[Show abstract][Hide abstract] ABSTRACT: Cylindrical Fe28Co67Cu5 nanowires modulated in diameter between 22 and 35 nm are synthesized by electroplating into the nanopores of alumina membranes. High-sensitivity MFM imaging (with a detection noise of 1 μN m(-1)) reveals the presence of single-domain structures in remanence with strong contrast at the ends of the nanowires, as well as at the transition regions where the diameter is modulated. Micromagnetic simulations suggest that curling of the magnetization takes place at these transition sites, extending over 10-20 nm and giving rise to stray fields measurable with our MFM. An additional weaker contrast is imaged, which is interpreted to arise from inhomogeneities in the nanowire diameter.
[Show abstract][Hide abstract] ABSTRACT: 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. 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.
Chemistry of Materials 09/2015; DOI:10.1021/acs.chemmater.5b02723 · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Structural and chemical properties of InAs/AlSb interfaces have been studied by transmission electron microscopy. InAs/AlSb multilayers were grown by molecular beam epitaxy with different growth sequences at interfaces. The out-of-plane strain, determined using high resolution microscopy and geometrical phase analysis, has been related to the chemical composition of the interfaces analyzed by high angle annular dark field imaging. Considering the local strain and chemistry, we estimated the interface composition and discussed the mechanisms of interface formation for the different growth sequences. In particular, we found that the formation of the tensile AlAs-type interface is spontaneously favored due to its high thermal stability compared to the InSb-type interface. We also showed that the interface composition could be tuned using an appropriate growth sequence.
[Show abstract][Hide abstract] ABSTRACT: Epitaxial films of SrMnO3 and bilayers of SrMnO3/La0.67Sr0.33MnO3 have been deposited by pulsed laser deposition on different substrates, namely, LaAlO3 (001), (LaAlO3)0.3(Sr2AlTaO6)0.7 (001), and SrTiO3 (001), allowing us to perform an exhaustive study of the dependence of antiferromagnetic order and exchange bias field on epitaxial strain. The Néel temperatures (TN) of the SrMnO3 films have been determined by low-energy muon spin spectroscopy. In agreement with theoretical predictions, TN is reduced as the epitaxial strain increases. From the comparison with first-principles calculations, a crossover from G-type to C-type antiferromagnetic orders is proposed at a critical tensile strain of around 1.6±0.1%. The exchange bias (coercive) field, obtained for the bilayers, increases (decreases) by increasing the epitaxial strain in the SrMnO3 layer, following an exponential dependence with temperature. Our experimental results can be explained by the existence of a spin-glass (SG) state at the interface between the SrMnO3 and La0.67Sr0.33MnO3 films. This SG state is due to the competition between the different exchange interactions present in the bilayer and favored by increasing the strain in the SrMnO3 layer.
Physical Review B 07/2015; 92(2):024419. DOI:10.1103/PhysRevB.92.024419 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Iron nanostructures grown by focused electron beam induced deposition (FEBID) are promising for applications in magnetic sensing, storage and logic. Such applications require a precise design and determination of the coercive field (H C), which depends on the shape of the nanostructure. In the present work, we have used the Fe2(CO)9 precursor to grow iron nanowires by FEBID in the thickness range from 10 to 45 nm and width range from 50 to 500 nm. These nanowires exhibit an Fe content between 80 and 85%, thus giving a high ferromagnetic signal. Magneto-optical Kerr characterization indicates that H C decreases for increasing thickness and width, providing a route to control the magnetization reversal field through the modification of the nanowire dimensions. Transmission electron microscopy experiments indicate that these wires have a bell-type shape with a surface oxide layer of about 5 nm. Such features are decisive in the actual value of H C as micromagnetic simulations demonstrate. These results will help to make appropriate designs of magnetic nanowires grown by FEBID.
[Show abstract][Hide abstract] ABSTRACT: Local perturbations in complex oxides, such as domain walls, strain and defects, are of interest because they can modify the conduction or the dielectric and magnetic response, and can even promote phase transitions. Here, we show that the interaction between different types of local perturbations in oxide thin films is an additional source of functionality. Taking SrMnO3 as a model system, we use nonlinear optics to verify the theoretical prediction that strain induces a polar phase, and apply density functional theory to show that strain simultaneously increases the concentration of oxygen vacancies. These vacancies couple to the polar domain walls, where they establish an electrostatic barrier to electron migration. The result is a state with locally structured room-temperature conductivity consisting of conducting nanosized polar domains encased by insulating domain boundaries, which we resolve using scanning probe microscopy. Our 'nanocapacitor' domains can be individually charged, suggesting stable capacitance nanobits with a potential for information storage technology.
[Show abstract][Hide abstract] ABSTRACT: Crystal structure and hydrogenation properties of Gd2Co7-type Y2Ni7 were investigated by X-ray diffraction (XRD), Sievert's method and neutron powder diffraction. Unlike Nd2Ni7 and La2Ni7, Y2Ni7 exhibits three plateau pressures and forms three hydrides at room temperature. The maximum hydrogen capacity reaches 8.9 H/f.u at 10 MPa at the first absorption cycle. The compound can be cycled by solid-gas route and structural properties are fully recovered after dehydrogenation. For the first time, structural properties of rhombohedral A2B7Dx deuterides were studied. They were investigated by neutron powder diffraction at three different states of charge (x=2.1, 4.1 and 8.8). Deuterium positions as well as variation of unit cell parameters are given and discussed in this paper.
The Journal of Physical Chemistry C 05/2015; 119(22):150512115958003. DOI:10.1021/acs.jpcc.5b03096 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report ab initio calculations and thermoelectric power experiments in
e-doped thin films of SrTiO$_3$ (STO) which demonstrate that the electronic
band degeneracy can be lifted through defect management during growth. We show
that even small amounts of cationic vacancies, combined with epitaxial stress,
produce a homogeneous tetragonal distortion of the films, resulting in a
Kondo-like resistance upturn at low temperature, large anisotropic
magnetoresistance, and non-linear Hall effect. This phenomenology, previously
observed in LaAlO$_3$/STO interfaces and magnetic STO quantum wells, is shown
here to be intrinsic to tetragonally distorted e-doped STO thin films.
[Show abstract][Hide abstract] ABSTRACT: A systematic study of the iron–silicon interfaces formed upon preparation of (Fe/Si) multilayers has been performed by combination of modern and powerful techniques. Samples were prepared by thermal evaporation under ultrahigh vacuum onto a Si(1 0 0) substrate. The morphology of these films and their interfaces was studied by a combination of scanning transmission electron microscopy, X-ray reflectivity, angle resolved X-ray photoelectron spectroscopy and hard X-ray photoelectron spectroscopy. The Si-on-Fe interface thickness and roughness were determined to be 1.4(1) nm and 0.6(1) nm, respectively. Moreover, determination of the stable phases formed at both Fe-on-Si and Si-on-Fe interfaces was performed using conversion electron Mössbauer spectroscopy on multilayers with well separated Si-on-Fe and Fe-on-Si interfaces. It is shown that while a fraction of Fe remains as α-Fe, the rest has reacted with Si, forming the paramagnetic c-Fe1−xSi phase and a ferromagnetic Fe rich silicide (DO3 type phase). We conclude that the paramagnetic c-Fe1−xSi silicide sublayer is identical in both Si-on-Fe and Fe-on-Si interfaces, whereas an asymmetry is revealed in the composition of the ferromagnetic silicide sublayer.
Journal of Alloys and Compounds 04/2015; 627. DOI:10.1016/j.jallcom.2014.12.019 · 3.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work the position-controlled growth of GaN nanowires (NWs) on diamond by means of molecular beam epitaxy is investigated. In terms of growth, diamond can be seen as a model substrate, providing information of systematic relevance also for other substrates. Thin Ti masks are structured by electron beam lithography which allows the fabrication of perfectly homogeneous GaN NW arrays with different diameters and distances. While the wurtzite NWs are found to be Ga-polar, N-polar nucleation leads to the formation of tripod structures with a zinc-blende core which can be efficiently suppressed above a substrate temperature of 870°C. A variation of the III/V flux ratio reveals that both axial and radial growth rates are N-limited despite the globally N-rich growth conditions, which is explained by the different diffusion behavior of Ga and N atoms. Furthermore, it is shown that the hole arrangement has no effect on the selectivity, but can be used to force a transition from nanowire to nanotube growth by employing a highly competitive growth regime.
[Show abstract][Hide abstract] ABSTRACT: We report on the synthesis and optical gain properties of regularly shaped lead iodide (PbI2) platelets with thickness ranging from 10 to 500 nm using chemical vapor deposition (CVD). The as-prepared single crystalline platelets exhibit a near band edge emission of ~500 nm. Whispering gallery mode (WGM) lasing from individual hexagonal shaped PbI2 platelets is demonstrated in the temperature range from 77 to 210 K - where the lasing modes are supported by platelets as thin as 45 nm. The FDTD simulation and the edge-length dependent threshold confirm the planar WGM lasing mechanism in such hexagonal shaped PbI2 platelet. Through a comprehensive study of power-dependent photoluminescence (PL) and time-resolved PL spectroscopy, we ascribe the WGM lasing to be biexcitonic in nature. Moreover, for different thickness of platelet, the lowest lasing threshold occurs in platelets of ~120 nm, attributing to the formation of a good Fabry-Pérot resonance cavity in the vertical direction between the top and bottom platelet surfaces that enhance the reflection. Our present study demonstrates the feasibility of planar light sources based on layered semiconductor materials and their thickness dependent threshold characteristic is beneficial for the optimization of layered material based optoelectronic devices.
[Show abstract][Hide abstract] ABSTRACT: We report on magnetotransport properties on La0.7Sr0.3MnO3/MgO/Fe tunnel junctions grown epitaxially on
top of (001)-oriented SrTiO3 substrates by sputtering. It is shown that the magnetoresistive response depends
critically on the MgO/Fe interfacial properties. The appearance of an FeOX layer by the interface destroys the
�1 symmetry filtering effect of the MgO/Fe system and only a small negative tunneling magnetoresistance
(TMR) (∼ −3%) is measured. However, in annealed samples a switchover from positive TMR (∼ +25% at
70 K) to negative TMR (∼ −1%) is observed around 120 K. This change is associated with the transition from
semiconducting at high T to insulating at low T taking place at the Verwey transition (TV ∼ 120 K) in Fe3O4, thus suggesting the formation of a very thin slab of magnetite at the MgO/Fe interface during annealing treatments. These results highlight the relevance of interfacial properties on the tunneling conduction process and how it can be substantially modified through appropriate interface engineering.
[Show abstract][Hide abstract] ABSTRACT: To explore the possible occurrence of new stacking sequences within the La–Mg–Ni system, two alloys in the composition range La15.6–16.8Mg6.3–7.8Ni76.5–76.9 (at. %) have been synthesized by Spark Plasma Sintering (SPS). The X-ray diffraction analyses show that the obtained alloys contain a mixture of A2Ni7 and A5Ni19 phases (A = La, Mg) with coexistence of hexagonal and rhombohedral polymorphs. Such structures can be described as a stacking along the c crystallographic axis of mAB5 units (labeled C) and nA2B4 units (labeled L). Up to now, compounds with 1 ≤ m ≤ 4, but only n = 1 were confirmed in the La–Mg–Ni system. One composition has been particularly studied here by High-Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM). A continuous change of the stacking sequences (from C + L to 4C + L) is observed within a 50 nm region. In addition, two types of stacking faults (twinning and dislocation) have been revealed. The expected stacking sequence with n > 1 has not been observed in the present work. The hydrogen storage capacity and the electrochemical capacity are not significantly affected by the phase compositions, while the cyclic stability appears particularly sensitive to the Mg content.
The Journal of Physical Chemistry C 12/2014; 118(48):27808-27814. DOI:10.1021/jp510313a · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Epitaxial strain alters the physical properties of thin films grown on single crystal substrates. Thin film oxides are particularly apt for strain engineering new functionalities in ferroic materials. In the case of La2/3Ca1/3MnO3 (LCMO) thin films, here we show the first experimental images obtained by electron holography demonstrating that epitaxial strain induces the segregation of a flat and uniform non-ferromagnetic layer with antiferromagnetic (AFM) character at the top surface of a ferromagnetic (FM) layer, the whole film being chemical and structurally homogeneous at room temperature. For different substrates and growth conditions the tetragonality of LCMO at room temperature, defined as τ = |c-a|/a, is the driving force for a phase coexistence above an approximate critical value of τC ~ 0.024. Theoretical calculations prove that the increased tetragonality changes the energy balance of the FM and AFM ground states in strained LCMO, enabling the formation of magnetically inhomogeneous states. This work gives the key evidences that open a new route to synthesize strain-induced exchanged-biased FM-AFM bilayers in single thin films, which could serve as building blocks of future spintronic devices.
[Show abstract][Hide abstract] ABSTRACT: A series of [FeCoCu/Cu(x)]10 (7 ≤ x ≤ 40 nm with FeCoCu layer thickness of 300 nm) and [FeCoCu(y)/Cu]10 (120 ≤ y ≤ 900 nm with Cu layer thickness of 15 nm) arrays of multilayer nanowires, 35 nm in diameter, were fabricated by electrodeposition into self-assembled pores of anodic alumina membranes. High-resolution transmission electron microscopy and X-ray diffraction analysis confirm the segregation of layered structures, with well-defined Cu layers (fcc cubic structure) separating FeCoCu-alloy segments (bcc cubic structure). Hysteresis loop measurements indicate an overall magnetization easy axis parallel to the nanowires in all the samples. For constant FeCoCu segment length, the coercivity, the remanence, and especially, the susceptibility increase with the Cu layer thickness, whereas for the series with constant Cu layer thickness, the susceptibility significantly decreases with FeCoCu segment length. ComplementaryHenkel curves indicate that the net inter/intrananowires magnetostatic interactions always contribute to the demagnetization of the nanowires. The variation of the susceptibility with FeCoCu and Cu layers thickness together with the Henkel plots data indicate that a reduced demagnetizing effect is achieved for multilayer nanowires with the thicker Cu layer and the shorter FeCoCu segment, for which a moderated reduction in saturation magnetization of around 11% is estimated compared to a continuous FeCoCu alloy nanowire array.
[Show abstract][Hide abstract] ABSTRACT: Progress in nanotechnology requires new approaches to materials synthesis that make it possible to control material functionality down to the smallest scales. An objective of materials research is to achieve enhanced control over the physical properties of materials such as ferromagnets, ferroelectrics and superconductors. In this context, complex oxides and inorganic perovskites are attractive because slight adjustments of their atomic structures can produce large physical responses and result in multiple functionalities. In addition, these materials often contain ferroelastic domains. The intrinsic symmetry breaking that takes place at the domain walls can induce properties absent from the domains themselves, such as magnetic or ferroelectric order and other functionalities, as well as coupling between them. Moreover, large domain wall densities create intense strain gradients, which can also affect the material's properties. Here we show that, owing to large local stresses, domain walls can promote the formation of unusual phases. In this sense, the domain walls can function as nanoscale chemical reactors. We synthesize a two-dimensional ferromagnetic phase at the domain walls of the orthorhombic perovskite terbium manganite (TbMnO3), which was grown in thin layers under epitaxial strain on strontium titanate (SrTiO3) substrates. This phase is yet to be created by standard chemical routes. The density of the two-dimensional sheets can be tuned by changing the film thickness or the substrate lattice parameter (that is, the epitaxial strain), and the distance between sheets can be made as small as 5 nanometres in ultrathin films, such that the new phase at domain walls represents up to 25 per cent of the film volume. The general concept of using domain walls of epitaxial oxides to promote the formation of unusual phases may be applicable to other materials systems, thus giving access to new classes of nanoscale materials for applications in nanoelectronics and spintronics.
[Show abstract][Hide abstract] ABSTRACT: We report magnetic and electronic transport measurements across epitaxial bilayers of ferromagnetic insulator LaCoO3 and half-metallic ferromagnet La2/3Sr1/3MnO3 (LCO/LSMO: 3.5 nm / 20 nm) fabricated by a chemical solution method. The I/V curves at room temperature and 4K measured with Conducting Atomic Force Microscopy (CAFM) on well defined patterned areas exhibit the typical features of a tunneling process. The curves have been fitted to the Simmons model to determine the height (φ) and width (s) of he insulating LCO barrier. The results show a φ=0.40 ± 0.05 eV (0.50 ± 0.01 eV) at room temperature (4K) and a s= 3 nm, in good agreement with the structural analysis. Our results demonstrate that this chemical method is able to produce epitaxial heterostructures with the quality required for this type of fundamental studies and applications.
[Show abstract][Hide abstract] ABSTRACT: Combination of mismatched materials in semiconductor nanowire heterostructures offers a freedom of bandstructure engineering that is impossible in standard planar epitaxy. Nevertheless, the presence of strain and/or structural defects directly control the optoelectronic properties of these nanomaterials. Understanding with atomic accuracy how mismatched heterostructures release or accommodate strain is therefore highly desirable. By using atomic resolution high angle annular dark field scanning transmission electron microscopy combined with geometrical phase analyses and computer simulations we are able to establish the relaxation mechanism (including both elastic and plastic deformations) involved in order to release the mismatch strain in axial nanowire heterostructures. Formation of misfit dislocations, diffusion of atomic species, polarity transfer and induced structural transformations are studied at atomic resolution at the intermediate ternary interfaces. Two nanowire heterostructure systems with promising applications (InAs/InSb and GaAs/GaSb) have been selected as key examples.