Room-temperature multiferroicity in NiFe2O4 and its magnetoelectric coupling intensified through defect engineering

The well-known ferromagnetic oxide, NiFe₂O₄, was studied as a potential candidate for room-temperature Type II magnetoelectrics. A spin canting as one of the essential requirements for Type II multiferroics was induced by breaking the stoichiometry, that is, intentionally subtracting Fe ions. We observed that Fe ions were first subtracted exclusively from the tetrahedral sites, leading to an increase in the magnetoelectric coupling owing to an increasing degree of spin canting. The enhancement in the magnetoelectric coupling culminated beyond the subtraction level of ~30 at.%, at which Fe ions started to be removed from the octahedral sites. Alongside, we observed that the subtraction of Fe ions gives rise to a ferroelectricity due to the formation of defect complexes that establish an internal bias field.     

Enhanced doping effects of multielement on anisotropic thermal expansion in ZrO2 with new compositions

Coefficient of thermal expansion (CTE) of a solid material plays a critical role for a variety of high temperature applications such as thermal barrier coating (TBC) systems during the thermal cycling process. Ceramics contain ionic bonds; hence they tend to exhibit lower CTE values than alloys/metals. Developing new ceramic thermal barrier materials using promising dopants and compositions that have higher CTE values than the conventional 6-8 wt% Y₂O₃ stabilized ZrO₂(8YSZ) will contribute to the decrease in thermal expansion mismatch between a typical ceramic 8YSZ (10 ~ 11 × 10⁻⁶°C⁻¹) top coat and a metal alloy based bond coat such as NiCrAlY (14 ~ 17×10⁻⁶°C⁻¹, Padture et al., Science, 2002;296:280–4; Liang et al., J Mater Sci Technol, 2011;27(5):408–14), which is highly desirable. This work reports design, modeling, synthesis, and characterization of promising new compositions based on Dy³⁺, Al³⁺, and Ce⁴⁺-doped YSZ that consist of the tetragonal structure and have an enhanced thermal expansion than 8YSZ. The intrinsic CTE at the atomic level has been investigated via molecular dynamics (MD) simulation. The atomic scale analysis provides new insights into the enhanced doping effects of multiple trivalent and tetravalent cations on the lattice structure, lattice energy, and thermal expansion in ZrO₂. The calculated lattice energy becomes smaller with the incorporation of Dy³⁺, Al³⁺, and Ce⁴⁺ions, which corresponds strongly to the increase in CTE. The crystalline size is reduced due to the incorporation of the Al³⁺ and Ce⁴⁺, whereas the sintering resistance is enhanced ascribed to the addition of Dy³⁺ and Al³⁺. Doping Dy³⁺, Al³⁺, and Ce⁴⁺ cations to YSZ increased the CTE value of YSZ and for Dy_0.03Y_0.075Zr_0.895O_1.948, the CTE is 12.494 × 10⁻⁶°C⁻¹ at 900°C, which has an 11% increase, as compared with that of 8YSZ.