| AVS 64th International Symposium & Exhibition | |
| Thin Films Division | Thursday Sessions |
| Session TF+MI+NS-ThA |
| Session: | ALD and Nanostructures |
| Presenter: | Jeffrey Chang, University of California at Los Angeles |
| Authors: | J. Chang, University of California at Los Angeles A. Rosenberg, Stanford University A. Buditama, University of California at Los Angeles E. Jin, Yale University L. Kornblum, Yale University C. Ahn, Yale University S.H. Tolbert, University of California at Los Angeles K.A. Moler, Stanford University J.P. Chang, University of California at Los Angeles |
| Correspondent: | Click to Email |
Multiferroic materials, which exhibit the coexistence and coupling between ferroelectricity and magnetism, are of great interest due to their potential for enabling next-generation memories. To overcome the scarcity and weak responses of single-phase multiferroics, composite strategies were proposed for a robust multiferroic behavior by coupling the functional properties through the interface. By interfacing magnetostrictive and piezoelectric materials, the strain-mediated coupling strategy holds great promise, while both the interfacial-area-per-volume and the quality of interfaces play important roles in attainable functional properties. To obtain applicable magnetoelectric (ME) coupling under such scheme, lead-free multiferroic BiFeO3 (BFO) and ferrimagnetic CoFe2O4 (CFO) were used in this study due to their robust functionality.
With the aim of enhancing the coupling by nanostructuring, atomic layer deposition (ALD), with its high quality and conformal film growth, was employed to achieve BFO/CFO composites with closely coupled interfaces and industrial scalability. BFO and CFO with were synthesized by radical enhanced ALD on SrTiO3 (001) substrates using tmhd-based organometallic precursors(tmhd=2,2,6,6-tetramethylheptane-3,5 dione). The use of oxygen radicals as the oxidant provides a low temperature processing capability at 200°C. Post-deposition thermal treatments promotes the crystallization of both the BFO and CFO films and thus comparable functional properties were obtained. 2D-2D BFO/CFO composites were then synthesized by combining the ALD processes, with an optimized functionality obtained by nanolayering. ME behaviors comparable to previously studied multiferroic composites were observed, and the scaling of materials leads to an enhancement in the coupling coefficient (≈64 Oe cm/kV). The change in magnetic domains upon electrical poling was characterized using scanning SQUID. The 2D-2D BFO/CFO on SrTiO3-buffered Si substrates exhibited comparable ME behaviors to the case on SrTiO3, illustrating a path to integrate multiferroic materials into current industrial processes by ALD.
0D-3D BFO/CFO was realized by incorporating ALD-grown BiFeO3 and mesoporous CFO substrates. Composite functionality as well as ME coupling were studied as a function of residual porosity after ALD filling. Higher BFO incorporation increases the overall ferroelectricity but limits the strain interaction due to a decreased mechanical flexibility. The strain interaction was verified by the change in lattice parameters observed using high-resolution XRD measurements. The results showed that the residual porosity is the key for ME coupling in a strain-mediated scheme.