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

Abstract

[1] In 1965, D. J. Dunlop showed that the joint distribution of particle volumes and microcoercivities f(V, H k0 ) can be determined for magnetically monomineralic, thermally stable single-domain (SSD) ensembles by taking advantage of the joint temperature and field dependence of relaxation time. We have developed a procedure that follows Dunlop's strategy to obtain f(V, H k0 ) for ensembles containing both superparamagnetic and SSD grains, based on backfield remanence curves measured over a range of temperatures. Each point on the derivative curves represents the integrated contribution from grains that lie along a corresponding blocking contour on the Néel plot. A suitable set of such line integral samples can be used to reconstruct the f(V, H k0 ) distribution using the methods of tomographic imaging. Samples of the basal Tiva Canyon Tuff have narrow size distributions of elongate Ti-poor titanomagnetite. Tomographic inversion of the low-temperature backfield spectra yield sharply peaked f(V, H k0 ) distributions, from which we calculate modal grain dimensions in good agreement with those observed by transmission electron microscopy. Analysis of synthetic samples containing bimodal populations clearly distinguishes the two modes. Because our simplified forward calculations incompletely account for the effects of orientation distribution, the width of the coercivity distribution at each temperature is underestimated, and consequently, the inverse calculations yield grain distributions that are overly broad. Frequency-and temperature-dependent susceptibilities calculated for the inverted f(V, H k0 ) distributions accord fairly well with measured susceptibilities for the weakly interacting Tiva Canyon samples, less well for a moderately interacting paleosol specimen, and poorly for a strongly interacting ferrofluid.Citation: Jackson, M., B. Carter-Stiglitz, R. Egli, and P. Solheid (2006), Characterizing the superparamagnetic grain distribution f(V, H k ) by thermal fluctuation tomography,

Magnetic susceptibility of cambisol profiles in the vicinity of the Vír dam, Czech Republic

“…No remarkable susceptibility increase towards very low temperatures, typical for paramagnetic minerals, can be observed, even though paramagnetic component is present but its effect must have been suppressed. A similar run of susceptibility, slightly more increasing to 300 K compared to our results was found by Jackson et al (2006) for a sample of palaeosol measured at frequency of 1 kHz. They measured set of curves of susceptibility of a palaeosol in the low temperature range from 0 to 300 K and in various frequencies up to 1 kHz and obtained for all of them the similar pattern with no Verwey but with gradually increasing trend with decreasing frequencies.…”

A New Tool for Separating the Magnetic Mineralogy of Complex Mineral Assemblages from Low Temperature Magnetic Behavior

“…In addition, the fact that these analyses are isothermal, (i.e., investigating changes in magnetization as a function of field strength at a single temperature) and that the analysis temperature is almost always room temperature, there is a high probability of incomprehensiveness of the characterization results, similarly to scalar derived hysteresis parameters discussed above. The thermal fluctuation tomography method proposed by Jackson et al (2006) addresses the limitation of exclusion of particles with SP behavior from room temperature-derived coercivity distribution, and provides a means of characterizing grain size distributions from variable low-temperature remanence coercivity spectrums. Unfortunately, the method has been seldom applied (e.g., Nie et al, 2013;Wang et al, 2013) likely due to the time consuming and relatively costly nature of the analysis.…”

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

“…It is a basic assumption of this theory that T C is a material constant, depending only on chemical composition and crystal structure of the remanence-carrying minerals, and that the function M S (T) is a single-valued and invariant material property, governing not only magnetization intensities but also the scaling of temperature-dependent anisotropies and energy-barrier distributions [44][45][46][47][48][49][50][51][52][53] . When the T C is itself a function of thermal history, and when M S (T) depends on the time-and temperaturedependent degree of cation ordering, additional complexity is required in quantitative modelling of remanence blocking and unblocking.…”