Abstract
Combining ferroelectric with antiferromagentic materials in nanometer scale is an effective method for exploring multiferroic materials. We preflent two kinds of systems to show the possibility of multiferroic properties in such nanometer composites. One is the artificial superlattice LaFeO3-YMnO3, and the other is the natural layered Aurivillius material Bi4Ti3O12 doped with different layers of LaFeO3, BiFeO3. Both materials were synthesized by pulsed laser deposition method on SrTiO3 substrates. Microstructural charterizations with XRD, TEM, and EELS in scanning transmission electron microscopy mode substantiate that the samples have atomically sharp interfaces between neighboring layers; this is important for producing possible magneto-electric coupling in multiferroic materials. Magnetic characterization proves that these materials have ferrimagnetic properties, in spite of their anti-ferromagnetic nature before coupling. Magnetic characterization also proves that there is 0.55-0.9 B remanant magnetization generated at LaFeO3-YMnO3 interface. And the 0.5 and 1.5LaFeO3-Bi4Ti3O12 samples show ferrimagnetism which can remain even up to room temperature. Ferroelectric tests prove that there is a large leakage current in LaFeO3-YMnO3 superlattice and BiFeO3-inserted Bi4Ti3O12, but 0.5LaFeO3-Bi4Ti3O12 shows ferroelectric hysteresis loops. It can be therefore concluded that 0.5LaFeO3-Bi4Ti3O12 is a multiferroic material. If more perovskite layers (3-layer SrTiO3 or 2.5-layer LaFeO3) are inserted, the Aurivillius structure of Bi4Ti3O12 may appear structural instability that can be observed in our HRTEM measureflent. Our first principles calculations show that the degeneracy of formation enthalpies is the reason why the intergrowth in these materials forms and their structures are not stable. Our work may provide some examples for exploring new multiferroics by means of nano-meter composite.
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