the low hyperfine field contributions) and the less sensitive technique was X-ray diffraction. The Curie temperature of the amorphous phase increases with the milling time due to a slow and progressive incorporation of boron into this phase and the nanocrystals from the boron inclusions.

AMMONIUM OXALATE AND CITRATE-ASCORBATE AS SELECTIVE CHEMICAL AGENTS FOR THE MINERALOGICAL ANALYSIS OF CLAY FRACTIONS OF AN ULTISOLAND ANDISOLS FROM SOUTHERN CHILE

ABSTRACT Two selective chemical treatments consisting of mixtures of (i) oxalic acid - ammonium oxalate at pH 3 (OX) and (ii) sodium citrate - ascorbic acid at pH 6 (CA) were used to selectively remove minerals from clay fractions of four volcanic soils: one from an Ultisol and three Andisols, of the southern Chile. Chemical analysis shows that CA is more effective in removing iron from Ultisol, and OX extracts it more efficiently from Andisols. 6 K 57 Fe-Mossbauer data indicate that OX tends to remove all magnetically ordered species with low hyperfine fields; CA has no clear effect on the removal of those or the identified iron oxides in these volcanic soil materials. INTRODUCTION Selective chemical dissolution treatments are useful, and in many circumstances essential, laboratory procedures in order to improve the mineralogical analysis of soil samples. However, no chemical extractive solution, whatever the chemical dissolution mechanism, is expected to completely dissolve certain groups of chemically related minerals, while leaving unaffected the other species. In soils, iron (oxyhydr)oxides very often occur as a continuum of inter-related mineral forms or assemblages of strongly cemented multiphase grains in heterogeneous systems, of a wide range of degree of crystallinity.Cornell and Schwertmann

Hyperfine interactions on iron in R2−xFe14+2xSi3 (R=Ce, Nd, Gd, Dy, Ho, Er, Lu, Y) compounds studied by Mössbauer spectroscopy

Abstract Compounds R 2− x Fe 14 + 2 x Si 3 with (R = Ce, Nd, Gd, Dy, Ho, Er, Lu, Y) were prepared by the modified Czochralski method and investigated by means of the 57 Fe 14.4 keV Mossbauer spectroscopy and powder X-ray diffraction. The magnetic moment was additionally measured for samples being single crystals. Iron experiencing low magnetic hyperfine field and electric quadrupole interaction was found on (4e) sites in the P 6 3 / mmc structure. Similar iron was found in the R 3 ¯ m structure as well. The non-stoichiometry parameter was determined as being close to 2 x = 0.5 for all compounds investigated. Compound with R = Ho crystallizes in both structures. Our results show that silicon avoids (4f) and (6g) sites for compounds with the P 6 3 / mmc structure (R = Dy, Ho, Er, Lu, Y), whereas it avoids (6c) and (9d) sites for those with R 3 ¯ m structure (R = Nd, Gd, Ho).

Comparative Mössbauer and magnetization study of 1%Sn119-dopedLa0.67Ca0.33MnO3andLa0.67Sr0.33MnO3

A detailed comparative M\"ossbauer study of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ and ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ samples doped with 1% Sn is presented. The spectra at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ can be analyzed by a single ferromagnetic component. However, at higher temperatures two separate ferromagnetic components and a paramagnetic one are required to fit the spectra. The ferromagnetic component with the lower hyperfine field is attributed to clusters of weakened double-exchange interactions due to competition by localization mechanisms. Clear differences between the two samples in regard to the critical behavior arise: (i) The hyperfine field of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{Mn}}_{0.99}{\mathrm{Sn}}_{0.01}{\mathrm{O}}_{3}$ disappears abruptly at ${T}_{c}$ in contrast to that of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}}_{0.99}{\mathrm{Sn}}_{0.01}{\mathrm{O}}_{3}$ which varies continuously and goes to zero smoothly at the critical temperature. (ii) The second ferromagnetic phase appears above $0.35{T}_{c}$ in the Ca sample but only above $0.83{T}_{c}$ in the Sr sample. The paramagnetic component appears for $T&gt;0.67{T}_{c}$ in the former compared to $T&gt;0.9{T}_{c}$ in the latter. The results support recent studies that show differences in the polaronic character, and critical behavior thereof, between the two materials.

Distribution of Fe-bearing compounds in an Ultisol as determined with selective chemical dissolution and Mössbauer spectroscopy

The distribution of Fe-bearing compounds in the silt and clay fractions from samples of a Chilean Ultisol developing on pyroclastic material was determined in this study. The mineralogical analysis was carried out mainly by Mossbauer spectroscopy at 298, 80 and 6 K, for samples before and after treatment with ammonium oxalate (OX). The 298 and 80 K-Mossbauer spectra reveal relatively complex mineral assemblages for all samples. An intense central doublet, due to (super)paramagnetic Fe3+ is observed. The incipient broad-line sextet may be due to coexisting magnetically ordered forms of iron oxides. Goethite, hematite and maghemite were identified from the Mossbauer spectral analysis of patterns obtained at 6 K, which were numerically fitted with model-independent hyperfine field distributions. Some low hyperfine fields are identified for both, the silt and clay samples, being more intense in the last one. These species are likely due to poorly crystalline iron oxyhydroxides, which are more easily removed with the OX treatment; maghemite tends to remain in the coarse fraction, whereas poorly crystalline forms do occur mainly in the clay fraction.

Effect of boron on structural and magnetic properties of the Fe 60Al 40 system prepared by mechanical alloying

A new nanostructured system, FeAlB, with high content of boron, was prepared by mechanical alloying in a high-energy ball mill. X-ray diffraction (XRD) and 57Fe Mössbauer spectroscopy were used to study structural and magnetic properties, respectively. Two crystalline phases were found, a majority one corresponding to an FeAl bcc phase and another in low proportion, corresponding to an Fe 2B phase type which appears due to the presence of boron. Such a phase is also well observed from Mössbauer spectra. An anisotropic model was applied to fit X-ray patterns, giving evidence of different shapes of the nanocrystalline grains in agreement with the milling process involving both confinement and atomic diffusion. Also it was possible to postulate from both X-ray diffraction and Mössbauer spectrometry that there appears an additional contribution at low hyperfine fields attributed to grain boundaries. This contribution increases as both milling time and boron composition increases.

NMR study of 59Co nuclei in EuBaCo 2O 5+ x ( x=0 and 0.5)

NMR measurements of 59Co nuclei in EuBaCo 2O 5+ x ( x=0.52 and 0.02) are carried out to clarify the spin state of Co ions. We observe in the antiferromagnetic state of the x=0.52 sample three groups of NMR lines with high, intermediate and low hyperfine fields. These results do not agree with the crystal structure at room temperature.

NMR evidence for two-phase character of ferromagnetic manganites

A thorough analysis of the spin–spin relaxation and 55Mn NMR line shape revealed that in the number of manganites, including La 0.7Ca 0.3MnO 3 and La 0.7Sr 0.3MnO 3, two different ferromagnetic phases with mobile holes coexisted below T C in a certain temperature interval, which depended on the composition. The phase with lower hyperfine field has a faster nuclear relaxation, and its volume decreases with decreasing temperature.

Two-phase character of metallic ferromagnetism in manganites.

(55)Mn NMR spectra of several ferromagnetic manganites, including La(0.7)Ca(0.3)MnO(3) and La(0.7)Sr(0.3) MnO(3), were measured as a function of temperature. A thorough analysis of the spin-spin relaxation and the NMR line shape revealed that in these compounds two different ferromagnetic phases coexist below T(C) in a certain temperature interval which depends on the composition. The phase with lower hyperfine field has faster nuclear relaxation, its characteristic dimension is a few nm, and its volume decreases with decreasing temperature. The NMR lines of both phases are motionally narrowed, i.e., the electron holes are mobile.

Hyperfine Study on Mechanically Alloyed Fe–Mn Systems

High-energy ball milling of Fe–Mn elemental powder mixtures has been carried out for different Mn concentrations, ranging from 0 to 90%. Mossbauer spectroscopy of 57Fe and perturbed angular correlations with 111In – implanted at 400 keV into pills made from the milled powders – have been used to investigate the structure of the milled samples. Clear changes of the hyperfine spectra occur at Mn concentrations of 15–20% and above 70%, indicating extended concentration ranges of α-Fe and γ-Fe(Mn), compared to the equilibrium phase diagram. Detailed information on additional lower hyperfine fields due to nearest neighbor Mn atoms has been obtained in the α-Fe phase containing up to 20% Mn .

Deformation induced transformations and grain boundary thickness in nanocrystalline B2 FeAl

Precise measurement of fundamental Bragg peak shifts during milling of nanocrystalline ordered B2 Fe 60Al 40 has allowed for the first time, the deconvolution of the 110 fundamental Bragg peak intensities of the b.c.c. disordered regions and of the ordered B2 regions from the start and before full disappearance of the latter. The evolution of the lattice parameter a 0 of the b.c.c. solid solution with milling time shows two characteristics. First a jump to higher values from the initial a 0 of the B2 phase, with a Δ a 0 change of the order of 1% corresponding to a volume per atom Δ V expansion of about 3%. Subsequently, a 0 continues to increase slowly with further milling at constant grain size D and in the absence of any B2 phase. This continuing change of a 0 with further milling up to at least 180 min is attributable to a reduction of chemical short-range order (CSRO) or the number of Al–Fe heteroatomic “bonds”. The appearance of two well-defined maxima in the hyperfine field (HF) distributions derived from the Mössbauer spectra indicated the presence of two ferromagnetic environments contributing to the broadened Mössbauer resonance sextet signal. The evolution of the second component of this Mössbauer signal scales with the grain size. Using the mean grain size D derived from X-ray peak profiles and TEM pictures together with the grain boundary thickness d gb of 1.25 nm determined by Fultz et al. ( J. appl. Phys., 1996, 7, 127) for b.c.c. Fe-based alloys, the fraction of grain boundary atoms n gb/ n total was estimated and found to be consistent with the fraction of Fe atoms contributing to the lower HF component of the Mössbauer sextet signal. The grain boundary atom count using both methods confirms that grain boundaries in materials nanocrystallized by heavy deformation are nearly as dense as in the bulk.