Hyperfine Magnetic Field Distribution Research Articles

Hyperfine interactions in nanocrystallized NANOPERM-type metallic glass containing Mo

NANOPERM-type alloy with chemical composition Fe76Mo8CuB15 was studied by combination of 57Fe Mossbauer spectroscopy and 57Fe(10B, 11B) nuclear magnetic resonance in order to determine distribution of hyperfine magnetic fields and evolution of relative concentration of Fe-containing crystalline phases within the surface layer and the volume of the nanocrystallized ribbons with annealing temperature. Differential scanning calorimetry revealed two crystallization stages at Tx1 ∼ 510 ∘C and Tx2 ∼ 640 ∘C, connected to precipitation of α-Fe and Fe(Mo,B) nanocrystals, respectively. The amorphous and partially crystalline state was obtained by annealing at several temperatures in the range 510-650 ∘C. The combination of conversion electron (CEMS) and transmission Mossbauer spectrometry (TMS) showed that annealing induces crystallization starting from both surfaces of the ribbons. For the as-quenched sample, scanning electron microscopy (SEM) and CEMS revealed significant differences in the “air” and “wheel” sides of the ribbons, crystallites were preferentially formed at the latter. While SEM micrographs of annealed samples showed various mean diameters of the crystals at opposite sides of the ribbons, the amounts of crystalline volume derived from the CEMS spectra approximately equaled. Mossbauer spectra of annealed samples contained narrow sextet ascribed to crystalline α-Fe phase, three sextets with distribution of hyperfine field assigned to the interface regions of the nanocrystals and the contribution of the amorphous phases. In-field TMS performed at 4.2 K with magnetic moments aligned by external magnetic field enabled to properly determine in particular the contribution of the amorphous phases in the samples. Resulting distributions of the hyperfine fields were compared with 57Fe(10B, 11B) nuclear magnetic resonance (NMR) spectra.

Iron oxide nanoparticles for plant nutrition? A preliminary Mössbauer study

One of the most important micronutrients for plants is iron. We have prepared iron(III) oxyhydroxide and magnetite nanoparticles with the aim to use them as possible nutrition source for plants. The iron(III)-oxide/oxyhydroxide nanoparticles prepared under our experimental conditions as colloidal suspensions proved to be 6-line ferrihydrite nanoparticles as verified by XRD, TEM/SAED and Mossbauer spectroscopy measurements. 57Fe Mossbauer spectra of magnetite nanoparticles prepared under different preparation conditions could be analyzed on the basis of a common model based on the superposition of four sextet components displaying Gaussian-shaped hyperfine magnetic field distributions.

Annealed FINEMET ribbons: Structure and magnetic anisotropy as revealed by the high velocity resolution Mössbauer spectroscopy

The high velocity resolution 57Fe Mössbauer spectroscopy was used in order to elucidate structural and compositional details of FINEMET (Fe73.5Si15.5Nb3B7Cu1) alloys obtained via the annealing (with and without external magnetic field) of rapidly quenched ribbons. The analysis of the measured Mössbauer spectra was carried out, on one hand, by considering the possibility of a random distribution of iron atoms substituting Si at the D sites in the well crystallized DO3 Fe-Si phase, on the other hand, by allowing for an arbitrary-shape hyperfine magnetic field distribution for the case of the amorphous matrix. The results refer to the influence of the next-nearest-neighbor configurations on the magnitude of iron magnetic moments at the D sites in the precipitated nanocrystalline Fe-Si phase. The applied analysis method enables us to draw conclusions regarding the relative occurrence of the various iron microenvironments in the nanocrystalline phase and amorphous matrix, and the associated Si concentration of the precipitated nanocrystalline DO3 Fe-Si phase. The studied samples provide further evidence concerning the correlation between the induced magnetic anisotropy and the magnetic permeability in annealed FINEMET ribbons.

Sensitivity of 57Fe emission Mössbauer spectroscopy to Ar and C induced defects in ZnO

Emission Mossbauer Spectroscopy (eMS) measurements, following low fluence (<1012 cm−2) implantation of 57Mn (t1/2 = 1.5 min.) into ZnO single crystals pre-implanted with Ar and C ions, has been utilized to test the sensitivity of the 57Fe eMS technique to the different types of defects generated by the different ion species. The dominant feature of the Mossbauer spectrum of the Ar implanted ZnO sample was a magnetic hyperfine field distribution component, attributed to paramagnetic Fe3+, while that of the C implanted sample was a doublet attributed to substitutional Fe2+ forming a complex with the C dopant ions in the 2− state at O vacancies. Magnetization measurements on the two samples, on the other hand, yield practically identical m(H) curves. The distinctly different eMS spectra of the two samples display the sensitivity of the probe nucleus to the defects produced by the different ion species.

Carbon-Substituted Hematite and Magnetite Nanoparticles

ABSTRACTGraphite-doped hematite and magnetite nanoparticles systems (∼50 nm) were prepared by mechanochemical activation for milling times ranging from 2 to 12 hours. Their structural and magnetic properties were studied by 57Fe Mössbauer spectroscopy. The spectra corresponding to the hematite milled samples were analyzed by considering two sextets, corresponding to the incorporation of carbon atoms into the iron oxide structure. For ball milling time of 12 hours a quadrupole split doublet has been added, representing the contribution of ultrafine particles. The Mössbauer spectra of graphite-doped magnetite were resolved considering a sextet and a magnetic hyperfine field distribution, corresponding to the tetrahedral and octahedral sublattices of magnetite, respectively. A quadrupole split doublet was incorporated in the fitting of the 12-hour milled sample. The recoilless fraction for all samples was determined using our previously developed dual absorber method. It was found that the recoilless fraction of the graphite-doped hematite nanoparticles decreases as function of ball milling time. The f factor of graphite-containing magnetite nanoparticles for the tetrahedral sites stays constant, while that of the octahedral sublattice decreases as function of ball milling time. These findings reinforce the idea that carbon atoms exhibit preference for the octahedral sites of magnetite.

Mössbauer Study of Graphite-Containing Iron Oxide Nanoparticles

Graphite-doped hematite and magnetite nanoparticles systems (~50 nm) were prepared by mechanochemical activation for milling times ranging from 2 to 12 hours. Their structural and magnetic properties were studied by 57Fe Mossbauer spectroscopy. The spectra corresponding to the hematite milled samples were analyzed by considering two sextets, corresponding to the incorporation of carbon atoms into the iron oxide structure. For ball-milling time of 12 hours a quadrupole split doublet has been added, representing the contribution of ultrafine particles. The Mossbauer spectra of graphite-doped magnetite were resolved considering a sextet and a magnetic hyperfine field distribution, corresponding to the tetrahedral and octahedral sublattices of magnetite, respectively. A quadrupole split doublet was incorporated in the fitting of the 12-hour milled sample. The recoilless fraction for all samples was determined using our previously developed dual absorber method. It was found that the recoilless fraction of the graphite-doped hematite nanoparticles decreases as function of ball-milling time. The f factor of graphite-containing magnetite nanoparticles for the tetrahedral sites stays constant, while that of the octahedral sublattice decreases as function of ball-milling time. These findings reinforce the idea that carbon atoms exhibit preference for the octahedral sites of magnetite.

Magnetic Behavior of Soft Magnetic FeMoCuB Metallic Glass at Low Temperature

We have studied a NANOPERM-type Fe76Mo8Cu1B15 metallic glass with emphasis upon its magnetic properties at low temperatures. Temperature evolution of magnetization measured under zero-field cooling and field cooling conditions upon samples with different amounts of nanocrystalline grains have unveiled a two-phase behavior in temperature range from 50 up to about 200 K. In order to understand the correlation between the structural arrangement and resulting magnetic properties, we have employed a local probe method of 57Fe Mossbauer spectrometry performed at 300 and 4.2 K. The latter experiments were also accomplished in an external magnetic field of 6 T. Magnetically distinct regions were unveiled in the amorphous structure of the as-quenched and annealed samples. They are demonstrated by the distributions of hyperfine magnetic fields.

Occurrence of two amorphous phases in an Fe40Co40B20 alloy

A process for the preparation of amorphous alloys is introduced to realize physical properties, which could not previously be achieved when using known methods. The method of preparation is based on the sputtering of metal–metalloid alloys on a substrate. Unlike presently known steady sputtering techniques, in this method of preparation, the shutter that is located between substrate and target frequently interrupts the plasma between the target and the substrate. Thereby, it possible to create interfaces, different phases, and defects between the layers, which leads to drastic changes in the physical characteristics of the sample. In order to demonstrate the effect of interfaces on the magnetic parameters, an amorphous Fe40Co40B20 alloy with a thickness of 200nm was prepared by repeated interruption of plasma between the target and the substrate. The interruption of plasma took place after the growth of each 2nm amorphous film (8 atomic layers). It means that for the preparation of an amorphous Fe40Co40B20 alloy with a thickness of 200nm, the plasma has to be interrupted 100 times. Each interruption duration was 30s. The X-Ray MicroDiffraction measurements confirm the formation of amorphous structures. The analyses of Conversion Electron Mössbauer Spectroscopy, Transmission Electron Microscopy (TEM), and magnetization measurements show that the sample is amorphous. As the result of the interruption, the structure of the amorphous alloy is affected. An interesting result of this kind of preparation is the occurrence of two amorphous states or atomic arrangements of amorphous Fe40Co40B20, which leads to two different magnetic hyperfine field distributions. The TEM image shows that the formation of two amorphous states is coherent. It means that the two states are connected together without the formation of grain boundaries or interfaces.

Temperature investigations of the spatial spin-modulated structure of multiferroic BiFeO3 by means of Mössbauer spectroscopy

The results from 57Fe Mossbauer spectroscopic studies of multiferroic BiFeO3 in a range of tem-peratures including that of the magnetic phase transition are presented. The Mossbauer spectra are processed and analyzed by reconstructing the hyperfine magnetic field distributions and interpreting the spectra with a cycloid-type spatial spin-modulated structure model. The temperature dependences of the hyperfine spectrum parameters (the Mossbauer line shift, the quadrupole shift, and the isotropic and anisotropic contributions to the hyperfine magnetic field) are obtained, along with the anharmonicity parameter of an incommensurate spin wave.

Low temperature behavior of hyperfine fields in amorphous and nanocrystalline FeMoCuB

Low temperature (4.2 K) magnetic behavior of Fe76Mo8Cu1B15 metallic glass was studied by 57Fe Mössbauer spectrometry (MS) and 57Fe NMR. Distributions of hyperfine magnetic fields P(B) were determined for as-quenched and annealed (nanocrystalline) samples with relative fraction of the grains about 43%. P(B) distributions were derived for both the amorphous matrix and nanocrystalline grains. NMR of alloys with natural and 57Fe enriched Fe enabled to assess the contribution of 11B to the total NMR signal. P(B) distribution of the as-quenched alloy derived from MS matches reasonably well the one from NMR of the enriched sample. NMR signal from the sample with natural Fe exhibits contributions from 11B nuclei. The principal NMR lines of the annealed alloys at 47 MHz correspond to bcc Fe nanocrystals. Small asymmetry of the lines towards higher frequencies might be an indication of possible impurity atoms in the bcc structure. The observed differences between natural and enriched samples are attributed to higher sensitivity in the latter. Positions of the lines attributed to bcc Fe nanocrystals obtained from MS and NMR are in perfect agreement.

Low temperature study of mechanically alloyed Fe67.5Ni32.5 Invar sample

The study at low temperatures of powder of the Invar alloy, Fe67.5Ni32.5, produced by mechanical alloying, shows that the sample presents two structural phases, the Fe–Ni BCC and the Fe–Ni FCC. The 57Fe Mössbauer spectra obtained in this sample at different temperatures were fitted considering two hyperfine magnetic field distributions. The first one having the larger mean field and only one peak (at ca. 35T, varying with T), is associated with the BCC phase, and the second one, presenting several broad peaks (distributed between 10 and 35T), is associated to the FCC phase. A singlet, which is associated to low spin Fe sites of the FCC phase, was also considered. The mean hyperfine magnetic field of the BCC phase increases monotonically as temperature decreases, while that of the FCC phase presents an anomaly near 75K. The real part of the ac magnetic susceptibility temperature scans presents a peak whose position increases from 31 to 39K, when the ac field frequency increases from 100 to 5000Hz. These results permit to associate the detected anomaly to the occurrence of a reentrant spin glass transition.

Mössbauer study of alloy Fe67.5Ni32.5, prepared by mechanical alloying

We present the study of effect of the particle size on the structural and magnetic properties of the Fe67.5Ni32.5 alloy, prepared by mechanical alloying (MA). After milling the powders during 10 hours they were separated by sieving using different meshes. The refinement of the X-ray patterns showed the coexistence of the BCC (Body Centered Cubic) and the FCC (Face Centered Cubic) phases in all samples with lattice parameters and crystallite sizes independent of the mean particle size. However, big particles presented bigger volumetric fraction of BCC grains. The Mossbauer spectra were fitted with a broad sextet corresponding to the ferromagnetic BCC phase, a hyperfine magnetic field distribution and a broad singlet which correspond to the ferromagnetic and paramagnetic sites of the FCC phase, respectively. Hysteresis loops showed a magnetically, soft behavior for all the samples, however, the saturation magnetization values are smaller for the original powder and for the powders with small, mean, particle size due to the dipolar magnetic interaction and the smaller mean magnetic moment, respectively. These effects were proved by Henkel plots that were made to the samples.

Mossbauer and XRD characterization of the phase transformations in a Fe-Mn-Al-C-Mo-Si-Cu as cast alloy during tribology test

In present study Fe-29.0Mn-6Al–0.9C-1.8Mo-1.6Si-0.4Cu (%w) alloy was obtained after melted in an induction furnace, and then molded as an ingot. From the as cast ingot it were cut samples for the different characterization measurements. The microstructure of the as-cast sample is of dendritic type and its XRD pattern was refined with the lines of the austenite, with a big volumetric fraction, and the lines of the martensite, with small volumetric fraction. The Mossbauer spectrum of the sample was fitted with a broad singlet which corresponds to disordered austenite. After the tribology test, its XRD pattern was refined with the lines of two austenite phases, one similar to the previous one and other with bigger lattice parameter. The total volumetric fraction of the austenite is smaller than that obtained for sample without wear. It was added the lines of the martensite phase with bigger volumetric fraction than that of the previous sample. The Mossbauer spectrum of the weared sample was fitted with two paramagnetic sites which correspond to the two Fe austenite phases and a hyperfine magnetic field distribution which is associated to the disordered original martensite and the new one which appears in the surface as a consequence of the wear process. These results show that during wear process the original austenite phase is transformed in martensite and in a new austenite phase. The increases of the martensitic phase improves mechanical properties and wear behavior.

Magnetic properties of Fe nano-clusters stabilized at grain boundaries of Yb films

The temperature dependence of the magnetic properties of Fe nano-sized clusters in (5.0 at. % Fe)Yb films, prepared by vapor co-deposition, has systematically been investigated using zero-field and in-field Mössbauer spectroscopy as well as AC and DC magnetization measurements. Room temperature Mössbauer spectra reveal two types of Fe clusters, with non-cubic symmetry, located at Yb grain boundaries. Spectra, taken at 4.2 K, show a complex distribution of magnetic hyperfine fields, which can be related to inhomogeneous inter-cluster interactions. In addition, zero-field and in-field Mössbauer experiments indicate that the interactions within the Fe-clusters are predominantly ferromagnetic-like and their average magnetic moments are about 80 μB. As suggested by Mössbauer and AC magnetization data, the Fe-clusters are superparamagnetic above ca. 20 K, while their magnetic behavior below this temperature is that of cluster-glass-like systems with weak inter-cluster interactions. The details of the freezing process as a function of frequency and magnetic field are discussed and magnetic anisotropy constants for these Fe clusters are estimated to be Keff ∼ 8–12 × 105 J m−3.

Local atomic arrangement in mechanosynthesized Co x Fe1−x−y Ni y alloys studied by Mössbauer spectroscopy

Mechanosynthesized Co x Fe1−x−y Ni y alloys were examined using X-ray diffraction (XRD) and Mössbauer spectroscopy. In order to explain the shape of hyperfine magnetic field (HMF) distributions for the alloys, a local environment model based on a multinomial distribution was proposed. The model was in agreement with the XRD data and confirmed that the studied alloys were disordered solid solutions. It was successfully applied to describe the samples with bcc and fcc crystalline lattice type within the relatively broad range of components concentration. The results showed that the change of the crystalline lattice type does not cause an abrupt change of the HMF value. Moreover, a mean number of unpaired spins for the first coordination sphere may be used as a parameter to describe the HMF value experienced by 57Fe nucleus. Finally, a set of the most probable atomic configurations and their corresponding contributions to the HMF distribution were obtained.

Structure and hyperfine interactions in Bi1−xNdxFeO3 solid solutions prepared by solid-state sintering

The series of polycrystalline ceramic powders, Bi1−xNdxFeO3 (x=0.1−1) was successfully synthesized by mixed oxide method followed by pressureless sintering. X-ray diffraction and Mössbauer spectroscopy were used as complementary methods to study structure and hyperfine interactions of the samples. It was found that with an increase of Nd content, within the range of x=0.2−0.3 a structural phase transition from rhombohedral to orthorhombic system occurs. The lattice parameters and unit-cell volume decrease with an increase of Nd concentration. The Mössbauer spectra registered for Bi1−xNdxFeO3 are characterized by sextets with slightly broadened lines and reveal distributions of hyperfine magnetic fields. It was found that the value of the hyperfine magnetic field induction monotonically increases with Nd content. The mean values of isomer shift are typical for Fe3+ ions in a high spin state and slightly decrease from 0.40mms−1 for x=0.3 to 0.37mms−1 for x=0.9. The small value of quadrupole shift confirms octahedral coordination of the iron ions. The structural transition is accompanied by an increase of isomer shift and a change of sign of quadrupole shift.

Heat treatment influence on the structural and magnetic properties of the intermetallic Fe56.25Al43.75 alloy prepared by mechanical alloying and arc-melted

Alloys of the Fe56.25Al43.75 system were prepared by mechanical alloying (MA) using a high energy planetary ball mill, with milling times in the range from 12 up to 96 h named MA0 samples. The sample milled for 48 hours was heat treated at 700 °C for 9 days. Then this sample was milled for times of 1, 4, 8, 12, 24, and 48 h, named MA1 samples. Additionally, and for comparison, it was prepared a Fe56.25Al43.75 sample by arc-melting method. For all samples, the structural and magnetic study was conducted by X-rays diffraction (XRD) and Mössbauer spectrometry (MS). The XRD results show that the system is nanostructured and the MA0 samples present only the BCC disordered phase, whose lattice parameter remains relatively constant with milling time. For MA1 samples it was identify the FeAl, Fe3Al, FeO and α-Fe phases. The Mössbauer spectra for all samples were fitted by using a hyperfine magnetic field distribution (HMFD), and a paramagnetic site for all the times used here. The ferromagnetism increases when milling time increases, and this is a consequence of the structural disorder induced by mechanical alloying.

Magnetic phase evolution and particle size estimation study on nanocrystalline Mg–Mn ferrites

The nanocrystalline spinel ferrite compositions of Mg x Mn1−xFe2O4 (x = 0.0, 0.2, 0.4 and 0.5) system have been synthesized by the chemical co-precipitation route. The structural and magnetic properties have been studied by means of X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and Mossbauer spectroscopic measurements. 57Fe Mossbauer spectra of three specimens, as prepared, annealed at 200 °C and sintered at 1,100 °C, of the studied compositions are recorded and analyzed to study the magnetic phase evolution. The Mossbauer spectra of as-prepared samples show a paramagnetic doublet, annealed samples exhibit simultaneous presence of a central paramagnetic doublet superimposed on two broad magnetic sextets while spectra for sintered samples show two well-resolved Zeeman split sextets corresponding to the Fe3+ ions at the tetrahedral sites and the other due to the Fe3+ ions at the octahedral sites of the spinel lattice along with presence of central doublet. The particle size estimated from the probability versus hyperfine magnetic field distribution curve is in agreement with those determine from XRD and TEM analysis, validates the method employed.

Hyperfine magnetic interactions of 57Fe nuclei in NaFeAs arsenide

The results of the Mossbauer effect studies of layered NaFeAs arsenide in a wide temperature range are presented. The measurements at T > TN demonstrate that the main part (∼90%) of iron atoms are in the low-spin state Fe2+. The other atoms can be attributed to the impurity NaFe2As2 phase or to the extended defects in NaFeAs. The structural phase transition (at TS ≈ 55 K) does not produce any effect on hyperfine parameters (δ, Δ) of iron atoms. At T < TN, the spectra exhibit the existence of a certain distribution of the hyperfine magnetic field (HFe) at 57Fe nuclei, indicating the inhomogeneity of the magnetic environment around iron cations. The analysis of the temperature behavior of the distribution function p(HFe) allows us to determine the temperature of the magnetic phase transition (TN = 46 ± 2 K). It has been found that the magnetic ordering in the iron sublattice has a two-dimensional type. The analysis of the HFe(T) dependence in the framework of the Bean-Rodbell model reveals a first-order magnetic phase transition accompanied by a drastic change in the electron contributions to the main component (VZZ) and the asymmetry parameter (η) of the tensor describing the electric field gradient at 57Fe nuclei.

Mössbauer studies of the states of Fe atoms in the antiferromagnetic Fe–Mn Invar alloys

A study of the magnetic hyperfine field distribution (HFD) for 57Fe in the antiferromagnetic Fe–Mn Invar alloys have been performed by Mössbauer spectroscopy technique. The component of HFD having anomalous large value of isomer shift was observed. The component is localized in the low-field part of HFD and its intensity is equal to 12(3)%. We argue that the observed anomalous isomer shift value due to a local volume effect. The observation of the pronounced temperature dependence of the isomer shift for the low-field component strongly supports a relationship between the appearance of the component and the Invar effect. Analogous features were found previously in the Fe–Ni and Fe–Al Invar alloys. On the basis of the results, we have reason to believe that the existence of the Fe sites having anomalous isomer shift values (and, as we suggest, the anomalous large local volume) is a common characteristic for Fe-based Invar alloys with the competing exchange interactions. In such systems, the radial dependences of the competing exchange interactions are the key factors in the determination of the ground-state properties of the different atomic configurations.