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

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.

Progress in the Development of Single-Phase Magnetoelectric Multiferroic Oxides

Magnetoelectric (ME) multiferroics manifesting the coexistence and the coupling of ferromagnetic and ferroelectric order are appealing widespread interest owing to their fascinating physical behaviors and possible novel applications. In this review, we highlight the progress in single-phase ME multiferroic oxides research in terms of the classification depending on the physical origins of ferroic properties and the corresponding examples for each case, i.e., material by material, along with their ME multiferroic properties including saturation magnetization, spontaneous polarization, (anti)ferromagnetic/ferroelectric transition temperature, and ME coefficient. The magnetoelectrically-active applications of high expectancy are presented by citing the representative examples such as magnetoelectric random-access-memory and multiferroic photovoltaics. Furthermore, we discuss how the development of ME multiferroic oxides should proceed by considering the current research status in terms of developed materials and designed applications. We believe that this short review will provide a basic introduction for the researchers new to this field.