François Roulland

Microwave properties and microstructures of LaTiO3 stabilized with NiO

Abstract (1– x )La 2/3 TiO 3 – x TiNiO 3 ceramics with x ranging from 0.01 to 0.2, were elaborated by conventional solid-state reaction starting from TiO 2 , La 2 O 3 , and NiO powders. These compositions have been identified as secondary phases detected in (Zr,Sn)TiO 4 ceramics sintered with La 2 O 3 and NiO. Microwave resonators were sintered at temperatures ranging from 1340 to 1380°C. These sintering temperatures have been determined taking into account thermodilatometric data. Microwave dielectric properties and microstructures were investigated. Perovskite phase La 2 3 TiO 3 was identified by XRD, SEM and EDS together with minor phases. Dielectric constants are in the 50–70 range and QF values, measured at 3 GHz, are close to 20 000 GHz. One significant composition is characterized by k =69 and QF =17,000 GHz at 3 GHz.

Raman scattering of magnetoelectric gallium ferrite thin films

Gallium ferrite, Ga(2-x)Fe(x)O(3) (GFO), is a promising magnetoelectric material as it exhibits both magnetic and electric orders close to room temperature. Here, we report a temperature-dependent investigation of GFO thin films with x = 1.0 and 1.4 by using Raman scattering. Our investigation suggests the absence of a structural phase transition of both films in the investigated 90-500 K temperature range, which is similar to earlier observations on bulk samples. We note, however, the occurrence of weak anomalies in the temperature-dependent band position of some phonons, which we attribute to spin-phonon coupling as the anomalies occur close to the Néel temperature of the materials.

Optical Transitions in Magnetoelectric Ga0.6Fe1.4O3 from 0.73 to 6.45 eV

The optical properties of polycrystalline Ga0.6Fe1.4O3 bulk are determined by spectroscopic ellipsometry from 0.73 to 6.45 eV. Complex dielectric function ɛ = ɛ1 + iɛ2 spectra are obtained from the multilayer analysis. The ellipsometric data exhibit numerous optical structures, and the transition energies are accurately obtained by analyzing the second-energy derivatives of the data. The origins of the optical structures are explained in terms of Fe3+ ligand field transitions and ligand-to-metal charge transfer transitions.

Tuning the conductivity type in a room temperature magnetic oxide: Ni-doped Ga0.6Fe1.4O3 thin films

Ni-Doped thin films of the room temperature ferrimagnetic oxide Ga0.6Fe1.4O3 were deposited by pulsed laser deposition and their electronic transport and structural and magnetic properties were studied. The actual insertion of the Ni cations within the Ga0.6Fe1.4O3 structure has been checked by resonant X-ray scattering. A clear extremum is noticed for all properties for the 2% Ni doping: extrema in the crystallographic cell parameters of the films, maximum in the Curie temperature, and maximum in the electric resistivity. We also observed a change of conductivity type for this dopant concentration, from n-type for Ni contents below 2% to p-type for Ni contents above 2%. We explain this behavior by the existence of oxygen vacancies in the pulsed laser deposited Ga0.6Fe1.4O3 thin films, which results in the reduction of some of the Fe3+ into Fe2+ cations, and n-type conduction via a hopping mechanism. The insertion of Ni2+ cations first deals with the presence of oxygen vacancies and reduces the number of n-type carriers in the films, in a compensation-like mechanism. When the number of introduced Ni2+ cations dominates the number of oxygen vacancies, conductivity becomes p-type and starts to increase again. We believe that the tunability of the conduction type and magnitude in thin films of a room temperature ferrimagnetic material paves the way towards new all oxide electronic devices.

Incorporation of cobalt ions into magnetoelectric gallium ferrite epitaxial films: tuning of conductivity and magnetization

Thin films of Ga0.6Fe1.4O3 show ferrimagnetism with a transition temperature at around 360 K but suffer from large charge conduction. Substituting Fe2+ with non-magnetic Mg2+ ions reduces the charge conduction but also lowers the magnetic transition temperature. Doping Ga0.6Fe1.4O3 thin films with magnetic Co2+ ions leads to a similar reduction in the charge conduction, which is significant by two orders of magnitude, and, on the other hand, does not lead to any modification of the ferrimagnetic transition. The remnant magnetization of the leakage currents free Co-doped Ga0.6Fe1.4O3 thin films is of 53 emu cm−3 at 300 K. These films, therefore, are promising materials with potential uses in magnetoelectric and multiferroic devices.

Effects of iron concentration and cationic site disorder on the optical properties of magnetoelectric gallium ferrite thin films

Room-temperature dielectric function e = e1 + ie2 spectra of magnetoelectric Ga2−xFexO3 (x = 0.9, 1.0, and 1.4) thin films are determined by spectroscopic ellipsometry (SE) as a function of Fe concentration x. The SE data are analysed by a multilayer model with a series of Tauc-Lorentz oscillators. While the threshold energies slightly decrease as x increases, the oscillator strength shows a strong composition-dependence for the major optical structure at ∼3.5 eV. The experimental data are compared to the e spectra obtained by density functional theory (DFT) calculations. Even though the overall shape of e spectra is consistent, the experimental data and calculated spectra show a clear discrepancy in the oscillators strength ratio of the two optical structures at ∼3.5 and ∼6.0 eV. The DFT calculations suggest that a significant disordering in the cationic (Ga and Fe) sites in Ga2−xFexO3 is present in thin films, which influences their optical properties. This work demonstrates a successful application of optical characterization for determining the cationic sites occupation in thin films, which in turn improves our understanding of Physics and Chemistry in Ga2−xFexO3 thin films and paves a pathway to the development of new multifunctional devices.

Determination of the cationic distribution in oxidic thin films by resonant X-ray diffraction: the magnetoelectric compound Ga2−xFexO3

The cationic distribution is decisive for both the magnetic and electric properties of complex oxides. While it can be easily determined in bulk materials using classical methods such as X-ray or neutron diffraction, difficulties arise for thin films owing to the relatively small amount of material to probe. It is shown here that a full determination of the cationic site distribution in thin films is possible through an optimized processing of resonant elastic X-ray scattering experiments. The method is illustrated using gallium ferrite Ga2−xFexO3 samples which have been the focus of an increasing number of studies this past decade. They indeed represent an alternative to the, to date, only room-temperature magnetoelectric compound BiFeO3. The methodology can be applied to determine the element distribution over the various crystallographic sites in any crystallized system.