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Ferroic orders of transition metal oxides, such as ferroelectricity and ferromagnetism, are determined by the intercoupling between spin, charge, lattice. Consequently, ferroic epitaxial thin films have attracted wide interest due to the profuse novel phenomena and the great application potentials for modern electronics. However, the mainstream of the study for transition metal oxides has focused on perovskite structure, limiting the understanding and discovery of novel phenomena associate with ferroic orders. In this thesis, the epitaxial films of hexagonal rare-earth ferrites (h-RFeO3) and fluorite-structure hafnia oxide (HfO2) are studied comprehensively, providing new experimental and theoretic insights for ferroic orders, especially the ferroelectricity in these novel systems.
Ferroelectricity in h-RFeO3 is improper since the polarization originates from the non-polar structural distortion. In ultrathin range, the phenomenological model of interfacial clamping is developed, matching with thickness-dependent paraelectric-to-ferroelectric phase transition experimentally. By controlling the interfacial reconstruction, the interfacial clamping effect can be effectively removed. The multidimensional energy landscape of h-RFeO3 enables more abrupt change of effective field than conventical ferroelectrics, which brings a substantial signature on transient negative capacitance effect in h-YbFeO3 films. Moreover, h-RFeO3 is multiferroic, it is found that the domain wall magnetoelectric (ME) coupling could induce the change of macroscopic magnetization, even if ME decoupling switching path is favored for bulk state.
The ferroelectricity of HfO2 has been attributed to the metastable orthorhombic Pca21 phase, which can be transferred from the Pbca phase under electric field. The single-crystalline 10%La-doped HfO2 (LHO) film with Pbca phase is thermodynamically stabilized on YSZ substrate. The antiferroelectric-like double hysteresis loop is obtained in LHO film even with one-unit-cell thickness. Moreover, Curie temperature, corresponding to tetragonal-to-orthorhombic phase transition increases when thickness decreases, reaches to 850 oC at two-dimensional limit. The orthorhombic-related distortion emerges in the initial monolayer of Hf with bulk-state value, corresponding to the absence of interfacial clamping effect. These results indicate the nanoscale stabilization of ferroelectric HfO2 can be attributed to the antiferroelectric stable and ferroelectric stable state from Kittle model.