X. J. Zheng

Eight logic states of tunneling magnetoelectroresistance in multiferroic tunnel junctions

We propose a theoretical model based on the concept of multiferroic tunnel junction. The model is capable of producing eight different logic states by combining the spin-filter effect and the screening of polarization charges between two electrodes through a general spintronic tunneling. The dependence of the conductance ratio with very large magnitude on electric polarization, exchange splitting, barrier width, and bias voltage is investigated. The result may provide some insights into the realization of octal data storage namely, the eight different logic states are used as octal code, which could lead to the tremendous increase of memory storage density.

Structure evolution and ferroelectric and dielectric properties of Bi3.5Nd0.5Ti3O12 thin films under a moderate temperature annealing

Bi3.5Nd0.5Ti3O12 (BNT) ferroelectric thin films were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates by chemical solution deposition. Structure evolution and ferroelectric and dielectric properties of the as-prepared thin films under a moderate temperature (600–750°C) annealing were studied in detail. The experimental results showed that the BNT thin films annealed at 700°C exhibit preferred (00l) orientation, and the remnant polarization (2Pr) and dielectric constant (er) are higher (the values of 2Pr and er at 100kHz are 54μC∕cm2 and 448, respectively) than those of the deposited films annealed at other temperatures. Additionally, the mechanism concerning the dependence of electrical properties of the BNT ferroelectric thin films on the annealing temperature was discussed.

Residual stress in PZT thin films prepared by pulsed laser deposition

Abstract In this investigation, ferroelectric thin films Pb(Zr x Ti 1− x )O 3 (PZT) with x =0.58 were deposited on Pt/Ti/Si(001) by pulsed laser deposition (PLD). The residual stresses in PZT thin films were measured by an X-ray diffractometer (XRD) and indentation fracture method. In the indentation fracture method, the surface crack model and half-penny crack model were adopted. For PZT thin films with thickness of 0.05 μm, 0.5 μm and 1.0 μm, the residual stresses determined by a half-penny crack model were −74.73 MPa, −22.86 MPa and −14.74 MPa and those measured by XRD method were −57.6 MPa, −48.5 MPa and −25.6 MPa, respectively. The residual stresses determined by a half-penny crack model were closer to the results measured by XRD. A theoretical model is proposed to predict the residual stress in PZT thin film and the analytical results are essentially consistent with the experimental results. The effect of residual stress on piezoelectric properties is also discussed. The good ferroelectrics behaviors were demonstrated ( P r =22–35 μC cm −2 , E c =25–315 kV cm −1 ) by the hysteresis loops.

Improved ferroelectric properties of bismuth titanate films by Nd and Mn cosubstitution

Thin films of Nd and Mn cosubstituted bismuth titanate, i.e., Bi3.15Nd0.85(Ti3−xMnx)O12 (BNTM) (x=0, 0.005, 0.01, 0.03, 0.05, and 0.1), were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates at 700°C by a chemical solution deposition technique. The structures of the films were analyzed using x-ray diffraction and Raman spectroscopy. These films possessed preferred (117) and (00l)-oriented polycrystalline structures. The ferroelectric properties of BNTM films were systematically investigated as a function of the Mn content. It is found that a low concentration substitution with manganese ions in Bi3.15Nd0.85Ti3O12 greatly enhances the remnant polarization (2Pr) and reduces the coercive field (2Ec) of the film. The 2Pr and 2Ec are 78μC∕cm2 and 205kV∕cm, respectively. No fatigue phenomenon is also observed for the BNTM film with x=0.01 up to 1.5×1010 switching cycles.

Effects of europium content on the microstructural and ferroelectric properties of Bi4−xEuxTi3O12 thin films

The effects of europium (Eu) content on the microstructure, fatigue endurance, leakage current density, and remnant polarization (2Pr) of Bi4−xEuxTi3O12 (BET) thin films prepared by metal-organic decomposition method at 700°C annealing temperature were studied in detail. The results showed that 2Pr (82μC∕cm2 under 300kV∕cm), fatigue endurance (2% loss of 2Pr after 9.0×109 switching cycles), and leakage current density (1×10−8A∕cm2 at 200kV∕cm) of BET thin film with x=0.85 are better than those of thin films with other contents. Additionally, the mechanism concerning the dependence of ferroelectric properties on Eu content was discussed.

Electrical properties of V-doped Bi3.15Nd0.85Ti3O12 thin films with different contents

Thin films of Nd3+-∕V5+-cosubstituted bismuth titanate, (Bi3.15Nd0.85)(Ti3−xVx)O12 (BNTV), were fabricated by chemical solution deposition technique. For different vanadium contents (x=0.03, 0.06, and 0.09), the crystallized phase and electrical properties of the films were investigated using x-ray diffraction, polarization hysteresis loops, leakage current-voltage, and capacitance butterfly loops. The authors found that increasing vanadium content leads to the decrease of coercive field and the increase of capacitance. The film of vanadium content x=0.09 exhibits fatigue-free and excellent leakage current characteristics with I=5.99×10−9A at applied voltage of 3V, which is much lower than that of Bi3.15Nd0.85Ti3O12 thin film [X. S. Gao and J. Wang, Thin Solid Films 515, 1683 (2006)]. The frequency dependence of the remanent polarization for the BNTV thin film was discussed.

Residual stresses in Pb(Zr0.52Ti0.48)O3 thin films deposited by metal organic decomposition

Abstract Pb(Zr 0.52 Ti 0.48 )O 3 (PZT) ferroelectric thin films were prepared by metal organic decomposition (MOD). The residual stress in the thin film was characterized by the X-ray diffraction. The indentation fracture method including the surface crack model and half-penny crack model was developed to analyze residual stress in the thin film.

Modeling of imprint in hysteresis loop of ferroelectric thin films with top and bottom interface layers

The imprint of a ferroelectric thin film capacitor is studied using an improved model consisting of two nonswitching thin interface layers near the top and bottom electrodes. The difference in electrical conductivity between the two interface layers induces voltage offset and deformation behaviors in hysteresis loops. Size dependence of shift effect of Bi4−xNdxTi3O12 thin film is explained qualitatively by taking into account the thickness ratio of the interface layer and the bulk film. Various shifts and anamorphic shapes with different electrodes and processes have been effectively reproduced through changing the layer conductivities. The simulated shifted hysteresis loops agree well with the experiment. Theoretical prediction based on this approach may provide a method to reduce imprint failure.

Eight-logic memory cell based on multiferroic junctions

A model is proposed for a device combining a multiferroic tunnel junction with a magnetoelectric (ME) film in which the magnetic configuration is controlled by the electric field. Calculations embodying the Greens function approach show that the magnetic polarization can be switched on and off by an electric field in the ME film due to the effect of elastic coupling interaction. Using a model including the spin-filter effect and screening of polarization charges, we have produced eight logic states of tunnelling resistance in the tunnel junction and have obtained corresponding laws that control them. The results provide some insights into the realization of an eight-logic memory cell.

A model for the polarization hysteresis loops of the perovskite-type ferroelectric thin films

A model has been developed for the P-E hysteresis behavior from the perspective of dipole switching. Hysteresis loops have been reproduced with the model to agree reasonably well with the experimental data measured from various ferroelectric thin films. The model can also predict asymmetric hysteresis loop measured under unconventional situation. Additionally, the mathematical description can be easily combined with electronic design automation software in circuit simulation of ferroelectric capacitor or ferroelectric field effect transistor.