Text

theses

This thesis contains several investigations of ferroelectricity and magnetoelectric effects in complex oxides. We start by reviewing the history and mechanisms of ferroelectricity and multiferroics, then give a brief introduction to proper ferroelectricity, improper ferroelectricity, hybrid improper ferroelectricity and polar magnets. In the hybrid improper ferroelectricity section, several major systems are explained in detail including (AA')B2O6 double perovskites, (ABO3)2/AO Ruddlesden-Popper (RP) phase and A'(AB2O7) Dion-Jacobson (DJ) phase. Next, our investigations are divided into two parts. The rst half focuses on the study of ferroelectricity and domain wall ( ferroelectric vs. ferroelastic, charged vs. noncharged) motion in Sr3Sn2O7 system (n=2, RP phase). We discover that layered Sr3Sn2O7 exhibits switchable polarization at room temperature, demonstrating that Sr3Sn2O7 is the rst room-temperature insulating ferroelectric containing Sn4+. The in situ poling results on Sr3Sn2O7 using focused electron beams in transmission electron microscopy unveil the intriguing ferroelectric domain switching kinetics: ferroelectric noncharged domain walls move fast while ferroelectric charged domain walls do not move, probably due to octahedral rotation switching is easier along the c axis. Furthermore, due to small coercivity, we could erase (shrink) and re-generate (expand) orthorhombic twin domains using different direction strains. The corresponding ferroelectric-ferroelastic domain patterns are observed under transmission polarized optical microscope (TPOM) and in-plane piezo-response force microscope (IP-PFM). These discoveries reveal the rich scienti c nature of the Sn-containing ferroelectric and provide potential application opportunities of it. In the second half of the thesis, we focus on the study of M2Mo3O8 (M=Fe, Mn, Zn, Ni, Co) polar magnets. In polar magnets, the absence of poling requirements due to the polar nature makes possible utilization of giant ME coefficients as necessary for applications. Our works unveil the magnetic properties and the ME coupling inside this system, especially the largest ME coefficient in Fe2Mo3O8 among all discovered polar magnets and the in-plane magnetic ordering in Ni2Mo3O8 for the rst time. In Fe2Mo3O8, hidden ferrimagnetism of the Fe-O layers strongly enhances the magnetic response in the transition eld, providing an explanation for the observed giant differential ME coefficients. Our results demonstrate the promise of polar magnets as ME system and indicate that their functional properties could be further enhanced by presence of a local ("hidden") magnetic moment that can be easily converted to macroscopic magnetization by an applied field.

ETD_8796

Ph.D.

Includes bibliographical references

by Yazhong Wang

doi:10.7282/T3BV7M38

ETD doctoral

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