Multiferroic Materials

contact person: Alicia Castro

Single-phase multiferroics for sustainable magnetoelectric technologies

Magnetoelectric multiferroics are compounds that show coexistence of electrical and magnetic orders, and a range of coupling phenomena between polarization and magnetization. Activity in this field is extensive and advances are being made towards obtaining room-temperature multiferroics with significant magnetoelectric effects. This would enable a range of novel, potentially disruptive technologies, such as voltage-tunable spintronic devices, uncooled magnetic sensors, and the long-searched magnetoelectric memory. Among a number of material approaches under investigation, ABO3 perovskite oxides with multiferroic morphotropic phase boundary (MPB) stand out as very promising route for obtaining room temperature magnetoelectric response. In these materials multiferroicity can be chemically engineered on a site-by-site basis by placing ferroelectrically and magnetically active cations in the A- and B-site, respectively. In EOSMAD we are currently developing novel Bi-based perovskite oxides chemically designed to show multiferroic MPBs, with the aim of at obtaining large phase-change responses at structural instabilities of the multiferroic state. A second set of highly innovative, recently proposed material approaches for magnetoelectricity are multiferroic layered perovskite oxides, consisting of two-dimensional perovskite slabs interleaved with cations (alkaline earth) or structural units (Bi2O2, Aurivillius phases). In these materials multiferroism is anticipated if magnetically active species are introduced in the B-site of the perovskite slab, starting from well-known ferroelectric phases. Furthermore, recently enforced environmental regulations worldwide like EU-Directive 2002/95/EC (RoHS) require the restriction of the use of lead, among a list of hazardous substances, in electrical and electronic equipment. This has fostered the search of environmental friendly materials, and EOSMAD is applying design concepts and synthesis approaches to this aim.
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Multilayer ceramic magnetoelectric composites with tailored interfaces

Alternatively to single-phase multiferroics, and with greater design flexibility, composite materials consisting of two dissimilar ferroic phases typically yield very large magnetoelectric coupling responses at room-temperature. These are combinations of piezoelectric and magnetostrictive (or piezomagnetic) components, for which magnetoelectricity is obtained as the product property of their piezoresponses. They are enabling materials for a range of potentially disruptive technologies like remotely- or self-rechargeable powering technologies with/from magnetic fields for wireless bio-implanted devices and network sensor nodes. Among the different material approaches considered, cofired ceramic composites offer improved reliability in applications, and are more adequate for free-forming and miniaturization. In EOSMAD we are currently working in advanced methods for the preparation of multilayer ceramic composites with tailored interfaces for enhanced response, by using nanopowders of the ferroic oxides obtained by mechanosynthesis or wet-chemistry routes, and cofiring them by spark plasma sintering. Emphasis is put on controlling chemical reactions at and interdiffusion across the interfaces between high-sensitivity piezoelectric perovskites and magnetostrictive spinel oxides. The control of sintering mismatch of the components allows tailoring the microstructure and residual stresses of the interfaces for optimum magnetoelectric responses. Additionally, and in order to facilitate the development of the related technologies and their final translation into products, environmentally friendly magnetoelectric composites made of lead- and nickel-free materials along with ceramic-metal alloy (cermet) composites with large functional responses are preferentially being investigated, suitable for device demonstration activities.
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