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  • 1
    Electronic Resource
    Electronic Resource
    The effects of substitutional impurities on the magnetic, electronic, and hyperfine interactions in ilvaite, CaFe3Si2O7O (OH), a mixed-valence iron silicate (abstract) : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 5345-5345 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ilvaite, a mixed-valence iron silicate, rivals magnetite in the complexity of the phenomena exhibited in connection with the simultaneous occurrence of Fe2+ and Fe3+ on equivalent sites. Inasmuch as the effects of impurities on the several phase transitions and electron delocalization have been explored little, if at all, an investigation of the effect of manganese impurities by means of measurements on a large suite of samples has been undertaken. Significant variations in magnetic properties are observed even for low impurity levels. For a Mn2+ content of 0.5 wt %, the low temperature magnetic phase transition is at ∼40 K, for a Mn2+ content of 1.2%, the transition temperature is less than 20 K and for a Mn2+ content of 4.5%, the transition appears to be well below 10 K. In addition to the quantitative variations, the 1/χ versus T curves are qualitatively different for samples with different concentrations of manganese. The 57Fe Mössbauer spectra are also qualitatively different at 298 K with pronounced differences in the resolution of the two Fe2+ patterns and in their relative intensities. In agreement with earlier reports, no evidence for magnetic ordering at 120 K has been observed in the susceptibility curves; the ordering is, however, exhibited in the 57Fe Mössbauer spectra obtained at 82 K. The exploration of the effect of Mn2+ impurities has permitted rare insights to be gained on the relationship between the bulk and local, site-specific properties of ilvaite. Further progress is expected to result from electrical conductivity and x-ray diffraction measurements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353723
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  • 2
    Electronic Resource
    Electronic Resource
    The structure–property relationships in M-type hexaferrites: Hyperfine interactions and bulk magnetic properties : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6295-6297 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 57Fe Mössbauer 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 from the libration of the 2b Fe3+ parallel to the c axis, the anisotropy is considerably larger for PbFe12O19 than for BaFe12O19 or SrFe12O19. It is remarkable that the bulk magnetic anisotropy follows the same order as the anisotropy in the dynamical displacement and crystalline electric field of the 2b site.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352675
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  • 3
    Electronic Resource
    Electronic Resource
    The 57Fe Mössbauer spectra of oriented single-crystal and powder PbFe12O19 (abstract) : 35th annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 6231-6231 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The recent determination of the crystal structure of PbFe12O19 by means of single-crystal x-ray diffraction has made it possible to carry out a definitive and comparative analysis of the structure-property relationship of PbFe12O19, SrFe12O19, and BaFe12O19. We have therefore performed 57Fe Mössbauer measurements on oriented, single-crystal, and powder samples of PbFe12O19 at 298 K. For a single-crystal oriented parallel to the c axis, five distinct patterns are observed. The hyperfine fields, isomer shifts, and quadrupole interactions are as follows: 12k: Heff=413 kOe; δ=0.33 mm s−1; Δ=0.35 mm s−1; 4f1: 487 kOe; 0.24 mm s−1; 0.13 mm s−1; 4f2: 514 kOe; 0.39 mm s−1; 0.29 mm s−1; 2a: 496 kOe; 0.30 mm s−1; 0.11 mm s−1; 2b: 399 kOe; 0.30 mm s−1; 2.00 mm s−1. The relative magnitude of each of these parameters is in good agreement with and exhibits the same trend observed for high-purity SrFe12O19. The relative intensity of the 2b pattern for the crystal oriented parallel to the c axis is 8.2%, which approaches its theoretical value; of the total number of iron ions in the structure, 8.33% of them occupy the 2b site. The value for the polycrystalline sample is only 5.4% of the total intensity. However, for a single crystal oriented perpendicular to the c axis, the 2b pattern exhibits a negligible intensity within the experimental error. This result confirms the dynamic disorder in the position of the 2b iron ions and suggests a stronger anisotropy for the case of PbFe12O19 than for SrFe12O19 and BaFe12O19. Even though the trend in hyperfine parameters at 298 K is similar to Sr, Ba, and Pb hexaferrites, PbFe12O19 exhibits hyperfine fields that are consistently lower than those of the Ba and Sr analogues. This result is discussed in terms of the Curie temperatures, magnetic exchange interactions, and structural parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348818
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  • 4
    Electronic Resource
    Electronic Resource
    Magnetic properties, compositions, and microstructures of high-energy product strontium hexaferrites (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 4601-4603 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 Mössbauer 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.344852
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  • 5
    Electronic Resource
    Electronic Resource
    57Fe Mössbauer spectroscopic investigation of complex magnetic structures in Ga, Sc, and In substituted M-type hexagonal ferrites (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 5229-5230 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 structure and interactions than indium or gallium. Further, at technically significant substitution levels, Heff of the different sublattices exhibit broad and overlapping distributions of values far removed from their distinctiveness in the pure hexaferrites. Thus, the net magnetization of Ga, Sc, and In-doped hexaferrites results from a complex interplay of magnetic dilution on the 2b site, enhancement of the magnetization through substitutions on the 4f2 site, and a complex influence from the substitution-induced 12k2 sublattice. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.369952
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  • 6
    Electronic Resource
    Electronic Resource
    The influence of heat treatments on order–disorder phenomena in a naturally occurring manganese-ferrite : 38th Annual Confrerence on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 5538-5540 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Naturally occurring samples of iwakiite, Mn1.1Fe1.9O4, have been annealed for 17 and 77 h at 1000 K in an investigation of order–disorder phenomena in Mn–Fe oxide spinels. Two distinct magnetic hyperfine patterns, which correspond to Fe- and Mn-rich phases, were observed in the 57Fe Mössbauer spectra for the two annealed samples. While the spectrum of the Fe-rich phase corresponds to a magnetically and crystallographically highly ordered α-Fe2O3-like phase, the Mn-rich phase exhibits considerable disorder, as indicated by very broad linewidths and a low value for Heff. Annealing for longer periods of times leads to a considerable increase in the ordering of the Mn-rich spinel phase, as evidenced by the considerable narrowing of the lines, from Γ=2.40 mm s−1 to Γ=1.17 mm s−1, and an increase in Heff from 318 to 417 kOe at 298 K. The Fe-rich, α-Fe2O3-like phase is not affected by further annealing, as expected. At 85 K, the 57Fe Mössbauer spectrum of the Mn-rich, spinel phase continues to exhibit rather broad lines, indicating the presence of significant cation disorder.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355681
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  • 7
    Electronic Resource
    Electronic Resource
    Order-disorder and magnetic exchange interactions in substituted strontium hexaferrite SrAxFe12−xO19(A=Ga, In) : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6643-6645 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 Mössbauer spectroscopy at 298 K. The bifurcation of the 12k Mössbauer 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 Mössbauer 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 magnetic hyperfine spectrum is not due simply to random disruptions of the exchange interactions. The intensities of the 12k1 and 12k2 subspectra have been used to model the exchange interactions at the 12k site.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.356881
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  • 8
    Electronic Resource
    Electronic Resource
    The effects of chemical composition on electron delocalization and magnetic ordering in ilvaite, Ca[Fe2+, Fe3+][Fe2+]Si2O7O(OH) (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 5234-5236 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Substitutional impurities are expected to have significant influences on long-range, order–disorder phenomena associated with magnetic ordering and electron delocalization in mixed-valence solids. These influences have been investigated by means of superconducting quantum interference device susceptometry and 57Fe Mössbauer spectroscopy on more than 40 mineral specimens of the mixed-valence solid ilvaite, Ca[Fe2+, Fe3+][Fe2+]Si2O7O(OH), with manganese contents between 0.2 and 10 wt %. At 295 K, the 57Fe Mössbauer spectra are quite diverse, exhibiting different numbers of lines, varying degrees of resolution and line broadening. The complexity of the spectra correlate well with increasing Mn content. At 160 K, the spectra are remarkably similar and are characterized by three overlapping subspectra. The Isomer shifts are: 1.09, 1.07, and 0.64 mm s−1, respectively, which may be unambiguously assigned to 2 Fe2+ and 1 Fe3+ species. The magnetic ordering transition near 120 K, associated with the ordering of the moments of the Fe2+ and Fe3+ ions involved in electron delocalization, is not evident in the susceptibility data. Further, the susceptibility versus temperature curves exhibited no qualitative changes with varying Mn content. The magnetic transition associated with the ordering of the magnetic moments of the Fe2+ ion, not involved in electron delocalization, is clearly evident in the susceptibility data and exhibits a systematic dependence on Mn content, occurring at 27, 26, 25, and 23 K for Mn contents of 0.23, 0.70, 0.90, and 0.97 wt %. For Mn contents above 1 wt %, this magnetic transition is not observed. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.369954
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  • 9
    Electronic Resource
    Electronic Resource
    Crystal/chemical structures and magnetic properties of naturally occurring Mn1.7Fe1.3O4 (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 4133-4135 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An unusually pure Mnx Fe3−x O4 sample (x(approximately-equal-to)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 Mössbauer 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 ε≈0 for the more intense subspectrum I, whereas for II Heff =403 kOe, δ=0.34 mm s−1 , and ε(approximately-equal-to)−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 physical properties of Mnx Fe3−x O4 .
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.340519
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  • 10
    Electronic Resource
    Electronic Resource
    High-pressure 57Fe Mössbauer study of Fe3O4 at 298 K (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 4352-4354 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: At 298 K, 57Fe Mössbauer 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 gradient tensor.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.338418
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