A brief review is given of electrical properties of magnetoelectric, multiferroic materials, with emphasis on magnetocapacitance effects, nanostructures, integration into real random access memories, and critical phenomena, including defect dynamics near phase transitions.

Optical second-harmonic generation (SHG) was applied to study the ferroelectric and magnetic properties of strained epitaxial BiFeO3 films deposited on (001) SrTiO3 substrates by metal organic chemical vapor deposition. A strong interplay between the ferroelectric and magnetic subsystems of bismuth ferrite films results in the pronounced temperature dependence of the SHG intensity. The decrease of the critical temperature of the ferroelectric phase transition and the expansion of the nonlinear transient area near Néel temperature were observed for the strained BiFeO3 films.

Coexistence of ferroelectric and ferromagnetic order parameters was observed at room temperature in Bi0.6Dy0.3La0.1FeO3 thin films grown on Pt/TiO2/SiO2/Si substrates using a pulsed laser deposition technique similar to that for Bi0.6Tb0.3La0.1FeO3. The coexistence of ferroelectric and magnetic domains in specific spatial area of the thin film was also confirmed by scanning probe imaging. As expected, the magnetization values obtained for Bi0.6Dy0.3La0.1FeO3 bulk and thin films were higher than those of Bi0.6Tb0.3La0.1FeO3 bulk and polycrystalline thin films because the magnetic moment of Dy is higher than that of Tb. However, preferentially oriented thin films of Bi0.6Tb0.3La0.1FeO3 exhibit much higher magnetization values. It is speculated that structural alignment caused by stress developed during deposition of these films could be responsible for enhancement in magnetization.

Measurements of inverse magnetoelectric (ME) effects in lead zirconate titanate (PZT)–Ni–PZT trilayers are reported. Traditional ME measurements involve the electrical response of a composite subjected to an alternating current (ac) magnetic field. In the case of an “inverse ME effect,” one measures the variation in the magnetic induction due to an external ac electric field applied to PZT. A pickup coil wound around the sample is used to measure the ME voltage due to the change in the magnetic induction in Ni. The measured static magnetic-field dependence of ME voltage has been attributed to the variation in the piezomagnetic coefficient for Ni. The frequency dependence of the ME voltage shows a resonance character due to radial acoustic modes in PZT. Theoretical estimates of ME susceptibility are in excellent agreement with values determined from data on induced voltage.

The ferroelectric and magnetic properties of the perovskite solid solution, (1 − x)LaCrO3–xBiCrO3, have been investigated. While pure LaCrO3 does not show ferroelectric hysteresis even at 77 K, the solid solution of La1−xBixCrO3 with x = 0.1, 0.2, 0.3, and 0.35 displays ferroelectric hysteresis, with the remanent polarization increasing with the increase of the Bi3+ content. Using a superconducting quantum interference device, the magnetization was measured versus temperature under field cooling (FC) and zero field cooling (ZFC) conditions. Magnetic hysteresis has been found in La1−xBixCrO3 (0.1 ⩽ x ⩽ 0.3) below the Néel temperature, TN. With the increase of Bi3+ content, TN decreases, while the magnetization below TN is enhanced. While the ferroelectric and magnetic properties could be due to different origins, the Bi substitution results in both ferroelectric and magnetic enhancements in the (1 − x)LaCrO3–xBiCrO3 solid solutions.

A thermodynamic approach was used to describe the formation and magnetoelectric response of composite multiferroic films. Experimental and theoretical results that address the origins of different phase morphologies in epitaxial spinel-perovskite nanostructures grown on differently oriented substrates are presented. A theoretical model of magnetoelectric coupling in multiferroic nanostructures that considers a microscopic mechanism of magnetization in single-domain magnetic nanorods is described. This model explains a discontinuous electromagnetic coupling, as observed experimentally, and predicts a hysteretic behavior of magnetization under external electric fields.

The structure, magnetic response, and dielectric response of the grown epitaxial thin films of the orthorhombic phase of YMnO3 oxide on Nb:SrTiO3 (001) substrates have been measured. We have found that a substrate-induced strain produces an in-plane compression of the YMnO3 unit cell. The magnetization versus temperature curves display a significant zero-field cooling (ZFC)-field cooling hysteresis below the Néel temperature (TN ≈ 45 K). The dielectric constant increases gradually (up to 26%) below the TN and mimics the ZFC magnetization curve. We argue that these effects could be a manifestation of magnetoelectric coupling in YMnO3 thin films and that the magnetic structure of YMnO3 can be controlled by substrate selection and/or growth conditions.

Epitaxial thin films of Bi2FeCrO6 (BFCO) have been synthesized by pulsed laser deposition on SrRuO3 on (100)- and (111)-oriented SrTiO3 substrates. Detailed x-ray diffraction and cross-section transmission electron microscopy analysis revealed a double perovskite crystal structure of the BFCO epitaxial films very similar to that of BiFeO3 along with a particularly noteworthy Fe3+/Cr3+ cation ordering along the [111] direction. The films contain no detectable magnetic iron oxide impurities and have the correct cationic average stoichiometry throughout their thickness. They however exhibit a slight modulation in the Fe and Cr compositions forming complementary stripe patterns, suggesting minor local excess or depletion of Fe and Cr. The epitaxial BFCO films exhibit good ferroelectric and piezoelectric properties, in addition to magnetic properties at room temperature, as well as an unexpected crystallographic orientation dependence of their room-temperature magnetic properties. Our results qualitatively confirm the predictions made using the ab initio calculations: the double perovskite structure of BFCO films exhibit a Fe3+/Cr3+ cation ordering and good multiferroic properties, along with the unpredicted existence of magnetic ordering at room temperature.

We present a method that allows changing the anisotropy and the magnetic characteristics of piezoelectric–ferromagnetic hybrid structures by electric fields, thereby suppressing the need for external or local magnetic fields. We have investigated the magnetic properties of single Co50Fe50 and CoFe80B20 magnetostrictive thin films as well as of high-quality bottom-pinned spin valves (SV) sputtered on piezoelectric substrates [lead zirconate titanate (PZT)] and patterned in an interdigitated transducer (IDT). Induction of a uniaxial anisotropy axis and the changes on coercivity and switching fields as a function of the applied bias voltage on the IDT are analyzed and interpreted. The down-scalability of this hybrid method supports the possibility of achieving low-power/low-voltage control of the switching fields and shows the feasibility of a hybrid ferroelectric magnetic memory cell.

Single crystals of the perovskite solid solution (1 − x)Pb(Fe2/3W1/3)O3–xPbTiO3, with x = 0, 0.07, 0.27, and 0.75, have been synthesized by the high-temperature solution growth using PbO as flux and characterized by x-ray diffraction and dielectric and magnetic measurements. The crystal structure at room temperature changes from a pseudocubic to a tetragonal phase with the PbTiO3 (PT) content increasing to x ⩾ 0.27. As the amount of PT increases, the relaxor ferroelectric behavior of Pb(Fe2/3W1/3)O3 (PFW) is transformed toward a normal ferroelectric state with sharp and nondispersive peaks of dielectric permittivity at TC. Two types of magnetic orderings are observed on the temperature dependence of the magnetization in the crystals with x ⩽ 0.27. This behavior is explained based on the relationships among the magnetic ordering, perovskite structure, composition, and relaxor ferroelectric properties. Furthermore, the macroscopic magnetization of the system was measured under the application of a magnetic field, which demonstrates different magnetic behavior associated with the weakly ferromagnetic, antiferromagnetic, and paramagnetic ordering in the temperature range of 2 to 390 K. Interestingly, the low-temperature ferromagnetism is enhanced by the addition of ferroelectric PT up to x = 0.27.

A novel multiferroic thin-film heterostructure exhibiting both ferromagnetic (FM) and ferroelectric (FE) properties, as well as magneto-optic (MO) and electro-optic (EO) properties, was fabricated via a wet chemical route. Oxide buffer layers were used to allow the growth of ferroelectric lanthanum modified lead magnesium niobate titanate (La:PMNT) layer on top of ferromagnetic bismuth and aluminum substituted yttrium iron garnet (BiAl:YIG). X-ray diffractometer (XRD) analysis confirmed the formation of both crystalline structures. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to examine the surface and cross-section morphologies of the resulted heterostructure. Multiferroic properties of the film were investigated.

The nature of magnetoelectric (ME) interactions has been investigated in lead zirconate titanate (PZT) and (111) or (110) single-crystal nickel zinc ferrites. Data on the dependence of low-frequency ME voltage coefficients on static magnetic field orientation show (i) highest ME coefficients for bias field H along [100] and the smallest for H parallel to [110] and (ii) strongest ME interactions for transverse fields and for samples with Zn concentration of 0.3. Measurements on frequency dependence of ME coefficients reveal resonance enhancement due to bending and radial acoustic modes. The highest voltage coefficient is measured for radial modes in a sample with Zn concentration of 0.2. Theoretical estimates of low-frequency and resonance ME parameters are in very good agreement with data.

Single crystals of the multiferroic (1 − x)BiFeO3–xPbTiO3 (BF–PT) solid solution with a nominal morphotropic phase boundary (MPB) composition were grown from flux. Structural characterization by x-ray diffraction shows the simultaneous existence of a tetragonal, an orthorhombic, and a rhombohedral perovskite phase in the crystals. A high ferroelectric Curie point of 660 °C was found in the BF–PT crystals by dielectric measurements. The variation of the magnetic moment as a function of temperature of the BF–PT crystals measured under zero field cooling mode reveals three anomalies with the highest one around 440 K, corresponding to the antiferromagnetic ordering temperatures of the rhombohedral, orthorhombic, and tetragonal phases, respectively. These results demonstrate the intrinsic relations between the MPB phase components and the macroscopic ferroic properties.

Multiferroic magnetoelectrics are materials that are both ferroelectric and ferromagnetic in the same phase. In addition, electric and magnetic polarizations are strongly coupled in some magnetoelectric multiferroic materials. In this work, by virtue of the image method, exact closed-form Green’s functions are derived for a uniaxial multiferroic full-space, half-space, and bimaterial space. While for the bimaterial space case the interface is assumed to be perfect, for the half-space case four different sets of surface conditions are considered. The point source can be either an electric or a magnetic charge. Numerical results are presented to demonstrate the differences among the infinite-space, half-space, and bimaterial space Green’s functions.

TbMnO3 exists in an orthorhombic phase in nature. Recently, we successfully grew TbMnO3 thin films in the hexagonal phase using epitaxial stabilization techniques. In this article, we will show the details of the deposition conditions that allow us to fabricate the hexagonal TbMnO3 films on Pt–Al2O3(0001) substrates. The artificial hexagonal phase can be easily formed above 850 °C, irrespective of the oxygen partial pressure. The hexagonal TbMnO3 films showed ferroelectric properties, which are significantly enhanced compared to those of the orthorhombic TbMnO3 bulk phase. We find interesting anomalies in the magnetic and magnetodielectric properties of the TbMnO3 films at around 45 K, which should be related with the Mn3+ spin reorientation. We also find spin-glass-like behaviors in the magnetic susceptibility, which could be attributed to the geometric frustration of antiferromagnetically coupled Mn spins with an edge-sharing triangular lattice. This work shows details of the growth and properties of hexagonal TbMnO3 films.

The magnetic phase diagrams of RMnO3 (R = Er, Yb, Tm, Ho) are investigated up to 14 T via magnetic and dielectric measurements. The stability range of the atomic force microscopy order below the Néel temperature of the studied RMnO3 extends to far higher magnetic fields than previously assumed. Magnetic irreversibility indicating the presence of a spontaneous magnetic moment is found near 50 K for R = Er, Yb, and Tm. At very low temperatures and low magnetic fields, the phase boundary defined by the ordering of the rare-earth moments is resolved. The sizable dielectric anomalies observed along all phase boundaries are evidence for strong spin-lattice coupling in the hexagonal RMnO3. In HoMnO3, the strong magnetoelastic distortions are investigated in more detail via magnetostriction experiments up to 14 T. The results are discussed based on existing data on magnetic symmetries and the interactions among the Mn-spins, the rare-earth moments, and the lattice.

In ferrite–piezoelectric bilayers, the magnetoelectric (ME) interaction is mediated by mechanical strain. The ME coupling is expected to be strong, particularly when the magnetic and electric subsystems show resonance. Here we address the effect of magnetic exchange interactions on ME coupling at magnetoacoustic resonance (MAR), i.e., at the coincidence of electromechanical resonance in the piezoelectric phase and ferromagnetic resonance in a tangentially magnetized ferrite. When exchange is ignored, the estimated ME coefficient versus frequency profile shows a giant magnetoelectric coefficient at MAR, about 75–100 V/cm Oe for yttrium–iron garnet (YIG)/lead zirconate–titanate (PZT) nano bilayers. The magnetic exchange is predicted to enhance the coupling at MAR and produce a secondary peak due to the excitation of magnetoacoustic modes. Estimates of the ME coefficient are provided as a function of thickness ratio of YIG and PZT.

The dielectric response of La- and Dy- doped BiFeO3 thin films at microwave frequencies (up to 12 GHz) has been monitored as a function of frequency, direct current (dc) electric field, and magnetic field in a temperature range from 25 to 300 °C. Both the real and imaginary parts of the response have been found to be non-monotonic (oscillating) functions of measuring frequency. These oscillations are not particularly sensitive to a dc electric field; however, they are substantially dampened by a magnetic field. The same effect has been observed when the volume of the characterized sample is increased. This phenomenon is attributed to the presence of a limited number of structural features with a resonance type response. The exact origin of these features is unknown at present. Leakage current investigations were performed on the whole set of films. The films were highly resistive with low leakage current, thereby giving us confidence in the microwave measurements. These typically revealed ‘N’-type I-V characteristics.

The photocatalyst BaZn1/3Nb2/3O3 with ABO3 perovskite structure has been synthesized by using a solid-state reaction process. It was characterized by x-ray diffraction and photoluminescence spectroscopy. The luminescence band centers around 285 nm and shows a large Stokes shift compared with the excitation spectrum, indicating a strong electron–phonon interaction in the photocatalyst BaZn1/3Nb2/3O3. The electronic structure of BaZn1/3Nb2/3O3 was calculated by using the pseudopotential method of the density function theory. It shows that the conduction band should be mainly composed of the Nb 4d states, and the valence band should be mainly composed of the O 2p state. The densities of the O 2p states and the Zn 4s states at the bottom of the conduction band are very low. The Zn 4s states show an expanded structure, which was proposed to be helpful for the migration of the photoexcited carriers, thus favoring the photocatalytic activity of BaZn1/3Nb2/3O3.