L. Q. Chen

Pennsylvania State University, University Park, Maryland, United States

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Publications (206)746.25 Total impact

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    ABSTRACT: We developed a computational model to investigate the magnetic and structural phase transitions in metamagnetic shape memory alloys. The model combined the phase-field method with micromagnetic simulations. The model was used to calculate the transition temperature from ferromagnetic austenite to antiferromagnetic martensite, the Curie temperature, and their response to an external magnetic field, for the typical metamagnetic alloy NiCoMnIn. The calculated magnetization curves at different temperatures are consistent with reported experimental measurements. The simulations show that the walls of martensite twins are superimposed with the 90° magnetic domain walls of the low-temperature martensite phase, because of magnetostructural order parameter coupling.
    Acta Materialia 01/2015; 83:333–340. DOI:10.1016/j.actamat.2014.10.014 · 3.94 Impact Factor
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    ABSTRACT: We investigated the high power spin-torque oscillator in a half metallic Hensler alloy Co2MnSi spin valve nanopillars with perpendicular magnetization under external magnetic held using micromagnetic simulations. Our simulations show that the narrow optimum current of magnetization precession in the Heusler-based spin valve is broadened by introducing the surface anisotropy. The linear decrease of frequency with the out-of-plane magnetic held is obtained in our simulation. Additionally, the in-plane magnetic held dependence of frequency shows a parabolic curve which is explained by the magnetization trajectory tilting. Furthermore, we also discussed the decrease of output power using the excitation of non-uniform magnetization precession in the in-plane magnetic fields.
    Journal of Magnetism and Magnetic Materials 01/2015; 373:105. DOI:10.1016/j.jmmm.2014.03.050 · 2.00 Impact Factor
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    ABSTRACT: Achieving 180° magnetization reversal with an electric field rather than a current or magnetic field is a fundamental challenge and represents a technological breakthrough towards new memory cell designs. Here we propose a mesoscale morphological engineering approach to accomplishing full 180° magnetization reversals with electric fields by utilizing both the in-plane piezostrains and magnetic shape anisotropy of a multiferroic heterostructure. Using phase-field simulations, we examined a patterned single-domain nanomagnet with four-fold magnetic axis on a ferroelectric layer with electric-field-induced uniaxial strains. We demonstrated that the uniaxial piezostrains, if non-collinear to the magnetic easy axis of the nanomagnet at certain angles, induce two successive, deterministic 90° magnetization rotations, thereby leading to full 180° magnetization reversals.
    Scientific Reports 12/2014; 4:7507. DOI:10.1038/srep07507 · 5.08 Impact Factor
  • Journal of The Electrochemical Society 11/2014; 161(11):F3164. DOI:10.1149/2.0171411jes · 2.86 Impact Factor
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    ABSTRACT: Effect of substrate misfit strain on current-induced in-plane magnetization reversal in CoFeB-MgO based magnetic tunnel junctions is investigated by combining micromagnetic simulations with phase-field microelasticity theory. It is found that the critical current density for in-plane magnetization reversal decreases dramatically with an increasing substrate strain, since the effective elastic field can drag the magnetization to one of the four in-plane diagonal directions. A potential strain-assisted multilevel bit spin transfer magnetization switching device using substrate misfit strain is also proposed. (C) 2014 AIP Publishing LLC.
    Applied Physics Letters 09/2014; 105(12):122407. DOI:10.1063/1.4896692 · 3.52 Impact Factor
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    ABSTRACT: A solution-based thermodynamic description of the ternary Ni–Al–Mo system is developed here, incorporating first-principles calculations and reported modeling of the binary Ni–Al, Ni–Mo and Al–Mo systems. To search for the configurations with the lowest energies of the N phase, the Alloy Theoretic Automated Toolkit (ATAT) was employed and combined with VASP. The liquid, bcc and γ-fcc phases are modeled as random atomic solutions, and the γʹ-Ni3Al phase is modeled by describing the ordering within the fcc structure using two sublattices, summarized as (Al,Mo,Ni)0.75(Al,Mo,Ni)0.25. Thus, γ-fcc and γʹ-Ni3Al are modeled with a single Gibbs free energy function with appropriate treatment of the chemical ordering contribution. In addition, notable improvements are the following: first, the ternary effects of Mo and Al in the B2-NiAl and D0a-Ni3Mo phases, respectively, are considered; second, the N-NiAl8Mo3 phase is described as a solid solution using a three-sublattice model; third, the X-Ni14Al75Mo11 phase is treated as a stoichiometric compound. Model parameters are evaluated using first-principles calculations of zero-Kelvin formation enthalpies and reported experimental data. In comparison with the enthalpies of formation for the compounds ψ-AlMo, θ-Al8Mo3 and B2-NiAl, the first-principles results indicate that the N-NiAl8Mo3 phase, which is stable at high temperatures, decomposes into other phases at low temperature. Resulting phase equilibria are summarized in the form of isothermal sections and liquidus projections. To clearly identify the relationship between the γ-fcc and γʹ-Ni3Al phases in the ternary Ni–Al–Mo system, the specific γ-fcc and γʹ-Ni3Al phase fields are plotted in x(Al)–x(Mo)–T space for a temperature range 1200–1800 K.
    Calphad 09/2014; 46. DOI:10.1016/j.calphad.2014.03.002 · 1.39 Impact Factor
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    ABSTRACT: The precipitate morphology in Mg–rare earth (RE) element binary alloys is predicted using a multi-scale modeling approach combining a three-dimensional (3-D) phase-field model and first-principles density functional theory calculations. First-principles calculations provide all the required input parameters for the phase-field model, including lattice parameters, elastic constants, formation energies and interfacial energies. This integrated model is applied to a Mg–Nd alloy as a model system. Quantitative 3-D phase-field simulations are performed to study the metastable β′ precipitate morphologies, habit plane formation and spatial distribution of the precipitates during isothermal aging. The predicted morphologies of β′ precipitates are in excellent agreement with existing experimental observations. The influence of the precipitate morphology on the mechanical properties is also evaluated using the Orowan equation. The results are expected to provide guidance for achieving desirable precipitate morphologies and thus mechanical properties in Mg alloys.
    Acta Materialia 09/2014; 76:259–271. DOI:10.1016/j.actamat.2014.05.002 · 3.94 Impact Factor
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    ABSTRACT: We demonstrate that charge ordering can be quantitatively predicted by analyzing the Born effective charge (BEC), resolving the long-standing discrepancy between first-principles charge analysis and the nominal concepts of charge disproportionation in Fe3O4 and CaFeO3. In particular, the BEC differences between the disproportionated Fe ions are calculated to be similar to 2e, being in excellent agreement with the nominal charge separation in CaFeO3 while suggesting the charge disproportionation in Fe3O4 is understood by the charge separation similar to 2e instead of the nominal separation of similar to 1e.
    Chemical Physics Letters 06/2014; 607. DOI:10.1016/j.cplett.2014.05.044 · 1.99 Impact Factor
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    H.B. Huang, X.Q. Ma, Z.H. Liu, L.Q. Chen
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    ABSTRACT: We investigated the high-power spin-torque oscillator in a half-metallic Heusler alloy Co2MnSi spin-valve nanopillars with perpendicular magnetization using micromagnetic simulations. A stable high output power spin transfer precession was obtained in the condition of zero external magnetic field, and the narrow current of oscillation due to the high spin polarization of Heusler alloy is significantly widened by introducing the surface anisotropy. Furthermore, we discussed the current dependence of oscillation frequency and explained the blue frequency shift using the trajectories and spatial magnetic domains.
    Journal of Alloys and Compounds 06/2014; 597:230–235. DOI:10.1016/j.jallcom.2014.01.235 · 2.73 Impact Factor
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    ABSTRACT: The structural and kinetic properties of Ni have been investigated between 300 and 2700 K using ab initio molecular dynamics within the framework of density-functional theory. Equations of state (EOS) are derived from the constant NVT ensembles with N being the number of atoms, V the volume, and T the temperature. From EOS fitting, the equilibrium volumes of Ni are predicted as a function of temperature, which are in good agreement with available experimental data. It is found that the solid-liquid phase transformation can be evaluated by the internal energy change and validated by the appearance of short-range ordering according to structural analysis. Additionally, the diffusion coefficient and shear viscosity are also predicted, in favorable accord with experimental data.
    Computational Materials Science 06/2014; 89. DOI:10.1016/j.commatsci.2014.03.031 · 1.88 Impact Factor
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    ABSTRACT: A phase-field model is developed to study local elastic coupling between magnetic and ferroelectric domains that show one-to-one pattern match. A multiferroic layered heterostructure of Co0.4Fe0.6/BaTiO3 is considered as an example. Dynamics of the local elastic coupling is investigated by simulating a time-dependent electric-field driven changes in local magnetization/polarization/strain distributions and by comparing the associated velocities of the magnetic and ferroelectric domain walls. It is found that the electric-field-driven dynamic magnetic domain evolution manifests itself as an alternating occurrence of local magnetization rotation and coupled motion of magnetic and ferroelectric domain walls with almost identical velocities.
    Applied Physics Letters 05/2014; 104(20):202402-202402-5. DOI:10.1063/1.4875719 · 3.52 Impact Factor
  • Y. Cao, J. Shen, C.A. Randall, L.Q. Chen
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    ABSTRACT: A self-consistent model has been proposed to study the switchable current-voltage (I-V) characteristics in Cu/BaTiO3/Cu sandwiched structure combining the phase-field model of ferroelectric domains and diffusion equations for ionic/electronic transport. The electrochemical transport equations and Ginzburg-Landau equations are solved using the Chebyshev collocation algorithm. We considered a single parallel plate capacitor configuration which consists of a single layer BaTiO3 containing a single tetragonal domain orientated normal to the plate electrodes (Cu) and is subject to a sweep of ac bias from −1.0 to 1.0 V at 25 °C. Our simulation clearly shows rectifying I-V response with rectification ratios amount to 102. The diode characteristics are switchable with an even larger rectification ratio after the polarization direction is flipped. The effects of interfacial polarization charge, dopant concentration, and dielectric constant on current responses were investigated. The switchable I-V behavior is attributed to the polarization bound charges that modulate the bulk conduction.
    Applied Physics Letters 05/2014; 104(18):182905-182905-5. DOI:10.1063/1.4875902 · 3.52 Impact Factor
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    ABSTRACT: We investigated the current-induced magnetization switching in a Heusler alloy Co2FeAl-based spin-valve nanopillar by using micromagnetic simulations. We demonstrated that the elimination of the intermediate state is originally resulted from the decease of effective magnetic anisotropy constant. The magnetization switching can be achieved at a small current density of 1.0 × 104 A/cm2 by increasing the demagnetization factors of x and y axes. Based on our simulation, we found magnetic anisotropy and demagnetization energies have different contributions to the magnetization switching.
    Journal of Applied Physics 04/2014; 115(13):133905-133905-5. DOI:10.1063/1.4870291 · 2.19 Impact Factor
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    ABSTRACT: Voltage-modulated magnetism in magnetic/BiFeO3 heterostructures can be driven by a combination of the intrinsic ferroelectric-antiferromagnetic coupling in BiFeO3 and the antiferromagnetic-ferromagnetic exchange interaction across the heterointerface. However, ferroelectric BiFeO3 film is also ferroelastic, thus it is possible to generate voltage-induced strain in BiFeO3 that could be applied onto the magnetic layer across the heterointerface and modulate magnetism through magnetoelastic coupling. Here, we investigated, using phase-field simulations, the role of strain in voltage-controlled magnetism for these BiFeO3-based heterostructures. It is predicted, under certain condition, coexistence of strain and exchange interaction will result in a pure voltage-driven 180° magnetization reversal in BiFeO3-based heterostructures.
    Scientific Reports 04/2014; 4:4553. DOI:10.1038/srep04553 · 5.08 Impact Factor
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    ABSTRACT: A phase-field model coupled with constitutive equations is formulated to investigate the magnetoelectric cross-coupling in magnetic-ferroelectric composites. The model allows us to obtain equilibrium piezoelectric, piezomagnetic, dielectric, and magnetoelectric properties under a given magnetic or electric field, from the local distributions of polarization, magnetization, and strain in the composites. As an example, effective magnetoelectric coupling coefficient, i.e., magnetic-field-induced voltage output (or changes in polarization), of the CoFe2O4-BaTiO3 composites is numerically calculated. Influences of the phase connectivity and the phase fraction of the composites on the magnetoelectric coupling coefficient are discussed.
    Applied Physics Letters 02/2014; 104(5):052904-052904-5. DOI:10.1063/1.4863941 · 3.52 Impact Factor
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    ABSTRACT: We studied the ionic/electronic transport and resistance degradation behavior of dielectric oxides by solving the electrochemical transport equations. Here, we took into account the non-periodical boundary conditions for the transport equations using the Chebyshev collocation algorithm. A sandwiched Ni|SrTiO3|Ni capacitor is considered as an example under the condition of 1.0 V, 1.0 μm thickness for SrTiO3 layer, and a temperature of 150 °C. The applied voltage resulted in the migration of ionic defects (oxygen vacancies) from anode towards cathode. The simulated electric potential profile at steady state is in good agreement with the recent experimental observation. We introduced the possibility of polaron-hopping between Ti3+ and Ti4+ at the electrode interface. It is shown that both the oxygen vacancy transport and the polaron-hopping contribute to the resistance degradation of single crystal SrTiO3, which is consistent with the experimental observations.
    Journal of Applied Physics 12/2013; 114(22):224102-224102-7. DOI:10.1063/1.4842836 · 2.19 Impact Factor
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    ABSTRACT: We study the strain effects on magnetic domain stability and dynamics in nanoscale magnetic thin films using phase-field simulations. Numerous strain-stabilized single-/multi-domain states are discovered, including various magnetic vortices with circular in-plane domains. Furthermore, a strain-domain stability map was constructed, displaying the stable magnetic domain and domain wall structures as a function of biaxial isotropic and anisotropic in-plane strains at room temperature. The present work provides useful guidelines for a precise engineering and experimental observation of domain structures in nanoscale magnetic thin films and a promising scheme towards a low-power and local control over magnetic domain structures.
    Journal of Applied Physics 10/2013; 114(16):164303-164303-9. DOI:10.1063/1.4826491 · 2.19 Impact Factor
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    ABSTRACT: We present a new algorithm to generate Special Quasirandom Structures (SQS), i.e., best periodic supercell approximations to the true disordered state for a given number of atoms per supercell. The method is based on a Monte Carlo simulated annealing loop with an objective function that seeks to perfectly match the maximum number of correlation functions (as opposed to merely minimizing the distance between the SQS correlation and the disordered state correlations for a pre-specified set of correlations). The proposed method optimizes the shape of the supercell jointly with the occupation of the atomic sites, thus ensuring that the configurational space searched is exhaustive and not biased by a pre-specified supercell shape. The method has been implemented in the “mcsqs” code of the Alloy Theoretic Automated Toolkit (ATAT) in the most general framework of multicomponent multisublattice systems and in a way that minimizes the amount of input information the user needs to specify and that allows for efficient parallelization.
    Calphad 09/2013; 42:13–18. DOI:10.1016/j.calphad.2013.06.006 · 1.39 Impact Factor
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    ABSTRACT: Domain stability and structures in Pb(Zr 0.3 Ti 0.7)O 3 /Pb(Zr 0.7 Ti 0.3)O 3 bilayer films under different substrate strains are studied using the phase field method. It is demonstrated that the domain structure of the bilayer film is very different from those of the corresponding single layer films grown on the same silicon substrate with an incoherent interface. Moreover, the predicted rhombohedral domains in the Pb(Zr 0.7 Ti 0.3)O 3 layer of the bilayer film have smaller sizes than those in the single layer case. These results are compared with experi-mental observations and previous thermodynamic analyses. The polarization distributions of the ferroelectric–paraelectric bilayer are analyzed as a function of the thickness of the bilayer film, where there is a "ferroelectric proximity effect" due to dipole–dipole interac-tions. The phase diagrams for both the bilayer and single layer films as a function of temperature and effective in-plane substrate strain are constructed.
    Acta Materialia 05/2013; 61(8):2909. DOI:10.1016/j.actamat.2013.01.038 · 3.94 Impact Factor
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    ABSTRACT: We show that, using phase-field simulations, large voltage-driven perpendicular magnetic domain switching can be realized in magnetic-ferroelectric nanoislands with relieved substrate constraint, which is difficult in continuous multiferroic layered thin films due to significant substrate clamping. The as-grown magnetic and ferroelectric domain structures in the heterostructured nanoislands can be tailored by engineering their respective geometric sizes and/or the underlying substrate strain. Influences of the lateral size of the island on the dynamic voltage-driven magnetic domain switching are addressed, whereby an optimum lateral size is identified for illustration. Thus, such three-dimensional multiferroic nanoislands should provide great flexibilities for designing novel high-density spintronic/microelectronic devices with purely voltage-driven means.
    Journal of Applied Physics 05/2013; 113(19). DOI:10.1063/1.4804157 · 2.19 Impact Factor

Publication Stats

6k Citations
746.25 Total Impact Points

Institutions

  • 1995–2015
    • Pennsylvania State University
      • Department of Materials Science and Engineering
      University Park, Maryland, United States
  • 2013–2014
    • Tsinghua University
      • • State Key Laboratory of New Ceramics and Fine Processing
      • • School of Materials Science and Engineering
      Peping, Beijing, China
    • Chonnam National University
      • Department of Material Science and Engineering
      Gwangju, Gwangju, South Korea
  • 1999–2012
    • William Penn University
      Worcester, Massachusetts, United States
  • 2011
    • National Chiao Tung University
      • Department of Material Science and Engineering
      Hsin-chu-hsien, Taiwan, Taiwan
  • 1995–2011
    • Technical Institute of Physics and Chemistry
      Peping, Beijing, China
  • 2010
    • University of Wisconsin, Madison
      • Department of Materials Science and Engineering
      Madison, MS, United States
  • 2009
    • Oak Ridge National Laboratory
      • Center for Nanophase Materials Sciences
      Oak Ridge, FL, United States
  • 2003–2007
    • Chinese Academy of Sciences
      • Institute of Physics
      Peping, Beijing, China