Zhenxing Bi

Thick lead-free ferroelectric films with high Curie temperatures through nanocomposite-induced strain

Ferroelectric materials are used in applications ranging from energy harvesting to high-power electronic transducers. However, industry-standard ferroelectric materials contain lead, which is toxic and environmentally unfriendly. The preferred alternative, BaTiO(3), is non-toxic and has excellent ferroelectric properties, but its Curie temperature of ∼130 °C is too low to be practical. Strain has been used to enhance the Curie temperature of BaTiO(3) (ref. 4) and SrTiO(3) (ref. 5) films, but only for thicknesses of tens of nanometres, which is not thick enough for many device applications. Here, we increase the Curie temperature of micrometre-thick films of BaTiO(3) to at least 330 °C, and the tetragonal-to-cubic structural transition temperature to beyond 800 °C, by interspersing stiff, self-assembled vertical columns of Sm(2)O(3) throughout the film thickness. The columns, which are 10 nm in diameter, strain the BaTiO(3) matrix by 2.35%, forcing it to maintain its tetragonal structure and resulting in the highest BaTiO(3) transition temperatures so far.

A New Class of Room-Temperature Multiferroic Thin Films with Bismuth-Based Supercell Structure

Intergrowth of two partially miscible phases of BiFeO(3) and BiMnO(3) gives a new class of room-temperature multiferroic phase, Bi(3) Fe(2) Mn(2) O(10+δ) , which has a unique supercell (SC) structure. The SC heterostructures exhibit simultaneously room-temperature ferrimagnetism and remanent polarization. These results open up a new avenue for exploring room-temperature single-phase multiferroic thin films by controlling the phase mixing of two perovskite BiRO(3) (R = Cr, Mn, Fe, Co, Ni) materials.

Extremely High Tunability and Low Loss in Nanoscaffold Ferroelectric Films

There are numerous radio frequency and microwave device applications which require materials with high electrical tunability and low dielectric loss. For phased array antenna applications there is also a need for materials which can operate above room temperature and which have a low temperature coefficient of capacitance. We have created a nanoscaffold composite ferroelectric material containing Ba(0.6)Sr(0.4)TiO(3) and Sm(2)O(3) which has a very high tunability which scales inversely with loss. This behavior is opposite to what has been demonstrated in any previous report. Furthermore, the materials operate from room temperature to above 150 °C, while maintaining high tunability and low temperature coefficient of tunability. This new paradigm in dielectric property control comes about because of a vertical strain control mechanism which leads to high tetragonality (c/a ratio of 1.0126) in the BSTO. Tunability values of 75% (200 kV/cm field) were achieved at room temperature in micrometer thick films, the value remaining to >50% at 160 °C. Low dielectric loss values of <0.01 were also achieved, significantly lower than reference pure films.

Microstructure, magnetic, and low-field magnetotransport properties of self-assembled (La0.7Sr0.3MnO3)0.5:(CeO2)0.5 vertically aligned nanocomposite thin films

Two-phase (La(0.7)Sr(0.3)MnO(3))(0.5):(CeO(2))(0.5) (LSMO:CeO(2)) heteroepitaxial nanocomposite films were grown on SrTiO(3) (STO) (001) by pulsed laser deposition (PLD). X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that LSMO:CeO(2) films epitaxially grow on STO as self-assembled vertically aligned nanocomposite (VAN). Magnetic and magnetotransport measurements demonstrate that the LSMO phase in the VAN structure behaves differently from its epitaxial single-phase counterpart, e.g. greatly enhanced coercivity (H(C)) and low-field magnetoresistance (LFMR). The enhanced properties in the VAN system are attributed to the interaction between the perovskite and the secondary phase or phase boundary. The results suggest that the growth of functional oxide in another oxide matrix with vertical heteroepitaxial form is a promising approach to achieve new functionality that may not be easily realized in the single epitaxial phase.

Epitaxial growth and metal-insulator transition of vanadium oxide thin films with controllable phases

Vanadium oxide thin films with well controlled phases such as rhombohedra V2O3 and monoclinic VO2 were synthesized on Al2O3 (0001) substrates by optimizing the processing parameters of a polymer assisted deposition technique. X-ray diffraction and high-resolution transmission electron microscopy studies revealed that both V2O3 and VO2 films can be well controlled with good epitaxial quality. The temperature dependency of electrical resistivity demonstrated sharp metal-insulator transitions (MITs) for V2O3 and VO2 films. The crystallinity and the strains in the films are believed to play critical roles in determining the MIT properties.

Microstructural and magnetic properties of (La0.7Sr0.3MnO3)0.7:(Mn3O4)0.3 nanocomposite thin films

Epitaxial (La0.7Sr0.3MnO3)0.7:(Mn3O4)0.3 (LSMO:Mn3O4) nanocomposite thin films were grown on SrTiO3 (001) substrate by a pulsed laser deposition technique. The nanocomposite structures vary from triangular domains, to vertically aligned columns, and finally to smaller spherical domains as the deposition frequency varies from 1, 5, to 10 Hz, respectively. The strain in LSMO is systematically tuned, but that of the Mn3O4 phase is relatively stable as the deposition frequency increases. The tunable strain is found directly related to the different domain and grain boundary (GB) structures. Physical properties including saturation magnetization, Curie temperature (TC), magnetoresistance and metal–insulator transition temperature (TMI), all show systematic trends as the deposition frequency varies. This study reveals that the domain/GBs tunability achieved in nanocomposite thin films can affect the lattice strain and further tune their ferromagnetic properties.

Strong room temperature magnetism in highly resistive strained thin films of BiFe0.5Mn0.5O3

We report highly resistive strongly ferromagnetic strained thin (∼30 nm) films of BiFe0.5Mn0.5O3 (BFMO) grown on (001) SrTiO3 substrates using pulsed laser deposition. The films are tetragonal with high epitaxial quality and phase-purity. The magnetic moment and coercivity values at room temperature are 90 emu/cc (0.58μB/B-site ion) at H=3 kOe and 274 Oe, respectively. The magnetic transition temperature is strongly enhanced up to ∼600 K, which is ∼500 K higher than for pure bulk BiMnO3. Strained BFMO is a potential room temperature spin filter material for magnetic tunnel devices.

Growth-controlled surface roughness in Al-doped ZnO as transparent conducting oxide

The surface morphology of Al(2)O(3)-doped ZnO (AZO, 2 wt%) thin films varies from a uniform layer to nanorod structure by simply controlling oxygen pressure during growth. All AZO films were deposited on sapphire(0001) substrates using a pulsed laser deposition (PLD) technique. In the low oxygen pressure regime (vacuum approximately 50 mTorr), AZO films grow as a smooth and uniform layer. In the high oxygen pressure regime (100-250 mTorr) AZO thin films with nanorods have formed. Detailed cross-sectional transmission electron microscopy (TEM) and x-ray diffraction (XRD) studies reveal that, besides the obvious variation in the film morphology, the in-plane d spacing of AZO film increases and the out-of-plane d spacing decreases, as oxygen pressure increases. A bilayer AZO film with a nanorod structure on top of a uniform layer was demonstrated by controlling the oxygen pressure for the two layers. Electrical resistivity and optical transmittance measurements were carried out to correlate with the microstructures obtained under different oxygen pressures. The bilayer AZO films could find applications as a transparent conducting oxide (TCO) with a unique light trapping function in thin film solar cells.

Tunable lattice strain in vertically aligned nanocomposite (BiFeO3)x:(Sm2O3)1−x thin films

Unique epitaxial two-phase vertically aligned nanocomposite (VAN) (BiFeO3)x:(Sm2O3)1−x thin films were deposited on SrTiO3(001) substrates by pulsed laser deposition. The VAN thin films exhibit a highly ordered vertical columnar structure with high epitaxial quality. We demonstrate that the strains of the two phases in both out-of-plane and in-plane directions can be tuned by the deposition parameters during growth, e.g., deposition frequency and film composition of the nanocomposite. The strain tunability is found to be directly related to the systematic variation in the column widths in the nanocomposite. The dielectric property measurement shows that increasing vertical strain control will lead to a systematic dielectric loss reduction in the VAN thin films. This study suggests a promising avenue in achieving tunable strain in functional oxide thin films by using VAN structures.

Strong room temperature exchange bias in self-assembled BiFeO3–Fe3O4 nanocomposite heteroepitaxial films

Self-assembled, nanocomposite heteroepitaxial films of BiFeO3 + Fe3O4 (x BiFeO3 + (1 − x) Fe3O4), where x = 0.5 or 0.9, were grown on (011) SrTiO3. Depending on the value of x and on the film thickness, either exchange bias or exchange enhancement of coercivity was demonstrated. In epitaxially and highly strained (7%) films of 250 nm thickness, and for x = 0.9, exchange bias (HEB) values of 40 Oe and HEB/HC ratios of 0.5 were achieved. Most crucially, these effects were measured at room temperature, showing the high potential of chemically compatible BiFeO3 + Fe3O4 for achieving room temperature magnetoelectricity.