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The wealth of complex polar topologies1–10 recently found in nanoscale ferroelectrics results from a delicate balance between the intrinsic tendency of the materials to develop a homogeneous polarization and the electric and mechanical boundary conditions imposed on them. Ferroelectric–dielectric interfaces are model systems in which polarization curling originates from open circuit-like electric boundary conditions, to avoid the build-up of polarization charges through the formation of flux-closure11–14 domains that evolve into vortex-like structures at the nanoscale15–17 level. Although ferroelectricity is known to couple strongly with strain (both homogeneous18 and inhomogeneous19,20), the effect of mechanical constraints21 on thin-film nanoscale ferroelectrics has been comparatively less explored because of the relative paucity of strain patterns that can be implemented experimentally. Here we show that the stacking of freestanding ferroelectric perovskite layers with controlled twist angles provides an opportunity to tailor these topological nanostructures in a way determined by the lateral strain modulation associated with the twisting. Furthermore, we find that a peculiar pattern of polarization vortices and antivortices emerges from the flexoelectric coupling of polarization to strain gradients. This finding provides opportunities to create two-dimensional high-density vortex crystals that would enable us to explore previously unknown physical effects and functionalities.The stacking of freestanding ferroelectric perovskite layers with controlled twist angles results in a peculiar pattern of polarization vortices and antivortices that emerges from the flexoelectric coupling of polarization to strain gradients. [ABSTRACT FROM AUTHOR]

In this work the study of the ferromagnetic resonance (FMR) and of the electrical conductivity in the perovskite sample, Sr[SUB2]FeMoO[SUB6], is presented. The sample has been manufactured by conventional ceramic method sintering in inert gas atmosphere. Electrical conductivity, of about 1000-1200 S/m, has been measured in the 125-300 K temperature range. FMR linewidths have been measured at 9.48 GHz from 125 to 300 K. A decrease of the linewidth with increasing temperature has been obtained. This behavior has been attributed to the presence of valence exchange mechanism, which takes places in samples with Fe[SUP2+] cations. In this way, FMR linewidth analysis has been revealed like a useful and clear technique in determining the cation distribution of the sample showing the presence of Fe[SUP2+] ions. This technique has also shown data about the microstructure of the sintered polycrystalline sample finding the porosity value of the sample. [ABSTRACT FROM AUTHOR]

Exploiting multiferroic BiFeO3 thin films in spintronic devices requires deterministic and robust control of both internal magnetoelectric coupling in BiFeO3, as well as exchange coupling of its antiferromagnetic order to a ferromagnetic overlayer. Previous reports utilized approaches based on multi-step ferroelectric switching with multiple ferroelectric domains. Because domain walls can be responsible for fatigue, contain localized charges intrinsically or via defects, and present problems for device reproducibility and scaling, an alternative approach using a monodomain magnetoelectric state with single-step switching is desirable. Here we demonstrate room temperature, deterministic and robust, exchange coupling between monodomain BiFeO3 films and Co overlayer that is intrinsic (i.e., not dependent on domain walls). Direct coupling between BiFeO3 antiferromagnetic order and Co magnetization is observed, with ~ 90° in-plane Co moment rotation upon single-step switching that is reproducible for hundreds of cycles. This has important consequences for practical, low power non-volatile magnetoelectric devices utilizing BiFeO3. [ABSTRACT FROM AUTHOR]

Multiferroic domain walls have recently been proposed as the active element in devices related to spintronics, data storage, and magnetic logic. Among multiferroics, BiFeO3 is by far the most studied material. Its domain walls have shown rich behaviours that include conductivity in an otherwise insulating crystal, and magnetotransport properties that are markedly different from those of the bulk. In this article we summarize the experimental evidence and the current models used to understand the interplay between elastic, electric, and magnetic properties of the domain walls. Starting from the considera- tion of antiferromagnetic domain structures on a background of ferroelectric domains, and emphasizing pinning effects, we proceed to discuss the microscopic behavior of ferroelectricity and magnetism at the walls. We describe how domain organization in BiFeO3 is caused by structural transformations, and scrutinize modelling works pinpointing their characteristic features. Finally, we summarize the recent progress and list open questions for future study on BiFeO3 domain structures. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

In recent years an increasing relevance of the graduate works in physics at our universities is being recognized. Unfortunately it is not easy to provide enough interesting ideas on experimental assignments to our students in order to satisfactorily develop these projects and often we resort to theoretical, bibliographic or computational works. Efforts should be made to increase the experimental project ratio including subjects that can be completed with an additional theoretical task or some numerical simulation. Here we propose a set of selected ideas that we have developed or are currently in progress in our laboratory of electromagnetism. There are basically studies on electromagnetic induction in conductive materials and electro-dynamic systems in which the presence of magnetic forces are relevant. A few examples are discussed here. The first one corresponds to the study of the damped fall of a magnet through a hollow conducting tube, in which, after a simple calculation of the induced current, we can calculate the conductivity of the tube according to its radius and wall thickness. The second is a proposal concerning with the measurement of electrical conductivity in metallic tubes based on the screening of the magnetic field inside it at low frequencies. Another is related to a study on magnetic levitation by induced ac currents in non-magnetic conductors at low frequencies allowing us to distinguish the two components of the current responsible for levitation and heating respectively. One more is related to a simple and precise method to measure the electrical conductivity in nonferromagnetic tubes, rods and slabs without electric contacts. The results are readily obtained by analyzing the eddy currents in the sample when it is subjected to an harmonic magnetic field and can be obtained from a measurement of the phase of the induced voltage in a short probe coil located inside the tube or around the rod or slab when the sample is axially excited by a long primary coil. Another interesting idea corresponds to the analysis of the behavior of a small-scale model of a magnetic levitation system based on the Inductrack concept. Drag and lift forces acting on our prototype moving above a continuous copper track are studied analytically following a simple low-speed approach. And finally we present a study on magnetic actuators based on rare-earth permanent magnets concerning the correct choice of the aspect ratio and volume. [ABSTRACT FROM AUTHOR]

Abstract: The design and analysis of a small prototype of a magnetic levitation system at low-speed using a Halbach-type magnet array is presented here. For that purpose, we have arranged a copper rim over a carbon fiber wheel, which is driven by an electric motor in presence of the magnet array, in such a manner that allows performing the experiment readily. The analysis of the system is undertaken under a two-dimensional (2D)-approach which permits computing and extending the study of our model to higher speeds. Our work is completed with a series of experimental measurements of lift and drag forces for different circumstances. Initially, the drag force is significant but after the compensation speed (when both forces balance) it slowly decreases. Conversely, the lift force becomes progressively bigger in such a manner that it attains quickly noteworthy values. We observe that the theoretical compensation speed is always minor than the experimental one and that the measured values for both forces are slightly smaller than the expected, although the main features of the experiment are well matched by our numerical simulation. [Copyright &y& Elsevier]

The analysis of the induced current distribution in conducting wires subjected to a harmonic axial voltage is important in designing many electrical devices such as transformers and transmission lines. The azimuthal magnetic field induces axial electric currents and therefore the impedance of the wire depends on the excitation frequency. The current density is increasingly confined to a thin layer at the boundary of the wire as the frequency increases. To minimize this effect at higher frequencies it is necessary to enhance the surface-to-volume ratio by using thin high-conductivity wires. The study of induction phenomena in conducting samples subjected to a harmonic longitudinal magnetic field has attracted less attention. The time-varying magnetic flux induces eddy currents, which flow perpendicularly to the axis of the sample. We study the electromagnetic field in samples of simple geometry, making the usual approximations in good conductors. The validity of our calculations extends to several GHz and allows us to propose a method for determining the electrical conductivity by measuring the phase angle of the complex mutual inductance between a primary coil, responsible for the external magnetic field, and a secondary winding around the sample. [ABSTRACT FROM AUTHOR]

In this paper, we present a precise technique for contactless measurement of resistivity in conductive tubes. The method is simply based on the determination of the phase angle of the complex mutual inductance between two coils working under low-frequency conditions. The first coil is placed around the tube, whereas the detecting coil is situated inside it. Study of the magnetic screening caused by the currents induced in the tube allows its conductivity to be determined. The use of low frequency guarantees the almost complete absence of the skin effect in such a way that the results are obtained in a very simple manner. [ABSTRACT FROM AUTHOR]

In multiferroic BiFeO3 thin films grown on highly mismatched LaAlO3 substrates, we reveal the coexistence of two differently distorted polymorphs that leads to striking features in the temperature dependence of the structural and multiferroic properties. Notably, the highly distorted phase quasiconcomitantly presents an abrupt structural change, transforms from a standard to a nonconventional ferroelectric, and transitions from antiferromagnetic to paramagnetic at 360 ± 20 K. These coupled ferroic transitions just above room temperature hold promises of giant piezoelectric, magnetoelectric, and piezomagnetic responses, with potential in many applications fields. [ABSTRACT FROM AUTHOR]

Abstract: Study of temperature effects on magneto impedance (MI) ratio in Co68.15Fe4.35Si12.5B15 amorphous wires is here studied. Amorphous wires of 130μm in diameter were prepared by in-rotating-water quenching technique. During MI measurements as a function of temperature, a decrease of MI ratio has been observed as the number of temperature sweeps increased. These wires have been subjected to different temperature sweeps beginning in 80K until 300K and slowly increasing the range until 400K in order to detect a critical temperature. Such critical temperature has been found close to 375K. The decrease of MI ratio is due to thermal diffusion and a change of the energy of the amorphous state as a consequence of the fact that the amorphous state is a meta stable state. It is believed that, at that temperature, atom diffusion and stress annealing take place and the state moves to a low-energy state following a dynamics in which the time when the sample is subjected to diffusion is important. [Copyright &y& Elsevier]