Magnetic Hysteresis Properties of Fine Particles of Monoclinic Pyrrhotite Fe7S8.

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  • Menyeh A.
    Department of Physics, University of Newcastle upon Tyne
  • O'Reilly W.
    Department of Physics, University of Newcastle upon Tyne

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  • Magnetic Hysteresis Properties of Fine

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The magnetic hysteresis properties have been studied of synthetic Fe7S8 separated into a suite of 8 particle size fractions in the range 1 to 30μm. Between-196°C to about 260°C, both saturation isothermal remanence (Mrs) and coercive force (Hc) can be represented as analytical functions of particle size (L) of the form (Mrs, Hc) ∝ L-n or (Mrs, Hc) ∝ exp(-kL1+2) although the particle size dependence of Mrs is much weaker than that of Hc. The parameters n and k show a small but systematic fall in values as temperature rises, but may be acceptably constant to furnish analytic functions for physical models of more complex magnetic properties such as thermoremanent magnetization or domain dynamics. The temperature dependence of Hc below room temperature is weaker than that above, possibly due to an increasing single-ion contribution to magnetocrystalline anisotropy. The inferred temperature dependence of magnetocrystalline anisotropy above room temperature up to about 250°C suggests that temperature-induced expulsion or nucleation of domain walls is not expected in this temperature range. The temperature dependence of Mrs and Hc can be expressed as (1 - T/Tc)q where q shows a small but systematic fall as particle size increases in the range 1-30μm. The several properties of the finest, monodomain, fraction (<1 μm) seem best explained if a grain size an order of magnitude smaller than that observed electron optically is assigned to the particles. This may indicate the presence of “nanocrystals” within the envelope of the particles.

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