Relaxor-like ferroelectric behaviour favoured by short-range B-site ordering in 10% Ba²⁺ substituted MgFe₂O₄

Highlights

• Mechanism driving polarization in MgFe₂O₄ is the Maxwell–Wagner polarization.

• But Raman studies confirm the existence of local P4₁22/P4₃22 symmetry in MgFe₂O₄.

• Ba²⁺ substitution increases ferroelectric ordering, ΔT_m span, and masks electronic contribution.

Abstract

Using the molten salt method, pristine and Ba²⁺ substituted MgFe₂O₄ are prepared. The relaxor-like behaviour observed in the dielectric dispersion indicates the existence of B-site short-range ordering with the local P4₁22/P4₃22 symmetry which is confirmed by the Raman spectroscopy. The paper further analyses the origin of polarization using Maxwell–Wagner fit and Nyquist plot. This work suggests a possible way to increase the relaxor-like ferroelectric ordering, larger span of relaxation temperature (ΔT_m) and the effective masking of electronic contribution by the substitution of Ba²⁺ ion.

Introduction

The spinel ferrites (AB₂O₄) are room temperature insulating materials which are useful because of their technological applications as a ferromagnetic electrode in magnetic tunnel junctions or as a component in multiferroic heterostructures [1], [2]. The studies on their electrical properties pointed out the key factor for its dielectric behaviour as the formation of Fe²⁺ ions and the transfer of electron between Fe³⁺ and Fe²⁺ ions occurring within the octahedral site. So the high permittivity observed even at high frequency has been attributed to the large polarization contributed by the extra electron present in Fe²⁺ ion by disturbing the symmetry of the electron cloud. At low frequencies, the large dielectric permittivity in ferrites is attributed to the Maxwell–Wagner polarization [3]. This can occur when various heterogeneities like depletion layer between sample and electrode, interfacial layer of a grain boundary and/or domain boundary exist in the material [4]. But George et al. [5] have reported a deviation of dielectric dispersion from Maxwell–Wagner effect for the fine powder of inverse CoFe₂O₄. However, the studies on this aspect are limited due to the lack of insight towards the existence of some other phenomenon responsible for the dielectric behaviour other than Maxwell–Wagner effect in ferrites [6], [7], [8], [9]. Recent work by Schrettle et al. [10] on the magnetite (Fe₃O₄) using dielectric spectroscopy and other methods like PE loop provides the evidence for the existence of relaxor-like polar ordering below the Verwey Transition (T_v ≈ 122 K). According to them, as the magnetite exhibits cubic inverse spinel structure at room temperature (RT), the Maxwell–Wagner effect dominates the dielectric behaviour of magnetite above T_v, while the relaxor behaviour originating below T_v is because of the transition of cubic to monoclinic and tetrahedral symmetries. Contemporary to this, the polarized Raman studies of NiFe₂O₄ single crystal [11] and thin films [12] have provided evidence for the existence of short-range B-site ordering even at room temperature. The occurrence of P4₁22/P4₃22 local symmetry which is non-centrosymmetric is said to be responsible for this B-site short-range ordering which is beyond the probing dimension of X-ray diffraction. The prerequisites for the existence of ferroelectricity are the structural distortion that removes the centre of symmetry and the d⁰-ness of the B-site cation [13]. Aforesaid work on the spinel ferrites pound the pavement for studying the ferroelectric nature of MgFe₂O₄ (MFO) which has a partially filled Fe³⁺/Fe²⁺ at A and B sites and Mg²⁺ with d⁰-ness at B-site and locally having a non-centrosymmetric P4₁22/P4₃22 symmetry. Though Ba²⁺ has the similar electronic configuration as that of Mg²⁺, the higher ionic radius Ba²⁺ substitution allows us to explore the role of Mg²⁺ in the dielectric property of MFO. In this light, we intended to study the dielectric behaviour of Ba²⁺ substituted MFO polycrystalline sample for the better understanding of the mechanism of dielectric polarization in spinel ferrite.