Vortex Domain Structures Research Articles

Giant electrocaloric effects in ferroelectric nanostructures with vortex domain structures

Electrocaloric effects of Bi4Ti3O12 (BIT) ferroelectric nanoparticles with vortex domain structures in a curled electric field are investigated using a phase field method. A large adiabatic temperature change (ΔT = 16.6 K) is observed in the BIT nanoparticles with the change in vorticity vector of the curled field ΔQ = 0.15 mV A−2 at 620 °C. This large ΔT is attributed to a large change in the toroidal moment which is associated with vortex domain structures in the nanoparticles under the curled electric field. These results indicate that ferroelectric nanostructures with vortex domain structures can be exploited for use in solid state refrigeration.

Vortex domain structures of an epitaxial ferroelectric nanodot and its temperature-misfit strain phase diagram

Phase field simulations were conducted to investigate the effect of misfit strain on the vortex domain structure (VDS) in a BaTiO3 nanodot. Taking into account the effect of inhomogeneous eletromechanical fields, ambient temperature and surface effects, our calculations demonstrate a strong effect of misfit strain on the orientation and magnitude of the polarization dipoles. As a consequence, fruitful equilibrium vortex domain patterns can be obtained by adjusting the epitaxial misfit strain between the substrate and the nanodot. While the nanodot exhibits a single transition from a paraelectric to a near-rhombohedral vortex state at zero misfit strain with the decrease of temperature, complicated transformations of vortex domain patterns are found under nonzero misfit strain. Typically, orthorhombic, tetragonal and several unreported vortex domain patterns (e.g., with zero toroidal moment) are found. Moreover, misfit strain-induced transformations into these domain patterns are also predicted for a ferroelectric nanodot with initial near-rhombohedral vortex state. Combining effects of the ambient temperature and misfit strain, a "temperature-misfit strain" phase-diagram depicting the fruitful vortex domain patterns of the nanodot was obtained. Our simulations indicate promising application of strain engineering in controlling the VDS in ferroelectric nanostructures.

Effects of the surface charge screening and temperature on the vortex domain patterns of ferroelectric nanodots

Based on the phase field simulations, effects of the surface charge screening and temperature on the vortex domain structure of the ferroelectric nanodot have been investigated. Our calculations show that the ferroelectric nanodot adopts a rhombohedral vortex domain pattern under an ideal open-circuit boundary condition. With the increase of the surface charge screening, the dipole vortex gradually rotates and appears rhombohedral-orthorhombic-tetragonal transformation. By adjusting the surface charge screening, the polar single domain and multi-vortices domain patterns with zero toroidal moment have been obtained near the phase transition temperature. More importantly, temperature and charge screening “T-C” phase diagram has been summarized, which indicates an effective method to control the vortex domain structure in the low-dimensional ferroelectric nanostructures.

Vortex Domain Structure in Ferroelectric Nanoplatelets and Control of its Transformation by Mechanical Load

Vortex domain patterns in low-dimensional ferroelectrics and multiferroics have been extensively studied with the aim of developing nanoscale functional devices. However, control of the vortex domain structure has not been investigated systematically. Taking into account effects of inhomogeneous electromechanical fields, ambient temperature, surface and size, we demonstrate significant influence of mechanical load on the vortex domain structure in ferroelectric nanoplatelets. Our analysis shows that the size and number of dipole vortices can be controlled by mechanical load, and yields rich temperature-stress (T-S) phase diagrams. Simulations also reveal that transformations between “vortex states” induced by the mechanical load are possible, which is totally different from the conventional way controlled on the vortex domain by the electric field. These results are relevant to application of vortex domain structures in ferroelectric nanodevices, and suggest a novel route to applications including memories, mechanical sensors and transducers.

Confinement in G(2) gauge theories using the thick center vortex model and domain structures

The thick center vortex model, with the idea of using domain structures, is used to calculate the potentials between two G(2) heavy sources in the fundamental, the adjoint, and the 27-dimensional representations. The potentials are screened at large distances. This behavior is expected from the thick center vortex model since G(2) has only a trivial center element which makes no contribution to the average Wilson loop at the large distance regime. A linear potential is obtained at intermediate distances for all representations. This behavior can be explained by the thickness of the vortices (domains) and by defining a flux for the trivial center element of G(2). The role of the SU(3) subgroup of G(2) in the linear regime is also discussed. The string tensions are in rough agreement with the Casimir operators of the corresponding representations.

Stable vortex magnetite nanorings colloid: Micromagnetic simulation and experimental demonstration

Magnetite nanoring with vortex domain structure may form stable magnetic colloid for biomedical applications due to its weak magnetic interaction without superparamagnetic (SPM) limitation. In the present study, we perform three-dimensional (3 D) Landau-Liftshitz-Gilbert (LLG) micromagnetics simulation for magnetite nanorings. The ground state phase diagram and stable vortex area (SVA) as a function of outer diameter (Dout), thickness (T), and inner to outer diameter ratios (β) within 100 nm are obtained. The influence of notch, eccentricity, and crystallographic orientation are taken carefully into consideration. In the SVA, the vortex state is not only the ground state but also the remanence state after in-plane is fully magnetized. In particular, the results suggest that a 20 nm inter-rings distance for a typical magnetite nanoring (Dout = 70 nm, T = 50 nm, and β = 0.6) can achieve the stable colloid based on vortex domain structure. Furthermore, these simulation results have been confirmed experimentally and demonstrated by using phosphorylated-mPEG modified magnetite nanorings. The optimization of magnetite nanorings from both simulation and experiments in this work pave the way to achieve such novel and stable vortex domain based magnetic suspension for various biomedical applications.

Phase field simulations of stress controlling the vortex domain structures in ferroelectric nanosheets

Effect of stress loads on the vortex domain structures (VDSs) in ferroelectric nanosheet has been investigated. Results of phase field simulations show that the different vortex domain structures can form in free-standing nanosheet from random perturbations. Applying stress loads on nanosheet, it is found that the domain morphology, especially the size and number of vortices, can be regularly controlled. These results indicate promising controlling the vortex domain structures in ferroelectric nanostructures by the mechanical loads.

Structural and Magnetic Properties of Cobal Ferrite - Barium Titanate Nanotube Arrays

ABSTRACTCobalt Ferrite/Barium Titanate nanotube arrays were obtained in anodic aluminum oxide templates (AAO) of 100 nm pore diameter by a two step sol-gel process. Each phase was grown in several wetting – drying cycles starting from the cobalt ferrite layers with the barium titanate on top. As-dried composite structures were sintered at 700 C. The composite nanotubes showed a fine polycrystalline microstructure with an average grain size of 5 nm. The formation of both spinel and perovskite structures was verified by High Resolution Transmission Electron Microscopy (HR-TEM) on isolated nanotubes. The growing rate by layer was found to be lower for the BaTiO3 on top of CoFe2O4 than the later on top of the AAO. Wall sizes were estimated by Z-contrast as 9.8 nm for one layer of CoFe2O4 and 6.6 nm for six layers of BaTiO3. Magnetic properties were studied by VSM. Samples showed ferromagnetic behavior with low coercive values. By means of a finite element model the deformation and stress on the piezoelectric phase was estimated and used to simulate the magnetization reversal under stress in the composite nanotubes, using an updated micromagnetic framework to include the magnetostriction effect. Simulation results showed that a curling mode is expected with opposite vortex states at the end of the nanotubes. The change in the vortex domain structure under voltage driven applied stress is presented and discussed.

Chapter 7 - Magnetic Tunnel Junctions and Spin Torques

Publisher Summary This chapter briefly examines the concept magnetic RAM (“MRAM”) and how it is expected to compete with the current high speed data transfer techniques. Magnetic RAM (MRAM) is expected to handle various shortcomings of data transfers, providing instant boot-up capability, with high speed access, low power consumption and high density. Early MRAM devices used magnetic hysteresis to store information. A basic storage mechanism is tied to the magnetic state of a tunnel junction. MRAM does not require any refreshing to maintain the magnetic state of a given bit, once it is magnetized, and hence the memory is non-volatile. Not only does this mean that it retains its memory with the power turned off, but also that there is no need for a constant power supply. A magnetic tunnel junction (MTJ) is different from the metallic sandwich structures discussed with regard to the GMR effect. A MTJ consists of two metallic layers, sometimes called electrodes, separated by an insulating layer thin enough to allow some tunneling current. The interest in such devices is at least partly due their potential applications in magnetoresistive random access memory (MRAM) devices and sensors. The chapter discusses the theological aspects of tunneling magnetoresistance (TMR), devices with large TMR values, double barriers, vortex domain structures, spin transfer torques in metallic multilayers, ultra-fast reversal of magnetization, and transistors based on spin orientation. The chapter illustrates two key features of the scattering process. The first is that the spin current along the direction of the magnetization is con-served. The second point is that the reflected and transmitted currents have no transverse components. When combined, these two results show that the spin transfer torque is approximately given by the absorption of the transverse component of the incident spin current.

Current-induced butterfly shaped domains and magnetization switching in magnetic tunnel junctions

Patterned magnetic tunnel junctions (MTJs) with the layer structure of Ta (5 nm)/Ni79Fe21 (5 nm)/Cu (20 nm)/Ni79Fe21 (5 nm)/Ir22Mn78 (10 nm)/Co75Fe25 (4 nm)/Al (0.8 nm)-oxide/Co75Fe25 (4 nm)/Ni79Fe21 (20 nm)/Ta (5 nm) were fabricated using magnetron sputtering deposition and lithography. High tunnelling magnetoresistance ratios of 22 and 50% were obtained at room temperature before and after annealing, respectively. The evolution of leaf shaped images was observed via Lorentz transmission electron microscopy (LTEM) on the MTJs, which were deposited on a patterned and carbon-coated transmission electron microscopy grid. These leaf-like LTEM images correspond to a butterfly shaped domain structure that was confirmed by a micromagnetics simulation. When a large DC current or bias voltage was applied across the MTJ, the butterfly-like vortex domain structures could be induced to form in the free layer of the MTJ, resulting in a significant decrease of magnetization in the free layer. The existence of these butterfly shaped domains could be one of the major causes of the bias voltage dependence of the TMR ratio.

Vortex domain structures and dc current dependence of magneto-resistances in magnetic tunnel junctions

Microfabrication and the magneto-transport characteristics of the magnetic tunnel junctions (MTJs) with a spin-valve-type structure of Ta (5nm)/Ni79Fe21 (25nm)/Ir22Mn78 (12nm)/Co75Fe25 (4nm)/Al(0.8nm) oxide/Co75Fe25 (4nm)/Ni79Fe21 (20nm)/Ta(5nm) were investigated in this paper. A series of experimental data measured with a MTJ was used to verify a magnon-assisted tunnelling model and theory. Furthermore, a micromagnetics simulation shows that the butterfly-like vortex domain structures can be formed under a current-induced Oersted field, which decreases the net magnetization values of the ferromagnetic electrodes under a large dc current (i.e., in high voltage regimes). It is one of the main reasons for the tunnel magnetoresistance ratios to decrease significantly at high voltage biasing.

Switching properties and dynamic domain structures in double barrier magnetic tunnel junctions

Double barrier magnetic tunnel junctions (DBMTJs) with the layer structures of Ta (5nm)/Ni79Fe21(40nm)/Ir22Mn78(10nm)/Co75Fe25(4nm)/Al(1nm)-oxide/Co75Fe25(8nm)/Al(1nm)-oxide/Co75Fe25(4nm)/Ir22Mn78(10nm)/Ni79Fe21(30nm)/Ta(5nm) on Si/SiO2 wafer were micro-fabricated using a TMR R&D magnetron sputtering system and lithography method. TMR ratios of 30.0% and 22.1%, resistance-area product RS of around 32.0 and 27.5kΩμm2, and free layer coercivity of 201 and 141Oe at 4.2K and room temperature (RT), respectively, were obtained for the MTJs with a size of 80×80μm2. Static and dynamic domain structures occur as the DC current increases and magnetization switching properties are simulated based on micromagnetics using the energy minimization method. TMR ratios in DBMTJs that are far lower than the expected theoretical values can be clarified based on micromagnetics simulations due to the vortex domain structures formed in the free layer.

Vortex configurations, matching, and domain structure in large arrays of artificial pinning centers.

High-resolution scanning Hall probe microscopy has been used to image vortex configurations in very large periodic arrays of artificial pinning sites. Strong matching effects are seen at fields where either one or two vortices can sit at a site; with three vortices per site, however, no clear matching is observed. Matching effects have also been observed at several fractional multiples of the matching field, including 1/5, 1/4, 1/3, 1/2, and 3/4. These fractional values are characterized by striking domain structure and grain boundaries.