Thermomagnetic recording fidelity of nanometer-sized iron and implications for planetary magnetism

Lesleis Nagy,Wyn Williams,Lisa Tauxe,Adrian R Muxworthy,Idenildo Ferreira

doi:10.1073/pnas.1810797116

Abstract

Paleomagnetic observations provide valuable evidence of the strength of magnetic fields present during evolution of the Solar System. Such information provides important constraints on physical processes responsible for rapid accretion of the protoplanetesimal disk. For this purpose, magnetic recordings must be stable and resist magnetic overprints from thermal events and viscous acquisition over many billions of years. A lack of comprehensive understanding of magnetic domain structures carrying remanence has, until now, prevented accurate estimates of the uncertainty of recording fidelity in almost all paleomagnetic samples. Recent computational advances allow detailed analysis of magnetic domain structures in iron particles as a function of grain morphology, size, and temperature. Our results show that uniformly magnetized equidimensional iron particles do not provide stable recordings, but instead larger grains containing single-vortex domain structures have very large remanences and high thermal stability-both increasing rapidly with grain size. We derive curves relating magnetic thermal and temporal stability demonstrating that cubes (>35 nm) and spheres (>55 nm) are likely capable of preserving magnetic recordings from the formation of the Solar System. Additionally, we model paleomagnetic demagnetization curves for a variety of grain size distributions and find that unless a sample is dominated by grains at the superparamagnetic size boundary, the majority of remanence will block at high temperatures ([Formula: see text]C of Curie point). We conclude that iron and kamacite (low Ni content FeNi) particles are almost ideal natural recorders, assuming that there is no chemical or magnetic alteration during sampling, storage, or laboratory measurement.

Highlights

Paleomagnetic observations provide valuable evidence of the strength of magnetic fields present during evolution of the Solar System
The evolution of domain structure with grain size determined from unconstrained 3D micromagnetic models follows the well-established evolution seen in other materials [21, 29, 30] whereby the smallest particles have relaxation times of order 102 s or less and are termed superparamagnetic (SP)
For equidimensional cubes and iron spheres at room temperature, the critical grain size d0 marks the transition from SP to SD, d0 that from stable SD to unstable SV, and d1 that from unstable to stable SV

Summary

Introduction

The exception of Winklhofer et al [12] and Fabian et al [13], all previously published Pullaiah curves found in the literature, e.g., Pullaiah et al [9] and Garrick-Bethell and Weiss [14], are based entirely on SD theory, which does not take into account more complex magnetic domain structures such as the flower and single-vortex (SV) states [15] We know such nonuniform structures are ubiquitous in the vast majority of iron particles found in planetary materials [16,17,18]. Given that the majority of magnetic remanence carriers in iron, and likely other minerals, are SV [22], the paleomagnetic recordings that they contain can be correctly understood only by a reevaluation of their thermomagnetic stability Can such iron particles record and retain magnetic remanences over geological timescales and do Pullaiah curves for vortex states in natural kamacite significantly deviate from those of SD grains?. For a magnetic mineral to retain an original magnetic remanence, the magnetic carriers must be thermally stable on geological timescales

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Discussion

Conclusion