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theses

This thesis covers the study of multiferroics and magnetoelectrics by utilizing a collection of scanning force microscopy to investigate the cross-coupled phenomena. We mainly focus on the multiferroic hexagonal REMnO$_3$ (RE = rare earth), an improper ferroelectrics with the coexistence of ferroelectricity ($T_extrm{C}$ = 1200 - 1500 K) and antiferromagnetism ($T_extrm{N}$ = 70 - 120 K), and explore in depth the magnetoelectric effect of this system microscopically. Using cryogenic magnetic force microscope (MFM), we observed uncompensated magnetic moment along antiferromagnetic domain walls, which coincides with the ferroelectric domain boundaries. This magnetism presents an alternating feature around the multiferroic vortex, and correlates with each other through the vortex network. The study of the magnetic field dependence of domain wall magnetism also provides a way to probe the intrinsic bulk domain state. To directly image the magnetoelectric domains, we developed a novel mesoscopic technique, namely, magnetoelectric force microscopy (MeFM), to probe the local electric field-induced magnetization based on MFM. The detail of the novel technique will be presented in Chapter 2. With the application of MeFM in hexagonal manganities, we observed that the magnetoelectric response changes sign at each structural domain wall, which provides compelling evidence for a lattice-mediated magnetoelectric coupling. More interestingly, the magnetoelectric effect diverges when approaching the tri-critical point in $T-H$ phase diagram, where critical fluctuation plays a crucial role. Our study suggests the phase competition as a possibility to enhance magnetoelectric coupling. The systematic study of hexagonal manganites, including ErMnO$_3$ and YbMnO$_3$, disentangles the contribution from Mn and rare earth sublattices, suggesting that the $3d-4f$ coupling and the Ising anisotropy of rare earth spins are the key ingredients to understand the re-entrant spin-reorientation phase boundary and the emergence of a tri-critical point.

ETD_6095

Ph.D.

Includes bibliographical references

by Yanan Geng

doi:10.7282/T3QZ2CPJ

ETD doctoral

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