Multiferroic properties of multilayered BaTiO3–CoFe2O4 composites via tape casting method

Multiferroic magnetoelectric (ME) BaTiO₃–CoFe₂O₄ (BTO–CFO) ceramic composites with different thicknesses were fabricated via tape casting technique. The interfacial morphology of the composite demonstrates the presence of plate-like grains with a thickness of ~400 nm. This could be associated with the residual stresses originated from lattice mismatch and different thermal expansion coefficients between BTO and CFO layers. The dielectric constant, piezoelectric constant, and ferroelectric properties of the multilayered composite are degraded in the presence of CFO layers in comparison with those of BTO bulk. Furthermore, the dielectric constant and polarization of the composite decrease with increasing frequency. The leakage current density and magnetic remanence ratio of the composite reach up to the order of 10^?6 A/cm² and 40 %, respectively. The direct and converse ME coefficients were measured to be 8.1 μV/cm Oe and 1.1 × 10^?3 G/V, respectively. Based on the converse ME effect, an electrically controlled ME inductor was designed using the composite as its core. The inductance and tunability of the inductor increase with increase of applied dc electric field.     

Magnetoelectric properties of Ba0.8Sr0.2TiO3–CoFe2O4 multilayered composite film via sol–gel method

Ba_0.8Sr_0.2TiO₃–CoFe₂O₄ (BST–CFO) multilayered composite films were prepared on Pt/Ti/SiO₂/Si substrates via a sol–gel method and spin-coating technique. Microstructures, electric property, magnetic property and magnetoelectric (ME) property of the composites were studied. Results show that the composite films calcined at 750 °C have BST and CFO phases and no obvious impurity phases were detected. Further, the composite films exhibit layered structures and a transition layer which is composed of interfacial delamination exists at the interface between BST and CFO layers. Ferroelectric and ferromagnetic properties were simultaneously observed in the films, evidencing the coexistence of the ferroelectric and ferromagnetic properties. Furthermore, the saturation magnetization value of the composite film is lower than that of the pure CFO film derived by the same processing as a result of the effect of the nonferromagnetic BST layers. Also, ferroelectric hysteresis loops reveal that the saturated polarization and remanent polarization of the composite film are lower than those of the pure BST films. In addition, the composite film exhibits a strong ME effect, which makes the composite film attractive for technological applications as devices.     

Magnetoelectric properties of CoFe2O4–Pb(Zr0.52Ti0.48)O3 multilayered composite film via sol–gel method

CoFe₂O₄–Pb(Zr_0.52Ti_0.48)O₃ (CFO–PZT) multilayered composite film was prepared on Pt/Ti/SiO₂/Si substrate via a sol–gel method and spin-coating technique. Results show that PZT and CFO phases exist in the composite film, calcined at 700 °C, besides substrate phase, and no obvious impurity phases can be detected. The composite film exhibits layered structure with obvious boundary between CFO and PZT films. Ferroelectric and ferromagnetic properties were simultaneously observed in the composite film, evidencing the ferroelectric and ferromagnetic properties in the composite film. The composite film exhibits both good magnetic and electric properties, as well as, magnetoelectric (ME) effect. The saturation magnetization value of the composite film is lower than that of the pure CFO film derived by the same processing as a result of the effect of the nonferromagnetic PZT layers. Ferroelectric hysteresis loops reveal that saturated polarization and remanent polarization of the composite film are lower than those of the pure PZT films. The composite film exhibits a very large ME effect, which makes the composite film attractive for technological applications as devices.     

Synthesis and Characterization of CoFe<Subscript>2</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript>/BaTiO<Subscript>3</Subscript> Multiferroic Composites

Three composite modes of CoFe₂ O ₄/BaTiO₃ (CFO/BTO) were created with appropriate stoichiometric proportion. They are nitrate solution of CFO mixed with BTO (SCB), self-propagating precursor of CFO mixed with BTO (PCB), and the made-up CFO mixed with BTO (MCB) separately. The microstructural, ferroelectric, and ferromagnetic properties of SCB, PCB, and MCB bulk composites were investigated. SCB, PCB, and MCB bulk composites with a molar ratio of 2:8 were calcined at 1020, 1120, and 1160 ^°C, respectively. And X-ray diffraction (XRD) analysis showed that they all correspond to the CFO with cubic spinel structure and the BTO with tetragonal perovskite structure. The formation temperature of BTO with hexagonal structure is related to the distance of inter-diffusion and Co ²⁺ concentration in the CFO/BTO bulk composite. The wet chemical routing is of benefit to inhibit the agglomeration of the BTO in the CFO/BTO bulk composite. The maximum polarization of 5.24 μC/cm ² was received in the MCB bulk composite sintered at 1160 ^°C with a molar ratio of 1:9. The maximum saturation magnetization of 31.609 emu/g and the remnant magnetization of 10.336 emu/g were obtained in the MCB bulk composite sintered at 1160 ^°C with a molar ratio of 4:6. The threshold ferromagnetic phase content of percolation which has an effect on the MCB bulk composite is less than 40 mol %.     

Influence of ferroelectric-electrode interfaces on the electrical properties of BaTiO3-CoFe2O4 magnetoelectric composites

In order to further investigate the influence of electrode types and preparation conditions on the electrical properties of magnetoelectric (ME) composite ceramics, layered BaTiO₃-CoFe₂O₄ (BTO–CFO) composites were prepared using tape casting method, and the effects of silver and aluminium electrodes on the electrical properties were studied. It is found that the different electrical properties of the ME composites are related to the ferroelectric-silver and ferroelectric-aluminium interfaces formed at the same annealing temperature, in which the former enhance the leakage current density, dielectric properties and AC conductivity, but lower the ferroelectric properties compared to the latter. Meanwhile, the electrical properties of the composite, especially leakage current and ferroelectric properties, are influenced by the silver electrode annealing temperature. In addition, space charges and polarons are responsible for the dielectric constant and AC conductivity of the ME composites, respectively.     

Effect of interface bonding on the phase-transformation-aided magnetoelectric effect in ferromagnetic/ferroelectric composites

Based on modified constitutive equations and finite element method, calculations have been performed to study the effect of interface bonding on the phase-transition-aided magnetoelectric (ME) response in a new kind of NiMnGa/lead–zirconate–titanate (PZT) multiferroic laminate composites. The results quantitatively show that the ME effect is remarkably dependent on both the interface layer characteristics and the interface layer thickness. Stiffer and thinner interface layers are apt to produce higher ME effect. Calculations are in good agreement with available experimental results. Furthermore, the theoretical approach was improved to consider the enhancement in the magnetostriction of martensites induced by pre-applied opposing stress. Predictions reveal that the usage of single crystal Fe₇Pd₃ as ferromagnetic phase to form magnetoelectric composite with PZT can produce a high ME up to ~1 V/cm Oe.     

Multiferroic magnetoelectric coupling effect of three-layer multiferroic (Ba0.6Sr0.4TiO3–Ni0.6Zn0.4Fe2O4)3 heterojunction fabricated by sol–gel process

Multiferroic materials have been widely used in novel multifunctional smart devices for their excellent magnetoelectric coupling properties. In this work, a three-layer multiferroic (Ba_0.6Sr_0.4TiO₃–Ni_0.6Zn_0.4Fe₂O₄)₃ heterojunction was fabricated using a sol–gel process followed with a rapid thermal process. The phase composition, microtopography as well as magnetoelectric coupling properties of the heterojunction were investigated at room temperature. It demonstrated that such a three-layer heterostructure shows not only excellent ferroelectricity and ferromagnetism, but also good magnetoelectric coupling. The maximum value of the in-plane magnetoelectric coupling coefficient is ~?55 mV cm^?1 Oe^?1 at 630 Oe. Meanwhile, the macro-magnetoelectric coupling characteristics of the heterojunction are verified by the dynamic processes of the ferroelectric and ferromagnetic domain structure induced by the stress exerted on dipoles in an applied field, making it more attractive in the design of modern multifunctional devices.

     

Enhanced magnetoelectric coupling in Pb(Zr0.52Ti0.48)O3 film-on-CoFe2O4 bulk ceramic composite with LaNiO3 bottom electrode

By chemically depositing a conductive LaNiO₃ (LNO) electrode layer onto dense CoFe₂O₄ (CFO) ceramics, the magnetoelectric (ME) response in Pb(Zr_0.52Ti_0.48)O₃ (PZT)/CFO film-on-substrate system was significantly enhanced. Structural, morphological, ferroelectric, and piezoelectric analysis was performed for the present PZT/LNO/CFO film-electrode-substrate composites, whereby improved electric-related output was demonstrated. It was further shown that the maximum ME coefficient α _E31 could reach up to 155 mV/cm Oe, which was about 2.6 times higher than that in PZT film-on-CFO ceramics without bottom electrode layer. Further enhancement of ME response could be expected by preparing high-quality electrode layer with optimal thickness and improved fabrication processing for such film-electrode-substrate composites.     

Resonant magnetoelectric interaction in asymmetric bimorphous ferromagnetic-ferroelectric structure

The magnetoelectric (ME) interaction in a planar asymmetric structure comprising a bimorphous piezoelectric plate of lead zirconate titanate sandwiched between ferromagnetic layers of an amorphous magnet and nickel with the opposite signs of magnetostriction has been studied. Owing to the effective excitation of bending oscillations at a resonance frequency of ∼5 kHz, a ME voltage coefficient of about 18 V/(Oe cm) has been obtained.     

Ionic Liquid Gating Control of Spin Reorientation Transition and Switching of Perpendicular Magnetic Anisotropy

Electric field (E‐field) modulation of perpendicular magnetic anisotropy (PMA) switching, in an energy‐efficient manner, is of great potential to realize magnetoelectric (ME) memories and other ME devices. Voltage control of the spin‐reorientation transition (SRT) that allows the magnetic moment rotating between the out‐of‐plane and the in‐plane direction is thereby crucial. In this work, a remarkable magnetic anisotropy field change up to 1572 Oe is achieved under a small operation voltage of 4 V through ionic liquid (IL) gating control of SRT in Au/[DEME]⁺[TFSI]^?/Pt/(Co/Pt)₂/Ta capacitor heterostructures at room temperature, corresponding to a large ME coefficient of 378 Oe V^?1. As revealed by both ferromagnetic resonance measurements and magnetic domain evolution observation, the magnetization can be switched stably and reversibly between the out‐of‐plane and in‐plane directions via IL gating. The key mechanism, revealed by the first‐principles calculation, is that the IL gating process influences the interfacial spin–orbital coupling as well as net Rashba magnetic field between the Co and Pt layers, resulting in the modulation of the SRT and in‐plane/out‐of‐plane magnetization switching. This work demonstrates a unique IL‐gated PMA with large ME tunability and paves a way toward IL gating spintronic/electronic devices such as voltage tunable PMA memories.     

Enhanced magnetoelectrical coupling in cobalt ferrite/lead lanthanum zirconate titanate 0-3 composites through phase boundary modification

We have developed a processing technique for producing cobalt ferrite (CFO)/lead lanthanum zirconate titanate (PLZT) 0-3 composites with enhanced piezoelectric, ferroelectric and magnetoelectrical (ME) coupling properties. It includes modifying the phase boundary with zirconia barrier layer and decreasing the sintering temperature with Li₂O + 0.5Bi₂O₃ additives. The CFO/PLZT 0-3 composites were successfully fabricated via conventional co-firing procedures. The obtained 0-3 composite ceramics can withstand a DC electric field of 6 kV mm⁻¹. The effects of CFO/PLZT ratio on the crystalline structure, piezoelectric, ferroelectric, dielectric and M–E properties were investigated. The M–E coefficients of the composites with CFO:PLZT volume ratio of 14.4:85.6, 27.4:72.6 and 39.3:60.7 were found to be 31, 52 and 143 mV (Oe cm)⁻¹ at 100 kHz under a bias field of ∼1 kOe.     

Effect of piezoelectric grain size on magnetoelectric coefficient of Pb(Zr0.52Ti0.48)O3–Ni0.8Zn0.2Fe2O4 particulate composites

This study investigates the variation of magnetoelectric (ME) coefficient as a function of the piezoelectric grain size in the composite system of 0.8 Pb(Zr_0.52Ti_0.48)O₃–0.2 Ni_0.8Zn_0.2Fe₂O₄. It was found that as the piezoelectric-phase grain size increases the overall resistivity, piezoelectric, dielectric, and ferroelectric property of the composite increases and saturates above 600 nm. Below 200 nm average grain size, piezoelectric and dielectric properties decrease rapidly. The ferroelectric Curie temperature was found to decrease from 377 to 356 °C as the average grain size decreases from 830 to 111 nm. ME coefficient of the composite showed a rapid change below grain size of 200 nm and was found to saturate above 600 nm to a value of 155 mV/cm.Oe.     

Processing and magneto-electric properties of sol–gel-derived Pb(Zr0.52Ti0.48)O3–Ni0.8Zn0.2Fe2O4 2-2 type multilayered films

Pb(Zr_0.52Ti_0.48)O₃–Ni_0.8Zn_0.2Fe₂O₄ (PZT–NZFO) multilayered thin films with various volume fractions of the PZT phase (100, 74, 58, 48, 33, and 0%) were prepared on Pt/Ti/SiO₂/Si substrates using sol–gel spin-coating method. X-ray diffraction shows polycrystalline structure and scanning electron microscopy reveals good multilayer morphology of the composite thin film as annealed at 700 °C in air. The thickness of the composite films was estimated in the range of ~400 to ~600 nm. The ferroelectric and magnetic properties were measured as function of the volume fractions of the PZT phase. The magnetoelectric (ME) effect was investigated under various bias magnetic fields. The maximum ME voltage coefficient (α _E  = dE/dH) is 278 mV/cmOe for the composite film with the volume fractions of the PZT phase of ~48%.     

Mn0.7Zn0.3Fe2O4 + BaTiO3 composites: structural,morphological, magnetic,M–E effect and dielectric properties

Here in the present investigation, constituent phases of (1 ? x)Mn_0.7Zn_0.3Fe₂O₄(MZFO)?+?(x)BaTiO₃(BTO) (where x?=?0.0, 0.25, 0.50, 0.75 and 1.0) composites have been synthesized by sol–gel auto-ignition route and the composite structure by ceramic route. The XRD analysis ensures that the composite structure consists of both cubic spinel piezomagnetic and perovskite piezoelectric phases. The average crystallite size estimated from Scherrer equation increases from 15.36 to 21.94 nm. The strain induced in individual phases has been investigated by W–H analysis and it is observed that the MZFO phase shows comprehensive type strain while BTO phase shows tensile type strain. Scanning electron micrographs confirm the microstructure of the sample with grain size ranges from 36.006 to 54 nm. Energy dispersive X-ray spectra and elemental color mappings of typical samples (x?=?0.0 and 0.75) clearly indicates the phase purity and stoichiometric proportion of the composites. Fourier transform infrared spectra showed five major absorption bands related to stretching vibrations of different kinds of metal ions and oxygen ions. Increasing percentage of BTO phase in the composite reduces the saturation magnetization, remnant magnetization and coercivity. Real and imaginary parts of permittivity show maximum values at lower frequency region and decrease with increase in applied frequency. For the composition (0.25)MZFO–(0.75)BTO, composite shows maximum value of magnetoelectric coupling coefficient (α_ME?=?20.45 mVcm^?1 Oe^?1). The improved magnetoelectric properties make MZFO–BTO composite applicable for electronic devices.

     

Strain-induced artificial multiferroicity in Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript>/Pb(Fe<Subscript>0.66</Subscript>W<Subscript>0.33</Subscript>)O<Subscript>3</Subscript> layered nanostructure at ambient temperature

Layered nanostructures (LNs) of the commercial ferroelectric Pb(Zr_0.53Ti_0.47)O₃ (PZT) and the natural ferroic relaxor Pb(Fe_0.66W_0.33)O₃ (PFW) were fabricated with a periodicity of PZT/PFW/PZT (~5/1/5 nm, thickness ~250 nm) on MgO substrates by pulsed laser deposition. The dielectric behavior of these LNs were investigated over a wide range of temperatures and frequencies, observing Debye-type relaxation with marked deviation at elevated temperatures (>400 K). High dielectric constant and very low dielectric loss were observed below 100 kHz and 400 K, whereas the dielectric constant decreases and loss increases with increase in frequency, similar to relaxor ferroelectrics. Asymmetric ferroelectric hysteresis loops across UP and DOWN electric field were observed with high remanent polarization (P_r) of about 33 μC/cm². High imprint (~5–7 V across 250 nm thin films) were seen in ferroelectric hysteresis that may be due to charge accumulation at the interface of layers or significant amount of strain (~3.21) across the layers. Room temperature ferromagnetic hysteresis was observed with remanent magnetization 5.32 emu/cc and a coercive field of ~550 Oe. Temperature and field dependent leakage current densities showed very low leakage ~10⁻⁷–10⁻⁵ A/cm² over 500 kV/cm. We observed imprint in hysteresis that may be due to charge accumulation at the interface of layers or active role of polar nano regions (PNRs) situated in the PFW regions.     

Low-temperature spin spray deposited ferrite/piezoelectric thin film magnetoelectric heterostructures with strong magnetoelectric coupling

We report low-temperature spin spray deposited Fe₃O₄/ZnO thin film microwave magnetic/piezoelectric magnetoelectric heterostructures. A voltage induced effective ferromagnetic resonance field of 14 Oe was realized in Fe₃O₄/ZnO magnetoelectric (ME) heterostructures. Compared with most thin film magnetoelectric heterostructures prepared by high temperature (>600 °C) deposition methods, for example, pulsed laser deposition, molecular beam epitaxy, or sputtering, Fe₃O₄/ZnO ME heterostructures have much lower deposition temperature (<100 °C) at a much lower cost and less energy dissipation, which can be readily integrated in different integrated circuits.     

Effect of working pressure on the properties of BaTiO3-CoFe2O4 composite films deposited on STO (100) by PLD

The BaTiO₃-CoFe₂O₄ (BTO-CFO) composite films were grown on SrTiO₃ (STO) (100) substrates at 750 °C under various working pressures by pulsed laser deposition. The composite film grew into a supersaturated single phase at the working pressure of 10 mTorr, BTO and CFO (00 l) oriented hetero-epitaxial films on STO (100) at 100 mTorr, and a polycrystalline composite film at 500 mTorr. The slow growth rate at high working pressure led to the phase separation in the composite film. The CFO was compressively strained along out-of-plane due to the lattice mismatch with the BTO matrix phase. The BTO-CFO composite film grown at 100 mTorr showed reversible switching of ferroelectric polarization and magnetic hysteresis with strong magnetic anisotropy.     

Enhanced electric field tunable magnetic properties of lead-free Na0.5Bi0.5TiO3–MnFe2O4 multiferroic composites

Strong magnetoelectric (ME) interaction was exhibited at both dc and microwave frequencies in a lead-free multiferroic particulate composites of Na_0.5Bi_0.5TiO₃ (NBT) and MnFe₂O₄ (MFO) multiferroic, which were prepared by sol–gel route. The room temperature permeability measurements were carried out in the frequency range of 1 MHz–1 GHz. A systematic study of structural, magnetic and ME properties were undertaken. The room temperature ferromagnetic resonance (FMR) was studied. Strong ME coupling is demonstrated in 70NBT–30MFO composite by an electrostatically tunable FMR field shift up to 428 Oe (at E = 4 kV/cm), which increases to a large value of 640 Oe at E = 8 kV/cm. Furthermore, these lead-free multiferroic composites exhibiting electrostatically induced magnetic resonance field at microwave frequencies provide great opportunities for electric field tunable microwave devices.     

Electric and magnetic properties of ferromagnetic/piezoelectric bilayered composite

One of the most promising ways for the realization of multi-functional materials is the integration of oxides with different properties in artificial heterostructures. In this paper, a novel piezoelectric–ferromagnetic heterostructure consisting of 0.92Na_0.5Bi_0.5TiO₃–0.08BaTiO₃ (abbreviated as BNT–BT_0.08) and CoFe₂O₄ layers is fabricated on Si–Pt substrate, by sol–gel method coupled with spin-coating technique. The composite thin film shows only perovskite Bi_0.5Na_0.5TiO₃-like rhombohedral phase and CoFe₂O₄ cubic phase. The thickness of CoFe₂O₄ and BNT–BT_0.08 layers is ~?280 and?~?400 nm, respectively. BNT–BT_0.08/CoFe₂O₄ heterostructure thin film shows a saturation magnetization of 0.11 emu/g at 5 K and 0.07 emu/g at 295 K, dielectric constant of 235 at 1 kHz and tunability of 70% at 1 kHz and an electric field E?=?110 kV/cm. The results reveal that the investigated hybrid piezoelectric/ferromagnetic structure shows piezoelectric behavior, good ferroelectric and ferromagnetic properties. This bilayer composite can be used in miniature low-frequency magnetic sensor and piezoelectric sensor for biomedical domain.     

Review of magnetoelectric perovskite–spinel self-assembled nano-composite thin films

Two-phase multiferroic nano-composite thin films have been a topic of research interests in the last few years. This is because of their expected magnetoelectric coupling, as well as potential applications. This review focuses on recent findings in self-assembled nano-structure composite thin films, and various efforts to realize and improve their magnetoelectricity. Topics include: (i) nano-pillar and maze structures, and their formation mechanisms, and a nano-belt structure oriented in-plane found by our research group; (ii) the ferroelectric properties of composite thin films, and how they can be enhanced by epitaxial engineering; (iii) a magnetic anisotropy that is induced by constraint stress, and by the nano-structures of the ferromagnetic phase; and (iv) a magnetoelectric coupling that was first observed via a change in magnetization near the Curie temperature of the ferroelectric phase, a magnetization switching assisted by electric field, and recently direct measurements using a magnetic cantilever method yielding values of 18 mV/cm Oe in BiFeO₃–CoFe₂O₄.