Thermal demagnetization of partial thermoremanent magnetization

Abstract

Three well‐characterized glass‐ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations (pTRMs) in the temperature interval 400°–350°C for varying lengths of time (10 min to 5 days) and in magnetic fields from 0.5 to 5 Oe (0.05–0.5 mT). The thermal demagnetization behavior of the pTRMs changes with domain state in the sense that with decreasing coercivity (increasing grain size), demagnetization curves become less steep and may not completely demagnetize until heated to the Curie temperature. Numerical calculations based on Néel's single domain (SD) theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures. We observe that experimental results do not match any of the theoretical predictions. There is no discernible field dependence of demagnetization curves for the SD and pseudo‐single domain samples and even for the “soft” multidomain (MD) sample in the field range investigated. An important consequence for thermally derived paleointensity data is that they may not be entirely reliable, especially if the sample remanence is due to MD grains.

Characterizing the superparamagnetic grain distribution f(V, Hk) by thermal fluctuation tomography

“…T C = 775 K (502 C), corresponding to an unoxidized composition of approximately TM12 [Hunt et al, 1995]. There is significant uncertainty in the best values for these parameters, since the measurements are all made far below the Curie point, but this composition agrees well with those estimated previously [Schlinger et al, 1988[Schlinger et al, , 1991Worm and Jackson, 1999]. The room temperature mass-specific saturation magnetization for this composition is $78.4 A m 2 /kg [Hunt et al, 1995], which for a density of $5150 kg/m 3 yields M S $ 404 kA/m at room temperature.…”

“…For magnetite g = 0.43 is commonly used above room temperature [Dunlop and Ö zdemir, 1997, section 2.5]. Below room temperature the variation in M S is weaker, and can be described by Bloch's ''T 3/2 law'' [e.g., Worm and Jackson, 1999] or by a lower value of g.…”

“…Thermomagnetic analyses indicate that these grains are lowtitanium titanomagnetites with a composition of Fe 3-x Ti x O 4 (x $ 0.10 or TM10). Careful transmission electron microscopy (TEM) studies [Schlinger et al, 1991] and numerous magnetic investigations [Schlinger et al, 1991;Rosenbaum, 1993;Worm and Jackson, 1999;Egli and Lowrie, 2002;Shcherbakov and Fabian, 2005] have shown that at each stratigraphic level the distribution of grain sizes and shapes is very narrow, and that it closely approximates a StonerWohlfarth population of identical, randomly oriented elongate grains. Sizes increase from approximately 5 Â 5 Â 15 nm (4.5 Â 10À25 m 3 ) to 25 Â 25 Â 250 nm (1.6 Â 10 À22 m 3 ) over a 3-m span near the base of the flow (Table 2) [Schlinger et al, 1991].…”

“…[5] In Néel theory the grain volume is a key variable, and various studies have used this as the basis for transforming the distribution of unblocking temperatures into a volume distribution [e.g., Worm et al, 1988;Worm and Jackson, 1999]. This requires us to make the assumption that the distribution of coercivities is much narrower than that of grain size (as pointed out by Dunlop [1965]), since the unblocking temperature (the SSD-SP transition in zero field) depends on the product of grain volume and coercivity (see section 2 for details).…”

Effect of grain size and domain state on thermal demagnetization tails

“…5% and 95% limits were used to arrive at these ranges, but the broadening of the TuB spectrum is obvious to the eye. It is clear that highTuB tails, which have been much discussed in the literature [Worm et al, 1988;Halgedahl, 1993 …”