Observation of magnetic vortex configuration in non-stoichiometric Fe3O4 nanospheres

Theoretical and micromagnetic simulation studies of magnetic nanospheres with vortex configurations suggest that such nanostructured materials have technological advantages over conventional nanosystems for applications based on high-power-rate absorption and subsequent emission. However, full experimental evidence of magnetic vortex configurations in spheres of submicrometer size is still lacking. Here, we report the microwave irradiation fabrication of Fe3O4 nanospheres and establish their magnetic vortex configuration based on experimental results, theoretical analysis, and micromagnetic simulations. Detailed magnetic and electrical measurements, together with Mössbauer spectroscopy data, provide evidence of a loss of stoichiometry in vortex nanospheres owing to the presence of a surface oxide layer, defects, and a higher concentration of cation vacancies. The results indicate that the magnetic vortex spin configuration can be established in bulk spherical magnetite materials. This study provides crucial information that can aid the synthesis of magnetic nanospheres with magnetically tailored properties; consequently, they may be promising candidates for future technological applications based on three-dimensional magnetic vortex structures.

Quantitative Visualization of Thermally Enhanced Perpendicular Shape Anisotropy STT-MRAM Nanopillars

Perpendicular shape anisotropy (PSA) offers a practical solution to downscale spin-transfer torque magnetoresistive random-access memory (STT-MRAM) beyond the sub-20 nm technology node while retaining thermal stability. However, our understanding of the thermomagnetic behavior of PSA-STT-MRAM is often indirect, relying on magnetoresistance measurements and micromagnetic modeling. Here, the magnetism of a NiFe PSA-STT-MRAM nanopillar is investigated using off-axis electron holography, providing spatially resolved magnetic information as a function of temperature. Magnetic induction maps reveal the micromagnetic configuration of the NiFe storage layer (∼60 nm high, ≤20 nm diameter), confirming the PSA induced by its 3:1 aspect ratio. In situ heating demonstrates that the PSA of the storage layer is maintained up to at least 250 °C, and direct quantitative measurements reveal a moderate decrease of magnetic induction. Hence, this study shows explicitly that PSA provides significant stability in STT-MRAM applications that require reliable performance over a range of operating temperatures.

P439Could regional electrogram desynchronization identified using mean phase coherence be potential ablation targets in persistent atrial fibrillation?

Funding Acknowledgements

This work was supported by the NIHR Leicester Biomedical Research Centre. XL was funded by MRC(MR/S037306/1) and BHF (PG/18/33/33780)

Background

It remains controversial as to whether rotors detected using phase mapping during persistent atrial fibrillation (persAF) represent main drivers of the underlying mechanism as others found rotors to be located near line of conduction block. Regional electrogram desynchronization (RED) has been suggested as successful targets for persAF ablation, but automatic tools and quantitative measures are lacking.

Purpose

We aim to use mean phase coherence (MPC) to automatically identify RED regions during persAF. This method was compared with phase singularity density (PSD) maps.

Methods

Patients undergoing left atrial (LA) persAF ablation were enrolled (n = 10). 2048-channel virtual electrograms (VEGMs) were collected from each patient using non-contact mapping (St Jude Velocity System, Ensite Array) for 10 seconds.  To remove far field ventricular activities, QRS onset and T wave end locations were detected from ECG lead I (Figure 1A) and only the VEGM segments from T end to QRS onset were included in the analysis. VEGMs were reconstructed using sinusoidal wavelets fitting and the phase of VEGMs determined using Hilbert transform. Phase singularities (PS) were detected using the topological charge method and repetitive PSD maps were generated. RED was defined as the average of MPC of each node against direct neighbouring nodes on the 3D mesh (Figure 1A-B). Linear regression analysis was used to compare the average MPC vs. PSD and vs. the standard deviation of MPC (MPC_SD).

Results

A total of 221,184 VEGM segments were analysed with mean duration of 364.2 milliseconds.  MPC has shown the ability to quantify the level of synchronisation between VEGMs (Figure 1B). Inverse correlation was found between PSD and average MPC values for all 10 patients (p < 0.0001, Figure 1C). Average MPC and MPC_SD were found to be inversely correlated (p < 0.0001, Figure 1C). Spatially, similar graphic patterns can be found from LA MPC maps and PSD maps for all patients (Figure 1D).

Conclusion

We have proposed a method to quantify the level of synchronisation between VEGMs. Phase density mapping showed a considerable agreement with RED regions reflecting regional conducting delays, which supports the previous finding where rotors found at conduction block. Inverse correlation between local average MPC and MPC_SD suggests that conduction delays of the identified regions are not heterogenous, posing directional preferences. Rather than solely looking for rotational activities, this method could identify comprehensive RED regions, which may also explain the conflicting results from different studies targeting rotational activities, where incomplete subsets of RED regions could have been targeted. Atrial RED regions can easily be identified with simultaneously collected electrograms from multi-polar catheters and should be targeted in future persAF studies.

Abstract Figure 1

Off-axis electron holography of bacterial cells and magnetic nanoparticles in liquid

The mapping of electrostatic potentials and magnetic fields in liquids using electron holography has been considered to be unrealistic. Here, we show that hydrated cells of Magnetospirillum magneticum strain AMB-1 and assemblies of magnetic nanoparticles can be studied using off-axis electron holography in a fluid cell specimen holder within the transmission electron microscope. Considering that the holographic object and reference wave both pass through liquid, the recorded electron holograms show sufficient interference fringe contrast to permit reconstruction of the phase shift of the electron wave and mapping of the magnetic induction from bacterial magnetite nanocrystals. We assess the challenges of performing in situ magnetization reversal experiments using a fluid cell specimen holder, discuss approaches for improving spatial resolution and specimen stability, and outline future perspectives for studying scientific phenomena, ranging from interparticle interactions in liquids and electrical double layers at solid-liquid interfaces to biomineralization and the mapping of electrostatic potentials associated with protein aggregation and folding.

Quantitative TEM imaging of the magnetostructural and phase transitions in FeRh thin film systems

Equi-atomic FeRh is a very interesting material as it undergoes a magnetostructural transition from an antiferromagnetic (AF) to a ferromagnetic (FM) phase between 75-105 °C. Its ability to present phase co-existence separated by domain walls (DWs) above room temperature provides immense potential for exploitation of their DW motion in spintronic devices. To be able to effectively control the DWs associated with AF/FM coexistence in FeRh thin films we must fully understand the magnetostructural transition and thermomagnetic behaviour of DWs at a localised scale. Here we present a transmission electron microscopy investigation of the transition in planar FeRh thin-film samples by combining differential phase contrast (DPC) magnetic imaging with in situ heating. We perform quantitative measurements from individual DWs as a function of temperature, showing that FeRh on NiAl exhibits thermomagnetic behaviour consistent with the transition from AF to FM. DPC imaging of an FeRh sample with HF-etched substrate reveals a state of AF/FM co-existence and shows the transition from AF to FM regions proceeds via nucleation of small vortex structures, which then grow by combining with newly nucleated vortex states into larger complex magnetic domains, until it is in a fully-FM state.

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

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.

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

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.

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.

KEY POINTS

Nonideal magnetic systems accurately record field directionWeak-field remanences more stable than strong-field remanences.

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.

Controlling role of pH and temperature on CoFe2O4 nanostructures produced by hydrothermal synthesis

The hydrothermal synthesis (HS) of CoFe2O4 nanoparticles (NPs) has been investigated as a function of reaction temperature and pH, using complementary characterisation techniques of transmission electron microscopy and X-ray diffractometry. The HS of CoFe2O4 NPs (< 25 nm) at pH - 8 proceeded through the formation and dissolution of intermediate Fe(OH)3 and [FeCo3(OH)8]+ x [Cl- x H2O]- phases with increasing reaction temperature. In contrast, HS of CoFe2O4 NPs (< 50 nm) at pH - 12 resulted in the formation of additional intermediate Co(OH)2, CoOOH and alpha-FeOOH phases, with residual alpha-Fe2O3 present in the final reaction product. This research demonstrates the size and phase purity of the CoFe2O4 NPs may be controlled through the formation and dissolution of the intermediate phases at various pH values in the alkaline pH regime.

Hydrothermal synthesis and near in situ analysis of NiFe2O4 nanoparticles

The hydrothermal synthesis (HS) of NiFe2O4 nanoparticles (NPs) has been investigated using a novel valve-assisted pressure autoclave. This approach has facilitated the rapid quenching of hydrothermal suspensions into liquid nitrogen, providing 'snapshots' representative of the near in situ physical state of the synthesis reaction products as a function of known temperature. The acquired samples were examined using complementary characterisation techniques of transmission electron microscopy and X-ray diffractometry (XRD). The HS of NiFe2O4 NPs (< 25 nm) at pH - 8 proceeded through the formation and dissolution of intermediate amorphous Fe(OH)3 and FeNi3Cl2(OH)8 x H2O sheets with increasing reaction temperature. The near in situ nature of the HS suspension resulted in the formation of NaCI by-product during drying in advance of XRD investigation, not during the HS process.

Hydrothermal growth mechanism of α-Fe₂O₃ nanorods derived by near in situ analysis

The hydrothermal growth mechanism of α-Fe₂O₃ nanorods has been investigated using a novel valve assisted pressure autoclave. This approach has facilitated the rapid quenching of hydrothermal suspensions into liquid nitrogen, providing 'snapshots' representative of the near in situ physical state of the synthesis reaction products as a function of known temperature. Examination of the acquired samples using complementary characterisation techniques of transmission electron microscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy (FT-IR) has provided fundamental insight into the anisotropic crystal growth mechanism of the lenticular α-Fe₂O₃ nanorods.An intermediate ß-FeOOH phase was observed to precipitate alongside small primary α-Fe₂O₃ nanoparticles. Dissolution of the ß-FeOOH phase with increasing temperature, in accordance with Ostwald's rule of stages, led to the release of Fe³+ anions back into solution to supply the growth of α-Fe₂O₃ nanoparticles, which in turn coalesced to form lenticular α-Fe₂O₃ nanorods. The critical role of the PO₄³⁻ surfactant on mediating the lenticular shape of the α-Fe₂O₃ nanorods is emphasised. Strong phosphate anion absorption on α-Fe₂O₃ crystal surfaces stabilised the primary α-Fe₂O₃ nanoparticle size to < 10 nm. FT-IR investigation of the quenched reaction products provided evidence for PO₄³⁻ absorption on the α-Fe₂O₃ nanoparticles in the form of mono or bi-dentate (bridging) surface complexes on surfaces normal and parallel to the crystallographic α-Fe₂O₃ c-axis, respectively. Monodentate PO₄³⁻ absorption is considered weaker and hence easily displaced during growth, as compared to absorbed PO₄³⁻ bi-dentate species, which implies the α-Fe₂O₃ c-planes are favoured for the oriented attachment of primary α-Fe₂O₃ nanoparticles, resulting in the development of filamentary features which act as the basis of growth, defining the shape of the lenticular α-Fe₂O₃ nanorods.