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Aurivillius layered oxides with general formula (Bi2O2)(Am−1BmO3m+1) stand out among room-temperature magnetoelectric multiferroics for their large magnetization. However, despite extensive research, there is an obvious lack of understanding of their magnetism. The chemical design strategy for obtaining multiferroism involves the incorporation of M3+ magnetic cations at the B-site of well-known ferroelectric compounds such as Bi4Ti3O12 (m = 3). We report here a study of the magnetism and dielectric properties of Aurivillius Bi4Ti3 − 2xNbxFexO12 phases with x ≥ 1 up to 1.2, which correspond to magnetic cation fractions at the B site between 0.33 and 0.4 above the threshold for percolation. This is a three-layer system, less prone to the formation of intergrowths, which nominally contains Fe3+ as single magnetic species. Despite that, a rich phenomenology is uncovered. Coexisting magnetic orders are present in the Aurivillius compounds, and a Morin-like transition takes place at low temperatures. The dielectric characterization does not show any associated anomaly that could indicate a polymorphic phase transition but the appearance of relaxor-like characteristics. Possible scenarios are discussed, which involve the presence of Fe2+, cation partitioning between nonequivalent B-sites, and the development of polar nanodomains within a ferroelectric phase at a spin reorientation transition. [ABSTRACT FROM AUTHOR]

Aurivillius compounds with the general formula (Bi2O2)(An−1BnO3n+1) are a highly topical family of functional layered oxides currently under investigation for room-temperature multiferroism. A chemical design strategy is the incorporation of magnetically active BiMO3 units (M: Fe3+, Mn3+, Co3+ ...) into the pseudo-perovskite layer of known ferroelectrics like Bi4Ti3O12, introducing additional oxygen octahedra. Alternatively, one can try to directly substitute magnetic species for Ti4+ in the perovskite slab. Previous reports explored the introduction of the M3+ species, which required the simultaneous incorporation of a 5+ cation, as for the Bi4Ti3−2xNbxFexO12 system. A larger magnetic fraction might be attained if Ti4+ is substituted with Mn4+, though it has been argued that the small ionic radius prevents its incorporation into the pseudo-perovskite layer. We report here the mechanosynthesis of Aurivillius Bi4Ti2−xMnxNb0.5Fe0.5O12 (n = 3) compounds with increasing Mn4+ content up to x = 0.5, which corresponds to a magnetic fraction of 1/3 at the B-site surpassing the threshold for percolation, and equal amounts of Mn4+ and Fe3+. The appearance of ferromagnetic superexchange interactions and magnetic ordering was anticipated and is shown for phases with x ≥ 0.3. Ceramic processing was accomplished by spark plasma sintering, which enabled electrical measurements that demonstrated ferroelectricity for all Mn4+-containing Aurivillius compounds. This is a new family of layered oxides and a promising alternative single-phase approach for multiferroism. [ABSTRACT FROM AUTHOR]

Aurivillius oxides with general formula (Bi2O2)(Am−1BmO3m+1) are being extensively investigated for room-temperature multiferroism and magnetoelectric coupling. The chemical design strategy behind current investigations is the incorporation of magnetically active BiMO3 units (M: Fe3+, Mn3+, Co3+...) to the pseudoperovskite layer of known ferroelectrics like Bi4Ti3O12, increasing m. The percolation of magnetic cations at the B-site sublattice is required for magnetic ordering and thus, phases with m ≥ 5 are searched. Alternatively, one can try to directly substitute magnetic species for Ti4+ in the perovskite slab, without introducing additional oxygen octahedra. We report here the mechanosynthesis of Aurivillius Bi4Ti3−2xNbxFexO12 phases with increasing x values up to 1. A maximum magnetic fraction of 1/3, surpassing the threshold for percolation, was reached. Preliminary structural analysis indicated a continuous solid solution, though hints of structural changes between x = 0.25 and 0.5 were found. Ceramic processing was accomplished by spark plasma sintering of the mechanosynthesized phases, including those with high-x ones with reduced thermal stability. This has enabled us to carry out full electrical characterization and to demonstrate ferroelectricity for all phases up to x = 1. Magnetic measurements were also carried out, and weak ferromagnetism was found for x = 1. Therefore, Bi4TiNbFeO12 is proposed to be a novel room-temperature multiferroic. [ABSTRACT FROM AUTHOR]

Compared to other sensors, such as radio-frequency identification sensors, microwires have a significant advantage due to their small dimensions and contact-free reading. The most important advantage is that a microwire is considered to be a biocompatible material due to the glass-coating insulator. This is particularly applicable in various titanium implants (dental, knee joint, replacement of damaged skull part, repair of the femur, etc.). The temperature dependence of the switching field can be employed to monitor inflammatory processes. In this contribution we present the influence of molybdenum (Mo) content on the magnetic properties and thermal treatment in order to obtain a strong variation of the switching field to temperature. We show that stress annealing of a FeMo9BCu microwire at 775 K under an applied axial stress of 309 MPa leads to the strongest variation in a temperature range from 300 to 315 K, which is highly desirable for biomedical applications. [ABSTRACT FROM AUTHOR]

Aurivillius compounds with the general formula (Bi 2 O 2 )(A n -1 B n O 3 n +1 ) are a highly topical family of functional layered oxides currently under investigation for room-temperature multiferroism. A chemical design strategy is the incorporation of magnetically active BiMO 3 units (M: Fe 3+ , Mn 3+ , Co 3+ …) into the pseudo-perovskite layer of known ferroelectrics like Bi 4 Ti 3 O 12 , introducing additional oxygen octahedra. Alternatively, one can try to directly substitute magnetic species for Ti 4+ in the perovskite slab. Previous reports explored the introduction of the M 3+ species, which required the simultaneous incorporation of a 5+ cation, as for the Bi 4 Ti 3-2 x Nb x Fe x O 12 system. A larger magnetic fraction might be attained if Ti 4+ is substituted with Mn 4+ , though it has been argued that the small ionic radius prevents its incorporation into the pseudo-perovskite layer. We report here the mechanosynthesis of Aurivillius Bi 4 Ti 2- x Mn x Nb 0.5 Fe 0.5 O 12 ( n = 3) compounds with increasing Mn 4+ content up to x = 0.5, which corresponds to a magnetic fraction of 1/3 at the B-site surpassing the threshold for percolation, and equal amounts of Mn 4+ and Fe 3+ . The appearance of ferromagnetic superexchange interactions and magnetic ordering was anticipated and is shown for phases with x ≥ 0.3. Ceramic processing was accomplished by spark plasma sintering, which enabled electrical measurements that demonstrated ferroelectricity for all Mn 4+ -containing Aurivillius compounds. This is a new family of layered oxides and a promising alternative single-phase approach for multiferroism.