Magnetoelectric Response of Laminated Cantilevers Comprising a Magnetoactive Elastomer and a Piezoelectric Polymer, in Pulsed Uniform Magnetic Fields.

The voltage response to pulsed uniform magnetic fields and the accompanying bending deformations of laminated cantilever structures are investigated experimentally in detail. The structures comprise a magnetoactive elastomer (MAE) slab and a commercially available piezoelectric polymer multilayer. The magnetic field is applied vertically and the laminated structures are customarily fixed in the horizontal plane or, alternatively, slightly tilted upwards or downwards. Six different MAE compositions incorporating three concentrations of carbonyl iron particles (70 wt%, 75 wt% and 80 wt%) and two elastomer matrices of different stiffness are used. The dependences of the generated voltage and the cantilever's deflection on the composition of the MAE layer and its thickness are obtained. The appearance of the voltage between the electrodes of a piezoelectric material upon application of a magnetic field is considered as a manifestation of the direct magnetoelectric (ME) effect in a composite laminated structure. The ME voltage response increases with the increasing total quantity of the soft-magnetic filler in the MAE layer. The relationship between the generated voltage and the cantilever's deflection is established. The highest observed peak voltage around 5.5 V is about 8.5-fold higher than previously reported values. The quasi-static ME voltage coefficient for this type of ME heterostructures is about 50 V/A in the magnetic field of ≈100 kA/m, obtained for the first time. The results could be useful for the development of magnetic field sensors and energy harvesting devices relying on these novel polymer composites.

Structure-Related Electronic and Magnetic Properties in Ultrathin Epitaxial NixFe3-xO4 Films on MgO(001).

Off-stoichiometric NixFe3-xO4 ultrathin films (x < 2.1) with varying Ni content x and thickness 16 (±2) nm were grown on MgO(001) by reactive molecular beam epitaxy. Synchrotron-based high-resolution X-ray diffraction measurements reveal vertical compressive strain for all films, resulting from a lateral pseudomorphic adaption of the film to the substrate lattice without any strain relaxation. Complete crystallinity with smooth interfaces and surfaces is obtained independent of the Ni content x. For x < 1 an expected successive conversion from Fe3O4 to NiFe2O4 is observed, whereas local transformation into NiO structures is observed for films with Ni content x > 1. However, angle-resolved hard X-ray photoelectron spectroscopy measurements indicate homogeneous cationic distributions without strictly separated phases independent of the Ni content, while X-ray absorption spectroscopy shows that also for x > 1, not all Fe2+ cations are substituted by Ni2+ cations. The ferrimagnetic behavior, as observed by superconducting quantum interference device magnetometry, is characterized by decreasing saturation magnetization due to the formation of antiferromagnetic NiO parts.

Real-Time Monitoring of Strain Accumulation and Relief during Epitaxy of Ultrathin Co Ferrite Films with Varied Co Content.

Ultrathin CoxFe3-xO4 films of high structural quality and with different Co content (x = 0.6-1.2) were prepared by reactive molecular beam epitaxy on MgO(001) substrates. Epitaxy of these ferrite films is extensively monitored by means of time-resolved (operando) X-ray diffraction recorded in out-of-plane geometry to characterize the temporal evolution of the film structure. The Co ferrite films show high crystalline ordering and smooth film interfaces independent of their Co content. All CoxFe3-xO4 films exhibit enhanced compressive out-of-plane strain during the early stages of growth, which partly releases with increasing film thickness. When the Co content of the ferrite films increases, the vertical-layer distances increase, accompanied by slightly increasing film roughnesses. The latter result is supported by surface-sensitive low-energy electron diffraction as well as X-ray reflectivity measurements on the final films. In contrast, the substrate-film interface roughness decreases with increasing Co content, which is confirmed with X-ray reflectivity measurements. In addition, the composition and electronic structure of the ferrite films is characterized by means of hard X-ray photoelectron spectroscopy performed after film growth. The experiments reveal the expected increasing Fe3+/Fe2+ cation ratios for a higher Co content.

Influence of Oxygen Plasma on the Growth and Stability of Epitaxial NiCo2O4 Ultrathin Films on Various Substrates.

In this work, we investigated the influence of oxygen plasma on the growth of nickel cobaltite (NiCo2O4) thin films compared to growth in a molecular oxygen atmosphere. The films were grown on MgO(001), MgAl2O4(001) and SrTiO3(001) substrates by oxygen plasma (atmosphere of activated oxygen)-assisted and reactive molecular beam epitaxy (molecular oxygen atmosphere). Soft X-ray photoelectron spectroscopy showed that only the use of oxygen plasma led to a spectrum characteristic of (NiCo2O4). Low energy electron diffraction measurements were conducted to obtain information on the structure of the film surfaces. The results proved the formation of a spinel surface structure for films grown with oxygen plasma, while the formation of a rock salt structure was observed for growth with molecular oxygen. To determine the film thickness, X-ray reflectivity measurements were performed. If oxygen plasma were used to grow (NiCo2O4) films, this would result in lower film thicknesses compared to growth using molecular oxygen although the cation flux was kept constant during deposition. Additional X-ray diffraction experiments delivered structural information about the bulk structure of the film. All films had a rock salt bulk structure after exposure to ambient conditions. Angle-resolved hard X-ray photoelectron spectroscopy revealed a homogeneous depth distribution of cations of the grown film, but no typical (NiCo2O4) spectrum anymore. Thus, on the one hand, (NiCo2O4) films with a spinel structure prepared using activated oxygen were not stable under ambient conditions. The structure of these films was transformed into NiCo oxide with a rock salt structure. On the other hand, it was not possible to form (NiCo2O4) films using molecular oxygen. These films had a rock salt structure that was stable under ambient conditions.

Real-Time Monitoring the Growth of Epitaxial CoxFe3-xO4 Ultrathin Films on Nb-Doped SrTiO3(001) via Reactive Molecular Beam Epitaxy by Means of Operando HAXPES.

In this work, we present a comprehensive study on real-time monitoring the growth of epitaxial CoxFe3-xO4 thin films grown on SrTiO3(001) substrates via reactive molecular beam epitaxy. The growth process was monitored during evaporation by means of time resolved operando hard X-ray photoelectron spectroscopy (HAXPES). We prepared ultrathin ferrite films using different oxygen partial pressures, showing pure metallic, light oxidic, and cobalt ferrite-like growth. Additional X-ray diffraction measurements confirm HAXPES results.

Reactive Additive Capillary Stamping with Double Network Hydrogel-Derived Aerogel Stamps under Solvothermal Conditions.

Integration of solvothermal reaction products into complex thin-layer architectures is frequently achieved by combinations of layer transfer and subtractive lithography, whereas direct additive substrate patterning with solvothermal reaction products has remained challenging. We report reactive additive capillary stamping under solvothermal conditions as a parallel contact-lithographic access to patterns of solvothermal reaction products in thin-layer configurations. To this end, corresponding precursor inks are infiltrated into mechanically robust mesoporous aerogel stamps derived from double-network hydrogels. The stamp is then brought into contact with a substrate to be patterned under solvothermal reaction conditions inside an autoclave. The precursor ink forms liquid bridges between the topographic surface pattern of the stamp and the substrate. Evaporation-driven enrichment of the precursors in these liquid bridges, along with their liquid-bridge-guided conversion into the solvothermal reaction products, yields large-area submicron patterns of the solvothermal reaction products replicating the stamp topography. For example, we prepared thin hybrid films, which contained ordered monolayers of superparamagnetic submicron nickel ferrite dots prepared by solvothermal capillary stamping surrounded by nickel electrodeposited in a second orthogonal substrate functionalization step. The submicron nickel ferrite dots acted as a magnetic hardener, halving the remanence of the ferromagnetic nickel layer. In this way, thin-layer electromechanical systems, transformers, and positioning systems may be customized.

Cationic Ordering and Its Influence on the Magnetic Properties of Co-Rich Cobalt Ferrite Thin Films Prepared by Reactive Solid Phase Epitaxy on Nb-Doped SrTiO3(001).

Here, we present the (element-specific) magnetic properties and cation ordering for ultrathin Co-rich cobalt ferrite films. Two Co-rich CoxFe3-xO4 films with different stoichiometry (x=1.1 and x=1.4) have been formed by reactive solid phase epitaxy due to post-deposition annealing from epitaxial CoO/Fe3O4 bilayers deposited before on Nb-doped SrTiO3(001). The electronic structure, stoichiometry and homogeneity of the cation distribution of the resulting cobalt ferrite films were verified by angle-resolved hard X-ray photoelectron spectroscopy. From X-ray magnetic circular dichroism measurements, the occupancies of the different sublattices were determined using charge-transfer multiplet calculations. For both ferrite films, a partially inverse spinel structure is found with increased amount of Co3+ cations in the low-spin state on octahedral sites for the Co1.4Fe1.6O4 film. These findings concur with the results obtained by superconducting quantum interference device measurements. Further, the latter measurements revealed the presence of an additional soft magnetic phase probably due to cobalt ferrite islands emerging from the surface, as suggested by atomic force microscope measurements.

Impact of Strain and Morphology on Magnetic Properties of Fe3O4/NiO Bilayers Grown on Nb:SrTiO3(001) and MgO(001).

We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe3O4/NiO bilayers grown on both MgO(001) and SrTiO3(001) substrates by reactive molecular beam epitaxy. These structures were investigated by means of X-ray photoelectron spectroscopy, low-energy electron diffraction, X-ray reflectivity and diffraction, as well as vibrating sample magnetometry. While the lattice mismatch of NiO grown on MgO(001) was only 0.8%, it was exposed to a lateral lattice mismatch of −6.9% if grown on SrTiO3. In the case of Fe3O4, the misfit strain on MgO(001) and SrTiO3(001) amounted to 0.3% and −7.5%, respectively. To clarify the relaxation process of the bilayer system, the film thicknesses of the magnetite and nickel oxide films were varied between 5 and 20 nm. While NiO films were well ordered on both substrates, Fe3O4 films grown on NiO/SrTiO3 exhibited a higher surface roughness as well as lower structural ordering compared to films grown on NiO/MgO. Further, NiO films grew pseudomorphic in the investigated thickness range on MgO substrates without any indication of relaxation, whereas on SrTiO3 the NiO films showed strong strain relaxation. Fe3O4 films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO3. The magnetite layers on both substrates showed a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in 100 directions. The coercive field was strongly enhanced for magnetite grown on NiO/SrTiO3 due to the higher density of structural defects, compared to magnetite grown on NiO/MgO.