Observation of thermally-induced magnetic relaxation in a magnetite grain using off-axis electron holography

A synthetic basalt comprising magnetic Fe3O4 grains (~ 50 nm to ~ 500 nm in diameter) is investigated using a range of complementary nano-characterisation techniques. Off-axis electron holography combined with in situ heating allowed for the visualisation of the thermally-induced magnetic relaxation of an Fe3O4 grain (~ 300 nm) from an irregular domain state into a vortex state at 550˚C, just below its Curie temperature, with the magnetic intensity of the vortex increasing on cooling

Hydrothermal synthesis and characterisation of α-Fe2O3 nanorods

The hydrothermal synthesis (HS) of α-Fe2O3 nanorods (NRs) is investigated using a combination of complementary analytical techniques. The construction of an HS ‘process map’ as a function of temperature, time and phosphate (PO43-) concentration provides insight into the nature of intermediate β-FeOOH NR precipitation, dissolution and subsequent α-Fe2O3 growth, along with the effect of PO43- anion concentration on the development of α-Fe2O3 particle shape. An HS processing temperature of 200˚C and an Fe3+ : PO43- molar ratio of 31.5 yielded crystalline acicular α-Fe2O3 NRs with an aspect ratio of ~ 7 (~ 420 nm long, ~ 60 nm wide). The additional effects of FeCl3 concentration, pH, stage of phosphate addition and α-Fe2O3 seed content on the growth of α-Fe2O3 NRs is investigated. The development of a novel valve-assisted pressure autoclave is described, facilitating the rapid quenching of hydrothermal suspensions into liquid nitrogen, providing ‘snapshots’ closely representative of the in situ physical state of the synthesis reaction products. Examination of the samples acquired as a function of reaction time and known reaction temperature provides fundamental insight into the anisotropic crystal growth mechanism of the acicular α-Fe2O3 NRs. It is considered that the release of Fe3+ ions back into solution through intermediate β-FeOOH dissolution supplies the nucleation and growth of primary α-Fe2O3 nanoparticles (NPs) (< 10 nm) which subsequently coalescence through a mechanism of oriented attachment (OA) with increasing temperature, into larger, acicular α-Fe2O3 NRs. Fourier transform infra-red spectroscopy investigation of the quenched reaction products provides evidence for PO43- absorption on the α-Fe2O3 NPs, in the form of mono or bi-dentate (bridging) surface complexes, on surfaces normal and parallel to the crystallographic α-Fe2O3 c-axis, respectively. The balance between bi-dentate and mono-dentate phosphate absorption is considered to be critical in mediating the acicular shape of the α-Fe2O3 NRs. A feasibility study on the incorporation of ferromagnetic cobalt, Co3O¬4 NPs or CoFe2O4 NPs into α-Fe2O3 NRs during HS is presented. In all cases, there is no evidence for the incorporation of cobalt within the α-Fe2O3 NRs or the formation of hetero-nanostructures with the Co3O4 or CoFe2O4 NPs. The overall growth mechanism of single crystalline acicular α-Fe2O3 NRs involves the anisotropic growth and dissolution of intermediate β-FeOOH NRs, governed by its crystallographic structure, and the OA of primary α-Fe2O3 NPs, mediated by the preferential absorption of phosphate surfactant

Focused Electron-Beam Induced Deposition, In Situ TEM And Off-Axis Electron Holography Investigation of Bi-Magnetic Core-Shell Nanostructures

No abstract available

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

Magnetic characterization of synthetic titanomagnetites: Quantifying the recording fidelity of ideal synthetic analogs

A series of four synthetic basalts comprising titanomagnetite (Fe3-xTixO4) grains of varied size and titanium content have been produced by a glass-ceramic method. Complementary characterization techniques of X-ray diffractometry, secondary electron microscopy, and transmission electron microscopy (TEM) demonstrate the reaction product composition consisted of mainly Fe3-xTixO4, pyroxene hedenbergite, fayalite, and SiO2. The samples exhibit bimodal distributions of larger (<2 µm) and smaller Fe3-xTixO4 particles (<50 nm in diameter), the latter found inside pyroxene crystals, as well as the sporadic occurance of dendritic Fe3-xTixO4 structures. Magnetic measurements show their bulk characteristics fall into two groups: Ti-rich titanomagnetite samples with varying Ti content; and near-stoichiometric magnetite. The TEM technique of off-axis electron holography allowed for visualization of the magnetic behavior of the synthetic Fe3-xTixO4 grains. Energy dispersive X-ray analysis and off-axis electron holography confirmed the small Fe3-xTixO4 grains (<50 nm) confined within glassy pyroxene regions to be Fe-rich and single domain, carrying strong magnetic signals, compared to the relatively magnetically weak larger Fe3-xTixO4 grains (x ∼ 0.6). The large grains in the pure magnetite sample are shown to be pseudo-single domain in nature. The quenching process involved in synthesis is considered similar to that of pillow basalts found at mid-ocean ridges and hence the reaction products are thought ideal in terms of characterization and understanding, for the purpose of studying natural systems

Paleomagnetic recording fidelity of nonideal magnetic systems

We carry out simple analytical calculations and Monte Carlo studies to better understand the impact of QCD radiation on some well-known jet substructure methods for jets arising from the decay of boosted Higgs bosons. Understanding differences between taggers for these signal jets assumes particular significance in situations where they perform similarly on QCD background jets. As an explicit example of this we compare the Y-splitter method to the more recently proposed Y-pruning technique. We demonstrate how the insight we gain can be used to significantly improve the performance of Y-splitter by combining it with trimming and show that this combination outperforms the other taggers studied here, at high $p_T$. We also make analytical estimates for optimal parameter values, for a range of methods and compare to results from Monte Carlo studies.Comment: 53 pages, 21 figures. Version published in JHEP: addressed several points raised by the refere

Differential Phase Contrast Imaging of the Magnetostructural Transition and Phase Boundary Motion in Uniform and Gradient-doped FeRh-based Thin Films

No abstract available

Quantitative Differential Phase Contrast Imaging of the Magnetostructural Transition and Current-driven Motion of Domain Walls in FeRh Thin Films

No abstract available

Paleomagnetic recording fidelity of nonideal magnetic systems

A suite of near-identical magnetite nanodot samples produced by electron-beam lithography have been used to test the thermomagnetic recording fidelity of particles in the 74–333 nm size range; the grain size range most commonly found in rocks. In addition to controlled grain size, the samples had identical particle spacings, meaning that intergrain magnetostatic interactions could be controlled. Their magnetic hysteresis parameters were indicative of particles thought not to be ideal magnetic recorders; however, the samples were found to be excellent thermomagnetic recorders of the magnetic field direction. They were also found to be relatively good recorders of the field intensity in a standard paleointensity experiment. The samples' intensities were all within ∼15% of the expected answer and the mean of the samples within 3% of the actual field. These nonideal magnetic systems have been shown to be reliable records of the geomagnetic field in terms of both direction and intensity even though their magnetic hysteresis characteristics indicate less than ideal magnetic grains