Houbing Huang

Purely Electric-Field-Driven Perpendicular Magnetization Reversal

If achieved, magnetization reversal purely with an electric field has the potential to revolutionize the spintronic devices that currently utilize power-dissipating currents. However, all existing proposals involve the use of a magnetic field. Here we use phase-field simulations to study the piezoelectric and magnetoelectric responses in a three-dimensional multiferroic nanostructure consisting of a perpendicularly magnetized nanomagnet with an in-plane long axis and a juxtaposed ferroelectric nanoisland. For the first time, we demonstrate a full reversal of perpendicular magnetization via successive precession and damping, driven purely by a perpendicular electric-field pulse of certain pulse duration across the nanoferroelectric. We discuss the materials selection and size dependence of both nanoferroelctrics and nanomagnets for experimental verification. These results offer new inspiration to the design of spintronic devices that simultaneously possess high density, high thermal stability, and high reliability.

Toward Wearable Cooling Devices: Highly Flexible Electrocaloric Ba0.67Sr0.33TiO3 Nanowire Arrays

Flexible lead-free ferroelectric ceramic nanowire arrays exhibit a unique combination of features that can contribute to the realization of wearable cooling devices, including an outstanding electrocaloric effect at low fields, high efficiency, bendability and stretchability, and robustness against mechanical deformations. Thermodynamic and phase-field simulations are carried out to validate their superior electrocaloric effect in comparison to thin films.

Experiment and first principles investigation on the hydrogen-hindered phase transition of ferroelectric ceramics

In this letter, hydrogen-hindered phase transition of ferroelectric ceramics from cubic to tetragonal has been studied by experiment and first principles calculation. The calculation shows that for hydrogenated tetragonal PbTiO3, double-lowest-energy sites of Ti along the c axis exist no longer and the only lowest energy site locates at the center of the cell. The calculation can explain the experiment that hydrogen charged above its Curie temperature can hinder phase transition of lead zirconate titanate from cubic paraelectricity to tetragonal ferroelectricity.

Anisotropy of Indentation Cracks Propagation of PZT-5H Ferroelectric Ceramics in Air and Water under Sustained Load

The propagation process and the anisotropy of the indentation cracks of poled PZT-5H ferroelectric ceramics in air and water under sustained load were investigated using Vickers indentation equipment. It is found that the length of the indentation cracks increased with increasing time to keep indentation load in air and water, thus, sub-critical crack growth or stress corrosion cracking (SCC) under sustained load occurred. For the cracks parallel to the poling direction, the threshold stress intensity factor for cracks arrest, K ISCC , is larger, whereas the crack propagation rate, da/dt, is smaller than those for the cracks perpendicular to the poling direction. The anisotropy of susceptibility to SCC is related to the anisotropy of fracture toughness. The larger the fracture toughness is, the more susceptibility to SCC is, that is, the smaller the K ISCC is, the larger the da/dt is.

Magnetically actuated functional gradient nanocomposites for strong and ultra-durable biomimetic interfaces/surfaces

Magnetically-actuated functional gradient nanocomposites can be locally modulated to generate unprecedented mechanical gradients applied to various interfaces and surfaces following the design principles of natural biological materials. Several thus-far-inaccessible biomimics including a strong and ultra-durable interface mimicking the tooth dentin–enamel junction, a wear-resistant and long-lasting surface coating mimicking biological skins, and flexible yet structurally-stable micropillars mimicking the adhesive setae of insects are demonstrated.

Numerical simulation of vortex dynamics in type-II superconductors in oscillating magnetic field using time-dependent Ginzburg–Landau equations

Time-dependent Ginzburg-Landau equations were solved by the finite difference scheme for a superconducting sample in steady and oscillating magnetic fields for 3D geometry. The dynamic behaviour of penetrating and leaving magnetic vortices in superconductor with the oscillating magnetic field was simulated. Carrier concentration density and the average magnetization of the sample were studied as a function of the external oscillating magnetic field. Anomalies in carrier concentrations at certain magnetic field values were observed and discussed. It was also observed that the area swept by magnetization versus external magnetic field is magnetic oscillation frequency dependent, which increases with increasing frequencies. It was suggested that this effect may cause instability in the superconducting characteristics of the sample over a number of cycles. Calculated energy patterns showed consistency with vortex patterns in the steady magnetic field. Magnetic oscillations initiated oscillations in energy components, ripples in superconducting energy are subjected to the entrance and leaving of vortices, while instability observed in interaction energy is referred to vortex relaxation time.

Phase-Field Simulation of Strain-Assisted Current-Induced Magnetization Dynamics in a Magnetic Tunnel Junction

We investigated the effect of substrate misfit strain theoretically by using a phase-field microelasticity model in a simple sandwich structure of a magnetic tunnel junction with perpendicular anisotropy. The critical current for magnetization flipping decreases with increasing tensile strain except in the region above a threshold strain, which results from magneto-elastic coupling interaction between perpendicular magnetic anisotropy and substrate strain. Magnetization reversal speeds with different substrate strains and switching currents are also presented, which suggest ways to improve the magnetization switching efficiency and decrease the power consumption of magnetic devices.

Micromagnetic simulation of electric field-modulation on precession dynamics of spin torque nano-oscillator

Understanding electric field effects on precession dynamics is crucial to the design of spin transfer torque devices for improving the performance in nano-oscillator. In this letter, the precession dynamics of a CoFeB/MgO multi-layer structured nano-oscillator under externally applied electric field is predicted using a micromagnetic simulation. It is revealed that the electric field can modify the range of oscillation spectra in single frequency mode. With the increase in electric field, there is a red-shift of the resonant frequency. When a positive electric field pulse is applied, a phase lag of the spin precession is induced, which is proportional to the pulse amplitude and duration. The present work is expected to stimulate future experimental efforts on designing devices with electric-field modulated spin transfer torque nano-oscillators.Understanding electric field effects on precession dynamics is crucial to the design of spin transfer torque devices for improving the performance in nano-oscillator. In this letter, the precession dynamics of a CoFeB/MgO multi-layer structured nano-oscillator under externally applied electric field is predicted using a micromagnetic simulation. It is revealed that the electric field can modify the range of oscillation spectra in single frequency mode. With the increase in electric field, there is a red-shift of the resonant frequency. When a positive electric field pulse is applied, a phase lag of the spin precession is induced, which is proportional to the pulse amplitude and duration. The present work is expected to stimulate future experimental efforts on designing devices with electric-field modulated spin transfer torque nano-oscillators.

Boundary Pinning Effects on the Frequency Spectra of Point-Contact Spin-Torque Oscillators

Micromagnetic simulation is used to investigate the frequency spectra of a perpendicularly magnetized nanocontact spin-torque oscillator subject to different boundary pinning conditions. Boundary confinement is critical to the output spectra: spectra under two limiting conditions are attributed to the different pinning strengths at the boundaries. The correlation between boundary pinning strength and output spectra may facilitate the design of future nanocontact microwave devices.

Accelerating large-scale phase-field simulations with GPU

A new package for accelerating large-scale phase-field simulations was developed by using GPU based on the semi-implicit Fourier method. The package can solve a variety of equilibrium equations with different inhomogeneity including long-range elastic, magnetostatic, and electrostatic interactions. Through using specific algorithm in Compute Unified Device Architecture (CUDA), Fourier spectral iterative perturbation method was integrated in GPU package. The Allen-Cahn equation, Cahn-Hilliard equation, and phase-field model with long-range interaction were solved based on the algorithm running on GPU respectively to test the performance of the package. From the comparison of the calculation results between the solver executed in single CPU and the one on GPU, it was found that the speed on GPU is enormously elevated to 50 times faster. The present study therefore contributes to the acceleration of large-scale phase-field simulations and provides guidance for experiments to design large-scale functional devices.A new package for accelerating large-scale phase-field simulations was developed by using GPU based on the semi-implicit Fourier method. The package can solve a variety of equilibrium equations with different inhomogeneity including long-range elastic, magnetostatic, and electrostatic interactions. Through using specific algorithm in Compute Unified Device Architecture (CUDA), Fourier spectral iterative perturbation method was integrated in GPU package. The Allen-Cahn equation, Cahn-Hilliard equation, and phase-field model with long-range interaction were solved based on the algorithm running on GPU respectively to test the performance of the package. From the comparison of the calculation results between the solver executed in single CPU and the one on GPU, it was found that the speed on GPU is enormously elevated to 50 times faster. The present study therefore contributes to the acceleration of large-scale phase-field simulations and provides guidance for experiments to design large-scale functional dev...