Exploring the Design of Multiferroic Materials by Nanocrystal Building Block Approach
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Multiferroic materials have been extensively studied as they exhibit both magnetic and electric properties simultaneously, and could be potential candidate materials for the new generation of magnetoelectric (ME) devices. Perovskite materials with room-temperature ferroelectric ordering and simple crystal structure have emerged as favorable building blocks for developing multiferroic devices. In this work, we investigated different approaches to introducing magnetic properties into ferroelectric BaTiO3 and PbTiO3 nanostructures synthesized by hydrothermal method. Specifically, we investigated doping of perovskite nanostructures with magnetic centers as an intrinsic approach, and the formation of nanocomposite with ferrimagnetic spinel Co0.6Fe2.4O4 nanocrystals as an extrinsic approach to multiferroic nanostructures with different morphologies. Investigation of morphology, crystal structure, and magnetic properties of nanoscale perovskite materials and their heterostructures shows promising results towards developing materials which exhibit room-temperature multiferroic properties. Mn-doped BaTiO3 nanoparticles (NPs) were synthesized with different size and doping concentrations. Small particle size and high doping concentration favor the crystal phase transformation from tetragonal to cubic crystal structure. Mn-doped BaTiO3 NPs prepared in ethanol with an average size of ca. 6.4 nm show evidence of tetragonal crystal structure with doping concentration up to 10%. UV-visible absorption spectrum of the colloidal sample indicates the existence of Mn3+ ions in BaTiO3 lattice which is further confirmed by Mn L-edge X-ray absorption spectrum. Additional magnetic and magneto-optical properties measurements could shed light on the single-phase multiferroic nanomaterials. Multiferroic composites were attempted by incorporating ferroelectric BaTiO3 and ferrimagnetic Co0.6Fe2.4O4 NPs through impregnation or spin coating. Multi-domain structure is observed in high resolution transmission electron microscopy (HRTEM) images of the nanocomposite consisting of BaTiO3 and Co0.6Fe2.4O4 nanostructures. X-ray diffraction and Raman spectroscopy measurements indicate the existence of tetragonality in both type of composites after annealing at 373 K. In addition, magnetic measurements suggest that the composites have enhanced coercivity compared to ferrimagnetic component alone. Transparent multiferroic multilayer structure was obtained by spin-coating transparent PbTiO3 thin films with Co0.6Fe2.4O4 nanocrystals. This structure provides an opportunity to study the ME coupling between two building blocks with different magneto-optical spectroscopies.