Coexistence of large negative and positive magnetodielectric response in Bi1−xCaxFe1−yTiyO3−δ nanoparticle ceramics

Abstract

Magnetodielectric (MD) properties of as-prepared (AP) and air-annealed ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ nanoparticle ceramics made by spark plasma sintering process are investigated as a function of temperature. Aliovalent ${\mathrm{Ca}}^{2+}$ substitution at ${\mathrm{Bi}}^{3+}$ site creates oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in the lattice disrupting the intrinsic spin cycloid of ${\mathrm{BiFeO}}_{3}$, which are suppressed when the charge compensating ${\mathrm{Ti}}^{4+}$ is co-substituted. In addition, cation substitution reduces the grain size and increases surface oxygen vacancies. These lattice and surface ${\mathrm{V}}_{\mathrm{O}}$ defects play a significant role in enhancing the magnetic properties. Zero-field-cooled magnetization curves of all AP samples show a sharp Verwey-like transition at \ensuremath{\sim}120 K, which weakens on air-annealing. A coexistence of positive and negative MD [MD = $\frac{\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)}{\ensuremath{\varepsilon}(H=0)}$; $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)=\ensuremath{\varepsilon}(H)\ensuremath{-}\ensuremath{\varepsilon}(H=0)$] response is observed, with the former dominating at 300 K and the latter at 10 K. As-prepared 5 at.% (10 at.%) Ca and Ca-Ti substituted ${\mathrm{BiFeO}}_{3}$ ceramics exhibit a maximum MD response of --10% (\ensuremath{\sim}+3%) at 10 K (300 K). Negative MD response diminishes for air-annealed ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ ceramics due to the reduction in ${\mathrm{V}}_{\mathrm{O}}$ concentration. Samples exhibiting dominant positive MD response show a similar trend for MD $vs$ H and ${M}^{2}$ vs H plots. This agreement between ${M}^{2}$ and $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)$ demonstrates a strong inherent MD coupling. On the contrary, negative MD does not follow this trend yet shows a linear relationship of MD vs ${M}^{2}$, suggesting a strong coupling between the magnetic and dielectric properties. Temperature-dependent MD studies carried out at 5 T show a gradual change from negative to positive values. Negative MD at low temperatures could be activated by the spin-lattice coupling, which dominates even at high frequency (1 MHz) under the applied field. Other contributions, including Verwey-like transition, magnetoresistance, and Maxwell-Wagner effects, do not influence the observed MD response. A prominent role of oxygen vacancies in altering the MD behavior of ${\mathrm{BiFeO}}_{3}$ is discussed in detail.