[Show abstract][Hide abstract]ABSTRACT: Strain is a novel approach to manipulating functionalities in correlated complex oxides. However, significant epitaxial strain can only be achieved in ultrathin layers. We show that, under direct lattice matching framework, large and uniform vertical strain up to 2% can be achieved to significantly modify the magnetic anisotropy, magnetism, and magnetotransport properties in heteroepitaxial nanoscaffold films, over a few hundred nanometers in thickness. Comprehensive designing principles of large vertical strain have been proposed. Phase-field simulations not only reveal the strain distribution but also suggest that the ultimate strain is related to the vertical interfacial area and interfacial dislocation density. By changing the nanoscaffold density and dimension, the strain and the magnetic properties can be tuned. The established correlation among the vertical interface—strain—properties in nanoscaffold films can consequently be used to tune other functionalities in a broad range of complex oxide films far beyond critical thickness.
[Show abstract][Hide abstract]ABSTRACT: Purely voltage-driven, repeatable magnetization reversal provides a tantalizing potential for the development of spintronic devices with a minimum amount of power consumption. Substantial progress has been made in this subject especially on magnetic/ferroelectric heterostructures. Here, we report the in situ observation of such phenomenon in a NiFe thin film grown directly on a rhombohedral Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-PT) ferroelectric crystal. Under a cyclic voltage applied perpendicular to the PMN-PT without a magnetic field, the local magnetization of NiFe can be repetitively reversed through an out-of-plane excursion and then back into the plane. Using phase field simulations we interpret magnetization reversal as a synergistic effect of the metastable ferroelastic switching in the PMN-PT and an electrically rotatable local exchange bias field arising from the heterogeneously distributed NiO clusters at the interface.
[Show abstract][Hide abstract]ABSTRACT: The dielectricphase transition behavior of imprinted lead magnesium niobate–lead titanate relaxor ferroelectricthin films was mapped as a function of temperature and dc bias. To compensate for the presence of internal fields, an external electric bias was applied while measuring dielectric responses. The constructed three-dimensional dielectric maps provide insight into the dielectric behaviors of relaxor ferroelectricfilms as well as the temperature stability of the imprint. The transition temperature and diffuseness of the dielectric response correlate with crystallographic disorder resulting from strain and defects in the films grown on strontium titanate and silicon substrates; the latter was shown to induce a greater degree of disorder in the film as well as a dielectric response lower in magnitude and more diffuse in nature over the same temperature region. Strong and stable imprint was exhibited in both films and can be utilized to enhance the operational stability of piezoelectric devices through domain self-poling.
Full-text Article · Mar 2016 · Applied Physics Letters
[Show abstract][Hide abstract]ABSTRACT: Scientific Reports 4 : Article number: 7507 10.1038/srep07507 ; published online: 16 December 2014 ; updated: 25 February 2016 . This Article contains a typographical error in a grant number in the Acknowledgements section.
[Show abstract][Hide abstract]ABSTRACT: The complex interplay of spin, charge, orbital and lattice degrees of freedom provides a plethora of exotic phases and physical phenomena. In recent years, complex spin topologies have emerged as a consequence of the electronic band structure and the interplay between spin and spin-orbit coupling in materials. Here we produce complex topologies of electrical polarization-namely, nanometre-scale vortex-antivortex (that is, clockwise-anticlockwise) arrays that are reminiscent of rotational spin topologies-by making use of the competition between charge, orbital and lattice degrees of freedom in superlattices of alternating lead titanate and strontium titanate layers. Atomic-scale mapping of the polar atomic displacements by scanning transmission electron microscopy reveals the presence of long-range ordered vortex-antivortex arrays that exhibit nearly continuous polarization rotation. Phase-field modelling confirms that the vortex array is the low-energy state for a range of superlattice periods. Within this range, the large gradient energy from the vortex structure is counterbalanced by the corresponding large reduction in overall electrostatic energy (which would otherwise arise from polar discontinuities at the lead titanate/strontium titanate interfaces) and the elastic energy associated with epitaxial constraints and domain formation. These observations have implications for the creation of new states of matter (such as dipolar skyrmions, hedgehog states) and associated phenomena in ferroic materials, such as electrically controllable chirality.
[Show abstract][Hide abstract]ABSTRACT: The real time microstructure evolutions of directional solidification in Mg-Gd alloys are obtained by synchrotron X-ray radiography, and the effects of different low cooling rates under a fixed thermal gradient are studied. Different from the organic alloys, the growth direction of columnar dendrites gradually rotates to the direction of the thermal gradient as the cooling rates increase, which is attributed to the difference of undercooling. Meanwhile, the interface velocity increases but the mean dendrite spacing decreases, and the morphology varies with implication for solute segregation.
[Show abstract][Hide abstract]ABSTRACT: The Solid electrolyte interphase (SEI), either naturally formed or artificially designed, plays a critical role in the stability and durability of Li-ion batteries (LIBs). It is even more important for high energy density electrodes such as Li metal anodes, which is subjected to large volumetric and interfacial variations due to Li deposition/stripping cycles during operation. Currently, there is a lack of understanding of the role of SEI/Li interfaces and their mechanical and electrochemical properties. In this paper, we present an interfacial study to evaluate the two major SEI components, LiF and Li2CO3, based on density functional theory (DFT) calculations. The calculated interfacial energy results show that the Li2CO3/Li interface has higher interfacial mechanical strength. The density of states (DOS) and electrostatic potential results demonstrate that the LiF/Li interface has higher electron tunneling energy barrier from Li metal to SEI. These results provide quantitative inputs for related meso-scale simulations and valuable insights for advanced electrode protective coating design.
Full-text Article · Jan 2016 · Journal of The Electrochemical Society
[Show abstract][Hide abstract]ABSTRACT: The enhancement of the functional properties of materials at reduced dimensions is crucial for continuous advancements in
nanoelectronic applications. Here, we report that the scale reduction leads to the emergence of an important functional property,
ferroelectricity, challenging the long-standing notion that ferroelectricity is inevitably suppressed at the scale of a few
nanometers. A combination of theoretical calculations, electrical measurements, and structural analyses provides evidence
of room-temperature ferroelectricity in strain-free epitaxial nanometer-thick films of otherwise nonferroelectric strontium
titanate (SrTiO3). We show that electrically induced alignment of naturally existing polar nanoregions is responsible for the appearance of
a stable net ferroelectric polarization in these films. This finding can be useful for the development of low-dimensional
material systems with enhanced functional properties relevant to emerging nanoelectronic devices.
[Show abstract][Hide abstract]ABSTRACT: Multiferroic magnetoelectric nanostructures with coupled magnetization and electric polarization across their interfaces have stimulated intense research activities over the past decade. Such interface-based magnetoelectric coupling can be exploited to significantly improve the performance of many devices such as memories, tunable radio-frequency/microwave devices, and magnetic sensors. In this article, we introduce a number of current or developing technologies and discuss their limitations. We describe how the use of magnetoelectric nanostructures can overcome these limitations to optimize device performance. We also present challenges that need to be addressed in pursuing practical applications of magnetoelectric devices.
[Show abstract][Hide abstract]ABSTRACT: We employ phase-field modeling to explore the elastic properties of artificially created 1-D domain walls in (001)p-oriented BiFeO3
thin films, composed of a junction of the four polarization variants, all with the same out-of-plane polarization. It was found that these junctions exhibit peculiarly high electroelastic fields induced by the neighboring ferroelastic/ferroelectric domains. The vortex core exhibits a volume expansion, while the anti-vortex core is more compressive. Possible ways to control the electroelastic field, such as varying material constant and applying transverse electric field, are also discussed.
[Show abstract][Hide abstract]ABSTRACT: Voltage controlled 180° magnetization reversal has been achieved in BiFeO3-based multiferroic heterostructures, which is promising for the future development of low-power spintronic devices. However, all existing reports involve the use of an in-plane voltage that is unfavorable for practical device applications. Here, we investigate, using phase-field simulations, the out-of-plane (i.e., perpendicular to heterostructures) voltage controlled magnetism in heterostructures consisting of CoFe nanodots and (110) BiFeO3 thin film or island. It is predicted that the in-plane component of the canted magnetic moment at the CoFe/BiFeO3 interface can be reversed repeatedly by applying a perpendicular voltage across the bottom (110) BiFeO3 thin film, which further leads to an in-plane magnetization reversal in the overlaying CoFe nanodot. The non-volatility of such perpendicular voltage controlled magnetization reversal can be achieved by etching the continuous BiFeO3 film into isolated nanoislands with the same in-plane sizes as the CoFe nanodot. The findings would provide general guidelines for future experimental and engineering efforts on developing the electric-field controlled spintronic devices with BiFeO3-based multiferroic heterostructures.
[Show abstract][Hide abstract]ABSTRACT: We present the results of a mixed-space approach, based on first-principles calculations, to investigate phonon dispersions and thermal properties of Mg2Si and Mg2Sn, including the bulk modulus, Grüneisen parameter, heat capacity, and Debye temperature. It is shown that good agreements are obtained between the calculated results and available experimental data for both phonon dispersions and thermal properties. The phonon dispersions are accurately calculated compared with experimental data due to the high-quality description of LO–TO splitting and transverse acoustic branches along the Γ-K-X symmetry line. We also calculate the heat capacity CP and Debye temperature of Mg2Si1−x
alloys (x = 0.375, 0.5, 0.625, 0.875). The CP values at high temperature range from 0.5 to 0.7 J/g/K and ΘD values at room temperature from 332 to 384 K as the Sn content decreases from 0.875 to 0.375.
Article · May 2015 · Journal of Materials Research
[Show abstract][Hide abstract]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.
[Show abstract][Hide abstract]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.
Full-text Article · Jan 2015 · Journal of Magnetism and Magnetic Materials
[Show abstract][Hide abstract]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.