VTechWorks staff will be away for the Thanksgiving holiday beginning at noon on Wednesday, November 27, through Friday, November 29. We will resume normal operations on Monday, December 2. Thank you for your patience.

Browsing by Author "Gao, Min"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Magnetoelectricity of CoFe2O4 and tetragonal phase BiFeO3 nanocomposites prepared by pulsed laser deposition
    Gao, Min; Viswan, Ravindranath; Tang, Xiao; Leung, Chung Ming; Li, Jiefang; Viehland, Dwight D. (Springer Nature, 2018-01-10)
    The coupling between the tetragonal phase (T-phase) of BiFeO3 (BFO) and CoFe2O4 (CFO) in magnetoelectric heterostructures has been studied. Bilayers of CFO and BFO were deposited on (001) LaAlO3 single crystal substrates by pulsed laser deposition. After 30 min of annealing, the CFO top layer exhibited a T-phase-like structure, developing a platform-like morphology with BFO. Magnetic hysteresis loops exhibited a strong thickness effect of the CFO layer on the coercive field, in particular along the out-of-plane direction. Magnetic force microscopy images revealed that the T-phase CFO platform contained multiple magnetic domains, which could be tuned by applying a tip bias. A combination of shape, strain, and exchange coupling effects are used to explain the observations.
  • Thumbnail Image
    Nanopillars with E-field accessible multi-state (N ≥ 4) magnetization having giant magnetization changes in self-assembled BiFeO3-CoFe2O4/Pb(Mg1/3Nb2/3)-38at%PbTiO3 heterostructures
    Tang, Xiao; Viswan, Ravindranath; Gao, Min; Leung, Chung Ming; Folger, Carlos; Luo, Haosu; Howe, Brandon; Li, Jiefang; Viehland, Dwight D. (Springer Nature, 2018-01-26)
    We have deposited self-assembled BiFeO3-CoFe2O4 (BFO-CFO) thin films on (100)-oriented SrRuO3-buffered Pb(Mg1/3Nb2/3)0.62Ti0.38O3 (PMN-38PT) single crystal substrates. These heterostructures were used for the study of real-time changes in the magnetization with applied DC electric field (E DC ). With increasing E DC , a giant magnetization change was observed along the out-of-plane (easy) axis. The induced magnetization changes of the CFO nanopillars in the BFO/CFO layer were about ΔM/M rDC  = 93% at E DC  = −3 kv/cm. A giant converse magnetoelectric (CME) coefficient of 1.3 × 10−7 s/m was estimated from the data. By changing E DC , we found multiple(N ≥ 4) unique possible values of a stable magnetization with memory on the removal of the field.
  • Thumbnail Image
    Structure-Property Relations on Strain-Mediated Multiferroic Heterostructures
    Gao, Min (Virginia Tech, 2019-11-20)
    Multiferroic thin-film heterostructures have attracted a great deal of attention due to the increasing demand for novel energy-efficient micro/nano-electronic devices. Both single phase multiferroic materials like BiFeO3 (BFO) thin films, and strain-mediated magnetoelectric (ME) nanocomposites, have the potential to fulfill a number of functional requirements in actual applications—principally, direct control of magnetization by the application of an electric field (E) and vice-versa. From the perspective of material science, however, it is essential to develop a fuller understanding of the complex fabrication-structure-property triangle relationship for these multiferroic thin films. Pulsed laser deposition (PLD) was used in this study to fabricate diverse epitaxial thin film heterostructures on top of single crystal substrates. The crystal structure, phase transition processes (amongst nanodomain distributions, dielectric phases, magnetic spin states, etc.), and various ME-related properties were characterized under different E or temperature environments. Resulting data enabled us to determine the structure-property relationships for a range of multiferroic systems. First, BFO-based heterostructures were studied. Epitaxial BFO thin films were deposited on top of (001)-oriented Pb(Mg1/3Nb2/3)O3-30PbTiO3 (PMN-30PT) single crystal substrates. The strain states of BFO and crystal structural phases were tunable by E applied on the PMN-30PT via both the in-plane and out-of-plane modes. The strain-mediated antiferromagnetic state changes of BFO were also studied using neutron diffraction spectroscopy under E. Then, CoFe2O4(CFO)/tetragonal BFO nanocomposites were successfully fabricated on top of (001)-oriented LaAlO3 single crystal substrates. The surface morphology, crystal structure, magnetic properties, and ME effects were evaluated and compared with CFO/rhombohedral BFO nanocomposites. To enhance the performance of ME heterostructures with PMN-PT substrates, PMN-30PT single crystals with nanograted electrodes were also studied, which evidenced an enhancement in piezoelectric properties and dielectric constant by 36.7% and 38.3%, respectively. X-ray diffraction reciprocal space mapping (RSM) was used to monitor E-induced changes in the apparent symmetry and domain distribution of near-surface regions for the nanograted PMN-30PT crystals. Finally, in order to add antiferroelectric thin films to the family of strain-mediated multiferroic nanocomposites, epitaxial antiferroelectric thin films were prepared. Epitaxial (Pb0.98La0.02)(Zr0.95Ti0.05)O3 (PLZT) thin films were deposited on differently oriented SrTiO3 single crystal substrates. A thickness dependent incommensurate/commensurate antiferroelectric-to-ferroelectric phase transition was identified. The crystal structure, phase transition characteristics and pathways, and energy storage behaviors from room temperature to 250 ℃ were studied, enabling a more systematic understanding of PLZT-based AFE epitaxial thin films. To summarize, a range of epitaxial thin films were prepared using PLD, whose crystal structures and multiferroic properties were related through the strain. Accordingly, properties such as dielectricity, antiferroelectricity, and antiferromagnetism could be adjusted by E. This study sheds further light on the potential for designing desirable strain-mediated multiferroic nano-/micro-devices in the future.