Origin of Perovskite Multiferroicity and Magnetoelectric-Multiferroic Effects—The Role of Electronic Spin in Spontaneous Polarization of Crystals

Isaac B Bersuker

doi:10.3390/magnetochemistry8010009

Isaac B Bersuker

Open Access

https://doi.org/10.3390/magnetochemistry8010009

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Abstract

In this semi-review paper, we show that the multiferroic properties of perovskite ABO3 crystals with B(dn), n > 0, centers are fully controlled by the influence of the electronic spin on the local dipolar instability that triggers the spontaneous polarization of the crystal. Contrary to the widespread statements, the multiferroicity of these crystals does not emerge due to the addition of unpaired electrons (carrying magnetic moments) to the spontaneously polarizing crystal; the spin states themselves are an important part of the local electronic structure that determines the very possibility of the spontaneous polarization. This conclusion emerges from vibronic theory, in which the ferroelectricity is due to the cooperative interaction of the local dipolar distortions induced by the pseudo-Jahn-Teller effect (PJTE). The latter requires sufficiently strong vibronic coupling between ground and excited electronic states with opposite parity but the same spin multiplicity. The detailed electronic structure of the octahedral [B(dn)O6] center in the molecular orbital presentation shows how this requirement plays into the dependence of the possible perovskite magnetic, ferroelectric, and multiferroic properties on the number of d electrons, provided the criterion of the PJTE is obeyed. Revealed in detail, the role of the electronic spin in all these properties and their combination opens novel possibilities for their manipulation by means of external perturbations and exploration. In particular, it is shown that by employing the well-known spin-crossover phenomenon, a series of novel effects become possible, including magnetic-ferroelectric (multiferroic) crossover with electric-multiferroic, magnetic-ferroelectric, and magneto-electric effects, some of which have already been observed experimentally.

Highlights

Published: 11 January 2022Multiferroic perovskite ABO3 crystals with coexisting magnetic and ferroelectric properties are presently well studied, but for a long period of time, it was assumed that ferroelectricity takes place only when the B atom is in the d0 configuration (“the d0 mystery” [3]), as in the well-known ferroelectric BaTiO3, with zero spin and no magnetism
Ferroelectricity and multiferroicity are interdependent and strongly dependent on the spin multiplicity of the ground and excited electronic states. This leads to a variety of possible magneto-electric-multiferroic effects. In this semi-review paper, we show in more detail the micromechanisms of the PJTEinduced multiferroicity in perovskite crystals, especially the role of the electronic spin in their spontaneous polarization, and the spin classification of ferroelectric and multiferroic perovskites with electronic configurations B(dn ), n = 0, 1, . . . 10 [6,7], as well as the occurring due to this effect novel properties in the interaction with external perturbations, including electric-multiferroic, magnetic-ferroelectric, and magneto-electric effects
Since its discovery and until recently, it was assumed that ferroelectricity is a macroscopic crystalline effect, with the widespread theories stating that the spontaneous polarization is due to the compensation of the ion repulsions in the local dipolar displacements with the cooperative dipole-dipole attractions in the bulk crystal

Summary

Introduction

Multiferroic perovskite ABO3 crystals with coexisting magnetic and ferroelectric properties are presently well studied (see, e.g., in [1,2]), but for a long period of time, it was assumed that ferroelectricity takes place only when the B atom is in the d0 configuration (“the d0 mystery” [3]), as in the well-known ferroelectric BaTiO3 , with zero spin and no magnetism. The role of the number of electrons in the B(dn ) ion in multiferroic properties became more clarified when it was taken into account that the spontaneous polarization of such perovskite crystals is triggered by the local dipolar instability at the B centers, induced by the pseudo-Jahn-Teller effect (PJTE) [4,5] This finding directly influences the possible dn configurations, which could allow for ferroelectric properties in the presence of unpaired electrons, meaning multiferroicity [6,7]. This, in turn, requires that the mixing local electronic states effect of this B center have opposite parity but the same spin multiplicity The latter condition is essential: the vibronic coupling terms in the Hamiltonian have no spin operators (nuclear displacements directly affect the orbital motion of the electrons, but electric-multiferroic, magnetic-ferroelectric, and m not their spin), and electronic states with different spin multiplicity are orthogonal.

Electronic Structure and PJTE in Perovskite A

Conclusions