Composite Multiferroics Research Articles

Effect of vacancies (O, Ti) on the interfacial bonding strength and magnetoelectricity in Fe/BaTiO3: A first-principles study

As the important structure elements, vacancies are believed to be a crucial factor in the composite multiferroics. In the work, first-principles calculations have been performed to study the effect of vacancies (O, Ti) on the structural, magnetoelectric and electronic properties of the Fe/BaTiO3 interface. The vacancy-type defects are introduced in the first interfacial TiO2 layer. It is found that the interfacial vacancies (O, Ti) will increase the bonding strength of the ferromagnetic/ferroelectric system while the magnetoelectric effect can still keep the same order of magnitude as perfect Fe/BaTiO3, which may be helpful for designing multiferroic thin film materials.

Coupling of ferroelectricity and ferromagnetism through Coulomb blockade in composite multiferroics

Composite multiferroics are materials exhibiting the interplay of ferroelectricity, magnetism, and strong electron correlations. Typical example --- magnetic nano grains embedded in a ferroelectric matrix. Coupling of ferroelectric and ferromagnetic degrees of freedom in these materials is due to the influence of ferroelectric matrix on the exchange coupling constant via screening of the intragrain and intergrain Coulomb interaction. Cooling typical magnetic materials the ordered state appears at lower temperatures than the disordered state. We show that in composite multiferroics the ordered magnetic phase may appear at higher temperatures than the magnetically disordered phase. In non-magnetic materials such a behavior is known as inverse phase transition.

Nonlinear magnetoelectric effect in composite multiferroics

The theoretical and experimental studies of the nonlinear magnetoelectric effect in composite multiferroics in the low-frequency spectral region and in the electromechanical resonance region have been performed. It has been shown that such structures demonstrate a nonlinear magnetoelectric effect, which is quadratic in ac magnetic field strength at weak magnetic fields. In the region of the electromechanical resonance, the resonance excitation of an electric field occurs by means of ac magnetic field at a frequency lower than the resonance frequency by a factor of two. In the low-frequency spectral region, there is a difference of amplitude values of two neighboring voltage maxima due to the superposition of signals from the linear and nonlinear effects, and the difference is proportional to the dc magnetic field strength in weak fields. The results of the experimental study of the two-layer permendur-lead zirconate titanate structure are presented.

Electric Field‐Induced Oxidation of Ferromagnetic/Ferroelectric Interfaces

Composite multiferroics are a new class of material where magneto‐electric coupling is achieved by creating an interface between a ferromagnetic and a ferroelectric compound. The challenge of understanding the chemical and magnetic properties of such interface is a key to achieve good magneto‐electric coupling. The unique possibilities offered by isotope sensitive techniques are used to selectively investigate the interface's chemistry and magnetism in Fe/BaTiO3 and Fe/LiNbO3 systems during the application of an electric field. With a large enough electric field, a strong oxidation of Fe is triggered, which creates a magnetically dead interface. This leads to an irreversible decrease of the magneto‐electric coupling properties. Material parameters are identified that determine under which electric field the interface may be modified. The results are confirmed on the two systems and are expected to be widespread in this new class of hybrid material.

Magnetoelectric coupling in a ferroelectric/ferromagnetic chain revealed by ferromagnetic resonance

Understanding the multiferroic coupling is one of the key issues in the field of multiferroics. As shown here theoretically, the ferromagnetic resonance (FMR) renders possibly an access to the magnetoelectric coupling coefficient in composite multiferroics. This we evidence by a detailed analysis and numerical calculations of FMR in an unstrained chain of BaTiO3 in the tetragonal phase in contact with Fe, including the effect of depolarizing field. The spectra of the absorbed power in FMR are found to be sensitive to the orientation of the interface electric polarization and to an applied static electric field. Here, we propose a method for measuring the magnetoelectric coupling coefficient by means of FMR.

Observation of strong magnetoelectric effects in Ba0.7Sr0.3TiO3/La0.7Sr0.3MnO3 thin film heterostructures

A robust magnetoelectric (ME) response is essential in multiferroic thin films for potential nonvolatile ME memory elements. We report strong ME effects in bilayers and superlattices of Ba0.7Sr0.3TiO3/La0.7Sr0.3MnO3 composite multiferroics. The low-frequency ME coefficient ranges from 35 to 300 mV/cm Oe, depending on thickness, superlattice periodicity, and bias magnetic field and ac magnetic field orientations. Data on ME coupling are fitted well with a modified free-body model to obtain information on magneto-mechanical coupling in the composites. The strong ME coupling in spite of anticipated substrate clamping is a step forward towards the realization of multistate memories and read-write elements.

Microstructure of the potentially multiferroic Fe/BaTiO3 epitaxial interface

The Fe/BaTiO3 thin-film layered structure is a prototype of charge-mediated composite multiferroics, which is a promising but challenging route to achieve a sizable magnetoelectric effect. The real structure of the interface between the ferromagnetic Fe and ferroelectric BaTiO3 layers is crucial. In this paper, epitaxial Fe layers were successfully grown on top of BaTiO3 layers by carefully controlling the pulsed laser deposition and magnetron sputtering procedures. A detailed study of interfacial structure and defects at the Fe/BaTiO3 interface was carried out by transmission electron microscopy (TEM). Electron diffraction patterns and diffraction contrast images reveal a definite epitaxial relationship between Fe and BaTiO3 (001) films and a semi-coherent interface with nearly periodic interfacial dislocations. Based on high-resolution TEM images from both [010] and [110] direction observations, the interfacial dislocations were found to be partial with Burgers vectors and line directions of ⟨010⟩. By employing high-resolution Z-contrast imaging, the positions of individual atoms columns were resolved. The formation mechanism of interfacial dislocations was proposed in terms of geometrical models of the interface structure. On the basis of the remaining strain analysis in each layer, the effects of both BaTiO3 thickness and the SrTiO3 substrates on the density of the interface defects were discussed.

Magnetoelectric effect in magnetostriction-piezoelectric multiferroics

The present stage in the study of magnetoelectric (ME) composite multiferroics is analyzed. The ME effect in materials of this kind is due to the magnetostriction and piezoelectric properties of the components. The elastic mechanical interaction between the magnetostriction and piezoelectric phases engenders a giant magnetoelectric response in magnetoelectric composite materials. The ME effect is more than 100 times stronger near an electromechanical resonance. The recently obtained nanostructural composites made from ferroelectric and magnetic oxides with dimensions of the order of nanometers made in the form of films on a substrate engender interest in the possibility of constructing integrated devices. The ME interaction between ferroelectrics and magnetic oxides with nanometer dimensions is the same as in ordinary composite materials. Just as for bulk ME composites, sensors, transducers and diverse reading/writing devices are some of the possible applications of the ME effect in nanocomposites

Ab initio study of magnetoelectricity in composite multiferroics

The coexistence of magnetism and ferroelectricity in the same crystalline phase of a so-called multiferroic (MF) material involves the opportunity of magnetoelectric (ME) coupling. ME coupling, in principle, offers magnetization switching by an electric field or polarization switching by a magnetic field. Since this phenomenon allows to store information in nanometer-sized memories with four logic states, the issues of MFs are of prime interest. In the single-phase MFs, however, the electric polarization and magnetization interact weakly with each other while ferromagnetism disappears far below room temperature. A more robust scenario of magnetoelectricity might occur in artificial MFs composed of ferromagnetic (FM) thin films which are grown epitaxially on a ferroelectric substrate. In the study of composite MFs, the results of ab initio calculations have shown an extremely promising direction for the next years. Although these calculations go ahead of experiment they explore the trends and basic physics of ME. Here, on the basis of first-principles calculations we predict that epitaxial ultrathin Fe films deposited on TiO2-terminated (001) surface of ATiO3 perovskites (, Ba) exhibit an unexpected change in their magnetic structure with increasing Fe-film thickness. The magnetic order changes from strongly FM for the single-monolayer Fe system to ferrimagnetic with almost vanishing magnetization upon deposition of a second Fe layer. FM order is restored for thicker Fe films. This effect can be understood in terms of hybridization of electronic states and structural relaxation. Additionally, we study the effect of iron oxidation on the ME coupling at the Fe2/ATiO3(001) interface. The oxygen coverage ranged between 0.5 and 2.0 adsorbed O atom per Fe atom. The magnetic properties of the Fe layer are gradually degraded with increasing O coverage. However, the change in magnetization which is induced by the electric polarization reversal remains robust for all energetically favorable compositions. Thus, the surface oxidation of composite MFs cannot destroy the switchable magnetoelectricity.

Composite multiferroics as magnetic field detectors: how to optimise magneto-electric coupling

We present a computer simulation and theoretical study of composite multiferroics acting as magnetic field detectors. Our set-up consists of a layer of piezoelectric sandwiched between two magnetostrictive layers. When magnetic field is applied the magnetostrictive strains and this is passed onto the piezoelectric producing a detectable voltage. We model the device using our own finite element code and calculate open circuit voltage and input impedance. Good agreement is shown with analytical formulas. We then optimise the coupling by altering the ratio of piezoelectric to magnetostrictive volume and the shape of the inner piezoelectric layer: the former effect is more important. We show that nearly equal amounts of piezoelectric and magnetostrictive give best coupling but the exact optimal ratio depends on the relative stiffnesses of the two materials. Most calculations carried out in previous literature have assumed that the magnetic field found in the device is simply the applied field. Here we show to what extent such an applied field can enter the device by simulating also the air region around the detector. The field in the device is calculated to be ∼17% less than the applied field.

Effect of oxidation of the ultrathin Fe electrode material on the strength of magnetoelectric coupling in composite multiferroics

We present an ab initio study of the effect of iron oxidation on magnetoelectric coupling which appears at the ferromagnet/ferroelectric interface, such as ${\text{Fe}}_{2}/A{\text{TiO}}_{3}(001)$ $(A=\text{Ba},\text{Pb})$. The oxygen coverage, $c$, ranging between one half and two adsorbed O per each Fe atom was simulated within a slab geometry. Since the change in magnetization induced by electric polarization reversal remains robust below $cl3/2$, we suggest that the surface oxidation of composite multiferroics will not significantly damage their switchable magnetoelectricity.

Verified finite element simulation of multiferroic structures: Solutions for conducting and insulating systems

Composite multiferroics are an exciting class of engineered materials with a wide range of existing and potential applications. This paper describes an original finite element (FE) simulation for composite multiferroic devices. Detailed analysis is given on the flow of electric/magnetic fields between the composite regions with attention given to the conductivity of the magnetostrictive layers (piezoelectric assumed to be insulating). The simulation is verified against an existing FE code and against a theoretical analysis, also presented here. The issue of boundary and continuity conditions is discussed in detail and approximations made in both FE and analytical treatments are specified. We derive the I-V curves of the composite device when acting as a magnetic field detector, in both conducting and insulating cases. The magnetic field detector acts as a voltage source: we calculate its open-circuit voltage and internal impedance in each case.

Preparation and multiferroic properties of Bi 0.8La 0.2FeO 3–CoFe 2O 4 ceramics

Composite multiferroics usually show larger magnetoelectric effect, which is important for actual application, than their single-phase counterparts. Here, (1− x)BiFeO 3– xCoFe 2O 4 and (1− x)Bi 0.8La 0.2FeO 3– xCoFe 2O 4 ( x = 0.1 , 0.2, and 0.3) multiferroic composite ceramics were prepared. The former system shows low resistivity, which prevents the intrinsic ferroelectric measurements, whereas for the later system, the composition with x = 0.1 shows well saturated polarization–electric field hysteresis loop at room temperature, the measured remnant polarization is 13.4 μC/cm 2. However, for the compositions with x = 0.2 and 0.3, the measured loops show negative curvature near saturation, which is attributed to the decreased resistivity. Further magnetic measurements reveal that at low temperature, 0.9Bi 0.8La 0.2FeO 3−0.1CoFe 2O 4 shows ferromagnetic nature whereas at high temperature antiferroelectric-like nature, which is discussed based on comparing its magnetic response with that of pure Bi 0.8La 0.2FeO 3 and CoFe 2O 4 ceramics. These results may be helpful for further understanding, and designing, new multiferroic composites.

Multiferroic CoFe2O4–Pb(Zr0.52Ti0.48)O3 nanofibers by electrospinning

Multiferroic materials possess two or more types of orders simultaneously that couple the electric and magnetic fields, and composite multiferroics have been widely explored for their excellent magnetoelectric coupling. In this letter, we report a strategy to hybrid multiferroicity at nanoscale. Multiferroic CoFe2O4–Pb(Zr0.52Ti0.48)O3 nanofibers are synthesized by sol-gel process and electrospinning, the spinel structure of CoFe2O4 (CFO) and perovskite structure of Pb(Zr0.52Ti0.48)O3 (PZT) are verified by x-ray diffraction and high resolution transmission electron microscopy, and the multiferroicity of the nanofibers are confirmed by piezoresponse force microscopy and magnetic hysteresis. Excellent ferroelectric and ferromagnetic properties have been observed, which could enable multiferroic devices at nanoscale.