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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Hyperfine interactions 49 (1989), S. 19-41 
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The Mössbauer effect provides a direct method for identifying the spin axis in magnetic crystals and observing magnetic phase transitions. The order of the transition may be inferred from the Mössbauer spectrum. Phase changes can occur as a function of temperature (e.g. when the anisotropy fieldB A changes sign) or as a function of applied magnetic field. In an antiferromagnet a field ≃(2B E B A)1/2 along the spin axis whereB E is the exchange field causes the spin-flop transition which is normally first order (sharp) whereas the transition to the paramagnetic phase which occurs at higher fields≃2B E is second order (continuous). In quasi-one-dimensional crystals Mössbauer spectra show that the spin-flop transition is first order locally but occurs over a range of fields throughout the crystal, so that the first order character is masked in a conventional magnetization measurement. In fields applied at a finite angle〉B A/2B E to the spin axis the transition becomes second order, i.e. a continuous rotation of the spins occurs. In canted antiferromagnets (or weak ferromagnets) the spin-flop transition is also continuous; in addition a “screw” re-orientation may be induced by fields applied perpendicular to the spin axis and arises from antisymmetric exchange. For crystals with lowT N the hyperfine field changes when a magnetic field is applied and has a minimum at a phase transition; this may be used to map out the magnetic phase diagram.
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  • 2
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Spin-flop transitions in K2FeF5 subject to an applied field (i) parallel to the easy axis, and (ii) directed at 33° to the easy axis are found to be (i) abrupt and (ii) smooth, in accordance with theoretical predictions.
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  • 3
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The spin flop phase transition is compared in K2FeF5 and K2Fe0.99Ga0.01F5 in terms of spin flop field, hysteresis and coexistence of phases.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Hyperfine interactions 90 (1994), S. 27-43 
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The Mössbauer effect enabled the magnetic hyperfine splitting (hfs) in ferromagnetic solids to be observed directly for the first time. It was quickly extended to measurements on antiferromagnets, ferrimagnets and paramagnets, and is now well established as a probe for the study of magnetic materials. Applications have ranged from the “old” problem of the state of the iron atoms in ferromagnetic alloys to the “new” magnetic materials, e.g. amorphous magnets, spin glasses, fine particle magnets and multilayers. Some examples where the Mössbauer effect has made an important contribution are described.
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  • 5
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Mössbauer spectra have been recorded at 4.2 and 300 K on the series La1−x Sr x FeO3, wherex varies from 0 to 1.0 in steps of 0.1. Neutron diffraction experiments have shown that the crystal structure is orthorhombic for 0≤x〈0.3, rhombohedral for 0.4≤x≤0.7, and cubic for 0.8〈x≤1.0. Mössbauer spectra at 4.2 K are composed of magnetic sextet components arising from different charge states of iron ions. In the orthorhombic and rhombohedral phases, the charge states Fe3+ and Fe5+ coexist. In the cubic phase, iron is present as Fe3+ and Fe4+ states. At 300 K, the samples are magnetically ordered in the range 0 ≤x≤0.3 and the coexistence of Fe3+ and Fe5+ remains. For samples 0.4≤x≤1.0, the samples are paramagnetic. Fits to these spectra require two components, one corresponding to an Fe4+ state, the other being best described as an Fe3+ ion forx≤0.7 but forx〉0.7 having a mean charge state which increases to 3.5 forx=1.0.
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  • 6
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Tin is not a major constituent of window glass, but is found at high concentrations in the lower surface of float glass which has been in contact with the molten tin bath. It does not extend far into the surface, but causes the physical and chemical behaviour to differ from that of the upper surface. It is important, therefore, to understand the structural role of tin in silicate glasses and thus its effect on various properties. Mössbauer spectra were taken of three series of glassy materials, namely binary glasses (SnO and SiO2) in varying proportions, re-melted float glass containing tin, and float glass from a production plant. The binary glasses contained between 20 and 70% tin which was found to be mainly Sn2+, with very small amounts of Sn4+ in some of them. The spectra showed a small decrease in isomer shift with increase in tin content, which is ascribed to the change in molar volume. The re-melted samples were float glass which was mixed with stannous oxalate in appropriate conditions to try and maintain tin in the 2+ state, and contained up to 15% tin by weight. The spectra show both Sn4+ and Sn2+ with rather more in the 4+ oxidation state. The change in the spectra as a function of temperature revealed a large difference in thef-factor (and hence the chemical binding) of the two states. A series of spectra was taken between 17.5 and 900 K for the sample containing 15% tin. From the absorption as a function of temperature thef-factor was determined for both oxidation states, and hence enabled the relative amounts of Sn4+ and Sn2+ present in each sample to be estimated. Measurements of the shift as a function of temperature were also made. The float samples were surface material produced by grinding away all but 0.1 mm of the lower surface of industrially produced float glass. The Mössbauer spectra showed them to be predominantly Sn2+, as expected from the reducing atmosphere in the float plant. The concentration and oxidation state of the tin may be estimated from the value of thef-factors and isomer shifts.
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  • 7
    ISSN: 1573-1480
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A global two-dimensional (altitude-latitude) chemistry transport model is used to follow the changes in the tropospheric distribution of the two major radiatively active trace gases, methane and ozone, following step changes to the sustained emissions of the short-lived trace gases methane, carbon monoxide and non-methane hydrocarbons. The radiative impacts were dependent on the latitude chosen for the applied change in emissions. Step change global warming potentials (GWPs) were derived for a range of short-lived trace gases to describe their time-integrated radiative forcing impacts for unit emissions relative to that of carbon dioxide. The GWPs show that the tropospheric chemistry of the hydrocarbons can produce significant indirect radiative impacts through changing the tropospheric distributions of hydroxyl radicals, methane and ozone. For aircraft, the indirect radiative forcing impact of the NO x emissions appears to be greater than that from their carbon dioxide emissions. Quantitative results from this two-dimensional model study must, however, be viewed against the known inadequacies of zonally-averaged models and their poor representation of many important tropospheric processes.
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  • 8
    ISSN: 1573-0662
    Keywords: global model ; three-dimensional ; Lagrangian tropospheric chemistry ; ozone ; NOX ; emission controls
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract A three-dimensional Lagrangian tropospheric chemistry modelis used toinvestigate the impact of human activities on the tropospheric distributionofozone and hydroxyl radicals. The model describes the behaviour of 50 speciesincluding methane, carbon monoxide, oxides of nitrogen, sulphur dioxide andnineorganic compounds emitted from human activities and a range of other sources.Thechemical mechanism involves about 100 chemical reactions of which 16 arephotochemical reactions whose diurnal dependence is treated in full. The modelutilises a five minute chemistry time step and a three hour advection timestepfor the 50,000 air parcels. Meteorological data for the winds, temperatures,clouds and so on are taken from the UK Meteorological Office global model for1994 onwards. The impacts of a 50% reduction in European NOXemissions onglobal ozone concentrations are assessed. Surface ozoneconcentrations decrease in summertime and rise in wintertime, but to differentextents.
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  • 9
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
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  • 10
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract 57Fe Mössbauer spectra for a natural single crystal of ludlamite have been recorded at temperatures between room temperature and 4.2 K. Computer analysis reveals that at 4.2 K the Fe2+ moments lie on two distinct magnetic sublattices separated by at least 35°.
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