Hena Das

BiFeO₃ is the most intensively investigated multiferroic material. It has a cycloidal spin modulation superimposed on G-type antiferromagnetic ordering of Fe³⁺ (S = 5/2), which prohibits the appearance of a net magnetization. We found that the simultaneous substitution of Ru⁴⁺ or Ir⁴⁺ for Fe³⁺ and Ca²⁺ for Bi³⁺ suppressed the cycloidal modulation and induced canted weak ferromagnetism at room temperature, while preserving the polar rhombohedral crystal structure. Moreover, the A- and B-site substitutions substantially lowered the ferroelectric transition temperature and resulted in a 1.77% volumetric negative thermal expansion around room temperature. These findings open an avenue for designing new BiFeO₃-based functional materials.

The order or disorder of electrons is fundamental to materials properties and also provides simple analogues to the different states of matter. A charge ordered (CO) insulating state, analogous to a crystalline solid, is observed in many mixed valence materials. On heating, this melts to a charge liquid (metallic) phase, often with interesting associated physics and functions such as the Verwey transition of Fe₃O₄, colossal magnetoresistances in manganites (e.g., La_0.5Ca_0.5MnO₃), and superconductivity in K-doped BaBiO₃. Here we report the observation of pressure induced charge amorphisation in a crystalline material. BiNiO₃ has charge distribution Bi³⁺ _0.5Bi⁵⁺ _0.5Ni²⁺O₃ with long range order of the Bi³⁺ and Bi⁵⁺ states at ambient pressure, but adopts another, structurally crystalline, but charge glassy, insulating phase at pressures of 4–5 GPa and temperatures below 200 K, before metallization above 6 GPa. This is analogous to the much-studied pressure induced amorphisations of many crystalline materials and melting is even observed at accessible pressure/temperature. BiNiO₃ provides fundamental insights to the study of amorphisation using charge states rather than atoms or molecules.

Spinel-type InGaMgO₄ with a = 8.56615(3) Å was prepared by treating layered YbFe₂O₄-type InGaMgO₄ at 6 GPa and 1473 K. DFT calculation and Rietveld analysis of synchrotron X-ray powder diffraction data revealed the inverse spinel structure with In³⁺:Ga³⁺/Mg²⁺ = 0.726:0.274 in the tetrahedral site and 0.137:0.863 in the octahedral site. InGaMgO₄ spinel is an insulator with an experimental band gap of 2.80 eV, and the attempt at hole doping by post-annealing in a reducing atmosphere to introduce an oxygen defect was unsuccessful. This is the first report of the bulk synthesis of AB₂O₄ compounds with both YbFe₂O₄ and spinel polymorphs.

We explore the impact of a magnetic field on the ferroelectric domain pattern in polycrystalline hexagonal ErMnO3 at cryogenic temperatures. Utilizing piezoelectric force microscopy measurements at 1.65 K, we observe modifications of the topologically protected ferroelectric domain structure induced by the magnetic field. These alterations likely result from strain induced by the magnetic field, facilitated by intergranular coupling in polycrystalline multiferroic ErMnO3. Our findings give insights into the interplay between electric and magnetic properties at the domain scale and represent a so far unexplored pathway for manipulating topologically protected ferroelectric vortex patterns in hexagonal manganites.