Hyperfine Magnetic Field Distribution Research Articles

Effect of pH on the Formation of Goethite and Hematite from Ferrihydrite

AbstractStorage of ferrihydrite in aqueous suspensions at 24°C and pHs between 2.5 and 12 for as long as three years resulted in the formation of goethite and hematite. The proportions and crystallinity of these products varied widely with the pH. Maximum hematite was formed between pH 7 and 8, and maximum goethite at pH 4 and at pH 12. The crystallinity of both products, as indicated by X-ray powder diffraction line broadening and magnetic hyperfine field values and distribution widths, was poorer, the lower the proportion of the corresponding product in the mixture. The existence of two competitive formation processes is suggested: goethite is formed via solution, preferably from monovalent Fe(III) ions [Fe(OH)2+ and Fe(OH)4−], and hematite by internal rearrangement and dehydration within the ferrihydrite aggregates. This concept relates the proportions of goethite and hematite to the activity of the Fe(III) ion species in solution, and implies that conditions favorable for the formation of goethite are unfavorable for that of hematite and vice versa.

Magnetic hyperfine field distribution and structural relaxation of amorphous (Fe81.5B14.5Si4) prepared with different quenching rates

The hyperfine field distributions of the amorphous (Fe81.5B14.5Si4) alloy are explained in terms of structural disorder. The increase in Curie temperature with isothermal relaxation may be attributed to the increase of mean number of nearest-neighbor iron atoms around Fe, and so to reduction of topological short-range disorder.

On the determination of the magnetic hyperfine field distribution in an amorphous alloy

The magnetic hyperfine field distribution P(H) in Fe32Ni36Cr14P12B6 has been determined by several methods. The P(H) curves obtained over the range 1.5 K to the ordering temperature Tc ≈ 233 K, confirm the existence of two distinct regions of strong and weak magnetic interactions.

Magnetic inhomogeneity in amorphous Fe-Zr alloys

Magnetic properties of amorphous Fe92Zr8 and Fe90Zr10 alloys have been studied by 57Fe Mössbauer and magnetization measurements. The magnetic hyperfine field distribution spreads over a wide range and the temperature dependence of the hyperfine fields shows magnetically heterogeneous behavior. Superparamagnetic behavior was observed on the magnetization curves above Tc.

61Ni Mössbauer spectroscopy in amorphous alloys

Amorphous alloys of Metglas type, (Fe 1− x Ni x ) 80B 20, (0.25 ⩽ x ⩽ 0.75), prepared by melt-spinning, and Ni-rich Gd x Ni 1− x alloys ( x = 0.2, 0.4), prepared by sputtering, were investigated by 61Ni Mössbauer spectroscopy in conjunction with magnetization measurements and Mössbauer spectroscopy with 57Fe and 155Gd, respectively. The results for the first class of samples confirm that Ni does contribute to the bulk magnetic moment, at least for Ni concentrations up to x = 0.75. The 61Ni nuclei experience a broad distribution of magnetic hyperfine fields whose average is proportional to the bulk moment. The average fields, however, are substantially smaller than those in crystalline FeNi alloys of comparable concentrations. Also in GdNi alloys, a distribution of hyperfine fields was observed. Measurements in applied fields up to 5 T have shown that this distribution is centered at a value which is equal to the Lorentz field. This result confirms that Ni in these alloys does not carry a magnetic moment. The observed distribution of hyperfine fields can be ascribed to transferred fields and to dipole fields due to the Gd moments.

Magnetic hyperfine field distribution in annealed amorphous glasses

Glassy ribbons of FeB and FeCP have been obtained by a spatcooling method, and annealed between 250°C and 600°C. Mössbauer spectra measurements before and after annealing showed distinct differences in the linewidths and lineshapes. In order to fit the spectra with such high diversity of Mössbauer lineshapes, a computer program was made. The program assumed both a continuous distribution of the hyperfine field and an anomaly of the classical Lorentz lineshape, as well as a correlation between certain parameters of the Mössbauer spectra. Thus, a description - by a continuous function - of the hyperfine field distribution was obtained, using up to five magnetic patterns.

Identification of Ferrihydrite in Soils by Dissolution Kinetics, Differential X‐ray Diffraction, and Mössbauer Spectroscopy

AbstractFerrihydrite is a poorly crystalline, natural Fe3+ oxide which occurs in ochreous spring precipitates and hydromorphic soils of humid temperate climates. The identification of ferrihydrite in soils is complicated by its association with goethite, quartz, and layer silicates.The following criteria were used to identify ferrihydrite in Fe‐oxide accumulations from soils: high solubility in acid oxalate, five to six broad x‐ray diffraction lines, and the existence of a typical magnetic hyperfine field distribution at 4K in Mössbauer spectra rather than a discrete field value. Identification of low concentrations of ferrihydrite (≲ 20% oxalate‐soluble Fe) by x‐ray diffraction was made possible by subtracting diffraction data obtained after oxalate treatment from data obtained before such a treatment (differential XRD). Oxalate treatment preferentially dissolves ferrihydrite over goethite. This led to an increase in the quadrupole splitting observed in Mössbauer spectra from ∼0.10 to ∼0.23 mm s−1, resulted in a significantly narrower field distribution, and intensified the goethite DTA peak.

A mössbauer study of the magnetic hyperfine field distribution in the amorphous alloy Fe32Ni36Cr14P12B6

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Investigations of Dy(Fe xAl 1−x) 2 laves phases by x-ray, magnetometric and Mössbauer effect methods

The crystal and magnetic properties of the Dy(Fe x Al 1- x ) 2 (0≤ x≤1.0) system are investigated with the X-ray, magnetometric and Mössbauer effect methods. The system constitutes two cubic C15 structure regions separated by hexagonal C14 structure region. The X-ray investigations indicate the structure disorder of the C15 type phases. All compounds of the investigated system are ferrimagnets. Some of them exhibit a pronounced magnetic anisotropy. Substitution of Al for Fe decreases the Curie temperature and the Fe hyperfine magnetic field. The distribution of hyperfine magnetic field at iron sites is correlated with their nearest neighbours configurations. The Mössbauer effect data indicate the existence of the appreciable amount of the covalent apart from the long-range metallic bond in the investigated phases.

Hyperfine field distributions in ferromagnetic amorphous FexB1−x thin films

Sputtered films of amorphous FexB1−x (0.40?x?0.50) were studied by Mössbauer spectroscopy. The films were characterized for composition and homogeneity by secondary-ion mass spectrometry (SIMS) and for lack of crystallinity by X-ray diffraction. Below the magnetic ordering temperature Tc, the hyperfine spectra consist of a large amplitude quadrupole doublet superimposed on a broad magnetic hyperfine field distribution. The spectra of these alloys are remarkably different from that of polycrystalline FeB and from those of liquid-quenched alloys which are restricted to compositions near the eutectic at x≊0.8. Comparison with the polycrystalline FeB spectrum clearly shows that for a significant fraction of the Fe atoms in the amorphous alloys, the local environment is substantially different from the Fe environment in FeB.

Iron pentacarbonyl decomposition over Grafoil. Production of small metallic iron particles

Mossbauer spectroscopy was the principal technique used to study the adsorption and thermal decomposition of iron pentacarbonyl on a high surface area, oriented graphite (Grafoil). The iron carbonyl physisorbs on the graphite at temperatures less than ca. 300 K, whereas decomposition of the carbonyl occurs upon heating to 378 K. The Mossbauer spectra of the decomposition product are sextuplets indicative of metallic iron. However, the room-temperature magnetic hyperfine splitting (ca. 300 kOe) is smaller and the peak widths (ca. 1.5 mm s-/sup 1/) are larger than those corresponding spectral parameters for bulk metallic iron (330 kOe and 0.28 mm s-/sup 1/, respectively). Computer simulations suggest that these differences are due to magnetic relaxation and to a distribution of magnetic hyperfine fields. For consistency with the Mossbauer spectra, it is proposed that the iron particles are present as thin rafts on the graphite surface. Transmission electron microscopy shows that these rafts are preferentially located along edges and steps on the graphite. Heating to 450 K leads to significant sintering of the iron particles.

A Mössbauer Study of the Amorphous System (FexNi1−x)75P16B6Al3

ABSTRACTIn the system (FeXNi1−x)75P16B6Al3 an unusual magnetic behaviour is found in the composition range 0.2<x<0.3. The drastic changes seen at low temperature in the thermomagnetic measurements is found not to affect the Mössbauer spectra. From the width of the magnetic hyperfine field distribution as function of temperature there seems to be a distribution in Curie temperatures. Due to the absence of spin texture it was not possible to make a distinction between the two proposed low temperature phases although the general results favour the spin glass alternative.

Hyperfine field distribution in disordered binary alloys

Mossbauer measurements were made on the disordered binary alloys Ni3Fe NiFe and FeCo at different temperatures and the distribution of hyperfine magnetic fields has been evaluated using a model-independent method of analysis proposed by Window (1971). The full width at half-maximum of the P(H) distribution is found to remain constant for Ni3Fe with respect to temperature. It increases for NiFe but decreases for FeCo with increase of temperature. The full width at half-maximum is correlated to the exchange interaction and the measurements show that the range of exchange interaction is shorter in NiFe than in Ni3Fe. In FeCo all the atoms are found to have the same magnetic ordering temperature. The reduced value of the average field for NiFe is fitted to the Brillouin function.

Hyperfine field distribution in amorphous (FexNi1-x)80P14B6

The magnetic hyperfine field distributions (P(H)) of amorphous (FexNi1-x)80P14B6 have been measured by 57Fe Mossbauer spectroscopy. The saturation P(H) and their concentration dependences have been determined using a Fourier series method. The reliability and shortcomings of this method are illustrated. For all samples of (FexNi1-x)80P14B6, the P(H) are well defined, single-maximum and asymmetric functions of H with a higher population at the low-field side. The P(H) shifts toward smaller H values for samples with decreasing Fe content, but the extrapolated P(H) for dilute Fe in amorphous Ni80P14B6 remains finite. The shape of P(H) is found to be insensitive to the Fe content and practically temperature-independent. For amorphous Fe80P14B6, the ratio of the average hyperfine field and the average magnetic moment is 150 kOe/ mu B, which is essentially the same as those observed in a large number of crystalline Fe metalloid compounds. This suggests that the Fe hyperfine field is likely to be proportional to its moment, due to a negligible conduction electron contribution. An appreciable amount of Fe atoms in samples with x>or approximately=0.3 has been found to have hyperfine fields larger than 340 kOe, the saturation value of alpha Fe.

A Mossbauer study of the magnetic properties of CuFe alloys

The Mossbauer effect has been used to investigate the magnetic properties of Cu-Fe alloys of concentration 0.24-4.6% Fe in the temperature range 4.2-100K and in applied magnetic fields of up to 8.0 T. Alloys containing more than about 0.5% Fe exhibited magnetic broadening at 4.2K and the transition temperatures at which the broadening first occurred have been determined. The distributions of hyperfine magnetic fields observed in these specimens below the transition temperatures have been studied as functions of iron concentration and of temperature. The saturation hyperfine fields for isolated iron atoms are found to vary with concentration, exhibiting a linear variation at high concentrations but falling more rapidly at lower concentration. This is interpreted as evidence for the formation of a Kondo state. Spectra obtained at high magnetic fields indicate some anisotropy in the impurity response to the applied field.

Magnetic hyperfine field distribution at the tin site in the Cl structure alloys IrMnSn and PtMnSn

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Hyperfine field distributions from Mossbauer spectra

A simple procedure for obtaining the distributions of magnetic hyperfine fields from Mossbauer spectra is outlined.

Theory of Modulation Effects in Resonant Nuclear Disorientation Experiments

An analysis is made of modulation effects in nuclear disorientation experiments on radioactive nuclei which have been polarized by hyperfine interaction at low temperatures in ferromagnetic host metals. The inhomogeneously broadened nuclear magnetic resonance is detected through the destruction of the $\ensuremath{\gamma}$ radiation anisotropy by a resonant-frequency-modulated rf field. It is shown that to a good approximation the observed line shape is determined solely by the modulation amplitude and the distribution of hyperfine magnetic fields; for large rf fields, the signal amplitude depends separately upon the modulation amplitude and frequency. For finite rf field intensities, the signal amplitude also depends upon a parameter $k$ which is closely related to the saturation behavior which would be observed in the absence of inhomogeneous broadening. At high modulation frequencies, the dependence of the signal amplitude upon $k$ is easily calculated, and this should permit experimental determinations of $k$ values. Independent determinations should also be possible from studies of the time rate of destruction of the $\ensuremath{\gamma}$ radiation anisotropy immediately after applying a resonant rf field.

The Range of Ferromagnetic Exchange Interactions

Experimental investigations on the characteristic length over which two magnetic atoms interact in a nonmagnetic metal have been carried out. Two methods are used: (a) measurement of the transition temperature as a function of the magnetic atom concentration in zero field, and (b) study of the magnetization distribution in alloys in the region just below the transition temperature as determined from the magnetic hyperfine field distribution. These experiments have been done using palladium as the nonmagnetic and iron as the magnetic atoms. In the FePd alloys it is found that the transition temperature and the nature of the magnetic transition depend strongly on the method of sample preparation and in particular on the annealing of the samples. Samples quenched from the melt to room temperature show a diffuse transition and a relatively slow rise in magnetization with decreasing temperature. Those which have been annealed show a relatively sharp transition and a narrower magnetization distribution below the transition temperature, indicating that the process of annealing produces a more homogeneous distribution of the iron atoms. Interpretation of this phenomenon and its relation to the range parameter are discussed. Assuming that the magnetic interaction between two iron atoms is due to the overlap of the matrix polarization, the range obtained in this investigation from a simple statistical model is in excellent accord with neutron diffraction measurements.