Direct observation of the thermal demagnetization of magnetic vortex structures in nonideal magnetite recorders

Geochem Geophys Geosyst

et al. 2018

Fine particles of titanomagnetites (Fe3‐xTixO4, x > 0.5) in the pseudo‐single‐domain (PSD) size (0.5–20 μm) are important carriers of natural remanent magnetization in basalts. Understanding the mechanism of magnetic recording in these grains has important implications for paleomagnetic studies. This study reports first observations of magnetic vortex states in intermediate titanomagnetite. We imaged magnetic structures of 109 synthetic titanomagnetite grains with x = 0.54 (TM54) and 1–4‐μm size using magnetic force microscopy. For six grains, we explored local energy minimum states after alternating field demagnetization and saturation isothermal remanent magnetization. According to the magnetic force microscopy results, 80% of TM54 grains display in‐plane magnetization with one to four domains, vortex‐like or flux‐closure structures, and Néel‐like domain walls. Electron backscatter diffraction data on six grains showed that their surface orientations are cutting planes of octahedral crystals and those with approximately square cross sections are within 15° of a (100) crystallographic plane. Magnetic force microscopy observations of magnetic structures in ~1.5‐μm grains agree well with numerical micromagnetic modeling of a pyramidal shaped grain with a (100) square base and displayed four discrete local energy minimum states: a single vortex as a ground state and three multivortex states with higher energy. Our observations show that vortex states in titanomagnetite grains (1–5 μm) occur at the lower end of the PSD size range in this mineral and corresponding to a size range known to carry stable and reliable remanence in natural titanomagnetites.

Section: 1029/2018gc007723supporting

confidence: 70%

mentioning

confidence: 99%

Magnetic Vortex States in Small Octahedral Particles of Intermediate Titanomagnetite

Khakhalova

Moskowitz

Geochem Geophys Geosyst

et al. 2018

“…Shinjo et al, 2000) for some years, we believe that the results of the present study is the first indication that these domains states have sufficient thermal and temporal stability to make them excellent carriers of palaomagnetic information. Indeed the high blocking temperatures of PSD particles predicted by this model are supported by recent high-temperature nanometer scale magnetic imaging (18,(20)(21)(22) of PSD magnetite particles.…”

Section: R a F Tsupporting

confidence: 55%

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

Nagy

Proc. Natl. Acad. Sci. U.S.A.

Muxworthy

et al. 2017

Self Cite

111

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...

“…As a result, the earliest micromagnetic models could only be validated against bulk observations such as assemblies of sized particle fractions or magnetosome observations (Fabian et al, 1996;Williams & Dunlop, 1995;Witt et al, 2005) in the case of natural materials, or on thin film and particulate man-made recording media (Labrune & Miltat, 1990;Silva & Bertram, 1990). However, results from MERRILL have been directly compared with nanoscale experimental data via electron holography with good agreement (Almeida et al, 2016).…”

Section: Model Validationmentioning

confidence: 99%

MERRILL: Micromagnetic Earth Related Robust Interpreted Language Laboratory

Conbhuí

Geochem Geophys Geosyst

Fabian

et al. 2018

Self Cite

Complex magnetic domain structures and the energy barriers between them are responsible for pseudo‐single‐domain phenomena in rock magnetism and contribute significantly to the magnetic remanence of paleomagnetic samples. This article introduces MERRILL, an open source software package for three‐dimensional micromagnetics optimized and designed for the calculation of such complex structures. MERRILL has a simple scripting user interface that requires little computational knowledge to use but provides research strength algorithms to model complex, inhomogeneous domain structures in magnetic materials. It uses a finite element/boundary element numerical method, optimally suited for calculating magnetization structures of local energy minima (LEM) in irregular grain geometries that are of interest to the rock and paleomagnetic community. MERRILL is able to simulate the magnetic characteristics of LEM states in both single grains, and small assemblies of interacting grains, including saddle‐point paths between nearby LEMs. Here the numerical model is briefly described, and an overview of the scripting language and available commands is provided. The open source nature of the code encourages future development of the model by the scientific community.