| AVS 60th International Symposium and Exhibition | |
| Thin Film | Wednesday Sessions |
| Session TF+MI-WeM |
| Session: | Magnetic Thin Films and Nanostructures |
| Presenter: | C. Pham, University of California at Los Angeles |
| Authors: | C. Pham, University of California at Los Angeles J.P. Chang, University of California at Los Angeles |
| Correspondent: | Click to Email |
Multiferroic materials are a class of material which exhibit two or more forms of ferroic order such as (anti)ferroelectricity, (anti)ferromagnetism, or ferroelasticity and have been proposed for use in future devices in which magnetism is switched upon the application of a electric field. While the existence of intrinsic multiferroic materials, such as BiFeO3, have been demonstrated, composite multiferroics offer improved switching performance, consisting of a piezoelectric and a magnetostrictive material coupled together via interfacial strain. In addition, nanoscale composites have been shown in literature to have even greater coupling when compared to other composites. For this project, atomic layer deposition (ALD) is used to enable the precise control of the composition and thickness by manipulating the pulsing sequence of the precursors. In addition, two approaches to multiferroic composites emphasize the flexibility of the ALD technique; for a 2D composite approach, the ability to deposit nanoscale laminates; while for a 3D composite approach, the ability to uniformly coat films over a nanoscale porous template.
In this work, BiFeO3 (BFO) and CoFe2O4 (CFO) were deposited by ALD to synthesize 2D nanoscale multiferroic composite multilayers. The ALD processes used the metallorganic precursors Bi(tmhd)3, Co(tmhd)2, and Fe(tmhd)3 alongside oxygen atoms produced from a microwave atomic beam source. The ALD BFO and CFO films were able to be grown with a stoichiometric ratio Bi:Fe close to unity and Co:Fe close to 1:2, respectively, and with a controlled linear growth rate. The ALD BFO and CFO processes were then combined to deposit multilayer nanolaminates which repeated between the two oxides at varying thicknesses between 5-20 nm and number of repeating layers. Additionally, ALD BFO was integrated with a 3D mesoporous CFO template consisting of approximately 14 nm diameter pores, which was synthesized using a di-block copolymer self-assembly technique. The conformal aspect of ALD deposition was demonstrated by covering the pores at varying thicknesses until the pores were completely filled.
To compare the material performance of the ALD enabled BFO/CFO films to previously reported benchmarks, measurements of magnetic and ferroelectric properties were accomplished using SQUID magnetometry and Sawyer-Tower circuit methods, respectively. For the 3D mesoporous composite, SEM and XPS confirmed that BFO was able to be deposited onto the nanoscale high aspect-ratio structure of the CFO conformally. The magnetoelectric coupling properties in the composite films were studied by taking magnetic measurements with and without an ex-situ electric poling.