Reconstruction of the polar interface between hexagonal LuFeO3 and intergrown Fe3O4 nanolayers

Interplay of Magnetic Properties and Doping in Epitaxial Films of h-REFeO3 Multiferroic Oxides

Multiferroic materials demonstrating coexistence of magnetic and ferroelectric orders are promising candidates for magnetoelectric devices. While understanding the underlying mechanism of interplaying of ferroic properties is important, tailoring their properties to make them potential candidates for magnetoelectric devices is challenging. Here, the antiferromagnetic Neel ordering temperature above 200 K is realized in successfully stabilized epitaxial films of (Lu,Sc)FeO₃ multiferroic oxide. The first-principles calculations show the shrinkage of in-plane lattice constants of the unit cells of the films on different substrates which corroborates well the enhancement of the Neel ordering temperature (T_N ). The profound effect of lattice strain/stress at the interface due to differences of in-plane lattice constants on out of plane magnetic properties and on spin reorientation temperature in the antiferromagnetic region is further elucidated in the epitaxial films with and without buffer layer of Mn-doped LuFeO₃ . Writing and reading ferroelectric domains reveal the ferroelectric response of the films at room temperature. Detailed electron microscopy shows the presence of lattice defects in atomic scale. First-principles calculations show that orbital rehybridization of rare-earth ions and oxygen is one of the main driving force of ferroelectricity along c-axis in thin films of hexagonal ferrites.

Deposition of Lu-Fe-O thin films on silica glass substrates by MOCVD

Thin films of the Lu-Fe-O system were deposited by aerosol assisted MOCVD on silica glass substrates. Hexagonal h-LuFeO3, garnet Lu3Fe5O12, perovskite o-LuFeO3 or hematite Fe2O3 phases were obtained, depending on the thermodynamic deposition conditions or post annealing temperature. Magnetic measurements confirm the ferromagnetic behaviour at room temperature of the thin films with garnet phase. An indirect bandgap of 1.78 eV was measured.

Nanoscale LuFeO3: shape dependent ortho/hexa-phase constitution and nanogenerator application

In multiferroic LuFeO₃ the hexagonal (-h) phase is an intermediate metastable phase encountered during the amorphous to orthorhombic (-o) transformation and is ferroelectric in nature. Thus far it has only been stabilized in a substrate-supported few layered ultrathin film form. Herein we show that the surface-induced strain field intrinsically present in nano-systems can self-stabilize this phase and the hexagonal to orthorhombic phase constitution ratio depends on the shape of the nanomaterial. Thus, nanoparticles (nanofibres) strain-stabilize the o : h ratio of about 75 : 25 (23 : 77). The inclusion of nano-LuFeO₃ into PDMS renders impressive nanogenerator performance, consistent with the ferroelectric phase content.

Electronic phase transitions in ultrathin magnetite films

Magnetite (Fe3O4) shows singular electronic and magnetic properties, resulting from complex electron-electron and electron-phonon interactions that involve the interplay of charge, orbital and spin degrees of freedom. The Verwey transition is a manifestation of these interactions, with a puzzling connection between the low temperature charge ordered state and the dynamic charge fluctuations still present above the transition temperature. Here we explore how these rich physical phenomena are affected by thin film geometries, particularly focusing on the ultimate size limit defined by thicknesses below the minimum bulk unit cell. On one hand, we address the influence of extended defects, such as surfaces or antiphase domains, on the novel features exhibited by thin films. On the other, we try to isolate the effect of the reduced thickness on the electronic and magnetic properties. We will show that a distinct phase diagram and novel charge distributions emerge under reduced dimensions, while holding the local high magnetic moments. Altogether, thin film geometries offer unique possibilities to understand the complex interplay of short- and long-range orders in the Verwey transition. Furthermore, they arise as interesting candidates for the exploitation of the rich physics of magnetite in devices that demand nanoscale geometries, additionally offering novel functionalities based on their distinct properties with respect to the bulk form.

The stability and surface termination of hexagonal LuFeO3

The surface termination and the nominal valence states for hexagonal LuFeO3 thin films grown on Al2O3(0 0 0 1) substrates were characterized by angle resolved x-ray photoemission spectroscopy. The Lu 4f, Fe 2p and O 1s core level spectra indicate that both the surface termination and the nominal valence depend on surface preparation, but the stable surface terminates in a Fe-O layer. This is consistent with the results of density functional calculations which predict that the Fe-O termination of LuFeO3(0 0 0 1) surface is energetically favorable and stable over a broad range of temperatures and oxygen partial pressures when it is reconstructed to eliminate surface polarity.

Polarity compensation in ultra-thin films of complex oxides: the case of a perovskite nickelate

We address the fundamental issue of growth of perovskite ultra-thin films under the condition of a strong polar mismatch at the heterointerface exemplified by the growth of a correlated metal LaNiO₃ on the band insulator SrTiO₃ along the pseudo cubic [111] direction. While in general the metallic LaNiO₃ film can effectively screen this polarity mismatch, we establish that in the ultra-thin limit, films are insulating in nature and require additional chemical and structural reconstruction to compensate for such mismatch. A combination of in-situ reflection high-energy electron diffraction recorded during the growth, X-ray diffraction, and synchrotron based resonant X-ray spectroscopy reveal the formation of a chemical phase La₂Ni₂O₅ (Ni²⁺) for a few unit-cell thick films. First-principles layer-resolved calculations of the potential energy across the nominal LaNiO₃/SrTiO₃ interface confirm that the oxygen vacancies can efficiently reduce the electric field at the interface.