Symmetry Analysis of Magnetoelectric Effects in Perovskite-Based Multiferroics

In this article, we performed symmetry analysis of perovskite-based multiferroics: bismuth ferrite (BiFeO3)-like, orthochromites (RCrO3), and Ruddlesden–Popper perovskites (Ca3Mn2O7-like), being the typical representatives of multiferroics of the trigonal, orthorhombic, and tetragonal crystal families, and we explored the effect of crystallographic distortions on magnetoelectric properties. We determined the principal order parameters for each of the considered structures and obtained their invariant combinations consistent with the particular symmetry. This approach allowed us to analyze the features of the magnetoelectric effect observed during structural phase transitions in BixR1−xFeO3 compounds and to show that the rare-earth sublattice has an impact on the linear magnetoelectric effect allowed by the symmetry of the new structure. It was shown that the magnetoelectric properties of orthochromites are attributed to the couplings between the magnetic and electric dipole moments arising near Cr3+ ions due to distortions linked with rotations and deformations of the CrO6 octahedra. For the first time, such a symmetry consideration was implemented in the analysis of the Ruddlesden–Popper structures, which demonstrates the possibility of realizing the magnetoelectric effect in the Ruddlesden–Popper phases containing magnetically active cations, and allows the estimation of the conditions required for its optimization.

In-Plane Current Induced Nonlinear Magnetoelectric Effects In Single Crystal Films of Barium Hexaferrite

This report is on the observation and analysis of nonlinear magnetoelectric effects (NLME) for in-plane currents perpendicularly to the hexagonal axis in single crystals and liquid phase epitaxy grown thin films of barium hexaferrite. Measurements involved tuning of ferromagnetic resonance (FMR) at 56-58 GHz in the multidomain and single domain states in the ferrite by applying a current. Data on the shift in the resonance frequency with input electric power was utilized to estimate the variations in the magnetic parameter that showed a linear dependence on the input electric power. The NLME tensor coefficients were determined form the estimated changes in the magnetization and uniaxial anisotropy field. The estimated NLME coefficients for in-plane currents are shown to be much higher than for currents flowing along the hexagonal axis. Although the frequency shift of FMR was higher for the single domain resonance, the multi-domain configuration is preferable for device applications since it eliminates the need for a large bias magnetic field. Thus, multidomain resonance with current in the basal plane is favorable for use in electrically tunable miniature, ferrite microwave signal processing devices requiring low operating power.

Magnetoelectricity in Jahn–Teller Elastics

The magnetoelectric effects in Jahn–Teller crystals are discussed on the basis of phenomenology and microscopic theory. New magnetoelectric effects—metamagnetoelectricity—are analyzed. Formation of multiferroic crystal states as the consequence of the cooperative Jahn–Teller effect is discussed.

MAGNETOELECTRIC EFFECTS IN MULTIFERROICS

Research of magnetoelectric effects and multiferroic materials, in which these effects are manifested, is among key areas in modern magnetism. This is associated with promising aspects of applying the results to spintronics, orbitronics, and new generation systems for information storage and processing. Despite a great many of already known materials which, in varying degrees, have magnetoelectric properties, the question regarding physical mechanisms and nature of magnetoelectric effects still remains open. Single-phase multiferroics with their magnetic and segnetoelectric properties implemented in a single crystalline phase are of especially great interest for studying. The most known "traditional multiferroics", such as bismuth ferrite, manganites, rare earthearth orthoferrites and orthochromites, fall into the class of perovskite multiferroics, i.e. the crystalline structure of ABO3 perovskites serve as their pre-phase. However, the difference between crystallographic distortions that results in the formation of various crystalline structures, for example, bismuth ferrite and rare-earth orthoferrites/orthochromites, leads to an essential difference in their physical properties, including magnetoelectric ones. Whereas bismuth ferrite characterized by the R3c symmetry space group is a segnetoelectric (i.e. it has spontaneous segnetoelectric polarization), the presence of segnetoelectric polarization in orthoferrites/orthochromites with the Pbnm symmetry space group is impossible from the symmetry standpoint. However, recent experimental and theoretical research works show that under certain conditions magnetoelectric properties are found in both classes of the said multiferroics. This paper is an overview by its nature and discusses magnetoelectric effects in various classes of single-phase multiferroics with the distorted perovskite structure: proper multiferroics exemplified by bismuth ferrite and improper multiferroics exemplified by rare-earth orthoferrites/orthochromites. Consideration is given to basic principles of the symmetry approach used to study magnetoelectric effects in multiferroics; calculations and analysis of magnetoelectric effects in rare-earth orthoferrites/orthochromites are performed through the methods of group-theoretical analysis.