Trevor P. Almeida

Visualized effect of oxidation on magnetic recording fidelity in pseudo-single-domain magnetite particles

Magnetite (Fe3O4) is an important magnetic mineral to Earth scientists, as it carries the dominant magnetic signature in rocks, and the understanding of its magnetic recording fidelity provides a critical tool in the field of palaeomagnetism. However, reliable interpretation of the recording fidelity of Fe3O4 particles is greatly diminished over time by progressive oxidation to less magnetic iron oxides, such as maghemite (γ-Fe2O3), with consequent alteration of remanent magnetization potentially having important geological significance. Here we use the complementary techniques of environmental transmission electron microscopy and off-axis electron holography to induce and visualize the effects of oxidation on the magnetization of individual nanoscale Fe3O4 particles as they transform towards γ-Fe2O3. Magnetic induction maps demonstrate a change in both strength and direction of remanent magnetization within Fe3O4 particles in the size range dominant in rocks, confirming that oxidation can modify the original stored magnetic information.

Observing thermomagnetic stability of nonideal magnetite particles: Good paleomagnetic recorders?

The thermomagnetic behavior of remanence-induced magnetite (Fe3O4) particles in the pseudo-single-domain (PSD) size range (~0.1–10 µm), which dominate the magnetic signature of many rock lithologies, is investigated using off-axis electron holography. Construction of magnetic induction maps allowed for the visualization of the vortex domain state in an individual Fe3O4 grain (~200 nm in diameter) as a function of temperature. Acquisition of a series of electron holograms at 100°C intervals during in situ heating up to 700°C demonstrates the vortex state of the Fe3O4 grain, in this instance, remains thermally stable close to its unblocking temperature and exhibits a similar in-plane remanent state upon cooling; i.e., the particle is effectively behaving like a uniaxial single-domain particle to temperatures near TC. Such particles are thought to be robust magnetic recorders. It is suggested that evidence for PSD behavior should therefore not preclude paleomagnetic investigation.

Resolving the Origin of Pseudo‐Single Domain Magnetic Behavior

The term ‘pseudo-single domain’ (PSD) has been used to describe the transitional state in rock magnetism that spans the particle size range between the single domain (SD) and multi-domain (MD) states. The particle size range for the stable SD state in the most commonly occurring terrestrial magnetic mineral, magnetite, is so narrow (~20-75 nm) that it is widely considered that much of the paleomagnetic record of interest is carried by ‘PSD’ rather than stable SD particles. The PSD concept has, thus, become the dominant explanation for the magnetization associated with a major fraction of particles that record paleomagnetic signals throughout geological time. In this paper, we argue that in contrast to the SD and MD states, the term ‘PSD’ does not describe the relevant physical processes, which have been documented extensively using three-dimensional micromagnetic modeling, and by parallel research in materials science and solid-state physics. We also argue that features attributed to ‘PSD’ behavior can be explained by nucleation of a single magnetic vortex immediately above the maximum stable SD transition size. With increasing particle size, multiple vortices, antivortices, and domain walls can nucleate, which produce variable cancellation of magnetic moments and a gradual transition into the MD state. Thus, while the term ‘PSD’ describes a well-known transitional state, it fails to describe adequately the physics of the relevant processes. We recommend that use of this term should be discontinued in favor of “vortex state”, which spans a range of behaviors associated with magnetic vortices.

Direct visualization of the thermomagnetic behavior of pseudo-single-domain magnetite particles

In situ electron microscopy is used to visualize the thermomagnetic behavior of vortex domain states within magnetic minerals. The study of the paleomagnetic signal recorded by rocks allows scientists to understand Earth’s past magnetic field and the formation of the geodynamo. The magnetic recording fidelity of this signal is dependent on the magnetic domain state it adopts. The most prevalent example found in nature is the pseudo–single-domain (PSD) structure, yet its recording fidelity is poorly understood. Here, the thermoremanent behavior of PSD magnetite (Fe3O4) particles, which dominate the magnetic signatures of many rock lithologies, is investigated using electron holography. This study provides spatially resolved magnetic information from individual Fe3O4 grains as a function of temperature, which has been previously inaccessible. A small exemplar Fe3O4 grain (~150 nm) exhibits dynamic movement of its magnetic vortex structure above 400°C, recovering its original state upon cooling, whereas a larger exemplar Fe3O4 grain (~250 nm) is shown to retain its vortex state on heating to 550°C, close to the Curie temperature of 580°C. Hence, we demonstrate that Fe3O4 grains containing vortex structures are indeed reliable recorders of paleodirectional and paleointensity information, and the presence of PSD magnetic signals does not preclude the successful recovery of paleomagnetic signals.

A valve-assisted snapshot approach to understand the hydrothermal synthesis of α-Fe2O3 nanorods

A novel valve-assisted pressure autoclave is described, facilitating ‘snapshot’ investigations of hydrothermal synthesis (HS) product suspensions rapidly quenched in liquid nitrogen. The approach gives near in situ descriptions of the HS reaction products, as characterised using transmission electron microscopy, and provides fundamental insight into the mechanisms of crystal growth. An investigation into the development of α-Fe2O3 nanorods from aqueous FeCl3 solution clarifies the role of intermediate β-FeOOH nanorod and α-Fe2O3 nanoparticle phases during α-Fe2O3 nanorod growth. It is shown that intermediate β-FeOOH precipitation and dissolution occurs alongside the anisotropic growth of α-Fe2O3 nanorods through the coalescence of primary α-Fe2O3 nanoparticles. It is considered that this valve-assisted technique will be widely applicable to the study of many other HS systems.

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

Significance When magnetic crystals form in rocks and meteorites, they can record the ancient magnetic field and retain this information over geological timescales. Scientists use these magnetic recordings to study the evolution of the Earth and the Solar System. Previous theories for the recording mechanisms of the magnetic crystals in rocks and meteorites are based on the idea that the magnetic structures within crystals are near uniform. However, from numerous studies we know this not to be true. The crystals are too large in size and display complex nonuniform “vortex” structures. In this study we have shown that vortex structures are capable of recording and retaining magnetic signals over billions of years. Interpretations of paleomagnetic 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 paleomagnetic signals in rocks are usually larger, nonuniformly 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 timescales. We examine in detail the thermal stability characteristics of equidimensional cuboctahedral magnetite and find that, contrary to previously published theories, such nonuniformly magnetized particles provide greater magnetic stability than their uniformly magnetized counterparts. Hence, nonuniformly magnetized grains, which are commonly the main remanence carrier in meteorites and rocks, can record and retain high-fidelity magnetic recordings over billions of years.

Insights from in situ and environmental TEM on the oriented attachment of α-Fe2O3 nanoparticles during α-Fe2O3 nanorod formation

Acicular α-Fe2O3 nanorods (NRs), at an intermediate stage of development, were isolated using a snapshot valve-assisted hydrothermal synthesis (HS) technique, for the purpose of complementary in situ transmission electron microscopy (iTEM) and environmental TEM (ETEM) investigations of the effect of local environment on the oriented attachment (OA) of α-Fe2O3 nanoparticles (NPs) during α-Fe2O3 NR growth. Observations of static snapshot HS samples suggested that α-Fe2O3 NPs undergo reorientation following initial attachment, consistent with an intermediate OA stage, prior to ‘envelopment’ with the developing NR to adopt a perfect single crystal. Conversely, the heating of partially developed α-Fe2O3 NRs up to 250 °C, under vacuum, during iTEM, demonstrated the progressive coalescence of loosely packed α-Fe2O3 NPs and the coarsening of α-Fe2O3 NRs, without any direct evidence for an intermediate OA stage. Direct evidence was obtained for the action of an OA mechanism prior to the consumption of α-Fe2O3 NPs at the tips of developing α-Fe2O3 NRs during ETEM investigation, under an He pressure of 5 mbar at 500 °C. However, α-Fe2O3 NPs more strongly attached to the side-walls of developing α-Fe2O3 NRs were more likely to be consumed through a local NP destabilisation and reordering process, in the absence of an OA mechanism. Hence, the emerging ETEM evidence suggests a competition between OA and diffusion processes at the α-Fe2O3 NP coalescence stage of acicular α-Fe2O3 NR crystal development, depending on whether the localised growth conditions facilitate freedom of NP movement.

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

Abstract The thermal demagnetization of pseudo‐single‐domain (PSD) magnetite (Fe3O4) particles, which govern the magnetic signal in many igneous rocks, is examined using off‐axis electron holography. Visualization of a vortex structure held by an individual Fe3O4 particle (~250 nm in diameter) during in situ heating is achieved through the construction and examination of magnetic‐induction maps. Stepwise demagnetization of the remanence‐induced Fe3O4 particle upon heating to above the Curie temperature, performed in a similar fashion to bulk thermal demagnetization measurements, revealed that its vortex state remains stable under heating close to its unblocking temperature and is recovered upon cooling with the same or reversed vorticity. Hence, the PSD Fe3O4 particle exhibits thermomagnetic behavior comparable to a single‐domain carrier, and thus, vortex states are considered reliable magnetic recorders for paleomagnetic investigations.

Multi-walled carbon/IF-WS2 nanoparticles with improved thermal properties

A unique new class of core-shell structured composite nanoparticles, C-coated inorganic fullerene-like WS2 (IF-WS2) hollow nanoparticles, has been created for the first time in large quantities, by a continuous chemical vapour deposition method using a rotary furnace. Transmission electron microscopy and Raman characterisations of the resulting samples reveal that the composite nanoparticles exhibited a uniform shell of carbon coating, ranging from 2-5 nm on the IF-WS2 core, with little or no agglomeration. Importantly, thermogravimetric analysis and differential scanning calorimetry analysis confirm that their thermal stability against oxidation in air has been improved by about 70 °C, compared to the pristine IF-WS2, making these new C-coated IF-WS2 nanoparticles more attractive for critical engineering applications.

Hydrothermal synthesis of mixed cobalt-nickel ferrite nanoparticles

The hydrothermal synthesis of CoxNi1−xFe2O4 nanoparticles (NPs) from mixed FeCl3 / CoCl2 / NiCl2 precursor solution has been investigated as a function of reaction temperature, using the complementary characterisation techniques of X-ray diffractometry, transmission electron microscopy, energy dispersive X-ray analysis and electron energy loss spectroscopy. At pH ~ 8, the HS of Co0.5Ni0.5Fe2O4 NPs, up to ~ 25 nm in size, was found to proceed through the formation and dissolution of intermediate Fe(OH)3 and [FeCoNi2(OH)8]+.[Cl−.H2O]− phases.