A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

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Multiferroic and magnetoelectric materials

BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Physics and Applications of Bismuth Ferrite

Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties — and in turn promise new device applications — as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.

Multiferroics: progress and prospects in thin films.

Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result, they have a spontaneous magnetization that can be switched by an applied magnetic field, a spontaneous polarization that can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature or have been synthesized in the laboratory. In this paper, we explore the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. In addition, we examine the properties of some known magnetically ordered ferroelectric materials. We find that, in general, the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Consequently, an additional electronic or structural driving force must be present for ferromagnetism and ferroelectricity to occur simultaneously.

Why Are There so Few Magnetic Ferroelectrics

New information on the magnetic ordering of the iron ions in bismuth ferrite BiFeO3 was obtained by a study with a high-resolution time-of-flight neutron diffractometer. The observed splitting of magnetic diffraction maxima could be interpreted in terms of a magnetic cycloidal spiral with a long period of 620+or-20 AA, which is unusual for perovskites.

https://iopscience.iop.org/article/10.1088/0022-3719/15/23/020/pdf

Spiral magnetic ordering in bismuth ferrite

The temperature dependence of structural and magnetic order parameters of ferroelectric-antiferromagnetic BiFeO3 with rhombohedral perovskite structure is investigated by means of neutron diffraction on a powder sample. The tilt angle of oxygen octahedra decreases from 12.5 degrees at 4.2K to 11.4 degrees at 878K. Associated with ferroelectricity the cation displacements diminish essentially with increasing temperature. Changes of cation shifts, tilt angle, distortion and strain of octahedra are discussed with respect to phase transitions.

https://iopscience.iop.org/article/10.1088/0022-3719/13/10/012/pdf

Temperature dependence of the crystal and magnetic structures of BiFeO3

Neutron powder diffraction has been performed on polycrystalline BiMnxFe1-xO3 (x=0, 0.1 and 0.2). The structures of the doped compounds are described by the space group R3c of ferroelectric BiFeO3. Refined structure parameters are presented. Mn doping generates microstructural changes manifested by broadening in the diffraction patterns. The lattice parameters show a non-linear behaviour from 4 K to 630 K. Mn doping results in a transformation of the long-range spiral spin modulation of BiFeO3 to a collinear antiferromagnetic structure with spins along c. The average magnetic moments and the ordering temperatures decrease with increasing Mn concentration.

Crystal structure and spiral magnetic ordering of BiFeO 3 doped with manganese

Origin of the long period magnetic ordering in BiFeO3

The parameters of the crystal structure of BiFeO3, described within the space group R3c, have been determined by high-resolution neutron powder diffraction for temperatures from 293 to 923 K. It was found that there is a local minimum for the rhombohedral angle αrh, near the Neel temperature TN ≃ 640 K, a gradual rotation of the FeO6 octahedra and an increase of the Fe—O—Fe angle. The displacement of the Bi3+ ions from the FeO6 octahedra which influence the electric polarization decreases with temperature. One of the Bi—Fe distances also has a local maximum near TN. The atomic vibrations of Bi3+ and O2− ions show a significant anisotropy.

Izabela Sosnowska

Papers

Spiral magnetic ordering in bismuth ferrite

Temperature dependence of the crystal and magnetic structures of BiFeO3

Crystal structure and spiral magnetic ordering of BiFeO 3 doped with manganese

Origin of the long period magnetic ordering in BiFeO3

Atomic displacements in BiFeO(3) as a function of temperature: neutron diffraction study.