Low Energy Cluster Beam Deposition Research Articles

Planar Hall Effect Modified by the Evolution of Magnetic Domain States

The origin of the planar Hall effect (PHE) decides an effective regulation of magnetic domain states on its performance. However, it has to suffer from the mutual interference between different kinds of domain states due to the narrow switching size range between multi-domain and single-domain states. Here, we fabricated nanostructured Ni0.8Fe0.2 alloy films assembled with the extremely narrow size distribution of clusters using the low-energy cluster beam deposition technique. It realizes an evolution from multi-domain and bi-domain to single-domain states by using its precise size-controllable characteristics. The PHE of films with the multi-domain state presents obvious hysteresis behavior, but it disappears for the single-domain state. The switching of the domain state also significantly affects the field- and angle-dependent magnitudes of the PHE. Such regulation behaviors are mainly originated from the effect of the pinning and depinning behaviors of domain walls on electron scattering. This work shows the regulation of the PHE by domain state switching in a powerful way, which is beneficial to promote its mechanism and application.

Chemical and magnetic order in mass-selected large FeRh nanomagnets embedded in a carbon matrix

In this paper, we present some specific chemical and magnetic order results obtained on bimetallic FeRh nanoparticles prepared under non-equilibrium conditions using mass-selected Low Energy Cluster Beam Deposition. Clusters around 7 nm incident diameter are in situ sandwiched between amorphous carbon films before transfer in air, with different surface coverage (from nearly isolated particles to percolating 2D films) in order to artificially increase diameters of the nanoparticles (NPs) by coalescence. On such FeRh@C samples, we observe different NPs morphologies and magnetic responses after UHV thermal treatments. By transmission electron microscopy (TEM), we show that after moderate annealing, nanoalloys evolved from a metastable structure towards the equilibrium CsCl-type (B2) chemically ordered phase with a more spherical shape. While from Superconducting Quantum Interference Device (SQUID) magnetometry measurements on 700 °C-annealed FeRh@C samples, we clearly evidence huge magnetization enhancement for such B2 nanoalloys with robust ferromagnetic (FM) signature even at very low temperature unlike their bulk counterparts.

Interplay between local structure and magnetic properties of graded exchange-coupled Co@FePt nanocomposite films

Magnetically-hard nanocomposites are attractive materials for integration in various microsys-tems and for building next generation permanent magnets. However, exploiting their full potential requires to control their microstructure at the nanometre scale. Studying these materials in model systems synthesised by nanofabrication routes gives interesting insights about the interplay between the microstructure and the magnetic performances. Here, by using a combination of mass-selected low energy cluster beam deposition and electron-beam evaporation, we have prepared nanocomposite films where Co nanoinclusions are integrated in a hard magnetic FePt matrix. Local atomic structures and element selective magnetic properties of such nanocomposites have been thoroughly investigated using polarisation dependent hard X-ray absorption spectroscopies. These results demonstrate that magnetically soft inclusions are stabilised at room temperature, emphasising the role of the interdiffusion in the preparation of nanocomposites.

Input of IBA for the study of plasmonic properties of doped ZnO nanocrystals

In this work, we used ion beam analyses (IBA) to characterize doped ZnO crystalline nanoparticles elaborated by different methods: (i) low energy cluster beam deposition (LECBD) for ZnO:Ga, (ii) metal organic framework (MOF) for ZnO:Li and (iii) sol-gel for ZnO/SiOx core-shell system.The analyses were performed either in RBS mode using 4He++ ions of 6 MeV energy (characterization of Ga and Si) or in NRA mode using 1H+ ions of 2.5 MeV energy (characterization of Li). The experimental data allowed determining the mean concentration of dopants (Ga, Li) or shell atoms (Si) and their areal masses. These elemental information, coupled with structural (TEM, X-ray) and optical (IR spectroscopy, photoluminescence) ones, were of prime importance permitting to better understand the optical properties of doped ZnO nanocrystals.

Switchable metal-insulator transition in core-shell cluster-assembled nanostructure films.

Fe/Fe3O4 core-shell-cluster-assembled nanostructured films were prepared using the low-energy cluster beam deposition technique. The temperature-dependent resistivity behaviors were investigated for the films with changing core-occupation ratio of clusters. Much interestingly and surprisingly, a switchable metal-insulator transition can be observed, featuring the rapid switching from the metal state to the insulation state and then back to the metal state, for films within a specific range of core-occupation ratio. Further, the resistivity change rate used to characterize the metal-insulator transition can reach as high as two orders of magnitude over a very narrow temperature region. The design of Fe/Fe3O4 core-shell clusters plays a decisive role in the mechanism of the switchable metal-insulator transition in these films. The assembled core-shell clusters in the films form current conduction channels that are switchable between the cores and shells of clusters as the temperature changes. The switching of the current conduction channels can be regulated by controlling the core-occupation ratios of clusters, which induce a switchable metal-insulator transition and can be verified by the effective medium theory over a specific core-occupation ratio range.

In-situ stress modulated ferroelectric photovoltaic effect in cluster-assembled TbFe2/Bi5Ti3FeO15 heterostructural films

TbFe2/Bi5Ti3FeO15 heterostructural films were prepared by inserting cluster-assembled TbFe2 microdiscs into a Bi5Ti3FeO15 matrix using low energy cluster beam deposition combined with sol-gel methods. The phase structure, ferroelectric properties, bandgap, photovoltaic spectral response, and performances of the ferroelectric photovoltaic effect were modulated by the in situ stress driven by magnetostriction of TbFe2 clusters under external magnetic fields. The short-circuit current, open-circuit voltage, and power conversation efficient increase with the in situ stress, reaching 0.026 mA/cm2, 9.5 V, and 5.88 × 10−2%, respectively, under a maximum in-stress of 0.075 GPa. So the high open-circuit voltage above bandgap is attributed to the distinct bandgap shifting and the effective separation of photogenerated electron-hole pairs derived from the in situ stress induced large built-in field. The in situ stress dominated symmetry breaking contributes to the improvement of the power conversation coefficient. The in situ dynamic internal stress provides a high efficient approach to modulate and improve ferroelectric photovoltaic effects.

Clusters Deposition on Surface an Atomic Scale Study by Computer Simulation Method

The investigation is generalized to clusters with sizes up to 3000 atoms, covering this way the range of sizes experimentally available for low energy cluster beam deposition. The atomic scale modeling is carried on by both Molecular Dynamics and Metropolis Monte Carlo. This represents a huge series of simulations (175 cases) to which further calculations are added by spot when finer tuning of the parameters is necessary. Analyzing the results is a major task which is still in progress. This way, not only a realistic range of sizes is covered, but also the whole range of compositions and the temperature range relevant to the solid and the liquid states.

Bottom-up strategies for the assembling of magnetic systems using nanoclusters

In the frame of the 20th Anniversary of the Journal of Nanoparticle Research (JNR), our aim is to start from the historical context twenty, years ago and to give some recent results and perspectives concerning nanomagnets prepared from clusters preformed in the gas phase using the Low Energy Cluster Beam Deposition (LECBD) technique. In this paper, we focus our attention on the typical case of Co clusters embedded in various matrices to study interface magnetic anisotropy and magnetic interactions as a function of volume concentrations, and on still current and perspectives through two examples of binary metallic 3d-5d TM (namely CoPt and FeAu) clusters assemblies to illustrate size-related and nanoalloy phenomena on magnetic properties in well-defined mass-selected clusters. The structural and magnetic properties of these cluster assemblies were investigated using various experimental techniques that include High Resolution Transmission Electron Microscopy (HRTEM), Superconducting Quantum Interference Device (SQUID) magnetometry, as well as synchrotron techniques such as Extended X-Ray Absorption Fine Structure (EXAFS) and X-Ray Magnetic Circular Dichroism (XMCD). Depending on the chemical nature of both NPs and matrix, we observe different magnetic responses compared to their bulk counterparts. In particular, we show how finite size effects (size reduction) enhance their magnetic moment and how specific relaxation in nanoalloys can impact their magnetic anisotropy.

Giant magnetoelectric effects in pseudo 1-3 heterostructure films with FeGa nanocluster-assembled micron-scale discs embedded into Bi5Ti3FeO15 matrices.

Nanocluster-assembled FeGa micron-scale discs prepared by low-energy cluster beam deposition were embedded into Bi5Ti3FeO15 matrices to form pseudo 1-3 heterostructure films. The microstructures and multiferroic properties were investigated. Good ferroelectric, piezoelectric, and ferromagnetic properties and giant magnetoelectric effects are achieved for the heterostructure films, which is ascribed to the depression of the clamped effect from the hard substrate for such pseudo 1-3 structures and the multi-interface coupling between the large magnetostrictive coefficient of FeGa micron-scale discs and the high piezoelectric coefficient of the circle surrounding the Bi5Ti3FeO15 matrices. Such a strong interface strain coupling between the micron discs and the inhomogeneously multiferroic matrices induces obvious magnetoelectric coupling behaviors. Besides, the interaction between the ferroelectric domains and the spin orders in the multiferroic matrices also contributes to the magnetoelectric output. The present work provides a potential way to fabricate clamped-free magnetoelectric films for microdevice applications.

Enhanced magnetostriction derived from magnetic single domain structures in cluster-assembled SmCo films

Cluster-assembled SmCo alloy films were prepared by low energy cluster beam deposition. The structure, magnetic domain, magnetization, and magnetostriction of the films were characterized. It is shown that the as-prepared films are assembled in compact and uniformly distributed spherical cluster nanoparticles, most of which, after vacuum in situ annealing at 700 K, aggregated to form cluster islands. These cluster islands result in transformations from superparamagnetic states to magnetic single domain (MSD) states in the films. Such MSD structures contribute to the enhanced magnetostrictive behaviors with a saturation magnetostrictive coefficient of 160 × 10−6 in comparison to 105 × 10−6 for the as-prepared films. This work demonstrates candidate materials that could be applied in nano-electro-mechanical systems, low power information storage, and weak magnetic detecting devices.

Structure and Magnetic Properties of FeCo Clusters: Carbon Environment and Annealing Effects

In this article, we study the thermally activated chemical order transition in FeCo nanoalloys prepared by low energy clusters beam deposition (LECBD). Due to the low Z-contrast (ΔZ = 1) between Fe and Co atoms, a particular strategy based on extended X-ray absorption edge fine structure (EXAFS) experiments and simulations at both Fe and Co K-edges has been developed as well as accurate magnetic characterizations on as-prepared and annealed FeCo clusters embedded in a carbon matrix. We show that upon annealing crystallographic order and magnetic properties are improved, but we are not able to detect without ambiguity the B2 (CsCl-type) chemical order as expected in bulk-like phase, due to competition between stable iron carbide formation and cobalt–carbon demixing in bimetallic nanoparticles.

Cubic chemically ordered FeRh and FeCo nanomagnets prepared by mass-selected low-energy cluster-beam deposition: a comparative study.

Near the point of equiatomic composition, both FeRh and FeCo bulk alloys exhibit a CsCl-type (B2) chemically ordered phase that is related to specific magnetic properties, namely a metamagnetic anti-ferromagnetic/ferromagnetic transition near room temperature for FeRh and a huge magnetic moment for the FeCo soft alloy. In this paper, we present the magnetic and structural properties of nanoparticles of less than 5 nm diameter embedded in an inert carbon matrix prepared by mass-selected low-energy cluster-beam deposition technique. We obtained a CsCl-type (B2) chemically ordered phase for annealed nanoalloys. Using different experimental measurements, we show how decreasing the size affects the magnetic properties. FeRh nanoparticles keep the ferromagnetic order at low temperature due to surface relaxation affecting the cell parameter. In the case of FeCo clusters, the environment drastically affects the intrinsic properties of this system by reducing the magnetization in comparison to the bulk.

Exchange bias of nanostructured films assembled with Co/CoO core–shell clusters

Nanostructured films assembled with Co/CoO core–shell clusters were prepared by low energy cluster beam deposition. The structure and magnetism of the films were investigated. It is shown that core–shell clusters are detected with the average diameter of 6.3nm. And films assembled with clusters present exchange bias effect, which is attributed to the exchange interaction at the interface of Co and CoO layers. The present nanostructured films assembled with Co/CoO core–shell clusters promote its applications in ultrahigh-density magnetic recording, giant magnetoresistance, and spin-electronic devices.

Temperature-Dependent Evolution of the Oxidation States of Cobalt and Platinum in Co1–xPtx Clusters under H2 and CO + H2 Atmospheres

Co1–xPtx clusters of 2.9-nm size with a range of atomically precise Pt/Co atomic ratios (x = 0, 0.25, 0.5, 0.75, 1) were synthesized using the mass-selected low-energy cluster beam deposition (LECBD) technique and soft-landed onto an amorphous alumina thin film prepared by atomic layer deposition (ALD). Utilizing ex situ X-ray photoemission spectroscopy (XPS), the oxidation state of the as-made clusters supported on Al2O3 was determined after both a 1-h-long exposure to air and aging for several weeks while exposed to air. Next, the aged cluster samples were characterized by grazing-incidence X-ray absorption spectroscopy (GIXAS) and then pretreated with diluted hydrogen and further exposed to the mixture of diluted CO and H2 up to 225 °C at atmospheric pressure, and the temperature-dependent evolutions of the particle size/shape and the oxidation states of the individual metal components within the clusters were monitored using in situ grazing-incidence small-angle X-ray scattering and X-ray absorption s...

Mechanism of phase transition, from vapor to solid: Transient liquid phase is between the two

The mechanism of phase transition, from vapor to solid, is studied by producing non-stoichiometric ZnO and CdS nanoclusters (NCs) by low-energy cluster beam deposition technique, and examining their morphological and compositional evolution over a long span of time. It is concluded that the transition of vapor to solid goes through a transient liquid phase: coagulation of a large number of atomic clusters first forms liquid NCs which then solidify. The nature of the material and the experimental conditions determine crystallinity and shape of the NCs during the solidification process.

Enhanced ferromagnetism of cluster-assembled BiFeO3 nanostructured films

Cluster-assembled BiFeO3 nanostructured films were prepared using low-energy cluster beam deposition method with our homemade cluster apparatus. It is shown that the nanostructured films are perovskite structure and assembled uniformly and compactly with monodisperse spherical clusters with average diameter of ~22nm. The enhanced ferromagnetism is observed for the as-prepared films, which is attributed to the uncompensated surface spins and the release of the latent magnetization locked within the cycloid induced by the size effect of the clusters with the smaller characteristic size than the long-range cycloid order of 62nm. The present work provides a way on enhancing ferromagnetism of single-phase multiferroics.

Mixing Patterns and Redox Properties of Iron-Based Alloy Nanoparticles under Oxidation and Reduction Conditions

The redox behavior of 5 nm Fe-Me alloyed nanoparticles (where Me = Pt, Au, and Rh) was investigated in situ under H2 and O2 atmospheres by near ambient pressure X-ray photoelectron and absorption spectroscopies (NAP-XPS, XAS), together with ex situ transmission electron microscopy (TEM) and XAS spectra simulations. The preparation of well-defined Fe-Me nanoalloys with an initial size of 5 nm was achieved by using the mass-selected low energy cluster beam deposition (LECBD) technique. The spectroscopic methods permit the direct observation of the surface segregation and composition under different gas atmospheres and annealing temperatures. The ambient conditions were found to have a significant influence on the mixing pattern and oxidation state of the nanoparticles. In an oxidative atmosphere, iron oxidizes and segregates to the surface, leading to the formation of core–shell nanoparticles. This structure persists upon mild reduction conditions, while phase separation and formation of heterostructured bi...

Mechanical properties of C58 materials and their dependence on thermal treatment

C58 fullerene cages made by electron-impact induced fragmentation of C60 fullerenes have been assembled into several micron thick solid films by low energy cluster beam deposition onto inert substrates held at room temperature under ultrahigh vacuum. The resulting as-prepared material, RT-C58, behaves as an amorphous wide-band semiconductor. Nanoindentation was used to measure its mechanical properties revealing that RT-C58 has a higher elastic modulus E and hardness H than the reference carbon allotropes solid C60 and Highly Ordered Pyrolytic Graphite (HOPG): E(RT-C58)=14GPa and H(RT-C58)=1.2GPa. This effect can be explained by the unique intrinsic “functionalization” of C58 cages: they comprise reactive surface sites constituted by annelated pentagon rings which give rise to covalently stabilized oligomers, –C58–C58–C58, under our deposition conditions. Annealing, thick RT-C58 films up to 1100K in ultrahigh vacuum results in HT-C58, a new material with considerably modified electronic and vibrational properties compared to the as-prepared RT-C58 film. The associated molecular transformations, including also partial cage–cage coalescence reactions, raise the overall mechanical hardness of the material: H(HT-C58)=3.9GPa.

Investigation of luminescent properties of ZnO nanoparticles for their use as a down‐shifting layer on solar cells

AbstractCommercially available solar cells (e.g. CdS/CdTe, CIGS, c‐Si) have a spectrally narrow absorption band when compared to the solar emission spectrum. The UV‐wavelength response of a solar cell (SC) can be improved by the application of a luminescent down‐shifting layer (LDSL) on the SC front side, permitting the conversion of short‐wavelength photons to the longer wavelength photons better matching the SC’s absorption spectrum. The ideal down‐shifting material must possess a large Stokes shift and have a high luminescence quantum yield. We propose the use of the LDSL containing ZnO nanoparticles of less than 5 nm in diameter able to absorb UV light (λ < 400 nm), where the solar cell spectral response (SR) is low, and re‐emitting at longer wavelengths (λ > 425 nm), where the typical SC’s SR increases. ZnO nanoparticles were synthesized by a low energy cluster beam deposition (LECBD) technique and their luminescent properties were studied as a function of the oxygen partial pressure (OPP) applied during the deposition process. The stoichiometry and crystallinity of ZnO nanoparticles can be controlled via the adjustment of the OPP. It was also observed that there exists an optimal value of the oxygen pressure introduced during the LECBD process, which permits to obtain the highest visible photoluminescence emission, necessary for an efficient down‐shifting. The yield of the down‐shifting in ZnO nanoparticle layer was determined varying the excitation wavelength using the photoluminescence excitation technique.

Oxidation mechanism in metal nanoclusters: Zn nanoclusters to ZnO hollow nanoclusters

Zn nanoclusters (NCs) are deposited by the low-energy cluster beam deposition technique. The mechanism of oxidation is studied by analysing their compositional and morphological evolution over a long span of time (three years) due to exposure to ambient atmosphere. It is concluded that the mechanism proceeds in two steps. In the first step, the shell of ZnO forms over Zn NCs rapidly up to a certain limiting thickness: within a few days—depending upon the size—Zn NCs are converted to Zn–ZnO (core–shell), Zn–void–ZnO, or hollow ZnO type NCs. Bigger than ∼15 nm become Zn–ZnO (core–shell) type: among them, NCs above ∼25 nm could retain their initial geometrical shapes (namely triangular, hexagonal, rectangular and rhombohedral), but ∼25 to 15 nm size NCs become irregular or distorted geometrical shapes. NCs between ∼15 to 5 nm become Zn–void–ZnO type, and smaller than ∼5 nm become ZnO hollow sphere type, i.e. ZnO hollow NCs. In the second step, all Zn–void–ZnO and Zn–ZnO (core–shell) structures are converted to hollow ZnO NCs in a slow and gradual process, and the mechanism of conversion proceeds through expansion in size by incorporating ZnO monomers inside the shell. The observed oxidation behaviour of NCs is compared with theory of Cabrera–Mott on low-temperature oxidation of metal.