B. J. Evans

The structure–property relationships in M‐type hexaferrites: Hyperfine interactions and bulk magnetic properties

57Fe Mossbauer spectroscopic measurements have been made at 296 K on single crystals of MFe12O19 (M=Ba, Sr, Pb) oriented parallel and perpendicular to the c axis, permitting the establishment of the systematics of the static and dynamical aspects of the hyperfine interactions, and their relationship to crystal/chemical structures for the five Fe3+ sublattices, and to the bulk magnetic properties. With the exception of the electric quadrupole interaction at the 2b site, and the dependence of the 2b intensity on the crystal orientation, the magnitude of the hyperfine interactions of a given Fe3+ site exhibits only small variations among the different hexaferrites. The magnitude of the quadrupole interaction at the 2b site varies by more than 10%, with the 2b site in PbFe12O19 exhibiting the smallest value. The relative intensity of the 2b subspectrum varies markedly among the three hexaferrites for observations parallel and perpendicular to the c axis. Although all display the expected anisotropy resulting ...

57Fe Mössbauer spectroscopic investigation of complex magnetic structures in Ga, Sc, and In substituted M-type hexagonal ferrites

Investigation of preliminary evidence that Ga, Sc, and In influence the bulk and dynamic magnetic properties of hexagonal ferrites by means other than those supported in existing models has been undertaken. In SrFe12−xMxO19 (M=Ga, In) and BaFe12−xScxO19, the predominant 12k sublattice exhibits a remarkable splitting into two distinct subpatterns, 12k1 and 12k2: At x=0.3, hyperfine fields, Heff for 12k1 are 411, 408, and 405 kOe, respectively, and for 12k2 are 358, 339, and 311 kOe, respectively. The 12k1 hyperfine field values are virtually unchanged from that of the pure hexagonal ferrites and are independent of substitution level. Contrastingly, the abruptness of the drop in the 12k2 hyperfine field and its dependence on the nature of the substituting cation are remarkable. The relative intensity of the 12k2 component correlates with the concentration of nonmagnetic species on the 2b and 4f2 sites and with the magnetic anisotropy. Scandium seems to have a more profound influence on the magnetic structur...

Order‐disorder and magnetic exchange interactions in substituted strontium hexaferrite SrAxFe12−xO19(A=Ga, In)

The unusual magnetic behavior of the 12k site in SrGaxFe12−xO19 (0.3≤x≤6.0) and SrInxFe12−xO19 (0.3≤x≤1.4) has been investigated by means of 57Fe Mossbauer spectroscopy at 298 K. The bifurcation of the 12k Mossbauer subspectrum into 12k1 and 12k2 components of widely differing intensities has been confirmed for both Ga3+ and In3+. The bifurcation is apparent at x values as low as 0.3, and the less intense 12k2 pattern corresponds to an abrupt drop in the hyperfine magnetic field Heff, of 50 and 75 kOe for Ga3+ and In3+, respectively. Heff for the more intense 12k1 pattern is virtually unchanged from that of pure SrFe12O19 for x values as high as 1.0. Room‐temperature Mossbauer spectra confirm that the Ga3+ and In3+ ions preferentially occupy the 4f2 octahedral site. There are indications, however, that the dopants also enter the 2b trigonal‐bipyramidal site. The change in Heff at the 12k sites indicates that complex and competing exchange interactions are present, and that the splitting of the 12k magneti...

Mössbauer Study of Local Molecular Fields, Cation Distributions, and Recoilless Fractions in Sb‐Substituted Lithium Ferrite

Previous studies of the B‐site hyperfine field distribution in spinel ferrites utilizing a local‐molecular‐field model have demonstrated that A‐site cation disorder has a pronounced effect on Heff(B) and virtually no influence on Heff(A). The influence of B‐site disorder on these hyperfine fields is less well known. We have made 57Fe Mossbauer measurements on Li1.2Fe4.6Sb0.2O8 in which B‐site disorder greatly predominates over A‐site disorder. From the near natural linewidths for the A and B‐site patterns we conclude that, in this material, B‐site cation disorder is not nearly as effective as A‐site cation disorder in producing inhomogeneities in Heff at either site. The area ratio of the two hyperfine field patterns, which are well resolved in an applied field, yields equal recoilless fractions for the two sites.

High‐pressure 57Fe Mössbauer study of Fe3O4 at 298 K

At 298 K, 57Fe Mossbauer spectra of Fe3O4 exhibit an increase in complexity with increasing pressure in the range 1×105 to 5×109 Pa. There is an incipient splitting of the B‐site pattern into two components with relative intensities of 3 : 1. For the more intense pattern, Hn is 462 kOe and 470 kOe at 1×105 and 5×109 Pa, respectively; the corresponding values for the less intense patterns are 454 and 448 kOe. The B‐site electric quadrupole interactions also exhibit appreciable changes. The A‐site pattern exhibits no qualitative changes in line profiles and only modest changes in the hyperfine interaction parameters are observed. For example, at 1×105 and 5×109 Pa, Hn is 492 and 490 kOe, respectively; the corresponding isomer‐shift values are 0.258 and 0.233 mm s−1. These results are consistent with the broadening of the lines of the B‐site patterns at ambient temperature and pressure arising from the different relative orientations of the direction of magnetization and of the axes of the electric field gra...

Magnetic properties, compositions, and microstructures of high-energy product strontium hexaferrites

Commercial, high‐energy product strontium hexaferrites have been characterized in terms of their chemical compositions, microstructures, and bulk and submicroscopic magnetic properties. Compared to earlier permanent magnetic hexaferrites, these materials have a unique composition, containing no elements other than Sr and Fe at appreciable concentrations; dopants for nucleation and microstructure control are apparently absent. The microstructures indicate that, in some cases, the materials may be fully reacted prior to forming the final ceramic bodies. The compositions are variable and tend to be nonstoichiometric despite the absence of impurity elements. These deviations from stoichiometry are reflected in the Mossbauer spectra which indicate that further refinements can be made in the definition of the different magnetic sublattices. These refinements may lead to further advances in the magnetic properties.

Investigation of Virgin Coals and Coals Subjected to a Mild Acid Treatment

AbstractA quantitative determination of the relative marcasite/pyrite contents in virgin coals is possible by means of 57Fe Mössbauer spectroscopy. Complications arise, however, when iron-containing silicates, carbonates, or other salts are present. The application of a mild chemical treatment involving hydrofluoric acid has been employed to remove these Fe-containing phases while leaving the iron-disulfide phases unaffected.Several South African coal samples with non-iron disulfide, Fe-containing phases ranging from 18 to 30 weight percent have been subjected to a hydrofluoric acid leaching at room temperature. The loss of mineral matter with HF leaching correlates well with the mineral matter residue following low temperature ashing. The 57Fe Mössbauer spectra of the resulting coal samples, collected at 297K, indicate that only FeS2 phases are present. The Mössbauer parameters for these samples (0.619 ≤EQ≤0.622 mm s−1; 0.306≤δ≤0.309 mm s−1) indicate the absence of appreciable quantities of marcasite in the coals. These Mössbauer parameters differ slightly, but systematically, from those of pyrite for which a quadrupole splitting of 0.6110 ± 0.0020 mm s−1 has been established. On the basis of previous studies, these increased EQ values suggest the presence of As substitution in the pyrite phases. 57Fe Mössbauer spectra of virgin coals exhibit phase assemblages comparable to those observed by x-ray diffraction (XRD), e.g. pyrite and (Ca,Fe)CO3, even though the presence of pyrite is less definite in the XRD data.

57Fe Mössbauer investigations of manganese-containing spinels

Cation distributions and related order-disorder phenomena in the naturally occurring Mn-spinels: iwakiite, galaxite and franklinite, have been investigated by means of57Fe Mössbauer spectroscopy. Previous measurements on an iwakiite sample indicated the existence of Mn-rich and Mn-poor regions. This interpretation has been confirmed in this investigation by further measurements on annealed iwakiite samples. The57Fe Mössbauer spectrum of galaxite from Bald Knob, NC, is a well-resolved quadrupole doublet and indicates the presence of a single crystallographic Fe3+ species. A “galaxite” sample from Thailand exhibited a complex spectrum of Fe2+ and Fe3+ quadrupole doublets: This sample has been misidentified and should be regarded as a member of the (Mg, Fe)(Al, Fe)2O4 series. The57Fe hyperfine parameters of a rare franklinite from Längban, Sweden, are very close to those for synthetic zinc ferrite, confirming electron microprobe results of an unusually high zinc content.

The influence of magnetic order on charge delocalization in pure and Zn‐doped Fe3O4

The unexpected simplicity of the 57Fe Mossbauer spectrum of Fe3O4 above its Neel temperature has been confirmed for pure Fe3O4 and Fe3−xZnxO4, x=0.01, 0.1, and 0.2. The abrupt change in the isomer shifts of the A and B site patterns is observed to take place near the Neel temperature for all samples, despite a 90 K variation in Neel temperatures. The abrupt change in isomer shifts is caused by electron delocalization from the B site to the A site as the spins on these two sites lose their antiparallel arrangement near TN. This facile electron exchange between the A and B sites is possible because of the high thermal energies available at TN and the delocalization mechanism is believed to be hopping in between localized sites.

Crystal/chemical structures and magnetic properties of naturally occurring Mn1.7Fe1.3O4

An unusually pure Mnx Fe3−x O4 sample (x≂1.3) from a regionally metamorphosed ore body is expected to exhibit an approach to thermodynamic equilibrium unattainable in synthetic materials. This expectation is confirmed by the observation of (1) a small tetragonal distortion and (2) a complete ordering of Fe on the B sublattice. 57Fe Mossbauer spectra, obtained at 298, 175, 125, and 92 K and in an external field of 30 kG at 92 K, exhibit magnetic hyperfine splitting requiring the fitting of two subspectra, I and II. At 298 K, Heff =418 kOe, α=0.36 mm s−1 , and e≊0 for the more intense subspectrum I, whereas for II Heff =403 kOe, δ=0.34 mm s−1 , and e≂−0.15 mm s−1 . The two subspectra are not split by a 30 kG external field. Subspectra I and II correspond to Fe3+ ions located on a single magnetic sublattice. Subspectrum II is assigned to Fe3+ ions in regions with locally, large tetragonal distortions as a consequence of Mn3+clustering, which has often been invoked to explain the crystal chemistry and physica...