Investigations on Magnetoelectric Bulk and Nanocomposite Materials

Magnetoelectric multiferroics (ME MFs) exhibit both magnetic and ferroelectric orders and show coupling between the two order parameters. These materials have potential for their applications in magnetic field sensors, energy harvesters, and novel spintronic devices. For these applications, ME MFs with strong ME coupling at room temperature are needed. However at present there are no suitable single phase materials for these applications. This dissertation uses the versatility of wet chemistry sol-gel based synthesis methods to explore two methods of improving the ME coupling and furthering the understanding of the operating mechanisms. Historically, single phase ME MFs have weak ME coupling at room temperature. Composites of piezoelectric and magnetostrictive materials achieve ME coupling through strain transfer between the phases. The first pathway toward technologically applicable ME MFs pursued in this dissertation is to develop novel synthesis methods of composites of PbZr0.52Ti0.48O3 and CoFe2O4 that optimize the nanoscale phase distribution and avoid parasitic effects like leakage current to maximize the ME coupling. The discovery of magnetically driven ferroelectricity has renewed interest in single phase ME MFs for spintronic applications. The rare-earth chromites, such as DyCrO3, are good candidates for magnetically driven ferroelectricity at much higher temperatures (~150 K for DyCrO3), than other similar materials. Utilizing the versatility of the sol-gel based synthesis to study the effects of chemical substitutions, the fundamental understanding of the magnetic order and magnetic exchange interactions and how they relate to the electronic properties has been enhanced. These substitutions are shown to cause novel magnetic properties, such as single phase exchange bias, which facilitates novel functionalities. The work in this dissertation improves the ME coupling and understanding of the underlying physical mechanisms in MF MFs to enable future device applications