Fauzia Khatkhatay

Strong oxygen pressure dependence of ferroelectricity in BaTiO3/SrRuO3/SrTiO3 epitaxial heterostructures

The oxygen pressure effect on the structural and ferroelectric properties have been studied in epitaxial BaTiO3 (BTO)/SrRuO3/SrTiO3 (001) heterostructures grown by pulsed laser deposition. It is found that oxygen pressure is a sensitive parameter, which can influence the characteristics of oxide films in many aspects. The out-of-plane lattice parameter, tetragonality, (c/a) and Ti/Ba ratio monotonously decrease as the oxygen pressure increases from 5 mTorr to 200 mTorr. Microstructural study shows that the growth of BaTiO3 varies from a dense large grained structure with a smooth surface to a small columnar grain structure with rough surface as the deposition pressure increases. Electrical measurements show that the 40 mTorr deposited BTO films present maximum remanent polarization (Pr) (14 μC/cm2) and saturation polarization (Ps) (27 μC/cm2) due to the stoichiometric cation ratio, very smooth surface, and low leakage current. These results demonstrate that the controlling of cation stoichiometry, surface...

Integration of Self-Assembled Vertically Aligned Nanocomposite (La0.7Sr0.3MnO3)1–x:(ZnO)x Thin Films on Silicon Substrates

Epitaxial (La0.7Sr0.3MnO3)(1-x):(ZnO)x (LSMO:ZnO) in vertically aligned nanocomposite (VAN) form was integrated on STO/TiN-buffered silicon substrates by pulsed-laser deposition. Their magnetotransport properties have been investigated and are systematically tuned through controlling the ZnO concentration. The composite film with 70% ZnO molar ratio exhibits a maximum magnetoresistance (MR) value of 55% at 70 K and 1 T. The enhanced tunable low-field MR properties are attributed to structural and magnetic disorders and spin-polarized tunneling through the secondary ZnO phase. The integration of LSMO:ZnO VAN films on silicon substrates is a critical step enabling the application of VAN films in future spintronic devices.

Strain relaxation and enhanced perpendicular magnetic anisotropy in BiFeO3:CoFe2O4 vertically aligned nanocomposite thin films

Self-assembled BiFeO3:CoFe2O4 (BFO:CFO) vertically aligned nanocomposite thin films have been fabricated on SrTiO3 (001) substrates using pulsed laser deposition. The strain relaxation mechanism between BFO and CFO with a large lattice mismatch has been studied by X-ray diffraction and transmission electron microscopy. The as-prepared nanocomposite films exhibit enhanced perpendicular magnetic anisotropy as the BFO composition increases. Different anisotropy sources have been investigated, suggesting that spin-flop coupling between antiferromagnetic BFO and ferrimagnetic CFO plays a dominant role in enhancing the uniaxial magnetic anisotropy.

Ferroelectric Properties of Vertically Aligned Nanostructured BaTiO3–CeO2 Thin Films and Their Integration on Silicon

Epitaxial (BaTiO3)0.5(CeO2)0.5 films have been deposited in vertically aligned nanocomposite form on SrTiO3/TiN buffered Si substrates to achieve high-quality ferroelectrics on Si. The thin TiN seed layer promotes the epitaxial growth of the SrTiO3 buffer on Si, which in turn is essential for the high-quality growth of the vertically aligned nanocomposite structure. X-ray diffraction and transmission electron microscopy characterization show that the films consist of distinct c-axis oriented BaTiO3 and CeO2 phases. Polarization measurements show that the BaTiO3-CeO2 films on Si are actually ferroelectric at room temperature, and the ferroelectric response is comparable to pure BaTiO3 as well as the BaTiO3-CeO2 films on SrTiO3 single-crystalline substrates. Capacitance-voltage measurements show that, instead of decreasing, the Curie temperature increases to 175 and 150 °C for the samples on SrTiO3 and Si substrates, respectively. This work is an essential step towards integrating novel nanostructured materials with advanced functionalities into Si-based devices.

Magnetotransport properties of quasi-one-dimensionally channeled vertically aligned heteroepitaxial nanomazes

A unique quasi-one-dimensionally channeled nanomaze structure has been self-assembled in the (La0.7Sr0.3MnO3)1−x:(ZnO)x vertically aligned nanocomposites (VANs). Significantly enhanced magnetotransport properties have been achieved by tuning the ZnO composition x. The heteroepitaxial VAN thin films, free of large angle grain boundaries, exhibit a maximum low-field magnetoresistance (LFMR) of 75% (20 K and 1 T). The enhanced LFMR close to the percolation threshold is attributed to the spin-polarized tunneling through the ferromagnetic/insulating/ferromagnetic vertical sandwiches in the nanomazes. This study suggests that the phase boundary in the nanomaze structure is an alternative approach to produce decoupled ferromagnetic domains and thus to achieve enhanced magnetoresistance.

Self-Assembled Epitaxial Au–Oxide Vertically Aligned Nanocomposites for Nanoscale Metamaterials

Metamaterials made of nanoscale inclusions or artificial unit cells exhibit exotic optical properties that do not exist in natural materials. Promising applications, such as super-resolution imaging, cloaking, hyperbolic propagation, and ultrafast phase velocities have been demonstrated based on mostly micrometer-scale metamaterials and few nanoscale metamaterials. To date, most metamaterials are created using costly and tedious fabrication techniques with limited paths toward reliable large-scale fabrication. In this work, we demonstrate the one-step direct growth of self-assembled epitaxial metal-oxide nanocomposites as a drastically different approach to fabricating large-area nanostructured metamaterials. Using pulsed laser deposition, we fabricated nanocomposite films with vertically aligned gold (Au) nanopillars (∼20 nm in diameter) embedded in various oxide matrices with high epitaxial quality. Strong, broad absorption features in the measured absorbance spectrum are clear signatures of plasmon resonances of Au nanopillars. By tuning their densities on selected substrates, anisotropic optical properties are demonstrated via angular dependent and polarization resolved reflectivity measurements and reproduced by full-wave simulations and effective medium theory. Our model predicts exotic properties, such as zero permittivity responses and topological transitions. Our studies suggest that these self-assembled metal-oxide nanostructures provide an exciting new material platform to control and enhance optical response at nanometer scales.

Magnetic properties of (CoFe2O4)x:(CeO2)1−x vertically aligned nanocomposites and their pinning properties in YBa2Cu3O7−δ thin films

Vertically aligned nanocomposites (VAN) combined ferrimagnetic CoFe2O4 with non-magnetic CeO2 ((CoFe2O4)x:(CeO2)1−x) in different phase ratios (x = 10%, 30% to 50%) have been grown by a pulsed laser deposition technique. Various unique magnetic domain structures form based on the VAN compositions and growth conditions. Anisotropic and tunable ferrimagnetic properties have been demonstrated. These ordered ferrimagnetic nanostructures have been incorporated into YBa2Cu3O7−δ thin films as both cap and buffer layers to enhance the flux pinning properties of the superconducting thin films. The results suggest that the ordered magnetic VAN provides effective pinning centers by both defect and magnetic nanoinclusions.

Enhanced tunable magnetoresistance properties over a wide temperature range in epitaxial (La0.7Sr0.3MnO3)1−x:(CeO2)x nanocomposites

Vertically aligned nanocomposite (VAN) (La0.7Sr0.3MnO3)1−x:(CeO2)x (LSMO:CeO2) thin films have been grown on SrTiO3 (001) substrates by pulsed laser deposition. Tunable magnetoresistance properties as well as microstructures are demonstrated in these VAN films by modulating the film composition (x = 0, 0.3, 0.4, 0.45, 0.5, and 0.55). The sample of x = 0.3 shows a large low-field magnetoresistance (LFMR) in a high temperature range, i.e., over 10% at the range of 280 K to 320 K under 1 T and with a peak value of ∼13.5% at 310 K. In addition, a vast enhancement of LFMR in a low temperature range of 20–150 K with peak of ≈34.3% at 45 K for 1 T could be achieved with x = 0.5. The enhanced LFMR properties can be attributed to both the phase boundary induced spin fluctuation and the magnetic tunneling effect through vertical ferromagnetic/insulator/ferromagnetic structures. The observed enhanced LFMR performance, especially at high temperatures, as well as its simple growth method, offers a great potential for ...

Perpendicular Exchange-Biased Magnetotransport at the Vertical Heterointerfaces in La(0.7)Sr(0.3)MnO3:NiO Nanocomposites.

Heterointerfaces in manganite-based heterostructures in either layered or vertical geometry control their magnetotransport properties. Instead of using spin-polarized tunneling across the interface, a unique approach based on the magnetic exchange coupling along the vertical interface to control the magnetotransport properties has been demonstrated. By coupling ferromagnetic La0.7Sr0.3MnO3 and antiferromagnetic NiO in an epitaxial vertically aligned nanocomposite (VAN) architecture, a dynamic and reversible switch of the resistivity between two distinct exchange biased states has been achieved. This study explores the use of vertical interfacial exchange coupling to tailor magnetotransport properties, and demonstrates their viability for spintronic applications.

Enhanced Flux Pinning Properties in Self-Assembled Magnetic

In this work, self-assembled magnetic CoFe2O4 (CFO) nanoparticles are introduced into YBa2Cu3O7-δ (YBCO) thin films as both cap and buffer layers to enhance the flux pinning properties. CFO nanoparticles are incorporated into the YBCO matrix by alternative laser ablation of the YBCO and CFO targets by employing the pulsed laser deposition method, which is compatible with the commercial processing method of coated conductors. By introducing ferromagnetic nanoinclusions, both magnetic and defect pinning can be incorporated into YBCO thin films to achieve stronger pinning effects under higher applied magnetic field regimes. Detailed microstructural studies are conducted to verify the morphologies of all doped samples. Both self-field and in-field performance (Jcsf and Jcin-field(H//c)) measured at various temperatures (65, 40, and 5 K) are studied. The results suggest that the CFO nanoparticle cap is an ideal approach to introduce ordered magnetic pinning centers in the YBCO thin films, which shows enhanced superconducting properties both at self-field and under high applied field regimes without degrading YBCOs intrinsic properties.