[en] Phonons are exquisitely sensitive to finite-length scale effects in complex materials because they are intimately connected to charge, polarizability, and structure, and a quantitative analysis of their behavior can reveal microscopic aspects of chemical bonding. To investigate these effects in a model correlated oxide, we measured the infrared vibrational properties of 8-nm particles of MnO, compared the results with the analogous bulk material, and quantified the phonon confinement with a calculation of the Born effective charge. Our analysis reveals that the Born effective charge decreases by ∼20%, compared to the bulk material. Moreover, this change impacts both ionicity and polarizability. Specifically, we find that MnO nanoparticles are ∼12% less ionic than the corresponding bulk. This discovery is important for understanding finite-length scale effects in this simple binary oxide and the more complicated functional oxides that emanate from this parent compound.

[en] The surface termination and the nominal valence states for hexagonal LuFeO_3 thin films grown on Al_2O_3(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 LuFeO_3(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. (paper)

[en] We have carried out the growth of h-RFeO₃ (0 0 1) (R  =  Lu, Yb) thin films on Fe₃O₄ (1 1 1)/Al₂O₃ (0 0 1) substrates, and studied the effect of the h-RFeO₃ (0 0 1)/Fe₃O₄ (1 1 1) interfaces on the epitaxy and magnetism. The observed epitaxial relations between h-RFeO₃ and Fe₃O₄ indicate an unusual matching of Fe sub-lattices rather than a matching of O sub-lattices. The out-of-plane direction was found to be the easy magnetic axis for h-YbFeO₃ (0 0 1) but the hard axis for Fe₃O₄ (1 1 1) in the h-YbFeO₃ (0 0 1)/Fe₃O₄ (1 1 1)/Al₂O₃ (0 0 1) films, suggesting a perpendicular magnetic alignment at the h-YbFeO₃ (0 0 1)/Fe₃O₄ (1 1 1) interface. These results indicate that Fe₃O₄ (1 1 1)/Al₂O₃ (0 0 1) could be a promising substrate for epitaxial growth of h-RFeO₃ films of well-defined interface and for exploiting their spintronic properties. (paper)

[en] One emergent property of ferroelectric nanoparticles is the sized-induced structural distortion to a high-symmetry paraelectric phase at small particle sizes. Finite length scale effects can thus be advantageously employed to elucidate ferroelectric transition mechanisms. In this work, we combine infrared spectroscopy with group theory and lattice dynamics calculations to reveal the displacive nature of the ferroelectric transition in BiFeO3, a room temperature multiferroic. Systematic intensity and frequency trends in selected vibrational modes show that the paraelectric phase is Pm3 m and the lowest frequency A1 feature is the soft mode that drives the first order transition. Finite length scale effects are also evident in the electronic structure with a red-shifted band gap in nanoscale BiFeO3 compared with that of the rhombohedral film, a result that can impact the development of ferroelectric photovoltaics and oxide-based electronics. Taken together, these findings demonstrate the foundational importance of size effects for enhancing the rich functionality and broad utility of transition metal oxides.

[en] We studied the crystal structure and magnetic properties of the rare-earth-free intermetallic compound Fe₃Co₃Nb₂, which has recently been demonstrated to have potentially high magnetic anisotropy, using temperature-dependent neutron powder diffraction. The temperature dependence of the diffraction spectra reveals a magnetic transition between 300 and 400 K, in agreement with the magnetometry measurements. According to the structural refinement of the paramagnetic state and the substantial magnetic contribution to the diffuse scattering in the ferromagnetic state, the Fe/Co anti-site mixing is so strong that the site occupation for Fe and Co is almost random. The projection of the magnetic moments turned out to be non-zero along the c axis and in the a–b plane of Fe₃Co₃Nb₂, most likely because of the exchange interactions between the randomly orientated nanograins in the samples. These findings suggest that future studies on the magnetism of Fe₃Co₃Nb₂ need to take the Fe/Co anti-site mixing into account, and the exchange interactions need to be suppressed to obtain large remanence and coercivity. (paper)

[en] Despite low resistivity (∼1 mΩ cm), metallic electrical transport has not been commonly observed in inverse spinel NiCo₂O₄, except in certain epitaxial thin films. Previous studies have stressed the effect of valence mixing and the degree of spinel inversion on the electrical conduction of NiCo₂O₄ films. In this work, we studied the effect of nanostructural disorder by comparing the NiCo₂O₄ epitaxial films grown on MgAl₂O₄ (1 1 1) and on Al₂O₃ (0 0 1) substrates. Although the optimal growth conditions are similar for the NiCo₂O₄ (1 1 1)/MgAl₂O₄ (1 1 1) and the NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films, they show metallic and semiconducting electrical transport, respectively. Post-growth annealing decreases the resistivity of NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films, but the annealed films are still semiconducting. While the semiconductivity and the large magnetoresistance in NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films cannot be accounted for in terms of non-optimal valence mixing and spinel inversion, the presence of anti-phase boundaries between nano-sized crystallites, generated by the structural mismatch between NiCo₂O₄ and Al₂O₃, may explain all the experimental observations in this work. These results reveal nanostructural disorder as being another key factor for controlling the electrical transport of NiCo₂O₄, with potentially large magnetoresistance for spintronics applications. (paper)

[en] The structural transition at about 1000 °C, from the hexagonal to the orthorhombic phase of LuFeO_3, has been investigated in thin films of LuFeO_3. Separation of the two structural phases of LuFeO_3 occurs on a length scale of micrometer, as visualized in real space using X-ray photoemission electron microscopy. The results are consistent with X-ray diffraction and atomic force microscopy obtained from LuFeO_3 thin films undergoing the irreversible structural transition from the hexagonal to the orthorhombic phase of LuFeO_3, at elevated temperatures. The sharp phase boundaries between the structural phases are observed to align with the crystal planes of the hexagonal LuFeO_3 phase. The coexistence of different structural domains indicates that the irreversible structural transition, from the hexagonal to the orthorhombic phase in LuFeO_3, is a first order transition, for epitaxial hexagonal LuFeO_3 films grown on Al_2O_3.

[en] The electronic structure for the conduction bands of both hexagonal and orthorhombic LuFeO_3 thin films have been measured using x-ray absorption spectroscopy at oxygen K (O K) edge. Dramatic differences in both the spectral features and the linear dichroism are observed. These differences in the spectra can be explained using the differences in crystal field splitting of the metal (Fe and Lu) electronic states and the differences in O 2p-Fe 3d and O 2p-Lu 5d hybridizations. While the oxidation states have not changed, the spectra are sensitive to the changes in the local environments of the Fe"3"+ and Lu"3"+ sites in the hexagonal and orthorhombic structures. Using the crystal-field splitting and the hybridizations that are extracted from the measured electronic structures and the structural distortion information, we derived the occupancies of the spin minority states in Fe"3"+, which are non-zero and uneven. The single ion anisotropy on Fe"3"+ sites is found to originate from these uneven occupancies of the spin minority states via spin–orbit coupling in LuFeO_3. (paper)

[en] We have studied the kinetics of the transitions between the Fe_2O_3 and Fe_3O_4 phases as thin epilayers (∼2.5 nm) on Al_2O_3 (001) substrates using time-resolved reflection high energy electron diffraction. The different iron oxide phases were identified using a combination of in-situ and ex-situ characterizations. The transition from an α-Fe_2O_3 (001) epilayer to a Fe_3O_4 (111) epilayer through thermal reduction was found to be determined by the Fe-O bonding energy, resulting in a long time scale. The oxidation at high temperature converts a Fe_3O_4 (111) epilayer to an α-Fe_2O_3 (001) epilayer quickly; at low temperature, a γ-Fe_2O_3 (111) epilayer was slowly generated instead. By repeating the deposition/thermal reduction processes, a thicker Fe_3O_4 (111) film was obtained, which exhibit high crystallinity and moderate magnetic coercivity.

[en] The spin crossover (SCO) transitions at both the surface and over the entire volume of the [Fe{H₂B(pz)₂}₂(bipy)] polycrystalline films on Al₂O₃ substrates have been studied, where pz  =  pyrazol-1-yl and bipy  =  2,2′-bipyridine. For [Fe{H₂B(pz)₂}₂(bipy)] films of hundreds of nm thick, magnetometry and x-ray absorption spectroscopy measurements show thermal hysteresis in the SCO transition with temperature, although the transition in bulk [Fe{H₂B(pz)₂}₂(bipy)] occurs in a non-hysteretic fashion at 157 K. While the size of the crystallites in those films are similar, the hysteresis becomes more prominent in thinner films, indicating a significant effect of the [Fe{H₂B(pz)₂}₂(bipy)]/Al₂O₃ interface. Bistability of spin states, which can be inferred from the thermal hysteresis, was directly observed using temperature-dependent x-ray diffraction; the crystallites behave as spin-state domains that coexist during the transition. The difference between the spin state of molecules at the surface of the [Fe{H₂B(pz)₂}₂(bipy)] films and that of the molecules within the films, during the thermal cycle, indicates that both cooperative (intermolecular) effects and coordination are implicated in perturbations to the SCO transition. (paper)