Hysteresis properties of titanomagnetites: Grain-size and compositional dependence

[1] Although most paleomagnetic and environmental magnetic papers incorporate a Day plot of the hysteresis parameters Mrs/Ms versus Hcr/Hc, a comprehensive theory covering superparamagnetic (SP), single-domain (SD), pseudo-single-domain (PSD), and multidomain (MD) (titano)magnetites is lacking. There is no consensus on how to quantify grain-size trends within the Day plot, how to distinguish MD from SP trends/mixtures, or whether magnetite, titanomagnetites, and other minerals have distinctive trends by which they might be identified. This paper develops the theory of the Day plot parameters for MD, MD + SD, PSD, and SP + SD grains of titanomagnetite (Fe 3– xTixO4) with compositions x = 0 (TM0 or magnetite) and x = 0.6 (TM60). MD grains have a separate trend that intersects the curve for SD + MD mixtures. SP + SD mixtures generate a variety of trends, depending on the SP grain size. All SP + SD curves lie much above those for MD or SD + MD trends, as has been proposed, but not demonstrated, previously. Data for PSD-size magnetites of many different origins fall along a single trend, but different levels of internal stress shift points for similar grain sizes along the ‘‘master curve.’’ In order to use the Day plot to determine grain size, one must have independent information about the state of internal stress. Theoretical model curves for SD + MD mixtures match the PSD magnetite and TM60 data quite well, although the SD!MD transition region in grain size is much narrower for TM60 than for magnetite. The agreement between PSD data and SD + MD mixing curves implies that PSD behavior is due to superimposed independent SD and MD moments, either in individual or separate grains, and not to exotic micromagnetic structures such as vortices. The theory also matches Mrs and Hc values in mechanical mixtures of very fine and very coarse grains, although nonlinear mixing theory is required to explain some Hcr and Hcr/Hc data. INDEX TERMS: 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques; 1533 Geomagnetism and Paleomagnetism: Remagnetization; 1512 Geomagnetism and Paleomagnetism: Environmental magnetism;

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JB000486

Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data

[1] In environmental magnetism, rock and mineral magnetic techniques are used to investigate the formation, transportation, deposition, and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes. All materials respond in some way to an applied magnetic field, and iron-bearing minerals are sensitive to a range of environmental processes, which makes magnetic measurements extremely useful for detecting signals associated with environmental processes. Environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics, chemistry, and biology and environmental science, including research on climate change, pollution, iron biomineralization, and depositional and diagenetic processes in sediments to name a few applications. Magnetic parameters are used to routinely scan sediments, but interpretation is often difficult and requires understanding of the underlying physics and chemistry. Thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities, complexities, and limitations to interpretations. In this review, we evaluate environmental magnetic parameters based on theory and empirical results. We describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations. We also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology, chemistry, and physics. Finally, we discuss directions in which environmental magnetism is likely to develop in the future.

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Environmental magnetism: Principles and applications

Magnetic anisotropy in rocks

It is demonstrated that the stability against af demagnetization of TRM carried by multidomain magnetite increases with increasing strength of the inducing field. This multidomain field-dependence trend of stability against af demagnetization is opposite to that of single-domain particles. A simple test to distinguish multidomain and single-domain carriers of remanence is designed that makes use of the distinctive af demagnetization characteristics. The test was applied to a variety of igneous rocks. None of these showed truly multidomain characteristics. Thus, although some TRM is undoubtedly multidomain in these rocks, the TRM is dominated by single-domain carriers. These results are consistent with the growing evidence of the importance of very fine grained magnetite as a carrier of NRM in many igneous rocks.

On the alternating field demagnetization characteristics of multidomain thermoremanent magnetization in magnetite

Single crystal magnetic grains are found to have appreciable permanent (pseudo-single domain) moments by virtue of the Barkhausen discreteness in the positions of their domain walls. These moments are proportional to the 3/2 power of grain diameter and are responsible for the low-field thermoremanence of grains with diameters between 0.03 μ and approximately 5 μ. Smaller grains are true single domains, larger ones behave as true multi-domains with properties accurately described by an earlier theory. A graph of thermoremanence against grain size shows a strong maximum at 0.3 μ, at which TRM is 40 times as strong as for large grains. Under demagnetizing conditions the permanent moments of grains up to several hundred microns across become important. They acquire saturation alignment in extremely small stray fields, giving observable specimen magnetizations quite out of proportion to the fields, and may also interact to cause mutual alignment even in the absence of stray fields. This is considered ...

A generalized theory of thermoremanence, covering the transition from single domain to multi-domain magnetic grains

The stress-dependence of magnetic susceptibility of igneous rock is calculated from the multidomain theory of magnetic grains, assuming that the magnetic mineral is pure magnetite whose saturation magnetization, magnetocrystalline anisotropy and saturation magnetostriction are known. The susceptibility χ measured in the direction of a linear compression σ is given by d(ln χ)/dσ = −1.0y × 10−4 cm2/kg, in excellent agreement with measurements on a number of rocks by S. P. Kapitsa who obtained values between −0.8 × 10−4 and −1.3 × 10−4 cm2/kg. The stress-dependence of susceptibility is sufficient to cause readily observable local geomagnetic disturbances during earthquakes.

F. D. Stacey

Papers

A generalized theory of thermoremanence, covering the transition from single domain to multi-domain magnetic grains

Theory of the magnetic susceptibility of stressed rock