Reda Moubah

Photoluminescence Investigation of Defects and Optical Band Gap in Multiferroic BiFeO3 Single Crystals

Optical measurements have been carried out on high-quality BiFeO3 single crystals in order to show the presence of electronic defect states and calculate the band gap. The photoluminescence spectra show an intense electronic transition peak at a wavelength of 410 nm. This peak is large and asymmetric, indicating the existence of defects inside the gap, which we attribute to oxygen vacancies. These defects are likely to originate from the slow heating rate and long sintering times necessary to synthesize BiFeO3 single crystals. The optical band gap is measured to be 3 eV, a larger value than those previously reported.

Optical and electronic properties of one-dimensional Ca3Co2O6 thin films: Influence of the oxygen pressure

Thin films of Ca3Co2O6 were fabricated on c-cut sapphire substrates by pulsed laser deposition. The films are polycrystalline and can be grown in a large interval of oxygen pressure with significant influence on the film texture. X-ray photoelectron spectroscopy analyses indicated that only Co3+ ions are present in our samples for both octahedral and trigonal prismatic sites. The band gap measured by ultraviolet-visible light absorption experiments was found about 1.35 eV and slightly larger when the oxygen partial pressure during the deposition is increased from 10−3 to 7×10−2 mbar. Temperature dependent transport measurements showed a semiconducting behavior of the films and the associated band gap was found similar to the one evaluated by optical measurements and reported by theoretical studies.

Extrinsic ferromagnetism in epitaxial Co-doped CeO2 pulsed laser deposited films

We report on the structural and magnetic properties of 5 at. % Co-doped CeO2 films grown on LaAlO3(001) substrates by pulsed laser deposition. A series of epitaxially grown samples made under different oxygen partial pressures ranging from 10−4 to 10−1 mbar showed a ferromagnetic signal at room temperature. This signal is independent on the oxygen partial pressure during deposition. X-ray photoelectron spectroscopy showed an increasing concentration of defects as the oxygen pressure during deposition decreases. Although x-ray diffraction and transmission electron microscopy observations could not prove the existence of spurious phases, the temperature dependent variation in the magnetization suggests the existence of small magnetic Co clusters with a large distribution of blocking temperatures.

Effect of La doping on the properties of Sr2−xLaxFeMoO6 double perovskite

We report on the carrier doping effect (achieved through La doping) on the electronic structure, crystalline structure, grain morphology, and electric and magnetic properties of the ferromagnetic perovskite Sr2FeMoO6. X-ray diffraction on Sr2−xLaxFeMoO6 (x=0, 0.25, 0.5, and 1) ceramic powders shows that all compounds have the tetragonal crystal structure with I4∕mmm symmetry. The cell parameter a and the volume of the tetragonal structure increase with the La doping and the grain size is halved with respect to that of undoped Sr2FeMoO6. Mossbauer spectroscopy shows that the presence of La reduces the degree of ordering on the Fe and Mo sites which causes a reduction of the saturation magnetization MS. However, the addition of electrons in the system results in an increase of the Curie temperature of the Sr2FeMoO6. All these results are supported by ab initio calculations which indicate that the half-metallic character is preserved upon doping and that the magnetization decreases for perfect samples by 1μB...

Defects analysis at the nanometric scale in Ca3Co4O9 thin films

We report on the nature and origin of structural defects at a nanometric scale in incommensurate Ca3Co4O9 thin films deposited by pulsed laser ablation on Al2O3(001) substrates. X-ray diffraction suggests that the deposited films have a well defined texture and that are free of spurious phases. However, cross section scanning high resolution transmission electron microscopy observations show the presence of regions with different kinds of stacking. Such regions present different chemical compositions from that of Ca3Co4O9 and are not detectable in diffraction mode. The local chemical analysis and the interplane distance measurement suggest that these defects correspond to the formation of the CaCo2O4 spurious phase. This phase has a similar structure and close lattice parameters with those of Ca3Co4O9. The origin of the formation of CaCo2O4 is discussed in terms of (i) strains due to the substrate which tend to suppress the incommensurability of the system, and (ii) local chemical nonstoichiometry.

Tunable giant magnetic anisotropy in amorphous SmCo thin films

SmCo thin films have been grown by magnetron sputtering at room temperature with a composition of 2–35 at. % Sm. Films with 5 at. % or higher Sm are amorphous and smooth. A giant tunable uniaxial in-plane magnetic anisotropy is induced in the films which peaks in the composition range 11–22 at. % Sm. This cross-over behavior is not due to changes in the atomic moments but rather the local configuration changes. The excellent layer perfection combined with highly tunable magnetic properties make these films important for spintronics applications.

Effect of the nanometric scale thickness on the magnetization steps in Ca3Co2O6 thin films

We report on the effect of the film thickness on the magnetic properties of Ca₃Co₂O₆films with an emphasis on the magnetization steps usually observed in the M-H curves below 10 K. Films with thicknesses between 35 and 200 nm all present two magnetic transitions at about T(C₁) = 22 K and T(C₂) = 10 K, corresponding to a 3D long range ferrimagnetic order and the transition to the formation of a frozen spin state, respectively. The magnetization curves at 10 K exhibit the expected stepped variation. However, by decreasing the thickness below a critical value of about 60 nm, no magnetization plateau is observed when the M-H curve is recorded at 2 K. Moreover, an additional transition in the susceptibility curve is observed at 45 K. These changes can be attributed to the reduced coherence length of the propagation vector along and perpendicular to the chains, and are supported by the magnetization relaxation measurements which indicate a reduction of the relaxation time. These results are helpful for understanding the origin of the magnetization steps in the one-dimensional Ca₃Co₂O₆ cobaltite and confront the theoretical models aimed at explaining the magnetic properties in this system.

Epitaxial growth of one-dimensional Ca3Co2O6 thin films prepared by pulsed laser deposition

We report on the growth and structural properties of Ca3Co2O6 thin films deposited by pulsed laser ablation on SrTiO3 substrates heated at 700°C. In situ reflection high-energy electron diffraction and ex situ atomic force microscopy observations reveal that Ca3Co2O6 grows in a three-dimensional (3D) mode with a surface roughness of about 1.5nm rms. X-ray diffraction and cross-section transmission electron microscopy characterizations show that the deposited films are epitaxial without secondary phases and with a preferential growth orientation perpendicular to the (220) plane. Temperature dependent magnetization measurements reveal that the ferrimagnetic-ferromagnetic transition in the Ca3Co2O6 film is shifted toward higher temperatures with respect to the bulk cobaltite.

Magnetization plateaus in Ca3Co2O6 thin films

Continuous Ca3Co2O6 thin films were fabricated on Al2O3(001) substrate by pulsed laser deposition. X-ray diffraction and cross-section transmission electron microscopy show that the deposited films are polycrystalline without any secondary phases. The magnetic properties of the films are similar to those reported for bulk polycrystalline samples. At low temperatures, the films exhibit a magnetic transition at 20 K towards a 3D long range ferrimagnetic order. At 10 and 2 K, steps of magnetization have been observed and related to the ferrimagnetic–ferromagnetic transition and to magnetization quantum tunneling phenomena, respectively. This behavior is promoted by the frustrated antiferromagnetic interaction between the Ising-type Co chains of Ca3Co2O6 and by the spin configuration of the octahedral Co3+ ions. Such layers with magnetic properties similar to those of the bulk were obtained here for the first time. This result opens new perspective for the integration of these materials in thermoelectric or spintronic devices.

Direct imaging of both ferroelectric and antiferromagnetic domains in multiferroic BiFeO3 single crystal using x-ray photoemission electron microscopy

In this work, we propose to study the magnetic and ferroelectric configurations in ferroelectric multidomain BiFeO3 single crystals. Using x-ray (magnetic) linear dichroism in a photoemission electron microscope (X-PEEM), we are able to directly image both the antiferromagnetic and ferroelectric domains. We find that inside one single ferroelectric domain several antiferromagnetic domains coexist. This is different from what was observed on epitaxial thin films, where the ferroelectric domains perfectly match the antiferromagnetic ones, but also from previous neutron measurements on ferroelectric monodomain single-crystals for which one single antiferromagnetic domain was identified. This underlines the fundamental differences between thin films, bulk samples, and single versus ferroelectric multidomain samples.