Stability of equidimensional pseudo–single-domain magnetite over billion-year timescales

Abstract

Interpretations of palaeomagnetic observations assume that naturally occurring magnetic particles can retain their primary magnetic recording over billions of years. The ability to retain a magnetic recording is inferred from laboratory measurements, where heating causes demagnetization on the order of seconds. The theoretical basis for this inference comes from previous models that assume only the existence of small, uniformly magnetized particles, whereas the carriers of palaeomagnetic signals in rocks are usually larger, non-uniformly magnetized particles, for which there is no empirically complete, thermally-activated model. This study has developed a thermally-activated numerical micromagnetic model that can quantitatively determine the energy barriers between stable states in nonuniform magnetic particles on geological time scales. We examine in detail the thermal stability characteristics of equidimensional cuboctahedral magnetite and find that contrary to previously published theories, such non-uniformly magnetized particles provide greater magnetic stability than their uniformly magnetized counterparts. Hence, non-uniformly magnetized grains, which are commonly the main remanence carrier in meteorites and rocks, can record and retain highfidelity magnetic recordings over billions of years.micromagnetics | paleomagnetism | geomagnetism S ince the 1900s magnetic recordings observed in rocks and meteorites have been studied to understand the evolution of the Earth and the Solar System. The validity of the findings from these studies depends on a theoretical understanding of rock-magnetic recordings provided by Néel (1, 2) and numerous experimental studies, for example, Strangway et. al. (3) and Evans and Wayman (4). The overwhelming evidence from these authors was that stable natural magnetic remanence (NRM) in rocks resides within ultrafine, uniformly magnetized particles, called single domain (SD) particles. Néel's theory (1, 2) for the behavior of thermally-activated SD particles describes a unique relationship between thermal and temporal stability and gave confidence that palaeomagnetic recordings that become unstable (unblocked) only at high temperatures, retain magnetic recordings from the time of their mineral crystallization, possibly as far back in time as four billion years ago.However, in the 1970s and 80s the widespread use of hysteresis parameters to characterize magnetic mineralogy (5) found that the majority of magnetic particles in rocks are not in uniform magnetic states, but are larger in size (80 -1000 nm) and contain complex magnetic states that are not described by either SD theory or the multidomain theory of micron-sized particles (2, 6). The term pseudo-single-domain (PSD) was coined for such particles and much effort was spent in determining the origin of their magnetic fidelity (Dunlop Psark, 1977, Moon and Merrill, 1985). Due to the complexity of the problem it has not been possible to determine the temporal stability of magnetisation in PSD grains on geological time scales fro...

Magnetic Vortex States in Small Octahedral Particles of Intermediate Titanomagnetite

“…Recent findings about extremely stable vortex states in magnetite (Almeida et al, 2014(Almeida et al, , 2016Nagy et al, 2017) combined with our observations of vortex states in titanomagnetite grains (1-5 μm) suggest that vortex states are probably a common micromagnetic state occurring at the lower end of the PSD size range (Roberts et al, 2017). The variation in domain structures we observed from single domain to four domains might then result from the different cutting planes of octahedral crystals and the resulting 3-D shape of the grains.…”

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Fine particles of titanomagnetites (Fe 3-x Ti x O 4 , x > 0.5) in the pseudo-single-domain (PSD) size (0.5-20 μm) are important carriers of natural remanent magnetization in basalts. These states are characterized by continuous magnetization states emerging as vortices, curls, and flux-closure structures as shown by numerical micromagnetic models (e.g., Nagy et al, 2017;Rave et al, 1998;Williams et al, 2006;Williams & Wright, 1998).According to recent micromagnetic calculations in magnetite, single-vortex states can carry an extremely stable and reliable remanence over billions of years (Nagy et al, 2017). This study reports first observations of magnetic vortex states in intermediate titanomagnetite.

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“…Yet the mechanism of magnetic recording in PSD grains is not completely understood (e.g., Dunlop & Özdemir, 1997;Fabian & Hubert, 1999;Nagy et al, 2017;Roberts et al, 2017). Fine particles of titanomagnetite in single-domain (SD) or pseudo-single-domain (PSD) states (0.5-20 μm in size) are the dominant carriers of natural remanent magnetization in these rocks.…”

Resolving the Origin of Pseudo‐Single Domain Magnetic Behavior

“…The persistence of weak but stable remanences and the gradual decline in remanence with increasing particle size through the PSD size range is likely explained by the imbalanced nature of magnetic moments in complex magnetic structures within magnetic particles in the vortex state (Einsle et al, ). Available evidence suggests that these structures can record paleomagnetically meaningful information over long geological periods (Almeida et al, , ; Nagy et al, ).…”

“…The most important question for paleomagnetic analysis of materials in the magnetic vortex state concerns their magnetic recording capability. Single vortex state particles have been demonstrated to have high field and thermal stability close to the Curie temperature of magnetite (Almeida et al, , ; Einsle et al, ; Muxworthy et al, ; Nagy et al, ), which suggests that they are capable of recording paleomagnetic information over long geological periods. This confirms the paleomagnetic recording fidelity of PSD particles that has been postulated for over 55 years.…”