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Zhou Y.,KTH Royal Institute of Technology | Akerman J.,KTH Royal Institute of Technology | Akerman J.,Gothenburg University | Sun J.Z.,IBM | Sun J.Z.,Hong Kong Institute of Technology
Applied Physics Letters | Year: 2011

We report on a numerical study of the micromagnetic switching process of a nanostructured spin torque device. We show that incoherent spin waves can be excited over a wide range of current and field even at zero temperature. These large amplitude, incoherent, and nonzero k spin wave modes are shown to alter the switching phase boundary from that calculated within a macrospin model. The presence of telegraphic transitions between different spin wave modes may also contribute to the so-called back-hopping phenomenon where the switching probability varies nonmonotonically with increasing bias current. © 2011 American Institute of Physics.

Zhou Y.,Hong Kong Institute of Technology
Solid State Communications | Year: 2010

I studied theoretically the enhancement of remanent polarization and dielectric permittivity of interfacial-coupled ferroelectric superlattices based on the LandauGinzburg theory. Our model adopts the LandauKhalatnikov equation to describe hysteresis behavior and takes the time-dependent space-charge-limited conductivity into account to investigate the ferroelectric and dielectric properties of ferroelectric superlattices. The results are in good agreement with recent experimental observations on the enhancement of remanent polarization and permittivity of BaTiO3SrTiO3 superlattices and heterolayered Pb(Zr,Ti)O3 thin films. © 2010 Elsevier Ltd. All rights reserved.

Sun H.-M.,National Tsing Hua University | Wang H.,National Tsing Hua University | Wang K.-H.,Hong Kong Institute of Technology | Chen C.-M.,National Tsing Hua University
IEEE Transactions on Computers | Year: 2011

As new vulnerabilities on Windows systems are reported endlessly, it is more practical to stop polymorphic malicious code from exploiting these vulnerabilities by building an behavior-based monitor, rather than adopting a signature-based detection system or fixing these vulnerabilities. Many behavior-based monitors have been proposed for Windows systems to serve this purpose. Some of them hook high-level system APIs to detect the suspicious behaviors of code. However, they cannot detect malicious code that directly invokes Native APIs. In this paper, we present a novel security scheme that hooks Native APIs in the kernel mode. This method effectively prevents malicious code calling Native APIs directly. It introduces an average eight percent computation overhead into the system. Analyses and a series of experiments are given in the paper to support our claims. © 2011 IEEE.

Zhou Y.,Hong Kong Institute of Technology
Nanotechnology | Year: 2011

A simple model is developed to investigate the potential profile changes due to mechanical stress at the ferromagnetic/ferroelectric interfaces of ferromagnetic-ferroelectric-ferromagnetic tunnel junctions with an ultrathin ferroelectric barrier. The potential changes associated with the polarization variation have significant effects on the tunneling conductance of the junctions. The discovered effect is illustrated by the example of a multiferroic tunnel junction in which approximately four orders of changes of the tunneling conductance and several-fold changes of the tunneling magnetoresistance (TMR) are observed due to the spin-flip induced nanomechanical stress. The TMR modulation effect is essential for realization of novel spintronics nano-devices as well as being useful for investigating fundamental aspects of the spin transfer. © 2011 IOP Publishing Ltd.

We have studied the effect of the fieldlike spin torque term bJ, present in magnetic tunneling junctions (MTJs) with perpendicular magnetic anisotropy in the free layer, on switching characteristics of the devices. We find that bJ has a strong impact on the switching current density and switching speed. The theoretical limit of the switching current density can be significantly reduced and the theoretical limit of the switching time will be lowered compare to the spin-valve (SV) devices. These results strongly suggest that the spin transfer torque random access memory based on the MTJs with perpendicularly magnetized free layer will likely have the multiple benefits of much larger intrinsic signal, smaller switching current, and faster switching speed than the SV devices. © 2011 American Institute of Physics.

Zhou Y.,Hong Kong Institute of Technology
Solar Energy | Year: 2012

In this work, we propose the compositionally graded ferroelectrics (CGF) as solar energy harvesting device. Such a novel geometrically frustrated system exhibits an intrinsic built-in potential which can be used to separate the hole and electron currents. It is shown that the CGF based photovoltaic devices can achieve orders of higher efficiency than bulk/non-graded ferroelectric thin films. The dependence of the photovoltaic effect on various device parameters have also been investigated. © 2011 Elsevier Ltd.

Li D.,Hong Kong Baptist University | Zhou Y.,Hong Kong Institute of Technology | Zhou C.,Hong Kong Baptist University | Hu B.,University of Houston
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Fractional synchronization is one of the most interesting collective behaviors in coupled or driving-response oscillators system, very important for both a deep understanding of a particular oscillator and for its applications. We numerically investigate the fractional synchronization of a spin-torque oscillator by injected ac current. Multiple p:q locking regions are found, which display some sophisticated overlaps. The system can be analyzed as a perturbed heteroclinic cycle rather than a phase oscillator. Both the modulations on the output frequency and power are mainly due to the modulation by the external signals on the distance between the dynamical orbit and the saddle point in phase space. By using this dynamical picture, we can well understand all the numerical results, including the variation of the locking region with the amplitude |Ja| or frequency f of the injected signal, the influence by noise, and the difference among the output powers of coexisting locking attractors. These understandings are significant for both potential applications in electronic communications and a deep investigation into this novel device. © 2011 American Physical Society.

Li D.,Hong Kong Baptist University | Zhou Y.,Hong Kong Institute of Technology | Hu B.,University of Houston | Zhou C.,Hong Kong Baptist University
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Following our Rapid Communication, we present a more detailed analysis of the synchronization of spin-torque oscillators (STOs), as well as some other nonlinear characteristics of STO, by using the perturbed heteroclinic cycle model rather than a linear phase oscillator model. We analyze four critical points of the system and numerically get the fifth one and meanwhile systematically study the bifurcation process of the system. Three of the fixed points limit the parameter regions for sustaining an oscillation, and another one is the boundary between a global oscillation (out-of-plane mode oscillation) and a local oscillation (in-plane mode oscillation). The frequency of the physically more relevant global oscillation is also estimated depending on the system parameters. The results are not only significant for the design of useful devices but also important for further understanding the response of an STO to an external signal and interpreting the difficulty of synchronizing serially connected STOs. We also discuss in detail several other synchronization schemes. © 2011 American Physical Society.

Li D.,Hong Kong Baptist University | Zhou Y.,Hong Kong Institute of Technology | Zhou C.,Hong Kong Baptist University | Zhou C.,Hong Kong Institute of Technology | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Previously, synchronization of spin-torque oscillator (STO) has usually been analyzed by phase oscillator model. Here we show that STO is more precisely described as a perturbed heteroclinic cycle where the frequency is sensitive to the distance between the cycle and a saddle. In the presence of external signal or coupling, the frequency changes mainly due to the modification on this distance rather than on the phase directly. Multiple attractors coexist and synchronization depends sensitively on the initial conditions when a STO is driven by an external ac. We reveal that these properties underlay the mechanisms why synchronization region in two serially connected STOs is quite small and why time delay can enhance synchronization. When more STOs are added, the parameter region for a globally attracting synchronization state may disappear. Our analysis suggests that initial conditions have to be controlled or different designs have to be fabricated in order to obtain robust synchronization of a large number of STOs for the purpose of applications. © 2010 The American Physical Society.

Sun H.-M.,National Tsing Hua University | Ting W.-C.,Industrial Technology Research Institute of Taiwan | Wang K.-H.,Hong Kong Institute of Technology
IEEE Transactions on Dependable and Secure Computing | Year: 2011

Security issues become more and more significant in RFID development. Recently, Chien proposed an ultralightweight RFID authentication protocol in order to achieve privacy and authenticity with limited computation and transmission resources. However, we find two desynchronization attacks to break the protocol. In order to repair the protocol, two patches that slightly modify the protocol are presented in the paper. © 2011 IEEE.

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