Aug 2012, Volume 7 Issue 4

Cover illustration

  • Multiferroelectric tunnel junctions (MFTJs), consisting of two ferromagnetic (FM) electrodes separated by a nanoscale ferroelectric (FE) barrier, exploiting the capability to control FM and FE orders via external electric and magnetic fields simultaneously, have potential applications not only in multi-state data storage due to the coexsistence of tunneling magnetoresistance and tunneling electroresistance effects, but also in electric field controlled spintronics as a result [Detail] ...

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    Jun-Ming Liu, Ce-Wen Nan

    Chun-Gang Duan (段纯刚)

    In this perspective paper, we discuss possible ways to control magnetism using electric-field. Special focus is given to interface/surface magnetoelectric effects, which will become important when the thickness of magnetic films drops to nanoscale.We show that significantly different mechanisms may lead to interface/surface magnetoelectric effects, providing great flexibility to apply such effects. As a result, we propose several protype devices utilizing these novel magnetoelectric effects, and strongly advocate experimental endeavors to realize such devices.

    Yue-Wei Yin, Muralikrishna Raju, Wei-Jin Hu, Xiao-Jun Weng, Ke Zou, Jun Zhu, Xiao-Guang Li, Zhi-Dong Zhang, Qi Li

    Magnetic tunnel junctions with ferroelectric barriers, often referred to as multiferroic tunnel junctions, have been proposed recently to display new functionalities and new device concepts. One of the notable predictions is that the combination of two charge polarizing states and the parallel and antiparallel magnetic states could make it a four resistance state device. We have recently studied the ferroelectric tunneling using a scanning probe technique and multiferroic tunnel junctions using ferromagnetic La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 as the electrodes and ferroelectric (Ba, Sr)TiO3 as the barrier in trilayer planner junctions. We show that very thin (Ba, Sr)TiO3 films can sustain ferroelectricity up till room temperature. The multiferroic tunnel junctions show four resistance states as predicted and can operate at room temperatures.

    Masashi Tokunaga

    In this article, studies on the magnetoelectric effects of multiferroic materials in high magnetic fields, particularly pulsed magnetic fields, are discussed and results for some representative materials are presented. In the discussions on representative materials, the relationship between the crystallographic symmetry and the linear magnetoelectric effect in Cr2O3 is introduced. Then drastic changes in polarization caused by magnetic transitions are discussed through a case study of manganites with a perovskite-type structure. In addition, high field studies on the magnetoelectric effects in BiFeO3, which is an exceptional multiferroic material, are presented and discussed in the framework of the Landau–Ginzburg theory.

    Shu-Hong Xie (谢淑红), Yun-Ya Liu(刘运牙), Jiang-Yu Li(李江宇)

    Multiferroic materials with two or more types of ferroic orders have attracted a great deal of attention in the last decade for their magnetoelectric coupling, and new ideas and concepts have been explored recently to develop multiferroic materials at nano-scale. Motivated by theoretical analysis, we synthesized single-phase BiFeO3 (BFO) nanofibers, Pb(Zr0.52Ti0.48)O3-CoFe2O4 (PZT-CFO) and Pb(Zr0.52Ti0.48)O3-NiFe2O4 (PZT-NFO) composite nanofibers, and CoFe2O4-Pb(Zr0.52Ti0.48)O3 (CFO-PZT) core-shell nanofibers using sol-gel based electrospinning. These nanofibers typically have diameters in the range of a few hundred nanometers and grain size in the range of 10s nanometers, and exhibits both ferroelectric and ferromagnetic properties. Piezoresponse force microscopy (PFM) based techniques have also been developed to examine the magnetoelectric coupling of the nanofibers, which is estimated to be two orders of magnitude higher than that of thin films, consistent with our theoretical analysis. These nanofibers are promising for a variety of multiferroic applications.

    Na Zhang, Shuai Dong, Jun-Ming Liu

    While the ferroelectricity in type-II multiferroic rare-earth manganites is believed to be generated by the inverse Dzyaloshinskii–Moriya (DM) interaction (spin–orbit coupling) associated with the Mn spiral spin order, recent results revealed the strong spin–lattice coupling arising from the Dy–Mn spin interaction in DyMnO3, which may also be an ingredient contributing to the ferroelectricity. In this work, we summarize our recent experiments on this issue by performing a series of rare-earth site nonmagnetic Y and magnetic Ho substitutions at Dy site for DyMnO3. It is demonstrated that the Dy–Mn spin interaction contributes to the ferroelectric polarization through the symmetric exchange striction mechanism (spin–lattice coupling). A coexistence of the spin–orbit coupling and spin–lattice coupling in one compound is confirmed. At the same time, the independent Dy antiferromagnetic spin order at low temperature can be effectively suppressed by the substitutions, beneficial to the polarization enhancement.

    D. V. B. Murthy, Gopalan Srinivasan

    A systematic study has been carried out on the effects of interface bonding on the strain mediated magnetoelectric (ME) coupling in ferromagnetic–ferroelectric bilayers. The technique used involves the static electric field E tuning of the ferromagnetic resonance (FMR) in yttrium iron garnet (YIG) and lead zirconate titanate (PZT) or lead magnesium niobate-lead titanate (PMN-PT). A broad band detection technique has been developed for studies over 1-40 GHz in three types of bilayers: epoxy bonded, eutectic bonded and YIG films directly grown onto piezoelectric substrate by electrophoretic deposition. The strength A of the converse ME effect (CME) defined as the ratio of the frequency shift δf in FMR in E, A = δf/E, varies over the range 0.8 to 4.3 MHz·cm/kV, and is the highest for eutectic bonded samples and is the weakest for epoxy bonded bilayers. The results presented here as of importance for dual electric and magnetic field tunable ferrite–ferroelectric microwave resonators and filters.

    Cheng-Liang Lu(陆成亮), Jun-Ming Liu(刘俊明), Tao Wu(吴韬)

    Highly compressively strained BiFeO3 thin films with different thickness are epitaxially grown on (001) LaAlO3 substrates and characterized using various techniques. The quasi-tetragonal phase with a giant axial ratio of ~1.25 and its thickness-dependent evolution are investigated. An interesting twining structure of the quasi-tetragonal phase is evidenced in thicker films through detailed reciprocal space mapping, which becomes more pronounced with increasing film thickness. Moreover, an interesting electric-field driven phase transition was evidenced in the film with a thickness of 38 nm, in which the quasi-tetragonal and rhombohedral phases are close to each other in energy landscape.

    Zhen Chen, Rui-Juan Xiao, Chao Ma, Yuan-Bin Qin, Hong-Long Shi, Zhi-Wei Wang, Yuan-Jun Song, Zhen Wang, Huan-Fang Tian, Huai-Xin Yang, Jian-Qi Li

    The electronic structure of multiferroic YMn2O5 material has been studied by use of the generalized gradient approximation (GGA). The results demonstrate that the oxygen 2p and manganese 3d orbitals are strongly hybridized. Considering the on-site Coulomb interaction U, we performed the GGA+U calculations for 0<U≤8 eV, and it is found that the increase of U could enlarge the band gap and, on the other hand, weaken the Mn–O hybridization. The experimental measurements of the electron energy-loss spectrometry (EELS) exhibit a rich variety of structural features in both O–K edge and Mn–L edges. A theoretical and experimental analysis on the O–K edge suggests that the on-site Coulomb interaction (U) in YMn2O5 could be less than 4 eV. Certain electronic structural features of LaMn2O5 have been discussed in comparison with those of YMn2O5.

    Jin-Jin Du (杜金锦), Wen-Fang Li (李文芳), Peng-Fei Zhang (张鹏飞), Gang Li (李刚), Jun-Min Wang (王军民), Tian-Cai Zhang (张天才)

    In this review article, the progress and recent developments in the measuring and controlling of single atom trajectories are reviewed. With the development of laser cooling and trapping technology, it is possible to achieve the measurement and control of single atom trajectory experimentally. The experiment of tracking a single atom trajectory with high resolution and the endeavor of eliminating the degeneracy of the trajectories are then introduced.

    Xiao-Yu Chen (陈小余)

    We exactly evaluate the entanglement of a six vertex and a nine vertex graph states which correspond to non “two-colorable” graphs. The upper bound of entanglement for five vertex ring graph state is improved to 2.9275, less than the upper bound determined by local operations and classical communication. An upper bound of entanglement is proposed based on the definition of graph state.

    Jiajun Ouyang, W. LiMing, Liangbin Hu

    The tunneling spectrum of an electron and a hole in metal–superconductor–metal junctions is computed using the Blonder–Tinkham–Klapwijk method. The incident and the outgoing currents finally balance each other by an interface charge inside the superconductor and metal junction. The present computation shows a more abundant structure compared to that on a metal–superconductor junction, such as the resonance at bias voltages above the energy gap of the superconductor. The density of the interface charge shows a quantum-like oscillation.

    Xiao-Dong Li, Hong Zhang, Yong-Jian Tang, Chao-Yang Wang

    The adsorption and separation of CH4/H2 in two covalently-linked organic-inorganic hybrid frameworks polyoctaphenylsilsesquioxane (JUC-Z1) were computationally studied using the Grand Canonical Monte Carlo (GCMC) simulations. The results show that JUC-Z1 with Linde type A (LTA) and polycubane (zeolite code ACO) net topologies can adsorb up to 20.32, 18.57 mmol/g of CH4 and 19.04, 17.89 mmol/g of H2 at 298 K and 10 MPa, respectively. For the adsorption of binary mixture, the selectivity of CH4 over H2 in LTA-JUC-Z1 decrease gradually with the increase of the pressure or the CH4 mole fraction of the mixture. As to ACO-JUC-Z1, the selectivity first increases at low pressure or CH4 mole fraction, and then begins to decrease with the further increase of the corresponding amount. Anyhow, the two materials both exhibit excellent adsorption and separation capacities of CH4/H2.

    Fa-Bo Feng

    The lensing effect of a cosmic string is studied, and some new methods are proposed to detect the cosmic string. The technique for using jets as extended gravitational lensing probes was first explored by Kronberg.We use the “alignment-breaking parameter” ηG as a sensitive indicator of gravitational distortion by a wiggly cosmic string. Then, we applied the non-constant deflection angle to jets, and ηG of a specific jet is just related to the projected slope of the jet. At least three jets in the sample of Square Kilometer Array (SKA) would have significant signals (ηG>10°) if the wiggly infinite cosmic string existed. The distortion of elliptical object is also studied and used to do a statistical research on the directions of axes and ellipticities of galaxies. In the direction of the string, we find that galaxies appear to be more elliptical for an observer and the distribution of apparent ellipticity changes correspondingly. The ellipticity distribution of current SDSS spiral sample has the signalto-noise ratio up to 8.48 which is large enough for astronomical observations. The future survey, such as Large Synoptic Survey Telescope (LSST) and Dark Energy Survey (DES) would weaken the requirement of special geometry in the data processing. As a result, all kinds of distributions, including ellipticity axis distribution, would serve as probes to detect wiggly strings in the near future. In brief, if a wiggly cosmic string existed, these signals would be convenient to be observed with the future weak lensing survey or other surveys in the deep space. If there was no lensing signal in these distributions, it would give the upper limit of the abundance of infinite strings.

    Wei-Xing Ma, Li-Juan Zhou, Tong-Quan Yuan, Jin-Song Peng, Xiao Lu, Ji-Huan Pan, Guang-Xiong Peng, Cheng-Ju Meng

    Based on the Veneziano ghost theory of QCD, we predict the cosmological constant Λ, which is related to energy density of cosmological vacuum by Λ=8πG3ρΛ. In the Veneziano ghost theory, the vacuum energy density ρΛ is expressed by absolute value of the product of quark vacuum condensate and quark current mass: ρΛ=2NfHmη'c|mq?0|q ˉq|0?|. We calculate the quark local vacuum condensates ?0|q ˉq|0? by solving Dyson–Schwinger Equations for a fully dressed confining quark propagator Sf(p) with an effective gluon propagator Gμνab(q). The quark current mass mq is predicted by use of chiral perturbation theory. Our theoretical result of Λ, with the resulting ?0|q ˉq|0?=-(235 MeV)3 and light quark current mass mq mq?3.29-6.15 MeV is in a good agreement with the observable of the Λ10-52 m-2 used widely in a great amount of literatures.

    Yan-Biao Gan, Ai-Guo Xu, Guang-Cai Zhang, Ying-Jun Li

    The aims of the present paper are threefold. First, we further study the fast Fourier transform thermal lattice Boltzmann (FFT–TLB) model for van der Waals (VDW) fluids proposed in Phys. Rev. E, 2011, 84(4): 046715. We analyze the merits of the FFT approach over the traditional finite difference scheme and investigate the effects of smoothing factors on accuracy and stability in detail. Second, we incorporate the VDW equation of state with flexible parameters into the FFT–TLB model. As a result, the revised model may be used to handle multiphase flows with various critical densities and temperatures. Third, we design appropriate boundary conditions for systems with solid walls. These improvements, from the views of numerics and physics, significantly extend the application scope of the model in science and engineering.