Measuring Electrical Resistivity at the Nanoscale in Phase-Change Materials.

Electrical resistivity is the key parameter in the active regions of many current nanoscale devices, from memristors to resistive random-access memory and phase-change memories. The local resistivity of the materials is engineered on the nanoscale to fit the performance requirements. Phase-change memories, for example, rely on materials whose electrical resistance increases dramatically with a change from a crystalline to an amorphous phase. Electrical characterization methods have been developed to measure the response of individual devices, but they cannot map the local resistance across the active area. Here, we propose a method based on operando electron holography to determine the local resistance within working devices. Upon switching the device, we show that electrical resistance is inhomogeneous on the scale of only a few nanometers.

Optimized Route for the Fabrication of MnAlC Permanent Magnets by Arc Melting.

The rare-earth-free MnAlC alloy is currently considered a very promising candidate for permanent magnet applications due to its high anisotropy field and relatively high saturation magnetization and Curie temperature, besides being a low-cost material. In this work, we presented a simple fabrication route that allows for obtaining a magnetically enhanced bulk τ-MnAlC magnet. In the fabrication process, an electric arc-melting method was carried out to melt ingots of MnAlC alloys. A two-step solution treatment at 1200 °C and 1100 °C allowed us to synthesize a pure room-temperature ε-MnAlC ingot that completely transformed into τ-MnAlC alloy, free of secondary phases, after an annealing treatment at 550 °C for 30 min. The Rietveld refinements and magnetization measurements demonstrated that the quenched process produces a phase-segregated ε-MnAlC alloy that is formed by two types of ε-phases due to local fluctuation of the Mn. Room-temperature hysteresis loops showed that our improved τ-MnAlC alloy exhibited a remanent magnetization of 42 Am2/kg, a coercive field of 0.2 T and a maximum energy product, (BH)max, of 6.07 kJ/m3, which is higher than those reported in previous works using a similar preparation route. Experimental evidence demonstrated that the synthesis of a pure room-temperature ε-MnAlC played an important role in the suppression of undesirable phases that deteriorate the permanent magnet properties of the τ-MnAlC. Finally, magnetic images recorded by Lorentz microscopy allowed us to observe the microstructure and magnetic domain walls of the optimized τ-MnAlC. The presence of magnetic contrasts in all the observed grains allowed us to confirm the high-quality ferromagnetic behavior of the system.

Defect-induced monopole injection and manipulation in artificial spin ice.

Lithographically defined arrays of nanomagnets are well placed for application in areas such as probabilistic computing or reconfigurable magnonics due to their emergent collective dynamics and writable magnetic order. Among them are artificial spin ice (ASI), which are arrays of binary in-plane macrospins exhibiting geometric frustration at the vertex interfaces. Macrospin flips in the arrays create topologically protected magnetic charges, or emergent monopoles, which are bound to an antimonopole to conserve charge. In the absence of controllable pinning, it is difficult to manipulate individual monopoles in the array without also influencing other monopole excitations or the counter-monopole charge. Here, we tailor the local magnetic order of a classic ASI lattice by introducing a ferromagnetic defect with shape anisotropy into the array. This creates monopole injection sites at nucleation fields below the critical lattice switching field. Once formed, the high energy monopoles are fixed to the defect site and may controllably propagate through the lattice under stimulation. Defect programing of bound monopoles within the array allows fine control of the pathways of inverted macrospins. Such control is a necessary prerequisite for the realization of functional devices, e. g. reconfigurable waveguide in nanomagnonic applications.

Key Signatures of Magnetofossils Elucidated by Mutant Magnetotactic Bacteria and Micromagnetic Calculations.

Magnetotactic bacteria (MTB) produce single-stranded or multi-stranded chains of magnetic nanoparticles that contribute to the magnetization of sediments and rocks. Their magnetic fingerprint can be detected in ancient geological samples and serve as a unique biosignature of microbial life. However, some fossilized assemblages bear contradictory signatures pointing to magnetic components that have distinct origin(s). Here, using micromagnetic simulations and mutant MTB producing looped magnetosome chains, we demonstrate that the observed magnetofossil fingerprints are produced by a mixture of single-stranded and multi-stranded chains, and that diagenetically induced chain collapse, if occurring, must preserve the strong uniaxial anisotropy of native chains. This anisotropy is the key factor for distinguishing magnetofossils from other populations of natural magnetite particles, including those with similar individual crystal characteristics. Furthermore, the detailed properties of magnetofossil signatures depend on the proportion of equant and elongated magnetosomes, as well as on the relative abundances of single-stranded and multi-stranded chains. This work has important paleoclimatic, paleontological, and phylogenetic implications, as it provides reference data to differentiate distinct MTB lineages according to their chain and magnetosome morphologies, which will enable the tracking of the evolution of some of the most ancient biomineralizing organisms in a time-resolved manner. It also enables a more accurate discrimination of different sources of magnetite particles, which is pivotal for gaining better environmental and relative paleointensity reconstructions from sedimentary records.

Single-Crystalline Body Centered FeCo Nano-Octopods: From One-Pot Chemical Growth to a Complex 3D Magnetic Configuration.

Single crystalline magnetic FeCo nanostars were prepared using an organometallic approach under mild conditions. The fine-tuning of the experimental conditions allowed the direct synthesis of these nano-octopods with body-centered cubic (bcc) structure through a one-pot reaction, contrarily to the seed-mediated growth classically used. The FeCo nanostars consist of 8 tetrahedrons exposing {311} facets, as revealed by high resolution transmission electron microscopy (HRTEM) imaging and electron tomography (ET), and exhibit a high magnetization comparable with the bulk one (Ms = 235 A·m2·kg-1). Complex 3D spin configurations resulting from the competition between dipolar and exchange interactions are revealed by electron holography. This spin structures are stabilized by the high aspect ratio tetrahedral branches of the nanostars, as confirmed by micromagnetic simulations. This illustrates how magnetic properties can be significantly tuned by nanoscale shape control.

Phase detection limits in off-axis electron holography from pixelated detectors: gain variations, geometric distortion and failure of reference-hologram correction.

We investigate the effect that recording off-axis electron holograms on pixelated detectors, such as charge-coupled devices (CCD) and direct-detection devices (DDD), can have on measured amplitudes and phases. Theory will be developed for the case of perfectly uniform interference fringes illuminating an imperfect detector with gain variations and pixel displacements. We will show that both these types of defect produce a systematic noise in the phase images that depends on the position of the holographic fringes with respect to the detector. Subtracting a reference hologram from the object hologram will therefore not remove the phase noise if the initial phases of the two holograms do not coincide exactly. Another finding is that pi-shifted holograms are much less affected by gain variations but show no improvement concerning geometric distortions. The resulting phase errors will be estimated and simulations presented that confirm the theoretical developments.

Customized MFM probes based on magnetic nanorods.

Focused Electron Beam Induced Deposition (FEBID) for magnetic tip fabrication is presented in this work as an alternative to conventional sputtering-based Magnetic Force Microscopy (MFM) tips. FEBID enables the growth of a high-aspect-ratio magnetic nanorod with customized geometry and composition to overcome the key technical limitations of MFM probes currently on the market. The biggest advantage of these tips, in comparison with CoCr coated pyramidal probes, lies in the capability of creating sharp ends, nearly 10 nm in diameter, which provides remarkable (topographic and magnetic) lateral resolution in samples with magnetic features close to the resolution limits of the MFM technique itself. The shape of the nanorods produces a very confined magnetic stray field, whose interaction with the sample is extremely localized and perpendicular to the surface, with negligible in-plane components. This effect can lead to a better analytical and numerical modelling of the MFM probes and to an increase in the sensitivity without perturbing the magnetic configuration of soft samples. Besides, the high-aspect ratio achievable in FEBID nanorod tips makes them magnetically harder than the commercial ones, reaching coercive fields higher than 900 Oe. According to the results shown, tips based on magnetic nanorods grown by FEBID can be eventually used for quantitative analysis in MFM measurements. Moreover, the customized growth of Co- or Fe-based tips onto levers with different mechanical properties allows MFM studies that demand different measuring conditions. To showcase the versatility of this type of probe, as a last step, MFM is performed in a liquid environment, which still remains a challenge for the MFM community largely due to the lack of appropriate probes on the market. This opens up new possibilities in the investigation of magnetic biological samples.

Exotic Transverse-Vortex Magnetic Configurations in CoNi Nanowires.

The magnetic configurations of cylindrical Co-rich CoNi nanowires have been quantitatively analyzed at the nanoscale by electron holography and correlated to local structural and chemical properties. The nanowires display grains of both face-centered cubic (fcc) and hexagonal close packed (hcp) crystal structures, with grain boundaries parallel to the nanowire axis direction. Electron holography evidences the existence of a complex exotic magnetic configuration characterized by two distinctly different types of magnetic configurations within a single nanowire: an array of periodical vortices separating small transverse domains in hcp-rich regions with perpendicular easy axis orientation and a mostly axial configuration parallel to the nanowire axis in regions with fcc grains. These vastly different domains are found to be caused by local variations in the chemical composition modifying the crystalline orientation and/or structure, which give rise to change in magnetic anisotropies. Micromagnetic simulations, including the structural properties that have been experimentally determined, allow for a deeper understanding of the complex magnetic states observed by electron holography.

One-Pot Seed-Mediated Growth of Co Nanoparticles by the Polyol Process: Unraveling the Heterogeneous Nucleation.

The one-step seed-mediated synthesis is widely used for the preparation of ferromagnetic metal nanoparticles (NPs) since it offers a good control of particle morphology. Nevertheless, this approach suffers from a lack of mechanistic studies because of the difficulties of following in real time the heterogeneous nucleation and predicting structure effects with seeds that are generated in situ. Here, we propose a complete scheme of the heteronucleation process involved in one-pot seed-mediated syntheses of cobalt nanoparticles in liquid polyols, relying on geometrical phase analysis (GPA) of high-resolution high-angle annular dark field (HAADF)-STEM images and in situ measurements of the molecular hydrogen evolution. Cobalt particles of different shapes (rods, platelets, or hourglass-like particles) were grown by reducing cobalt carboxylate in liquid polyols in the presence of iridium or ruthenium chloride as the nucleating agent. A reaction scheme was established by monitoring the H2 evolution resulting from the decomposition of metal hydrides, formed in situ by ß-elimination of metal alkoxides, and from the polyol dehydrogenation, catalytically activated by the metal particles. This is a very good probe for both the noble metal nucleation and the heterogeneous nucleation of cobalt, showing a good separation of these two steps. Ir and Ru seeds with a size in the range 1-2 nm were found exactly in the center of the cobalt particles, whatever the cobalt particle shape, and high-resolution images revealed an epitaxial growth of the hcp Co on fcc Ir or hcp Ru seeds. The microstructure analysis around the seeds made evident two different ways of relaxing the lattice mismatch between the seeds and the cobalt, with the presence of dislocations around the Ir seeds and compression zones of the cobalt lattice near the Ru seeds. The relationship between the nature of the nucleating agent, the reaction steps, and the microstructure is discussed.

Magnetic imaging using geometrically constrained nano-domain walls.

Magnetic nanostructures, as part of hybrid CMOS technology, have the potential to overcome silicon's scaling limit. However, a major problem is how to characterize their magnetization without disturbing it. Magnetic force microscopy (MFM) offers a convenient way of studying magnetization, but spatial resolution and sensitivity are usually boosted at the cost of increasing probe-sample interaction. By using a single magnetic domain wall (DW), confined in a V-shape nanostructure fabricated at the probe apex, it is demonstrated here that the spatial resolution and the magnetic sensitivity can be decoupled and both enhanced. Indeed, owing to the nanostructure's strong shape anisotropy, DW-probes have 2 high and 2 low magnetic moment states with opposite polarities, characterised by a geometrically constrained pinned DW, and curled magnetization, respectively. Electron holography studies, supported by numerical simulations, and in situ MFM show that the DW-probe state can be controlled, and thus used as a switchable tool with a low/high stray field intensity.

Nanoparticle Ripening : A Versatile Approach for the Size and Shape Control of Metallic Iron Nanoparticles.

A novel approach for the synthesis of Fe(0) nanoparticles (NPs) with tunable sizes and shapes is reported. Ultrasmall Fe(0) NPs were reacted under mild conditions in the presence of a mixture of palmitic acid and amine ligands. These NPs acted not only as preformed seeds but also as an internal iron(II) source that was produced by the partial dissolution of the NPs by the acid. This fairly simple approach allows the strict separation of the nucleation and the growth steps. By changing the acid concentration, a fine tuning of the relative ratio between the remaining Fe(0) seeds and the iron(II) reservoir was achieved, giving access to both size (from 7 to 20 nm) and shape (spheres, cubes or stars) control. The partial dissolution of the ultrasmall Fe(0) NPs into iron(II) source and the successive growth was further studied by using combined TEM and Mössbauer spectroscopy. The successive corrosion, coalescence, and ripening observed could be understood in the framework of an environment-dependent growth model.

Magnetic-field induced rotation of magnetosome chains in silicified magnetotactic bacteria.

Understanding the biological processes enabling magnetotactic bacteria to maintain oriented chains of magnetic iron-bearing nanoparticles called magnetosomes is a major challenge. The study aimed to constrain the role of an external applied magnetic field on the alignment of magnetosome chains in Magnetospirillum magneticum AMB-1 magnetotactic bacteria immobilized within a hydrated silica matrix. A deviation of the chain orientation was evidenced, without significant impact on cell viability, which was preserved after the field was turned-off. Transmission electron microscopy showed that the crystallographic orientation of the nanoparticles within the chains were preserved. Off-axis electron holography evidenced that the change in magnetosome orientation was accompanied by a shift from parallel to anti-parallel interactions between individual nanocrystals. The field-induced destructuration of the chain occurs according to two possible mechanisms: (i) each magnetosome responds individually and reorients in the magnetic field direction and/or (ii) short magnetosome chains deviate in the magnetic field direction. This work enlightens the strong dynamic character of the magnetosome assembly and widens the potentialities of magnetotactic bacteria in bionanotechnology.

Air-Stable Anisotropic Monocrystalline Nickel Nanowires Characterized Using Electron Holography.

Nickel is capable of discharging electric and magnetic shocks in aerospace materials thanks to its conductivity and magnetism. Nickel nanowires are especially desirable for such an application as electronic percolation can be achieved without significantly increasing the weight of the composite material. In this work, single-crystal nickel nanowires possessing a homogeneous magnetic field are produced via a metal-organic precursor decomposition synthesis in solution. The nickel wires are 20 nm in width and 1-2 µm in length. The high anisotropy is attained through a combination of preferential crystal growth in the ⟨100⟩ direction and surfactant templating using hexadecylamine and stearic acid. The organic template ligands protect the nickel from oxidation, even after months of exposure to ambient conditions. These materials were studied using electron holography to characterize their magnetic properties. These thin nanowires display homogeneous ferromagnetism with a magnetic saturation (517 ± 80 emu cm-3), which is nearly equivalent to that of bulk nickel (557 emu cm-3). Nickel nanowires were incorporated into carbon composite test pieces and were shown to dramatically improve the electric discharge properties of the composite material.

Platinum tripods as nanometric frequency multiplexing devices.

Electrical and structural characterization studies of nano-particles are very important steps to determine their potential applications in microelectronics. In this paper, we address the crystallographic and electric transport properties of soft-chemistry-grown nanometric Pt tribranches. We report that Pt nanostars grown from the reduction of H2PtCl6 salt in pure oleylamine present a remarkable crystalline structure and deeply metallic character despite being grown under mild conditions. We demonstrate that such devices are able to operate at current densities surpassing 200 MA cm-2, actuating as highly compact frequency multiplexers in the non-ohmic regime.

Quantitative Nanoscale Magnetic Study of Isolated Diameter-Modulated FeCoCu Nanowires.

The comprehension of the magnetic configuration in FeCoCu nanowires with a diameter-modulated cylindrical geometry will allow controlling the domain wall motion in this low-dimensional system under the application of magnetic fields and/or the injection of current pulses. Here we perform a quantitative magnetic characterization of isolated diameter-modulated FeCoCu nanowires by combining nanoscale magnetic characterization techniques such as electron holography, magnetic force microscopy, and micromagnetic simulations. Local reconstructions of the magnetic distribution show the diameter-modulated geometry of the wires induces the formation of vortex-like structures and magnetic charges in the regions where the diameter is varied. Vortex-like structures modify the axial alignment of the magnetization in large-diameter segments. Moreover, the magnetic charges control the demagnetizing field distribution, promoting a flux-closure stray field configuration around large-diameter segments and keeping the demagnetizing field parallel to the NW's magnetization around small diameter segments. The detailed description of the remanent state in diameter-modulated cylindrical FeCoCu nanowires allows us to provide a clear explanation of the origin of bright and dark contrast observed in magnetic force microscopy images, which have the same feature of magnetic domain walls. This work establishes the primary knowledge required for future magnetization reversal studies with the aim of searching efficient modulated geometries that allow an optimum and controlled domain wall propagation.

Size-Specific Spin Configurations in Single Iron Nanomagnet: From Flower to Exotic Vortices.

The different spin configurations in the vicinity of the single-domain/vortex transition are reported in isolated magnetic nanoparticles. By combining chemical synthesis, electron holography in a dedicated transmission electron microscope and micromagnetic simulations, we establish the "magnetic configurations vs size" phase diagram of Fe single-crystalline nanocubes. Room temperature high resolution magnetic maps reveal the transition between single-domain and vortex states for Fe nanocubes from 25 to 27 nm, respectively. An intermediate spin configuration consisting of an ⟨111⟩ vortex is for the first time evidenced.

Air- and water-resistant noble metal coated ferromagnetic cobalt nanorods.

Cobalt nanorods possess ideal magnetic properties for applications requiring magnetically hard nanoparticles. However, their exploitation is undermined by their sensitivity toward oxygen and water, which deteriorates their magnetic properties. The development of a continuous metal shell inert to oxidation could render them stable, opening perspectives not only for already identified applications but also for uses in which contact with air and/or aqueous media is inevitable. However, the direct growth of a conformal noble metal shell on magnetic metals is a challenge. Here, we show that prior treatment of Co nanorods with a tin coordination compound is the crucial step that enables the subsequent growth of a continuous noble metal shell on their surface, rendering them air- and water-resistant, while conserving the monocrystallity, metallicity and the magnetic properties of the Co core. Thus, the as-synthesized core-shell ferromagnetic nanorods combine high magnetization and strong uniaxial magnetic anisotropy, even after exposure to air and water, and hold promise for successful implementation in in vitro biodiagnostics requiring probes of high magnetization and anisotropic shape.

Crystal growth of bullet-shaped magnetite in magnetotactic bacteria of the Nitrospirae phylum.

Magnetotactic bacteria (MTB) are known to produce single-domain magnetite or greigite crystals within intracellular membrane organelles and to navigate along the Earth's magnetic field lines. MTB have been suggested as being one of the most ancient biomineralizing metabolisms on the Earth and they represent a fundamental model of intracellular biomineralization. Moreover, the determination of their specific crystallographic signature (e.g. structure and morphology) is essential for palaeoenvironmental and ancient-life studies. Yet, the mechanisms of MTB biomineralization remain poorly understood, although this process has been extensively studied in several cultured MTB strains in the Proteobacteria phylum. Here, we show a comprehensive transmission electron microscopy (TEM) study of magnetic and structural properties down to atomic scales on bullet-shaped magnetites produced by the uncultured strain MYR-1 belonging to the Nitrospirae phylum, a deeply branching phylogenetic MTB group. We observed a multiple-step crystal growth of MYR-1 magnetite: initial isotropic growth forming cubo-octahedral particles (less than approx. 40 nm), subsequent anisotropic growth and a systematic final elongation along [001] direction. During the crystal growth, one major {111} face is well developed and preserved at the larger basal end of the crystal. The basal {111} face appears to be terminated by a tetrahedral-octahedral-mixed iron surface, suggesting dimensional advantages for binding protein(s), which may template the crystallization of magnetite. This study offers new insights for understanding magnetite biomineralization within the Nitrospirae phylum.

3D Magnetic Induction Maps of Nanoscale Materials Revealed by Electron Holographic Tomography.

The investigation of three-dimensional (3D) ferromagnetic nanoscale materials constitutes one of the key research areas of the current magnetism roadmap and carries great potential to impact areas such as data storage, sensing, and biomagnetism. The properties of such nanostructures are closely connected with their 3D magnetic nanostructure, making their determination highly valuable. Up to now, quantitative 3D maps providing both the internal magnetic and electric configuration of the same specimen with high spatial resolution are missing. Here, we demonstrate the quantitative 3D reconstruction of the dominant axial component of the magnetic induction and electrostatic potential within a cobalt nanowire (NW) of 100 nm in diameter with spatial resolution below 10 nm by applying electron holographic tomography. The tomogram was obtained using a dedicated TEM sample holder for acquisition, in combination with advanced alignment and tomographic reconstruction routines. The powerful approach presented here is widely applicable to a broad range of 3D magnetic nanostructures and may trigger the progress of novel spintronic nonplanar nanodevices.