We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO ₃ -CoFe ₂ O ₄ ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO ₃ (001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe ₂ O ₄ nanopillars embedded in a BaTiO ₃ matrix. The CoFe ₂ O ₄ nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases.

In ${\mathrm{BiFeO}}_3$ films, it has been found that epitaxial constraint results in the destruction of a space modulated spin structure. For ${\left(111\right)}_c$ films, relative to corresponding bulk crystals, it is shown (i) that the induced magnetization is enhanced at low applied fields; (ii) that the polarization is dramatically enhanced; whereas, (iii) the lattice structure for ${\left(111\right)}_c$ films and crystals is nearly identical. Our results evidence that eptiaxial constraint induces a transition between cycloidal and homogeneous antiferromagnetic spin states, releasing a latent antiferromagnetic component locked within the cycloid.

A magnetoelectric (ME) laminate composite consisting of a symmetric longitudinally poled piezoelectric $\mathrm{Pb}\left({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}\right){\mathrm{O}}_3–{\mathrm{PbTiO}}_3$ crystal and two longitudinally magnetized magnetostrictive ${\mathrm{Tb}}_{1-x}{\mathrm{Dy}}_x{\mathrm{Fe}}_2$ layers has been developed that has a notably superior ME voltage coefficient, relative to previous laminate configurations. The symmetric nature of the longitudinally poled piezoelectric layer allows for operation in a push-pull mode that optimizes elastic coupling between layers. Our small laminate has a giant ME voltage coefficient of $\sim 1.6\mathrm{V}∕\mathrm{Oe}$ at low frequencies, a significant enhancement of this coefficient to $\sim 20\mathrm{V}∕\mathrm{Oe}$ under resonance drive, and an exceptional low-level magnetic field sensitivity of $\sim {10}^{-12}\mathrm{T}$ at $f=f_0$ .

The structural phase transformations of $0.7\mathrm{Pb}\left({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}\right){\mathrm{O}}_3\text{-}0.3{\mathrm{PbTiO}}_3$ $\left(\mathrm{PMN}\text{-}30\%\mathrm{PT}\right)$ 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 $\left(\mathrm{C}\right)\to$ tetragonal $\left(\mathrm{T}\right)\to$ rhombohedral (R) in the zero-field-cooled (ZFC) condition; (ii) $\mathrm{C}\to \mathrm{T}\to$ monoclinic $\left({\mathrm{M}}_{\mathrm{C}}\right)\to$ monoclinic $\left({\mathrm{M}}_{\mathrm{A}}\right)$ in the field-cooled (FC) condition; and (iii) $\mathrm{R}\to {\mathrm{M}}_{\mathrm{A}}\to {\mathrm{M}}_{\mathrm{C}}\to \mathrm{T}$ with increasing field at fixed temperature beginning from the ZFC condition. Upon removal of the field, the ${\mathrm{M}}_{\mathrm{A}}$ 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 ${\mathrm{M}}_{\mathrm{A}}$ phase, (ii) a difference in lattice parameters between inside bulk and outside layer regions, and (iii) a difference in the phase transition temperature between XRD and dielectric data.

The domain structures of annealed (001)-oriented $\mathrm{Pb}\left({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}\right){\mathrm{O}}_3\text{-}x\%{\mathrm{PbTiO}}_3(\mathrm{PMN}\text{-}x\%\mathrm{PT})$ crystals for $x=10$ , 20, 30, 35, and $40\mathrm{at.}\%$ have been investigated by polarized optical microscopy and scanning force microscopy in the piezoresponse mode. The results demonstrate the presence of a domain hierarchy on various length scales ranging from $40\mathrm{nm}$ to $0.1\mathrm{mm}$ , which varies with $x$ .

Structural phase transformations of [110] electric-field-cooled $\mathrm{Pb}\left({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}\right){\mathrm{O}}_3$-30%$\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ (PMN-30%PT) crystals have been performed by x-ray diffraction. A phase sequence of cubic $\left(C\right)\to$ tetragonal $\left(T\right)\to$ orthorhombic $\left(O\right)\to$ monoclinic $\left(M_{\mathit{B}}\right)$ was found on field cooling (FC); and a rhombohedral $\left(R\right)\to M_B\to O$ 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 $\left(E\right)$ is applied. Only when $E$ is applied along [110] are intermediate $O$ and $M_B$ phases observed.