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

The direct conversion of electrical energy to mechanical work by a material is relevant to a number of applications. This is illustrated by ferroelectric 'relaxors' such as Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) (PMN-PT; refs 5, 6): these materials exhibit a giant electromechanical (piezoelectric) response that is finding use in ultrasonic and medical applications, as well as in telecommunications. The origins of this effect are, however, still unclear. Here we show that the giant electromechanical response in PMN-PT (and potentially other ferroelectric relaxors) is the manifestation of critical points that define a line in the phase diagram of this system. Specifically, in the electric-field-temperature-composition phase diagram of PMN-PT (the composition being varied by changing the PT concentration), a first-order paraelectric-ferroelectric phase transition terminates in a line of critical points where the piezoelectric coefficient is maximum. Above this line, supercritical evolution is observed. On approaching the critical point, both the energy cost and the electric field necessary to induce ferroelectric polarization rotations decrease significantly, thus explaining the giant electromechanical response of these relaxors.

The giant electromechanical response in ferroelectric relaxors as a critical phenomenon

Advantages and Challenges of Relaxor-PbTiO3 Ferroelectric Crystals for Electroacoustic Transducers- A Review.

The last decade has witnessed a significant growth of research into materials with coupled magnetic and electric properties. Reviewed here are the main types and mechanisms of magnetoelectric interactions and conditions of their origin. Special attention is given to potentially practical materials that display magnetoelectric properties at room temperature. Example applications of magnetoelectric materials and multiferroics in information and energy saving technologies are discussed.

https://iopscience.iop.org/article/10.3367/UFNe.0182.201206b.0593/pdf

Magnetoelectric and multiferroic media

Relaxor-based ferroelectric single crystals: growth, domain engineering, characterization and applications.

It has been found that aliovalent-substituted $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$-based polycrystalline materials have significantly enhanced multiferroic properties. Relative to unmodified $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$, our results for modified $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$-based ceramics reveal: (i) a dramatic increase in the electric-field-induced polarization; and (ii) the establishment of a remanent magnetization. The results evidence the destruction of a space-modulated spin structure in bulk materials, via substituent effects, releasing a latent magnetization locked within the cycloid.

Multiferroic properties of modified BiFeO3-PbTiO3-based ceramics: Random-field induced release of latent magnetization and polarization

The structural phase transformations of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-30%PT) have been studied using x-ray diffraction (XRD) and neutron scattering as a function of temperature and electric field. We observe the phase transformational sequence (i) cubic (C)-> tetragonal (T)-> rhombohedral (R) in the zero-field-cooled (or ZFC) condition; (ii) C->T-> monoclinic (MC)-> monoclinic (MA) in the field-cooled (or FC) condition; and (iii) R->MA->MC->T with increasing field at fixed temperature beginning from the ZFC condition. Upon removal of the field, the MA phase is stable at room temperature in the FC condition, and also in the ZFC condition with increasing field. Several subtleties of our findings are discussed based on results from thermal expansion and dielectric measurements, including (i) the stability of the MA phase; (ii) a difference in lattice parameters between inside bulk and outside layer regions; and (iii) the diffuse nature of the MA and MC phase transition.

/pdf/x-ray-and-neutron-diffraction-investigations-of-the-4vgoe54dap.pdf

X-ray and Neutron Diffraction Investigations of the Structural Phase Transformation Sequence under Electric Field in 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 Crystals

Structural phase transformations of [110] electric-field-cooled Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-30%PbTiO{sub 3} (PMN-30%PT) crystals have been performed by x-ray diffraction. A phase sequence of cubic (C){yields} tetragonal (T){yields} orthorhombic (O){yields} monoclinic (M{sub B}) was found on field cooling (FC); and a rhombohedral (R){yields}M{sub B}{yields}O sequence was observed with increasing field, beginning from the zero-field-cooled condition at room temperature. The application of the [110] electric field induced a dramatic change in the phase sequence in the FC condition, compared to the corresponding data for PMN-30%PT crystals in a [001] field, which shows that the phase sequence in the FC condition is altered by the crystallographic direction along which a modest electric field (E) is applied. Only when E is applied along [110] are intermediate O and M{sub B} phases observed.

/pdf/intermediate-ferroelectric-orthorhombic-and-monoclinic-m-sub-15s99ed8j6.pdf

Intermediate ferroelectric orthorhombic and monoclinic M{sub B} phases in [110] electric-field-cooled Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-30%PbTiO{sub 3} crystals

Naigang Wang

Papers

Multiferroic properties of modified BiFeO3-PbTiO3-based ceramics: Random-field induced release of latent magnetization and polarization

X-ray and Neutron Diffraction Investigations of the Structural Phase Transformation Sequence under Electric Field in 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 Crystals

Intermediate ferroelectric orthorhombic and monoclinic M{sub B} phases in [110] electric-field-cooled Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-30%PbTiO{sub 3} crystals