Search

[EN] A promising approach for obtaining a large magnetoelectric response at room temperature in single-phase materials is to make use of phase-change phenomena at structural instabilities of a multiferroic state. This is the case of perovskites showing the morphotropic phase boundary (MPB) between ferroelectric polymorphs with differentiated magnetic orderings, like in BiFeO3-PbTiO3. Indeed, distinctive magnetoelectric effects were recently shown in Mn-modified BiFeO3-PbTiO3, associated with the electric field-induced phase transition between the coexisting MPB phases. We report here analogous effects under the magnetic field in 0.7BiFeO3-0.3PbTiO3, which shed light on the reversibility of the phase-change phenomena and the role of the magnetic ordering in the phase coexistence at the structural instability characteristic of the MPB. Results demonstrate magnetic field-driven phase-change phenomena.

[EN] Aurivillius compounds with the general formula (Bi2O2)(A(n-1)B(n)O(3n+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+ horizontal ellipsis ) 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.

The ferromagnetic perovskite oxide BiMnO is a highly topical material, and the solid solutions it forms with antiferromagnetic/ferroelectric BiFeO and with ferroelectric PbTiO result in distinctive polar/nonpolar morphotropic phase boundaries (MPBs). The exploitation of such a type of MPBs could be a novel approach to engineer novel multiferroics with phase-change magnetoelectric responses, in addition to ferroelectrics with enhanced electromechanical performance. Here, the interplay among crystal structure, point defects, and multiferroic properties of the BiMnO–BiFeO–PbTiO ternary system at its line of MPBs between polymorphs of tetragonal P4mm (polar) and orthorhombic Pnma (antipolar) symmetries is reported. A strong dependence of the phase coexistence on thermal history is found: phase percentage significantly changes whether the material is quenched or slowly cooled from high temperature. The origin of this phenomenon is investigated with temperature-dependent structural and physical property characterizations. A major role of the complex defect chemistry, where a Bi/Pb-deficiency allows Mn and Fe ions to have a mixed-valence state, in the delicate balance between polymorphs is proposed, and its influence in the magnetic and electric ferroic orders is defined.

Phase-change functional responses can be obtained in perovskite solid solutions at structural instabilities involving multiferroic states, a very promising approach for realizing magnetoelectric effects at room temperature. A line of multiferroic morphotropic phase boundaries (MPBs) has been described within the phase diagram of the BiFeO-BiMnO-PbTiO ternary system, in which an evolution from Cc/P4mm-type (polar/polar) to Pnma/P4mm-type (antipolar/polar) MPBs takes place. Coexistence of the three polymorphs results in a small region, where distinctive phase-change magnetoelectric responses are anticipated. In this work, all three polymorphs were obtained, either isolated or in phase coexistence with varying percentages, by tailoring the cooling profile after high-temperature annealing in a small compositional range. The perovskite structure was characterized, and temperature-dependent magnetic and electrical characterizations were carried out to define the ferroic orders and crystal physicochemical properties of the three polymorphs. Results revealed the close interplay among the crystal structure, point defects, and multiferroism in this small region, where the delicate balance among the three polymorphs is shown. Additionally, the effect of size reduction across the submicron range, down to the nanoscale, is described and discussed.

There is a growing activity in the search of novel single-phase multiferroics that could finally provide distinctive magnetoelectric responses at room temperature, for they would enable a range of potentially disruptive technologies, making use of the ability of controlling polarization with a magnetic field or magnetism with an electric one (for example, voltage-tunable spintronic devices, uncooled magnetic sensors and the long-searched magnetoelectric memory). A very promising novel material concept could be to make use of phase-change phenomena at structural instabilities of a multiferroic state. Indeed, large phase-change magnetoelectric response has been anticipated by a first-principles investigation of the perovskite BiFeO -BiCoO solid solution, specifically at its morphotropic phase boundary between multiferroic polymorphs of rhombohedral and tetragonal symmetries. Here, we report a novel perovskite oxide that belongs to the BiFeO -BiMnO -PbTiO ternary system, chemically designed to present such multiferroic phase boundary with enhanced ferroelectricity and canted ferromagnetism, which shows distinctive room-temperature magnetoelectric responses.

Recently, large phase-change magnetoelectric response has been anticipated by a first-principles investigation of phases in the BiFeO-BiCoO perovskite binary system, associated with the existence of a discontinuous morphotropic phase boundary (MPB) between multiferroic polymorphs of rhombohedral and tetragonal symmetries. This might be a general property of multiferroic phase instabilities, and a novel promising approach for room temperature magnetoelectricity. We review here our current investigations on the identification and study of additional material systems, alternative to BiFeO-BiCoO that has only been obtained by high pressure synthesis. Three systems, whose phase diagrams were, in principle, liable to show multiferroic MPBs have been addressed: the BiMnO-PbTiO and BiFeO-PbTiO binary systems, and the BiFeO-BiMnO-PbTiO ternary one. A comprehensive study of multiferroism across different solid solutions was carried out based on electrical and magnetic characterizations, complemented with mechanical and electromechanical measurements. An in-depth structural analysis was also accomplished when necessary.

A highly topical set of materials are those ABO3 perovskite oxides, in which multiferroicity is chemically engineered by placing ferroelectrically and magnetically active cations in A- and B-site, respectively. This is the case of the BiFeO3 and BiMnO3 perovskites, and also of the solid solutions they form with PbTiO3. Interest in these binary systems is fostered by the presence of distinctive morphotropic phase boundaries (MPBs), multiferroic in the case of BiFeO3-PbTiO3, for which a high magnetoelectric response has been anticipated. Here, new compositions belonging to the ternary system BiFeO3-BiMnO3-PbTiO3, and specifically along the line that joins the former MPBs, have been prepared by mechanosynthesis to accomplish a thorough analysis of their multiferroic nature. Nanocrystalline powders with a perovskite-type structure were obtained in the entire range of compositions, which all exhibited polymorphic phase coexistence allowing a line of MPBs to be established. The variation of the perovskite structural characteristics along this line has been defined, and correlated with those of the magnetic and electrical properties. A set of novel and promising multiferroic materials have been found for BiFeO3 rich compositions.