Periodic Stripe Domain Research Articles

Impact of Mn doping on the ferroelectric photovoltaic effect in multidomain BiFeO3 thin films under above-bandgap illumination

While recent studies have revealed that chemical doping can enhance the photovoltaic (PV) response of ferroelectrics under light with photon energy (hν) lower than the bandgap energy (E g), its impact under above-bandgap illumination (hν > E g) is not fully understood. In this study, the PV response at hν > E g of Mn-doped BiFeO3 (BFO) thin films with a periodic stripe domain pattern is investigated. The short-circuit photocurrent density (J sc) and open-circuit voltage decrease with increasing Mn content. Analyses of the light-polarization angle dependence of J sc indicate that the PV response arises primarily from the non-centrosymmetric nature of crystal lattices in both the bulk and domain wall (DW) regions. In contrast, the contribution of local electric fields (E local) across DWs which dominates the PV response of undoped BFO films is significantly suppressed by doping. We consider that a screening effect by charged defects leads to a suppressed E local in the Mn-doped films.

Reconfigurable Magnonic Crystals Based on Imprinted Magnetization Textures in Hard and Soft Dipolar-Coupled Bilayers.

Reconfigurable magnetization textures offer control of spin waves with promising properties for future low-power beyond-CMOS systems. However, materials with perpendicular magnetic anisotropy (PMA) suitable for stable magnetization-texture formation are characterized by high damping, which limits their applicability in magnonic devices. Here, we propose to overcome this limitation by using hybrid structures, i.e., a PMA layer magnetostatically coupled to a low-damping soft ferromagnetic film. We experimentally show that a periodic stripe-domain texture from a PMA layer is imprinted upon the soft layer and induces a nonreciprocal dispersion relation of the spin waves confined to the low-damping film. Moreover, an asymmetric bandgap features the spin-wave band diagram, which is a clear demonstration of collective spin-wave dynamics, a property characteristic for magnonic crystals with broken time-reversal symmetry. The composite character of the hybrid structure allows for stabilization of two magnetic states at remanence, with parallel and antiparallel orientation of net magnetization in hard and soft layers. The states can be switched using a low external magnetic field; therefore, the proposed system obtains an additional functionality of state reconfigurability. This study offers a link between reconfigurable magnetization textures and low-damping spin-wave dynamics, providing an opportunity to create miniaturized, programmable, and energy-efficient signal processing devices operating at high frequencies.

Flexosensitive polarization vortices in thin ferroelectric films

We present a theoretical study on the influence of the flexoelectric coupling on the spatial distribution and temperature behavior of the spontaneous polarization for several types of stable domain structure in thin ferroelectric films, such as stripe domains and vortices. Finite-element modeling (FEM) for ${\mathrm{BaTiO}}_{3}$ films and analytical calculations within the Landau-Ginzburg-Devonshire approach reveals that an out-of-plane polarization component can be very sensitive to the flexoelectric coupling for periodic quasi-2D stripe domains and 3D vortex-antivortex structures. However, the influence is rather different for these structures. The flexoelectric coupling increases significantly the amplitude of a small out-of-plane polarization component in the stripe domains, and the ``up'' or ``down'' direction of the component is defined by the sign of the flexoelectric coefficients. Concerning the vortex-antivortex pairs, their antivortices with in-plane anti\cyrchar\cyrs{}irculation have smooth wide dipolar cores through the entire film, whose shape and other features are almost insensitive to the coupling. The vortices with in-plane vorticity have spikelike cores with an out-of-plane quadrupolar moment induced by the flexoelectric coupling. The cores are located near the film--dead-layer interfaces. FEM results corroborated by analytical calculations prove that a change of the flexoelectric coefficient sign leads to a reorientation of the core axial polarization, making the flexosensitive 3D vortices similar to the recently introduced ``flexons'' in cylindrical nanoparticles. The relatively wide temperature range (from 200 to 400 K) of the flexosensitive vortices' existence gives us confidence that they can be observed experimentally in thin ferroelectric films by scanning-probe and nonlinear optical microscopy methods.

Influence of thermal annealing on the morphology and magnetic domain structure of Co thin films

Centimeter scale cobalt films with various thicknesses (8 nm ∼ 100 nm) were deposited by electron beam evaporation (EBE) and then annealed in a gas mixture of Ar and H2 at temperatures ranging from 200 °C to 500 °C. Advanced characterization techniques (e.g., XRD, SEM, AFM and MFM) were employed to investigate the influence of annealing on the morphology, crystal structures and magnetic domain structures of Co thin films. The results of SEM and AFM suggest that there is no obvious change in the morphology of Co film before and after annealing especially for thicker films and the root-mean-square roughness of Co film surface is slightly reduced after annealing. The influence of thermal annealing on the magnetic domain structure of EBE Co thin films was investigated by magnetic force microscopy (MFM) for the first time. It is found that even if there is no clear domain structure in the as-deposited films, it is possible to obtain periodic stripe domains with perpendicular magnetic anisotropy (PMA) by thermal annealing owing to the development of HCP Co phase, which was confirmed by XRD analysis. The correlations between the film morphology, thickness and magnetic domain structure are discussed qualitatively. Based on the periodic stripe magnetic domains, the domain-wall energy density of annealed cobalt films is calculated and this study found that the annealing has almost no effect on the energy density of the domain-wall. This work provides an effective way to obtain the perpendicular magnetic anisotropy (PMA) for practical applications.

Optimizing the piezoelectric vibration of Pb(Mg1/3Nb2/3)O3-0.25PbTiO3 single crystal by alternating current polarization for ultrasonic transducer

Alternate current poling (ACP) is an effective method to enhance the piezoelectric performance of Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) single crystals dramatically. For their application in piezoelectric transducers for medical imaging, the beam mode vibration of PMN-0.25PT single crystal resonators after ACP was investigated by combining experimental impedance measurements and finite element simulation. The complete material coefficients of PMN-0.25PT single crystals were obtained by using pulse-echo ultrasound and inverse impedance spectroscopy. Compared to traditional direct current poling (DCP), the beam mode electromechanical coupling coefficient (k33′) of PMN-0.25PT for ACP reaches up to 89.1% from 84.5% of DCP. As the attribution of the special periodic stripe domain structure (109° domain), the lateral vibration will be suppressed effectively in the array transducer. This indicates that AC-poled PMN-0.25PT single crystals can be an efficient strategy to obtain higher sensitivity and a larger bandwidth in ultrasonic transducers.

Light-Induced Currents at Domain Walls in Multiferroic BiFeO3.

Multiferroic BiFeO3 (BFO) films with spontaneously formed periodic stripe domains can generate above-gap open circuit voltages under visible light illumination; nevertheless the underlying mechanism behind this intriguing optoelectronic response has not been understood to date. Here, we make contact-free measurements of light-induced currents in epitaxial BFO films via detecting terahertz radiation emanated by these currents, enabling a direct probe of the intrinsic charge separation mechanisms along with quantitative measurements of the current amplitudes and their directions. In the periodic stripe samples, we find that the net photocurrent is dominated by the charge separation across the domain walls, whereas in the monodomain samples the photovoltaic response arises from a bulk shift current associated with the non-centrosymmetry of the crystal. The peak current amplitude driven by the charge separation at the domain walls is found to be 2 orders of magnitude higher than the bulk shift current response, indicating the prominent role of domain walls acting as nanoscale junctions to efficiently separate photogenerated charges in the stripe domain BFO films. These findings show that domain-wall-engineered BFO thin films offer exciting prospects for ferroelectric-based optoelectronics, as well as bias-free strong terahertz emitters.

Dielectric and piezoelectric properties of Pb[(Mg1/3Nb2/3)0.52(Yb1/2Nb1/2)0.15Ti0.33]O3 single-crystal rectangular plate and beam mode transducers poled by alternate current poling

The dielectric, piezoelectric and physical properties of the [001]-oriented rectangular plate and beam mode of Pb[(Mg1/3Nb2/3)0.52(Yb1/2Nb1/2)0.15Ti0.33]O3 (PMN-PYbN-PT) ternary single crystals (SC) poled by alternate current poling (ACP) were investigated. The dielectric permittivity and average piezoelectric coefficient d33 were 6800 and 2490 pC/N, respectively, for the rectangular plate sample after ACP, which were 31% and 41% larger than those of the same sample after conventional direct current poling. The highest value of d33 reached 2750 pC N−1. The dielectric loss was reduced by 31%; nevertheless, the d33 improved after ACP. The beam mode electromechanical coupling factor () was 91% by using ACP, showing the highest value among all relaxor-PT SC systems reported so far. The regular periodic stripe domains were observed in ACP samples, showing high domain wall density and uniform domain size. This work demonstrates that the high performance after ACP is an effective method of improving the relaxor-PT SC transducers.

Tunable magnetic domains and depth resolved microstructure in Gd-Fe thin films

This paper reports the tunability of magnetic domains and its correlation with depth-resolved microstructure in 100 nm thick Gd-Fe films after rapid thermal processing. The as-deposited films are amorphous in nature and display elongated periodic stripe domains, signifying the development of perpendicular magnetic anisotropy. Rapid thermal processing at 550oC for different time intervals viz. 5, 10, 15 and 20 min induces the crystallization of Fe over the amorphous Gd-Fe matrix and the presence of wider reaction zone has been evidenced at the film-substrate interface. While the magnetization measurements reveal a spin re-orientation along the plane of the film surface, the stripe domains are no longer observed for the processed films. 3D micromagnetic simulations have been performed to complement the experimental findings. The consideration of in-plane tilt of anisotropy axis along with anisotropy reduction successfully demonstrate the experimentally observed changes in magnetic properties and domain structure.

Thickness-Dependent Evolution of Piezoresponses and Stripe 90° Domains in (101)-Oriented Ferroelectric PbTiO3 Thin Films.

High-index ferroelectric films as (101)-orientated ones exhibit enhanced dielectric responses, piezoelectric responses, and exotic ferroelectric switching behaviors, which are potential candidates for applications in memories and capacitors. However, possible domain patterns and domain wall structures in (101)-oriented ferroelectric thin films are still elusive, which results in difficulties in understanding the origin and further modulating their special properties. In this work, a series of PbTiO3 (PTO) thin films with 35, 50, 60, and 70 nm in thickness were grown on (101)-oriented (LaAlO3)0.29(SrTa1/2Al1/2O3)0.71 (LSAT(101)) substrates by pulsed laser deposition and investigated by both piezoresponse force microscopy (PFM) and (scanning) transmission electron microscopy ((S)TEM). PFM measurements reveal that periodic stripe domains are dominant in 50 nm thick PTO films. Besides stripe domains, a/ c domains appear in films with thickness more than 60 nm. A thickness-dependent evolution of piezoresponse amplitude indicates that the 50 nm thick PTO films demonstrate a superior piezoresponse. Electron diffraction and contrast analysis clarify that all these (101)-oriented PTO films contain periodic stripe ferroelectric 90° domains. The domain periods increase with the film thickness following Kittel's law. Aberration-corrected STEM imaging reveals that the stripe ferroelectric 90° domains have an alternate arrangement of wide and narrow c domains with polarization directions along [100] for c1 domains and [001̅] for c2 domains, forming a "head-to-tail" polarization configuration. Further strain analysis reveals that stripe domains have uniform strain distributions and distinct lattice rotations around domain walls. It is proposed that the periodic arrangement of high-density stripe 90° domains in 50 nm thick PTO films is the main contributor to the superior piezoresponse behavior. These results are expected to provide useful information to understand the domain structures in (101)-oriented PTO thin films and thus facilitate further modulation of the properties for potential applications.

Straight motion of half-integer topological defects in thin Fe-N magnetic films with stripe domains

In thin magnetic films with perpendicular magnetic anisotropy, a periodic “up-down” stripe-domain structure can be originated at remanence, on a mesoscopic scale (~100 nm) comparable with film thickness, by the competition between short-range exchange coupling and long-range dipolar interaction. However, translational order is perturbed because magnetic edge dislocations are spontaneously nucleated. Such topological defects play an important role in magnetic films since they promote the in-plane magnetization reversal of stripes and, in superconductor/ferromagnet hybrids, the creation of superconducting vortex clusters. Combining magnetic force microscopy experiments and micromagnetic simulations, we investigated the motion of two classes of magnetic edge dislocations, randomly distributed in an {{rm{N}}}_{2}^{+}-implanted Fe film. They were found to move in opposite directions along straight trajectories parallel to the stripes axis, when driven by a moderate dc magnetic field. Using the approximate Thiele equation, analytical expressions for the forces acting on such magnetic defects and a microscopic explanation for the direction of their motion could be obtained. Straight trajectories are related to the presence of a periodic stripe domain pattern, which imposes the gyrotropic force to vanish even if a nonzero, half-integer topological charge is carried by the defects in some layers across the film thickness.

Stripe domains in epitaxial BiFeO3 thin films on (100) SrTiO3 substrates

Highly crystallized ferroelectric BiFeO3 thin films were deposited on various miscut (100) SrTiO3 substrates. The domain structure of the films varies with substrate miscut angles and atomic-layer termination at the surface. The BiFeO3 thin films grown on 4° miscut substrates exhibit 71° periodic stripe domains consisting of two downward polarization variants. In contrast, four downward variants coexist in the films deposited on 0.2° miscut substrates, regardless of atomic-layer termination at the surface of the substrate. The introduction of an additional SrTiO3 repair layer on the same substrate results in a step-flow growth mode of the film. It is believed that the improved mobility of the BiFeO3 atomic species during the film growth leads to the appearance of preferred ferroelectric variants at the step edges. These preferred variants eventually form stripe domains. Our results reveal that both the miscut angle and the step-flow growth promote the formation of the two variant striped domains in BiFeO3 films deposited on decorated isotropic SrTiO3 substrates.

Nanoscale domain patterns and a concept for an energy harvester

The current work employs a phase-field model to test the stability of nanoscale periodic domain patterns, and to explore the application of one pattern in an energy harvester device. At first, the stability of several periodic domain patterns with in-plane polarizations is tested under stress-free and electric field-free conditions. It is found that simple domain patterns with stripe-like features are stable, while patterns with more complex domain configurations are typically unstable at the nanoscale. Upon identifying a stable domain pattern with suitable properties, a conceptual design of a thin film energy harvester device is explored. The harvester is modeled as a thin ferroelectric film bound to a substrate. In the initial state a periodic stripe domain pattern with zero net charge on the top electrode is modeled. On bending the substrate, a mechanical strain is induced in the film, causing polarized domains to undergo ferroelectric switching and thus generate electrical energy. The results demonstrate the working cycle of a conceptual energy harvester which, on operating at kHz frequencies, such as from vibrations in the environment, could produce an area power density of about 40 W m−2.

Single-domain epitaxial silicene on diboride thin films

Epitaxial silicene, which forms spontaneously on ZrB2(0001) thin films grown on Si(111) wafers, has a periodic stripe domain structure. By adsorbing additional Si atoms on this surface, we find that the domain boundaries vanish, and a single-domain silicene sheet can be prepared without altering its buckled honeycomb structure. The amount of Si required to induce this change suggests that the domain boundaries are made of a local distortion of the silicene honeycomb lattice. The realization of a single domain sheet with structural and electronic properties close to those of the original striped state demonstrates the high structural flexibility of silicene.

Visualization and Interpretation of Magnetic Configurations Using Magnetic Charge

The calculation and graphic imaging of magnetic charge density provides fresh insights into old problems in micromagnetics. (1) The full saturation magnetization $M_{\mathrm{s}}$ of polycrystalline iron can be used in a transformer when magnetic charge at the grain boundaries suppresses the adverse effects of anisotropy. A single domain wall separates two regions of opposite magnetization $\vert {{\mathbf M}} \vert = M_{\mathrm{s}}$ in toroidal geometry. This is a three-dimensional expression of the ripple effect, long known in polycrystalline thin films of Permalloy. (2) Helical patterns of ${{\mathbf M}}$ result from interaction of regions of opposite magnetic charge developed by the introduction of some $\nabla \cdot {{\mathbf M}}$ when the exchange energy primarily invokes $\nabla \times {{\mathbf M}}$ to accommodate to the boundary conditions set by geometry and surface charge density ${{\mathbf n}} \cdot {{\mathbf M}}$ . The helical structures resolve 40-year-old mysteries in the appearance of periodic stripe domains associated with the propagation of edge solitons in a hexagonal [1 1 1] oriented iron whisker.

Local control of magnetic anisotropy in transcritical permalloy thin films using ferroelectric BaTiO3 domains

We investigated the local coupling between dense magnetic stripe domains in transcritical permalloy (tPy) thin films and ferroelectric domains of BaTiO3 single crystals in a tPy/BaTiO3 heterostructure. Two distinct changes in the magnetic stripe domains of tPy were observed from the magnetic force microscopy images after cooling the heterostructure from above the ferroelectric Curie temperature of BaTiO3 (120 °C) to room temperature. First, an abrupt break in the magnetic stripe domain direction was found at the ferroelectric a-c-domain boundaries due to an induced change in in-plane magnetic anisotropy. Second, the magnetic stripe domain period increased when coupled to a ferroelectric a-domain due to a change in out-of-plane magnetic anisotropy. Micromagnetic simulations reveal that local magnetic anisotropy energy from inverse magnetostriction is conserved between in-plane and out-of-plane components.

Vortex Confinement in Planar Superconductor/Ferromagnet Hybrid Structures

We use low temperature magnetic force microscopy and global magnetometry measurements to study the influence of magnetic domains on the Abrikosov vortex pinning in planar superconducting/ferromagnet bilayers. The superconducting/ferromagnet bilayers consist of a 200 nm superconducting Nb film covering a Permalloy film, with an insulating layer in between to avoid proximity effect. The periodic stripe domain in the Permalloy film produces a potential for directing vortex motion in the adjacent superconducting film. We observed an enhancement of vortex pinning by a factor of 3 that occurs in bilayers with a magnetic stripe domains <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</i> ≈ 500 nm, close to the superconducting critical temperature (T/T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 0.9) . At lower temperatures, when T/T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 0.6 the channeled vortex motion and the intrinsic pinning favor vortex avalanches.

Spontaneous Periodic Deformations in Nonchiral Planar-Aligned Bimesogens with a Nematic-Nematic Transition and a Negative Elastic Constant

Hydrocarbon linked mesogenic dimers are found to exhibit an additional nematic phase below the conventional uniaxial nematic phase as confirmed by x-ray diffraction. The phase produces unusual periodic stripe domains in planar cells. The stripes are found to be parallel to the rubbing direction (in rubbed cells) with a well-defined period equal to double the cell gap. The stripes appear without external electromagnetic field, temperature or thickness gradients, rubbing or hybrid alignment treatments. Simple modeling proposes a negative sign for at least one of the two elastic constants: splay and twist, as a necessary condition for the observed pattern.

Spin wave quantization in continuous film with stripe domains

We investigated the spin wave dynamics of CoFeSiB film, which has a stripe domain structure at a low magnetic field region (&amp;lt;1 kOe). We measured the spin wave excitation spectra by employing Brillouin light scattering. Abnormal field dependence and dispersion relations were observed, and they are similar to spin wave quantization in laterally confined magnetic structures such as arrays of magnetic nanowires. The observed spin wave excitation spectra must be interpreted with spin wave quantization such as Damon–Eshbach mode separation. It was found that the spin wave quantization is related to the stripe magnetic domain structure in continuous film. The physical origin of the quantization is the partial reflection of the propagating spin wave at the periodic stripe domain boundaries.

Temperature dependence of magnetic stripe domain period in ultrathin films

In ultrathin films, due to the thermal activation and temperature dependencies of the magnetic parameters, magnetization reversal processes are strongly affected by thermal effects. We analyze changes of domain periods of ultrathin cobalt and L 1 0 films in a wide temperature range. With regard to the temperature dependencies of the film magnetic parameters we calculate the equilibrium stripe period as a function of temperature. It is shown that on film heating the equilibrium domain structure (DS) period decreases and at the reorientation phase transition (RPT) approaches its minimal value corresponding to the temperature independent period of the sinusoidal domain structure. Just below the RPT temperature (or thickness) the stripe domain period was found to exponentially decrease with temperature. Irreversible temperature changes of the domain period affected by coercivity are also discussed.

Domain evolution ofBaTiO3ultrathin films under an electric field: A first-principles study

A first-principles-derived method is used to study the morphology and electric-field-induced evolution of stripe nanodomains in (001) BaTiO3 (BTO) ultrathin films, and to compare them with those in (001) Pb(Zr,Ti)O3 (PZT) ultrathin films. The BaTiO3 systems exhibit 180o periodic stripe domains at null electric field, as in PZT ultrathin films. However, the stripes alternate along [1-10] in BTO systems versus [010] in PZT systems, and no in-plane surface dipoles occur in BTO ultrathin films (unlike in PZT materials). Moreover, the evolution of the 180o stripe domains in the BaTiO3 systems, when applying and increasing an electric field along [001], involves four regions: Region I for which the magnitude of the down dipoles (i.e., those that are antiparallel to the electric field) is reduced, while the domain walls do not move; Region II in which some local down dipoles adjacent to domain walls switch their direction, resulting in zigzagged domain walls - with the overall stripe periodicity being unchanged; Region III in which nanobubbles are created, then contract along [110] and finally collapse; and Region IV which is associated with a single monodomain. Such evolution differs from that of PZT ultrathin films for which neither Region I nor zigzagged domain walls exist, and for which the bubbles contract along [100]. Discussion about such differences is provided.