Electric-field-driven resistive transition in multiferroic SrCo$_2$Fe$_{16}$O$_{27}$/Sr$_3$Co$_2$Fe$_{24}$O$_{41}$composite

We report observation of electric-field-driven resistive transition at a characteristic threshold field $E_{th}(T)$ across a temperature range 10-200 K in an off-stoichiometric composite of (~80 vol%) W- and (~20 vol%) Z-type hexaferrites. The dielectric constant $ε$ and the relaxation time constant $τ$ too exhibit anomalous jump at $E_{th}(T)$. The $E_{th}(T)$, the extent of jump in resistivity ($Δρ$), and the hysteresis associated with the jump [$ΔE_{th}(T)$] are found to decrease systematically with the increase in temperature ($T$). Several temperature-driven phase transitions have also been noticed in low and high resistive states (LRS and HRS). The temperature-driven conduction turns out to be governed by activated hopping of small polarons at all the phases with electric ($E$) and magnetic ($H$) field dependent activation energy $U(E,H)$. Interestingly, as the temperature is raised, the $E$-driven conduction at a fixed temperature evolves from $\textit{Ohmic}$ to $\textit{non-Ohmic}$ across 10-200 K and within 110-200 K, $ρ$ follows three-dimensional variable range hopping (3D-VRH) with stretched exponential $\sim$ $exp[(E_0/E)^4]$ or power law $\sim$ $(E_0/E)^m$ ($m$ varies within $\sim$0.6-0.7 and $\sim$0.6-0.8 at LRS and HRS, respectively) dependence depending on the localization length ($ζ_E$) to diffusion length ($d_E$) ratio associated with $E$-driven conduction. The $ρ(E,T)$ follows universal scaling only at LRS within 10-110 K but not at higher temperature or at HRS. The entire set of observations has been discussed within the framework of structural evolution of the point-defect (cation vacancies or oxygen excess) network. This comprehensive map of esoteric $ρ-E-T-H$ and $ε-E-T-H$ patterns provides insights on defect driven effects in a composite useful for tuning both the resistive transition and multiferroicity.

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