Non-d$^0$ Electric Dipole in FeO$_5$ bipyramid: a New Resource for Quantum Paraelectrics, Ferroelectrics and Multiferroics

TL;DR

This paper reveals a novel mechanism for local electric dipoles in FeO$_5$ bipyramids involving magnetic Fe$^{3+}$ ions, challenging the traditional d$^0$ rule and opening new avenues for multiferroic materials.

Contribution

It uncovers a new mechanism for electric dipoles based on Fe$^{3+}$ ions violating the d$^0$ rule, expanding the understanding of ferroelectricity in magnetic oxides.

Findings

Fe$^{3+}$ ions can induce local electric dipoles by off-center displacement.

The mechanism applies to ferrimagnetic hexaferrites, creating magnetic quantum paraelectrics.

Abstract

Electric polarization in conventional ferroelectric oxides usually involves nonmagnetic transition-metal ions with an empty d shell (the d$^0$ rule). Here we unravel a new mechanism for local electric dipoles based on magnetic Fe$^{3+}$ (3d$^5$) ion violating the d$^0$ rule. The competition between the long-range Coulomb interaction and short-range Pauli repulsion in a FeO$_5$ bipyramid with proper lattice parameters would favor an off-center displacement of Fe$^{3+}$ that induces a local electric dipole. The manipulation of this kind of non-d$^0$ electric dipoles opens up a new route for generating unconventional dielectrics, ferroelectrics, and multiferroics. As a prototype example, we show that the non-d$^0$ electric dipoles in ferrimagnetic hexaferrites (Ba,Sr)Fe$_{12}$O$_{19}$ lead to a new family of magnetic quantum paraelectrics.