We show that ordinary bananas exhibit closed loops of switched charge versus applied voltage that are nearly identical to those misinterpreted as ferroelectric hysteresis loops in crystals. The 'ferroelectric' properties of bananas are contrasted with those of the real ferroelectric material Ba2NaNb5O15, often nicknamed 'bananas'.

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Ferroelectrics go bananas

Research efforts addressing spin waves (magnons) in micro- and nanostructured ferromagnetic materials have increased tremendously in recent years. Corresponding experimental and theoretical work in magnonics faces significant challenges in that spin-wave dispersion relations are highly anisotropic and different magnetic states might be realized via, for example, the magnetic field history. At the same time, these features offer novel opportunities for wave control in solids going beyond photonics and plasmonics. In this topical review we address materials with a periodic modulation of magnetic parameters that give rise to artificially tailored band structures and allow unprecedented control of spin waves. In particular, we discuss recent achievements and perspectives of reconfigurable magnonic devices for which band structures can be reprogrammed during operation. Such characteristics might be useful for multifunctional microwave and logic devices operating over a broad frequency regime on either the macro- or nanoscale.

https://iopscience.iop.org/article/10.1088/0953-8984/26/12/123202/pdf

Review and prospects of magnonic crystals and devices with reprogrammable band structure

Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadmap edition follows a similar general layout, even if its focus is naturally shifted, and a different group of experts and, thus, viewpoints are being collected and presented. More importantly, key developments have changed the research landscape in very relevant ways, so that a novel view onto some of the most crucial developments is warranted, and thus, this 2017 Magnetism Roadmap article is a timely endeavour. The change in landscape is hereby not exclusively scientific, but also reflects the magnetism related industrial application portfolio. Specifically, Hard Disk Drive technology, which still dominates digital storage and will continue to do so for many years, if not decades, has now limited its footprint in the scientific and research community, whereas significantly growing interest in magnetism and magnetic materials in relation to energy applications is noticeable, and other technological fields are emerging as well. Also, more and more work is occurring in which complex topologies of magnetically ordered states are being explored, hereby aiming at a technological utilization of the very theoretical concepts that were recognised by the 2016 Nobel Prize in Physics. Given this somewhat shifted scenario, it seemed appropriate to select topics for this Roadmap article that represent the three core pillars of magnetism, namely magnetic materials, magnetic phenomena and associated characterization techniques, as well as applications of magnetism. While many of the contributions in this Roadmap have clearly overlapping relevance in all three fields, their relative focus is mostly associated to one of the three pillars. In this way, the interconnecting roles of having suitable magnetic materials, understanding (and being able to characterize) the underlying physics of their behaviour and utilizing them for applications and devices is well illustrated, thus giving an accurate snapshot of the world of magnetism in 2017. The article consists of 14 sections, each written by an expert in the field and addressing a specific subject on two pages. Evidently, the depth at which each contribution can describe the subject matter is limited and a full review of their statuses, advances, challenges and perspectives cannot be fully accomplished. Also, magnetism, as a vibrant research field, is too diverse, so that a number of areas will not be adequately represented here, leaving space for further Roadmap editions in the future. However, this 2017 Magnetism Roadmap article can provide a frame that will enable the reader to judge where each subject and magnetism research field stands overall today and which directions it might take in the foreseeable future. The first material focused pillar of the 2017 Magnetism Roadmap contains five articles, which address the questions of atomic scale confinement, 2D, curved and topological magnetic materials, as well as materials exhibiting unconventional magnetic phase transitions. The second pillar also has five contributions, which are devoted to advances in magnetic characterization, magneto-optics and magneto-plasmonics, ultrafast magnetization dynamics and magnonic transport. The final and application focused pillar has four contributions, which present non-volatile memory technology, antiferromagnetic spintronics, as well as magnet technology for energy and bio-related applications. As a whole, the 2017 Magnetism Roadmap article, just as with its 2014 predecessor, is intended to act as a reference point and guideline for emerging research directions in modern magnetism.

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The 2017 Magnetism Roadmap

The methodology for adapting a standard micromagnetic code to run on graphics processing units (GPUs) and exploit the potential for parallel calculations of this platform is discussed. GPMagnet, a general purpose finite-difference GPU-based micromagnetic tool, is used as an example. Speed-up factors of two orders of magnitude can be achieved with GPMagnet with respect to a serial code. This allows for running extensive simulations, nearly inaccessible with a standard micromagnetic solver, at reasonable computational times.

https://iopscience.iop.org/article/10.1088/0022-3727/45/32/323001/pdf

Micromagnetic simulations using Graphics Processing Units

Lithographic processing and film growth technologies are continuing to advance, so that it is now possible to create patterned ferroic materials consisting of arrays of sub-1 μm elements with high definition. Some of the most fascinating behaviour of these arrays can be realised by exploiting interactions between the individual elements to create new functionality. The properties of these artificial ferroic systems differ strikingly from those of their constituent components, with novel emergent behaviour arising from the collective dynamics of the interacting elements, which are arranged in specific designs and can be activated by applying magnetic or electric fields. We first focus on artificial spin systems consisting of arrays of dipolar-coupled nanomagnets and, in particular, review the field of artificial spin ice, which demonstrates a wide range of fascinating phenomena arising from the frustration inherent in particular arrangements of nanomagnets, including emergent magnetic monopoles, domains of ordered macrospins, and novel avalanche behaviour. We outline how demagnetisation protocols have been employed as an effective thermal anneal in an attempt to reach the ground state, comment on phenomena that arise in thermally activated systems and discuss strategies for selectively generating specific configurations using applied magnetic fields. We then move on from slow field and temperature driven dynamics to high frequency phenomena, discussing spinwave excitations in the context of magnonic crystals constructed from arrays of patterned magnetic elements. At high frequencies, these arrays are studied in terms of potential applications including magnetic logic, linear and non-linear microwave optics, and fast, efficient switching, and we consider the possibility to create tunable magnonic crystals with artificial spin ice. Finally, we discuss how functional ferroic composites can be incorporated to realise magnetoelectric effects. Specifically, we discuss artificial multiferroics (or multiferroic composites), which hold promise for new applications that involve electric field control of magnetism, or electric and magnetic field responsive devices for high frequency integrated circuit design in microwave and terahertz signal processing. We close with comments on how enhanced functionality can be realised through engineering of nanostructures with interacting ferroic components, creating opportunities for novel spin electronic devices that, for example, make use of the transport of magnetic charges, thermally activated elements, and reprogrammable nanomagnet systems.

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Artificial ferroic systems: novel functionality from structure, interactions and dynamics

Multiferroic thin films with the general formula BiFe1−xScxO3 (x=0.0, 0.1, 0.3, and 0.5mol%) (BFS) were synthesized on Pt∕Ti∕SiO2∕Si substrates through a sol-gel deposition method. From the x-ray diffraction (XRD) analysis, it was observed that the unit cell volume increased upon Sc doping up to 0.3mol%. Impure phase appeared for the BFS (Sc: 0.5mol%) films. Leakage current, ferroelectric, and magnetic properties were also found to improve for Sc doping up to 0.3mol%. Room temperature magnetic coercive field for in-plane orientation of films showed the lowest value for BFS (Sc: 0.3mol%) films, which indicates that BFS (Sc: 0.3mol%) films were more magnetically soft. Room temperature value of the magnetodielectric effect in the presence of magnetic field of 8kOe was found to have maximum of 3.36% for the BFS (Sc: 0.3mol%) films. From the x-ray photoelectron spectroscopy measurements, it was observed that the Fe has mixed valency states of 2+ and 3+. The ratio of Fe2+ and Fe3+ was found to decrease slightly...

Sc modified multiferroic BiFeO3 thin films prepared through a sol-gel process

Magnetoplasmonic crystals are spatially periodic nanostructured magnetic surfaces combining the features of surface plasmon polariton excitation and magneto-optical tunability. Here we present a comprehensive experimental and theoretical work demonstrating that in magnetoplasmonic crystals the coupling of free space radiation to surface plasmon polariton modes in conjunction with the inherent magneto-optical activity, enable cross-coupling of propagating surface plasmon polariton modes. We have explored the consequences of this unique magnetoplasmonic crystal optical feature by studying the light reflected from a two-dimensional periodic array of cylindrical holes in a ferromagnetic layer illuminated at oblique incidence and magnetized in the sample plane, namely, in the so-called longitudinal Kerr effect geometry. We observe that the magneto-optical spectral response arises from all the excitable surface plasmon polariton modes in the magnetoplasmonic crystal irrespective of the incoming light polarizati...

Resonant Enhancement of Magneto-Optical Activity Induced by Surface Plasmon Polariton Modes Coupling in 2D Magnetoplasmonic Crystals

We present a systematic study of the structural, magnetic, and magnetotransport properties of Co-doped Fe3O4 films deposited on MgO (100) substrates by cosputtering technique Transmission electron microscopy images suggest that the undoped and Co-doped Fe3O4 films are polycrystalline in nature and consist of a well defined grain boundary network The temperature dependence of resistance also shows that the transport mechanism in our films is dominated by electron tunneling across antiferromagnetically coupled grain boundaries We observed that the magnetic properties of the doped films are markedly sensitive to the Co doping concentration, with the magnetization curves showing drastic changes in coercivity with increasing doping concentration In-plane magnetoresistance curves show linear magnetic field dependence for the undoped Fe3O4 films while a reduction in magnetoresistance and a departure from linear field dependence are observed for the Co-doped films

Magnetic and transport properties of Co-doped Fe3O4 films

Effect of Sc substitution on the structure, electrical, and magnetic properties of multiferroic BiFeO3 thin films grown by a sol–gel process

Microwave magnetization dynamics on two-dimensional periodic arrays of nanoscale magnetic antidots with magnetic field applied perpendicular to the lattice plane have been studied using ferromagnetic resonance spectroscopy. Linear dependence of the resonant mode frequency on the applied field was observed experimentally. Theoretical calculations show that this linear dependence originates from the high symmetry imposed by applying the field perpendicular to the plane of the antidot lattice. The calculated mode profiles exhibit a fourfold symmetry in contrast to the twofold symmetry typical for the in-plane magnetization direction. From the calculated Bloch wave dispersion the group velocities along the [10] and [11] directions and close to the center of the first Brillouin zone are found to be the same, which demonstrates a very high degree of isotropy of magnonic modes for the center of this zone in this case. Perpendicular standing spin-wave modes due to microwave shielding were also observed on the antidot lattices.

D. Tripathy

Papers

Sc modified multiferroic BiFeO3 thin films prepared through a sol-gel process

Resonant Enhancement of Magneto-Optical Activity Induced by Surface Plasmon Polariton Modes Coupling in 2D Magnetoplasmonic Crystals

Magnetic and transport properties of Co-doped Fe3O4 films

Effect of Sc substitution on the structure, electrical, and magnetic properties of multiferroic BiFeO3 thin films grown by a sol–gel process

High-symmetry magnonic modes in antidot lattices magnetized perpendicular to the lattice plane