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.

We present time-resolved high-energy x-ray diffraction, time-resolved hard-x-ray photoelectron spectroscopy (tr-HAXPES), and time-resolved grazing incidence small-angle x-ray scattering (tr-GISAXS) data of the reactive molecular beam epitaxy of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ ultrathin films on various substrates. Reciprocal space maps are recorded during the deposition of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ on ${\mathrm{SrTiO}}_3\left(001\right)$, MgO(001), and NiO/MgO(001) in order to observe the thickness-dependent evolution of Bragg reflections sensitive to the octahedral and tetrahedral sublattices of the inverse spinel structure of ${\mathrm{Fe}}_3{\mathrm{O}}_4$. Rock salt and spinel-exclusive reflections appear at different thicknesses, revealing that first, the iron oxide film grows with ${\mathrm{Fe}}_{1-\delta }\mathrm{O}$ rock salt structure with exclusive occupation of octahedral lattice sites. After reaching a film thickness of $1.1\mathrm{nm}$, further growth of the iron oxide film proceeds in the inverse spinel structure, with both octahedral and tetrahedral lattice sites being occupied. In addition, iron oxide on ${\mathrm{SrTiO}}_3\left(001\right)$ initially grows with none of these structures. Here, the formation of the rock salt structure starts when reaching a film thickness of $1.5\mathrm{nm}$. This is confirmed by tr-HAXPES data obtained during growth of iron oxide on ${\mathrm{SrTiO}}_3\left(001\right)$, which demonstrate an excess of ${\mathrm{Fe}}^{2+}$ cations compared to ${\mathrm{Fe}}_3{\mathrm{O}}_4$ in growing films thinner than $3.2\mathrm{nm}$. This rock salt phase only appears during growth and vanishes after the supply of the Fe molecular beam is stopped. Thus, it can be concluded that the rock salt structure of the interlayer is a property of the dynamic growth process while the film is still exposed to oxygen. The tr-GISAXS data link these structural results to an island growth mode of the first 2–3 nm on both MgO(001) and ${\mathrm{SrTiO}}_3\left(001\right)$ substrates.

Off-stoichiometric Ni_xFe_3−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 ${\mathrm{Fe}}^{2+}$ cations are substituted by ${\mathrm{Ni}}^{2+}$ 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.

Here, we present the (element-specific) magnetic properties and cation ordering for ultrathin Co-rich cobalt ferrite films. Two Co-rich ${\mathrm{Co}}_x{\mathrm{Fe}}_{3-x}{\mathrm{O}}_4$ 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/Fe${}_3$O${}_4$ bilayers deposited before on Nb-doped SrTiO${}_3$(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 ${\mathrm{Co}}^{3+}$ cations in the low-spin state on octahedral sites for the ${\mathrm{Co}}_{1.4}{\mathrm{Fe}}_{1.6}{\mathrm{O}}_4$ 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.

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.

We present an investigation on the structural and magnetic properties of the interfaces of ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{MgO}\left(001\right)$ and ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{NiO}/\mathrm{MgO}\left(001\right)$ by extracting cation-selective magneto-optical depth profiles by means of x-ray resonant magnetic reflectivity in combination with charge-transfer multiplet simulations of x-ray magnetic circular dichroism data. For ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{MgO}\left(001\right)$, the magneto-optical depth profiles at the ${\mathrm{Fe}}_{\mathrm{oct}}^{2+}$ and the ${\mathrm{Fe}}_{\mathrm{oct}}^{3+}$ resonant energies follow exactly the structural profile, while the magneto-optical depth profile at the ${\mathrm{Fe}}_{\mathrm{tet}}^{3+}$ resonance is offset by $3.2\pm 1.3$ Å from the interface, consistent with a $B$-site interface termination of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ with fully intact magnetic order. In contrast, for ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{NiO}\left(001\right)$, the magneto-optical depth profiles at the ${\mathrm{Fe}}_{\mathrm{oct}}^{2+}$ and the ${\mathrm{Ni}}^{2+}$ resonances agree with the structural profile, but the interface positions of the magneto-optical depth profiles at the ${\mathrm{Fe}}_{\mathrm{oct}}^{3+}$ and the ${\mathrm{Fe}}_{\mathrm{tet}}^{3+}$ resonances are spatially shifted by $3.3\pm 1.4$ and $2.7\pm 0.9$ Å, respectively, not consistent with a magnetically ordered stoichiometric interface. This may be related to an intermixed $(\mathrm{Ni},\mathrm{Fe})\mathrm{O}$ layer at the interface. The dichroic depth profile at the Ni $L_3$ edge might hint at uncompensated magnetic moments throughout the NiO film.