# Ferroelectric order associated with an ordered occupancy at the octahedral site of the inverse spinel structure of multiferroic NiFe2O4

## TL;DR

This study discovers a ferroelectric phase transition in NiFe2O4 at 98 K linked to ordered Ni and Fe occupancy, revealing significant magnetoelectric coupling and classifying it as a new type-II multiferroic material.

## Contribution

It demonstrates the emergence of ferroelectric order in NiFe2O4 due to ordered occupancy at octahedral sites, a novel finding for this inverse spinel structure.

## Key findings

- Ferroelectric order appears at ~98 K with high polarization (~0.29 μC/cm²).
- Structural transition to a noncentrosymmetric phase at 98 K confirmed by synchrotron diffraction.
- Significant magnetoelectric coupling observed with polarization reduction under magnetic field.

## Abstract

We report a ferroelectric order at ~ 98 K for NiFe2O4, which carries an inverse spinel structure with a centrosymmetric Fd3m structure at room temperature. The value of spontaneous electric polarization is considerably high as ~ 0.29 {\mu}C/cm2 for 5 kV/cm poling field. The electric polarization decreases considerably (~ 17 %) around liquid nitrogen temperature upon application of 50 kOe field, proposing a significant magnetoelectric coupling. The synchrotron diffraction studies confirm a structural transition at ~ 98 K to a noncentrosymmetric structure of P4122 space group. The occurrence of polar order is associated with an ordered occupancy of Ni and Fe atoms at the octahedral sites of the P4122 structure, instead of random occupancies at the octahedral site of the inverse spinel structure. The results propose that NiFe2O4 is a new type-II multiferroic material.