Multiferroic Layers Research Articles

Tunable chiral photonic cavity based on multiferroic layers.

Realization of externally tunable chiral photonic sources and resonators is essential for studying and functionalizing chiral matter. Here, oxide-based stacks of helical multiferroic layers are shown to provide a suitable, electrically-controllable medium to efficiently trap and filter purely chiral photonic fields. Using analytical and rigorous coupled wave numerical methods we simulate the dispersion and scattering characteristics of electromagnetic waves in multiferroic heterostructures. The results evidence that due to scattering from the spin helix texture, only the modes with a particular transverse wavenumber form standing chiral waves in the cavity, whereas all other modes leak out from the resonator. An external static electric field enables a nonvolatile and energy-efficient control of the vector spin chirality associated with the oxide multilayers, which tunes the photonic chirality density in the resonator.

Thermally Induced Flexo‐Type Effects in Nanopatterned Multiferroic Layers

AbstractThe difficulty to generate and control large strain gradients in materials hinders the investigation and application of flexoelectricity and flexomagnetism. This work demonstrates that thermal expansion can be used to induce very large non‐uniform strains at the nanoscale, resulting in giant strain gradients at moderate temperatures. This is demonstrated in a nanopatterned multiferroic hybrid layer consisting of a regular array of ferromagnetic metallic nanocylinders embedded in a ferroelectric polymer matrix. The thermally‐induced strain gradients can fully depolarize the ferroelectric component, and modify the magnetization of the ferromagnetic component via flexoelectric and flexomagnetic effects, respectively. Finite‐element analysis provides a quantitative view on thermal expansion‐induced strains and strain gradients supporting the experimental findings. This work shows that nanoscale thermal strain engineering provides an additional degree of freedom to control electrical polarization and magnetization, which paves the way for the design and operation of novel functional devices and nanostructures.

Voltage Control of Two-Magnon Scattering in Multiferroic Layers for Tunable Magnetoelectric Devices

In this paper, multiferroic heterostructures have been prepared by growing MnZn ferrite films on ferroelectric single substrates using pulsed laser deposition technology. An enhanced magnetoelectric coupling has been achieved at the critical two-magnon scattering (TMS) angles, where the ferromagnetic resonance field shifts −653 and 211 Oe. The contribution of TMS effects is much larger than that of the conventional strain effect. This heterostructure provides an alternative route for novel integrated multiferroic materials and devices.

Self-Poling-Induced Magnetoelectric Effect in Highly Strained Epitaxial BiFeO3/La0.67Sr0.33MnO3-δ Multiferroic Heterostructures.

Highly strained epitaxial BiFeO3/La0.67Sr0.33MnO3-δ (BFO/LSMO) heterostructures were fabricated on LaAlO3 substrates by magnetron sputtering. The as-grown downward self-polarization of BFO capping layers was confirmed by piezoelectric force microscopy. Using the electrostatic field-induced charge screening, a hole depletion state was induced in ultrathin (8 nm) LSMO films. As a result of the interfacial charge coupling, appreciable saturated magnetization (MS) increase of about 500 and 100% can be observed in LSMO with BFO capping at 5 and 300 K, respectively. Besides, LSMO phase translations can be revealed by the BFO thickness-related exchange bias field (HE) and MS of the BFO/LSMO heterostructures. The results established a new approach in achieving interfacial magnetoelectric couplings with thin self-polarized multiferroic layers.

Multiferroic Domain Walls in Ferroelectric PbTiO3 with Oxygen Deficiency.

Atomically thin multiferroics with the coexistence and cross-coupling of ferroelectric and (anti)ferromagnetic order parameters are promising for novel magnetoelectric nanodevices. However, such ferroic order disappears at a critical thickness in nanoscale. Here, we show a potential path toward ultrathin multiferroics by engineering an unusual domain wall (DW)-oxygen vacancy interaction in nonmagnetic ferroelectric PbTiO3. We demonstrate from first-principles that oxygen vacancies formed at the DW unexpectedly bring about magnetism with a localized spin moment around the vacancy. This magnetism originates from the orbital symmetry breaking of the defect electronic state due to local crystal symmetry breaking at the DW. Moreover, the energetics of defects shows the self-organization feature of oxygen vacancies at the DW, resulting in a planar-arrayed concentration of magnetic oxygen vacancies, which consequently changes the deficient DWs into multiferroic atomic layers. This DW-vacancy engineering opens up a new possibility for novel ultrathin multiferroic.

Application of a generalized matrix averaging method for the calculation of the effective properties of thin multiferroic layers

It is proposed to use a generalized matrix averaging (GMA) method for calculating the parameters of an effective medium with physical properties equivalent to those of a set of thin multiferroic layers. This approach obviates the need to solve a complex system of magnetoelectroelasticity equations. The required effective characteristics of a system of multiferroic layers are obtained using only operations with matrices, which significantly simplifies calculations and allows multilayer systems to be described. The proposed approach is applicable to thin-layer systems, in which the total thickness is much less than the system length, radius of curvature, and wavelengths of waves that can propagate in the system (long-wave approximation). Using the GMA method, it is also possible to obtain the effective characteristics of a periodic structure with each period comprising a number of thin multiferroic layers.

Interfacial magnetic coupling in ultrathin all-manganite La0.7Sr0.3MnO3-TbMnO3 superlattices

We report the growth and magnetic properties of all-manganite superlattices composed of ultrathin double-exchange ferromagnetic La0.7Sr0.3MnO3 and noncollinear multiferroic TbMnO3 layers. Spontaneous magnetization and hysteresis loops are observed in such superlattices with individual La0.7Sr0.3MnO3 layers as thin as two unit cells, which are accompanied by pronounced exchange bias and enhanced coercivity. Our results indicate substantial interfacial magnetic coupling between spin sublattices in such superlattices, providing a powerful approach towards tailoring the properties of artificial magnetic heterostructures.

Intermixing and Magnetoelectric Coupling in Ferroelectric/Multiferroic Superlattices

We develop a simple model of superlattices consisting of alternate ferroelectric and multiferroic layers with interface intermixing based on the Landau-Ginzburg theory. Multiferroic system is described using a Landau-Ginzburg potential with a positive biquadratic coupling between magnetization and polarization. Intermixing at interfaces forms intermixed layers with electrical properties different from its constituent layers. Effects of intermixing and magnetoelectric coupling on multiferroic phase transitions in the ferroelectric/multiferroic superlattices is calculated and discussed.

Electromagnetic Effects and Layer Thickness Influence on Natural Characteristics of Multiferroics Composite Structure

Based on the elastic theory of anisotropic materials multilayers, using the finite element method, the dynamic characteristics are investigated for the composite structure which is composed of the electromagnetic multiferroic layer and isotropic metal layer. Firstly, the finite element model of the multilayers structure is built with shell and solid elements capable of contributing anisotropic materials. Then, based on the model analyses, the natural characteristics of the composite structure are obtained with considering the electric-magnetic-elastic effect and the layer thickness. It is also shown that the electromagnetic effects have a certain influence on the natural frequencies vibration patterns, and the thickness of the multiferroic layers has more important influence.

Bismuth ferrite bilayered thin films of different constituent layer thicknesses

Bilayered thin films consisting of (Bi 0.90La 0.10)(Fe 0.85Zn 0.15)O 3 and (Bi 0.90La 0.10)(Fe 0.90Zn 0.10)O 3 layers have been fabricated by radio frequency sputtering. Both multiferroic layers are well retained in these bilayers. Their leakage current, multiferroic properties, and fatigue behavior are largely dependent on the thicknesses of (Bi 0.90La 0.10)(Fe 0.85Zn 0.15)O 3. With an increase of the thickness in the (Bi 0.90La 0.10)(Fe 0.85Zn 0.15)O 3 layer, the leakage current density of bilayers is degraded due to different grain growth modes and an increase in oxygen vacancies, the dielectric constant ( ɛ r) becomes larger due to the introduction of (Bi 0.90La 0.10)(Fe 0.85Zn 0.15)O 3 with a high ɛ r value, and their magnetic properties are deteriorated with increasing the thickness ratios of (Bi 0.90La 0.10)(Fe 0.85Zn 0.15)O 3 with a weaker magnetization. All bilayers exhibit a good ferroelectric behavior regardless of varying thicknesses of the (Bi 0.90La 0.10)(Fe 0.85Zn 0.15)O 3 layer, while their coercive field decreases with increasing the thickness of the (Bi 0.90La 0.10)(Fe 0.85Zn 0.15)O 3 layer. An anomalous enhancement in switchable polarization is demonstrated by these bilayers, owing to the involvement of space charges accumulated at the interfaces between two constituent layers.

Constrained ferroelectric domain orientation in (BiFeO3)m(SrTiO3)n superlattice

Ferroelectric domains were investigated using piezoresponse force microscopy in superlattices composed of multiferroic BiFeO3 and SrTiO3 layers. Compared to single BiFeO3 thin films, a reduction in the domains size and a suppression of the in-plane orientation of domains are observed in a superlattice of (BiFeO3)4(SrTiO3)8, suggesting a constrained ferroelectric domain orientation along the out-of-plane ⟨001⟩ direction. Such modification of domain size and orientation in BiFeO3-based heterostructures could play a vital role on engineering the domains and domain wall mediated functional properties necessary for device applications.

Coaxial Multiferroic Nanorod Arrays

We have developed a facile yet versatile method for fabricating coaxial nanowires of carbon nanotubes (CNTs) with multiferroic layers, such as BaTiO3 and CoFe2O4. Oxide coatings were deposited by pulsed laser deposition, and a variety of deposition parameters were studied. Arrays of such multiferroic-coated CNTs offer a mean to design novel nanostructures with high surface areas.

Impedance spectroscopy of epitaxial multiferroic thin films

Temperature dependent impedance spectroscopy enables the many contributions to the dielectric and resistive properties of condensed matter to be deconvoluted and characterized separately. We have achieved this for multiferroic epitaxial thin films of BiFeO_(3) (BFO) and BiMnO_(3) (BMO), key examples of materials with strong magnetoelectric coupling. We demonstrate that the true film capacitance of the epitaxial layers is similar to that of the electrode interface, making analysis of capacitance as a function of film thickness necessary to achieve deconvolution. We modeled non-Debye impedance response using Gaussian distributions of relaxation times and reveal that conventional resistivity measurements on multiferroic layers may be dominated by interface effects. Thermally activated charge transport models yielded activation energies of 0.60± 0.05 eV (BFO) and 0.25± 0.03 eV (BMO), which is consistent with conduction dominated by oxygen vacancies (BFO) and electron hopping (BMO). The intrinsic film dielectric constants were determined to be 320± 75 (BFO) and 450± 100 (BMO).