Some theoretical aspects of rock-magnetism

Abstract

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Inferred time- and temperature-dependent cation ordering in natural titanomagnetites

“…Ferrimagnetism in these materials arises from an antiferromagnetic coupling of the A and B sublattices of the inverse spinel structure, with a net spontaneous magnetization that is determined by the proportions and the valence states of the magnetic cations in the A (tetrahedral) and B (octahedral) sites [1][2][3][4][5][6][7][8][9][10][11][12] . The Curie temperature (T C ) is directly related to the strength of the exchange interactions within and especially between the sublattices, which in turn depend on the site occupancy of the magnetic cations [13][14][15] .…”

“…Intermediate composition titanomagnetites (0oxo1) are complex spinels, with three different metal cations and valence states (Fe 2 þ , Fe 3 þ and Ti 4 þ ), and significant work has been done with synthetic samples to deduce the degree, form and temperature dependence of cation order and to understand its effects on fundamental magnetic properties [1][2][3][4][5][6][7][8][9]12,[19][20][21][22] . In natural titanomagnetites, additional complexity is added by the common presence of substituted cations (for example, Al, Mg and Mn) and variable degrees of cation deficiency, and we are unaware of any previous studies linking changes in fundamental properties such as T C to reordering of the cation distribution in natural titanomagnetites.…”

Magnetic force microscopy reveals meta-stable magnetic domain states that prevent reliable absolute palaeointensity experiments

“…The majority (B70-80%) of the magnetic remanence is carried by PSD to MD grains of the second generation of titanomagnetites (2-20 mm). Furthermore, small, SD grains are both theoretically 13,14 and empirically 15 known to yield good results in palaeointensity experiments; erroneous palaeointensity results are often attributed to transdomain processes in larger grains. We therefore choose to image 5-7 grains per sample ranging between 2 and 420 mm-in the PSD to MD range.…”

“…This primary TRM is then compared with a laboratory-imparted partial TRM (pTRM) in a known magnetic field. Palaeointensity techniques therefore rely on the assumption that the pTRMs are acquired analogously to the NRM of the samples, which is valid only for very small magnetic grains 13,14 that are single domain (SD), as typically found in baked archaeological artefacts 15 . Larger grains, however, are divided into more magnetic domains separated by domain walls; the NRM configuration of these magnetic domains is not necessarily reproduced after imparting a pTRM.…”

Saw‐toothed variations of relative paleointensity and cumulative viscous remanence: Testing the records and the model

“…The temperature dependence of single domain viscosity is well predicted by Nrel's theory [Ndel, 1949[Ndel, , 1955 We followed the same approach with the experimental data from site 851. Forty samples from hole 851E were subjected to stepwise acquisition of thermoremanent magnetization.…”