Self-assembled Nanocomposite Thin Films Research Articles

Tailoring physical functionalities of complex oxides by vertically aligned nanocomposite thin-film design

Self-assembled nanocomposite thin films couple two materials into a single film, typically, in the form of vertically aligned nanopillars embedded in a matrix film. High-density vertical heterointerfaces provide a great platform for engineering new physical properties and novel multifunctionalities, as well as for nanoscale device integration. Tremendous research efforts have been devoted to developing different nanocomposite systems. In this article, we summarize recent progress on vertically aligned nanocomposite thin films for enhanced functionalities such as ferroelectricity, tunable magnetoresistance, multiferroicity, dielectricity, magnetic anisotropy, perpendicular exchange bias, novel electrical/ionic properties, interfacial conduction, and resistive switching. Using specific examples, we discuss how and why the fundamental physical properties can be significantly tuned/improved in vertically aligned nanocomposites. Finally, we propose future research directions to achieve further enhanced performance as well as practical devices.

Self‐Assembled Room Temperature Multiferroic BiFeO3‐LiFe5O8 Nanocomposites

AbstractMultiferroic materials have driven significant research interest due to their promising technological potential. Developing new room‐temperature multiferroics and understanding their fundamental properties are important to reveal unanticipated physical phenomena and potential applications. Here, a new room temperature multiferroic nanocomposite comprised of an ordered ferrimagnetic spinel α‐LiFe5O8 (LFO) and a ferroelectric perovskite BiFeO3 (BFO) is presented. It is observed that lithium (Li)‐doping in BFO favors the formation of LFO spinel as a secondary phase during the synthesis of LixBi1−xFeO3 ceramics. Multimodal functional and chemical imaging methods are used to map the relationship between doping‐induced phase separation and local ferroic properties in both the BFO‐LFO composite ceramics and self‐assembled nanocomposite thin films. The energetics of phase separation in Li doped BFO and the formation of BFO‐LFO composites are supported by first principles calculations. These findings shed light on Li's role in the formation of a functionally important room temperature multiferroic and open a new approach in the synthesis of light element doped nanocomposites for future energy, sensing, and memory applications.

Interfacial spin glass mediated spontaneous exchange bias effect in self-assembled La0.7Sr0.3MnO3:NiO nanocomposite thin films

Zero field cooled spontaneous exchange bias (SEB) is observed in self-assembled nanocomposite thin films of La0.7Sr0.3MnO3 (LSMO):NiO grown on single crystal line SrTiO3 (001) substrate by pulsed laser deposition. SEB is displayed by the asymmetry in the hysteresis loop of the composite film along with the field cooled conventional exchange bias (CEB) effect. The training effect shows that exchange bias relaxation is disorder mediated. It is revealed from DC magnetization measurements that such nanocomposite film divulges spin glass like behaviour, arising due to competing ferromagnetic and antiferromagnetic interactions at the assorted interfaces of ferromagnetic LSMO and antiferromagnetic NiO. X-ray magnetic circular dichroism (XMCD) measurements discern that SEB is mainly originating due to ferromagnetic coupling of unstable interfacial antiferromagnetic spin due to NiO with the ferromagnetic LSMO at the disordered interface. These results of self-assembled thin films provide a useful input to realize and understand microscopic origin of SEB for device application.

Magnetic property modulation of sputter-grown BiFeO3CoFe2O4 nanocomposite thin films

Abstract Self-assembled nanocomposite thin films in which ferromagnetic CoFe2O4 nanopillars grow in a ferroelectric BiFeO3 matrix have attracted tremendous interest for use as next-generation multiple state memory devices. To take advantage of the various magnetic properties of nanocomposites, it is necessary to control the strain state and shape of the CoFe2O4 pillars. Herein two methods are reported that can modulate the magnetic properties of nanocomposite thin films: changing the CoFe2O4 content in BiFeO3 and changing the aspect ratios of the CoFe2O4 pillars. The out-of-plane magnetic anisotropy of nanocomposites increased with decreasing CoFe2O4 ratio, and the anisotropy agreed with the value predicted from the strain effect. Nanocomposites with high-aspect-ratio CoFe2O4 pillars exhibited increased out-of-plane magnetic anisotropy as a result of their changing shape and increasing out-of-plane strain. Strain effects on the magnetic anisotropy of nanocomposites were confirmed by removing the BiFeO3 matrix via chemical etching.

Multifunctional, self-assembled oxide nanocomposite thin films and devices

Abstract

New strain states and radical property tuning of metal oxides using a nanocomposite thin film approach

Auxetic-like strain states were generated in self-assembled nanocomposite thin films of (Ba0.6Sr0.4TiO3)1−x − (Sm2O3)x(BSTO − SmO). A switch from auxetic-like to elastic-like strain behavior was observed for x > 0.50, when the SmO switched from being nanopillars in the BSTO matrix to being the matrix with BSTO nanopillars embedded in it. A simple model was adopted to explain how in-plane strain varies with x. At high x (0.75), strongly enhanced ferroelectric properties were obtained compared to pure BSTO films. The nanocomposite method represents a powerful new way to tune the properties of a wide range of strongly correlated metal oxides whose properties are very sensitive to strain.

Magneto-Electro-Chemical Behavior of BaTiO3-CoFe2O4 Self-Assembled Thin Films

Two-phase CoFe2O4 (CFO) and BaTiO3 (BTO) self-assembled nanocomposite thin films were fabricated by pulsed laser deposition. The epitaxial relationships between the CFO and BTO phases, and the substrate, were observed by transmission electron microscopy. Magnetoelectric (ME) coupling between the two phases was studied. We found a magneto-electro-chemical behavior, where induced charge can be used to deposit metal nanoparticles on the surface of films placed in an ionic solution.

Review of magnetoelectric perovskite–spinel self-assembled nano-composite thin films

Two-phase multiferroic nano-composite thin films have been a topic of research interests in the last few years. This is because of their expected magnetoelectric coupling, as well as potential applications. This review focuses on recent findings in self-assembled nano-structure composite thin films, and various efforts to realize and improve their magnetoelectricity. Topics include: (i) nano-pillar and maze structures, and their formation mechanisms, and a nano-belt structure oriented in-plane found by our research group; (ii) the ferroelectric properties of composite thin films, and how they can be enhanced by epitaxial engineering; (iii) a magnetic anisotropy that is induced by constraint stress, and by the nano-structures of the ferromagnetic phase; and (iv) a magnetoelectric coupling that was first observed via a change in magnetization near the Curie temperature of the ferroelectric phase, a magnetization switching assisted by electric field, and recently direct measurements using a magnetic cantilever method yielding values of 18 mV/cm Oe in BiFeO3–CoFe2O4.

Characterization of electrostatically self-assembled nanocomposite thin films

Multilayer thin films of metallic nanoclusters, polymers and other molecules have been formed using a novel electrostatic self-assembly method and analysed by multiple characterization techniques. Nanocluster size measurements, ellipsometry and UV-visible absorption spectroscopy have been used to confirm the linear build-up of the thin film thickness with the number of deposited nanocluster, polymer and other molecular layers. Auger electron spectroscopy allowed verification of the distribution of molecular species through thick films with multilayer segments containing different elements. Field emission scanning electron microscopy and atomic force microscopy permitted visualization of the morphologies of the outermost layers of the deposited films. Together, such characterization allows improved understanding and the basis for the design of multilayer thin film materials engineered to have specific molecular level structures and macroscopic functionalities.