Tien-Kan Chung

Robust bi-stable memory operation in single-layer graphene ferroelectric memory

With the motivation of realizing an all graphene-based circuit for low power, we present a reliable nonvolatile graphene memory device, single-layer graphene (SLG) ferroelectric field-effect transistor (FFET). We demonstrate that exfoliated single-layer graphene can be optically visible on a ferroelectric lead-zirconate-titanate (PZT) substrate and observe a large memory window that is nearly equivalent to the hysteresis of the PZT at low operating voltages in a graphene FFET. In comparison to exfoliated graphene, FFETs fabricated with chemical vapor deposited (CVD) graphene exhibit enhanced stability through a bi-stable current state operation with long retention time. In addition, we suggest that the trapping/de-trapping of charge carriers in the interface states is responsible for the anti-hysteresis behavior in graphene FFET on PZT. V C 2011 American Institute of Physics. [doi:10.1063/1.3619816] Graphene is considered to be an exceptional material with high potential for future electronics, owing to its excellent electronic properties; 1 linear electron energy dispersion, and high room temperature mobility. If feasible, an all graphene-based circuit, including logic, analog, and memory devices, would be of great interest to further extend the performance of current Si-based electronics. Among various device applications, graphene based memory structures are still in their infancy in comparison to its logic and analog applications. To date, graphene memory has been demonstrated through chemical modification, 2 filament-type memristor, 3 nanomechanical switch, 4 and graphene FFETs. 5‐7 In graphene FFETs, however, the ambipolar conduction leads to undesirable on/off states for memory applications. Moreover, the absence of an electronic bandgap and controlled doping makes it difficult to resolve such issues. Therefore, a systematic study of graphene FFET is beneficial to realize graphene-based memory structures. In this Letter, we investigate graphene/PZT FFET structures using exfoliated- and CVD-SLG and their mechanism of operation. We show that exfoliated SLG can be optically identified on a PZT substrate and exhibit a hysteresis of the Vshaped conductance with a large memory window at low operating gate voltages. We compare exfoliated- with CVDSLG FFETs and show that devices made of CVD-SLG exhibit a robust bi-stable current state with a long retention time. In order to construct the SLG FFET, we first engineered a ferroelectric substrate to identify SLG. Previously, we have demonstrated that SLG is invisible under the optical micro

Electric-field-induced reversible magnetic single-domain evolution in a magnetoelectric thin film

We report experimental results on a Ni-nanobar/lead zirconate titanate-film magnetoelectric device demonstrating control of a metastable magnetic single domain with an electric field due to the converse magnetoelectric effect (i.e., coupling of piezoelectric effect, mechanical coupling, and magnetostriction). The reversible single-domain evolution from an initial single-domain state to a transitional S-shape domain state with an electric field was experimentally observed with magnetic force microscopy. Upon removal of the electric field, the single domain reverts to its original domain configuration. These results confirm change of a single domain in the nanoscale magnetoelectric/multiferroic device is achievable and subsequent control of local magnetic field is possible.

Reversible magnetic domain-wall motion under an electric field in a magnetoelectric thin film

We report direct microscopic measurements that confirm the magnetic stripe-domain patterns can be reversibly changed under an electric field due to the converse magnetoelectric effect in a bilayer thin film ferromagnetic-Ni/ferroelectric-lead zirconate titanate (100nm∕1.28μm) heterostructure. Electric field-induced curving, bending, branching, and elongation of magnetic stripe-domain patterns in the Ni layer are observed with the use of magnetic force microscopy. Upon removal of the electric field, the magnetic stripe-domain patterns return to their original configuration, i.e., reversible.

Influence of electric voltage bias on converse magnetoelectric coefficient in piezofiber/Metglas bilayer laminate composites

Experimental data on a piezofiber/Metglas bilayer composite subjected to both a dc electric voltage bias and a dc magnetic field bias while exciting it with an ac electric driving voltage are presented. As reported in previous studies, a dc magnetic field bias exists to maximize the converse magnetoelectric coefficient. Experimental data show that the optimum dc magnetic field bias is a function of applied dc electric voltage. Furthermore, it is revealed that an optimum dc electric voltage bias exists to further maximize the converse magnetoelectric coefficient.

Comparison of Effective Direct and Converse Magnetoelectric Effects in Laminate Composites

A theoretical analysis is used to prove that a piezoelectric/magnetostrictive laminate composite has equivalent effective direct and converse magnetoelectric (ME) coefficients. Experimentally a PZT/Terfenol-D/PZT trilayer laminate composite was fabricated and both direct and converse ME coefficients were characterized by measuring the magnetic-field-induced polarization and electric-field-induced magnetization. The laminate sample exhibits similar DC magnetic bias dependence and resonance response for both direct and converse ME effects. The magnitudes of effective ME coefficients are of the same order but significantly different and both are smaller than theoretical prediction. This raises the question as to whether these coefficients are equivalent in a layered composite or to the accuracy of ME coefficient data measurement.

Electrical and Mechanical Manipulation of Ferromagnetic Properties in Polycrystalline Nickel Thin Film

The ferromagnetic properties of a 35 nm polycrystalline nickel thin film deposited on a single-crystal, optical, ferroelectric lithium niobate substrate are magnetoelectrically tunable. The coercive field of the nickel film, which has low magnetocrystalline anisotropy and appropriate magnetoelasticity, changes by about 80% owing to the anisotropic strain produced by the substrate. In addition, mechanical strain changes the coercive field by about 260% and increases the normalized remanence bm Mbmr/bm Mbms from 0.3 to 1.0. This giant tunability would be achievable by combining polycrystalline Ni thin film with a ferroelectric substrate having large anisotropic piezoelectric coefficients.

Strain-induced magnetization change in patterned ferromagnetic nickel nanostructures

We report strain-induced coercive field changes in patterned 300u2009×u2009100u2009×u200935 nm3 Ni nanostructures deposited on Si/SiO2 substrate using the magnetoelastic effect. The coercive field values change as a function of the applied anisotropy strain (∼1000 ppm) between 390 and 500 Oe, demonstrating that it is possible to gradually change the coercive field elastically. While the measured changes in coercive field cannot be accurately predicted with simple analytical predictions, fairly good agreement is obtained by using a micromagnetic simulation taking into account the influence of nonuniform strain distribution in the Ni nanostructures. The micromagnetic simulation includes a position dependant strain-induced magnetic anisotropy term that is computed from a finite element mechanical analysis. Therefore, this study experimentally corroborates the requirement to incorporate mechanical analysis into micromagnetic simulation for accurately predicting magnetoelastic effects in patterned ferromagnetic nanostructures.

Atomic layer deposition of Pb(Zr,Ti)Ox on 4H-SiC for metal-ferroelectric-insulator-semiconductor diodes

Atomic layer deposited (ALD) Pb(Zr,Ti)Ox (PZT) ultra-thin films were synthesized on an ALD Al2O3 insulation layer on 4H-SiC substrate for metal-ferroelectric-insulator-semiconductor (MFIS) device applications. The as-deposited PZT was amorphous but crystallized into a perovskite polycrystalline structure with a preferred [002] orientation upon rapid thermal annealing (RTA) at 950u2009°C. The capacitance-voltage and current-voltage characteristics of the MFIS devices indicate carrier injection to the film induced by polarization and Fowler-Nordheim (FN) tunneling when electric field was high. The polarization-voltage measurements exhibited reasonable remanent and saturation polarization and a coercive electrical field comparable to that reported for bulk PZT. The piezoresponse force microscope measurements confirmed the polarization, coercive, and retention properties of ultra-thin ALD PZT films.

Electrical control of magnetic remanent states in a magnetoelectric layered nanostructure

We report experimental results on electrical control of magnetic remanent states (i.e., nanoscale remanent domain patterns) in a magnetoelectric layered nanostructure, Ni nanobar/lead zirconate titanate film. First, with application of different external magnetic fields as a baseline characterization, the magnetic single domain in the Ni nanobar presents time-dependent nanoscale remanent domain patterns which were observed under a magnetic force microscope. Based on this baseline characterization, we further successfully demonstrate that these time-dependent nanoscale magnetic domain patterns could be instantaneously controlled with an application of electric fields due to the converse magnetoelectric effect. The magnetic-field-induced changes are correlated with the electric-field-induced changes.

Design, Simulation, and Fabrication of a Novel Vibration-Based Magnetic Energy Harvesting Device

In this paper, we have described a vibration-based magnetic energy harvesting device (VMEHD). The VMEHD converts mechanical energy from the environment to electrical energy by using the piezoelectric effect and frequency rectification. Magnetic arrays are used to rectify the incoming frequency to a higher frequency using non-contact mechanisms. The finite element analysis is used to simulate the VMEHD. Testing results show that the output voltage is between 8 volts to 12 volts with input frequency of 10 Hz and a rectified frequency of 22 Hz. This provides large power densities to be obtained in a mechanical energy harvesting device.