Step‐wise and continuous thermal demagnetization and theories of thermoremanence

P. W. Schmidt*, D. A. Clark

*Corresponding author for this work

    Research output: Contribution to journalArticle

    14 Citations (Scopus)

    Abstract

    Summary. The difference between step‐wise and continuous thermal demagnetization of rock specimens is examined with particular reference to the presence of multi‐component magnetizations. The effect of the thermal dependence of the intensity of spontaneous magnetization is clearly evident in recordings of continuous demagnetization when a least‐squares analysis of a two component system is performed. The differences between spontaneous magnetization, saturation magnetization and high‐field magnetization are discussed and it is shown that the normalized thermal dependence of spontaneous magnetization can be determined with negligible error by measuring magnetization as a function of temperature in sufficiently high fields (> 0.1 T for magnetite, > 1 T for pyrrhotite). Using experimentally determined thermal dependence of high‐field magnetization to correct the continuously observed intensities leads to analyses comparable to those of step‐wise demagnetization data. Experiments designed to compare the temperature dependence of an apparent spontaneous magnetization, derived from the observed thermal decay of saturation remanence carried by a multi‐domain (MD) magnetite bearing sample, with the true spontaneous magnetization of magnetite reveal a systematic difference, with the apparent spontaneous magnetization decreasing more rapidly than the true spontaneous magnetization. This difference is minor, however, compared to the thermal decay of spontaneous magnetization and to a first‐order approximation the use of the thermal decay of saturation magnetization to correct intensities should usually be adequate. Similar conclusions are supported by experiments with MD pyrrhotite bearing samples. These experiments serve to constrain models of MD remanence. The approximate proportionality of blocked remanence (TRM and saturation remanence) to spontaneous magnetization which is observed for these samples, which contain predominantly MD grains, resembles the behaviour expected for non‐interacting single domain (SD) grains. Conventional MD theories assume that domain structure remains stable below the blocking temperature and attribute remanence carried by MD grains to Barkhausen discreteness of domain wall positions. However, if the domain multiplicity does not change with temperature, remanence due to Barkhausen moments should exhibit very strong temperature dependence below the blocking temperature, with significant departures from proportionality to spontaneous magnetization, when the reversible movement of domain walls in response to the self‐demagnetizing field is taken into account. Published experimental results show that partial TRM of assemblages of large magnetite grains exhibit anomalous behaviour, reflecting domain structure readjustment, during initial cooling. Furthermore, isothermal application of a moderate field preferentially activates true MD Barkhausen moments and the resulting remanence is found not to vary proportionally to spontaneous magnetization. This evidence suggests that Barkhausen moments, which show anomalous temperature dependence, make a negligible contribution to the TRM and saturation remanence of our samples. We propose that TRM and saturation remanence of assemblages containing MD grains is predominantly carried by a fraction of metastable nucleation‐controlled SD grains and by MD grains of anomalously low domain multiplicity with undemagnetizable moments. According to this hypothesis the PSD moments are independent of MD processes because they are associated with distinct grains. The spontaneous moments of the MD grains reflect equilibrium domain wall positions, in the absence of pinning, for grains with an odd number of domains and for irregular grains with an even, but asymmetrically disposed, number of domains. Provided the domain walls are compressed relative to their equilibrium width in bulk material, a condition which is favoured if the grain occupies a local energy minimum state with fewer than the equilibrium number of domains, such domain structures are very stable to temperature change. Below the unblocking temperature remanence carried by such grains is proportional to the spontaneous magnetization. Unblocking of remanence carried by these grains occurs upon nucleation of a new domain wall.

    Original languageEnglish
    Pages (from-to)731-751
    Number of pages21
    JournalGeophysical Journal of the Royal Astronomical Society
    Volume83
    Issue number3
    DOIs
    Publication statusPublished - 1985

    Keywords

    • multidomains
    • pseudo‐single domains
    • spontaneous magnetization
    • thermal demagnetization
    • TRM

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