Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe2O4

Abstract

Ferroelectric materials are widely used in modern electric devices such as memory elements, filtering devices and high-performance insulators. Ferroelectric crystals have a spontaneous electric polarization arising from the coherent arrangement of electric dipoles (specifically, a polar displacement of anions and cations). First-principles calculations and electron density analysis of ferroelectric materials have revealed that the covalent bond between the anions and cations, or the orbital hybridization of electrons on both ions, plays a key role in establishing the dipolar arrangement. However, an alternative model-electronic ferroelectricity-has been proposed in which the electric dipole depends on electron correlations, rather than the covalency. This would offer the attractive possibility of ferroelectric materials that could be controlled by the charge, spin and orbital degrees of freedom of the electron. Here we report experimental evidence for ferroelectricity arising from electron correlations in the triangular mixed valence oxide, LuFe(2)O(4). Using resonant X-ray scattering measurements, we determine the ordering of the Fe(2+) and Fe(3+) ions. They form a superstructure that supports an electric polarization consisting of distributed electrons of polar symmetry. The polar ordering arises from the repulsive property of electrons-electron correlations-acting on a frustrated geometry.

Intrinsic electrical properties of LuFe2O4

“…19) μC cm −2 , do not agree either between them or with the reported value of near 30 μC cm −2 deduced from pyroelectric current measurements. 4 On the contrary, a P (E) curve obtained in another recent work for a polycrystal sample at 140 K and rather high frequency of 80 kHz (Ref. 17) shows a nearly linear response suggesting the lack of ferroelectricity.…”

“…2D charge correlations are observed below 500 K, while the ferroelectric phase transition is proposed to coincide with a 3D Fe 3+ /Fe 2+ CO below 320 K. 4,5,10 This is followed by ferrimagnetic order below T N ∼ 240 K. 2,[11][12][13] The general claim about ferroelectricity in LuFe 2 O 4 has been based on the following experimental results: (i) the observation of giant dielectric constant at temperatures above 150 K, [4][5][6] and (ii) the measurement of the pyroelectric current after field polarization. 4,5,14 The colossal dielectric properties reported for LuFe 2 O 4 (Refs. 4-6) were accepted as originating from the motion of the ferroelectric domain boundary due to electron exchange between Fe 2+ and Fe 3+ .…”

“…From the structural point of view, recent singlecrystal x-ray-diffraction data agree with a structure of charged Fe nonpolar bilayers, 22 which is in contrast with the proposed CO with polar bilayers. 4 At this point, the only evidence for ferroelectricity in LuFe 2 O 4 is that provided by the pyroelectric current detection experiments. 4 However, as we will explain hereafter, there is not a possibility of reproducing such an experiment due to the high conductivity of LuFe 2 O 4 above the CO transition temperature.…”

“…3 Among the multiferroics, LuFe 2 O 4 is considered as a prototype material where ferroelectricity is driven by the electronic process of frustrated (Fe 2+ /Fe 3+ ) charge ordering (CO) at T CO ≈ 320 K, which is also coupled to magnetism and magnetic field. [4][5][6][7] LuFe 2 O 4 belongs to the rare-earth-iron oxide family RFe 2 O 4 (R = Ho-Lu and Y) adopting a layered structure. 8 It is a mixed valence oxide since the formal iron valence is 2.5.…”

First-principles study of ferroelectricity induced by p–d hybridization in ferrimagnetic NiFe 2 O 4

“…These compounds, however, have antiferromagnetic order, which makes their application strongly limited because of zero magnetization (and therefore uncontrollability), and what is worse, they have quite small electric polarization (≤0.1 µC/cm 2 ). On the other hand, it was found from first-principles and experiments that, in some ferrites including magnetite Fe 3 O 4 [21][22][23] and rare earth ferrites RFe 2 O 4 (R = Y, Yb or Lu) [20,[25][26][27], charge ordering or spin-charge ordering induces a spontaneous polarization of remarkably large value: up to 5 µC/cm 2 in monoclinic Fe 3 O 4 [23] and even tens of µC/cm 2 in hexagonal LuFe 2 O 4 [26], comparable to those of conventional perovskite ABO 3 -type ferroelectrics as well as proper multiferroics. In these ferrites, moreover, the charge and spin degrees of freedom of electrons can be controlled simultaneously, because they have ferrimagnetic order and thus may have non-zero magnetization.…”