Ph. Boullay

Absence of long-range Ni/Mn ordering in ferromagnetic La2NiMnO6 thin films

Epitaxial La2NiMnO6 thin films have been grown on (001)-orientated SrTiO3 using the pulsed laser deposition technique. The thin film samples are semiconducting and ferromagnetic with a Curie temperature close to 270K, a coercive field of 920Oe, and a saturation magnetization of 4.85μB/formula unit. Transmission electron microscopy, conducted at room temperature, reveals a majority phase having an “I-centered” structure with a≈c≈asub2 and b≈2asub along with minority phase domains having a “P-type” structure (asub being the lattice parameter of the cubic perovskite structure). A discussion on the absence of Ni∕Mn long-range ordering, in light of recent literature on the ordered double-perovskite La2NiMnO6 is presented.

Incipient ferroelectricity in 2.3% tensile-strained CaMnO3 films

Epitaxial CaMnO3 films grown with 2.3% tensile strain on (001)-oriented LaAlO3 substrates are found to be incipiently ferroelectric below 25 K. Optical second harmonic generation (SHG) was used for the detection of the incipient polarization. The SHG analysis reveals that CaMnO3 crystallites with in-plane orientation of the orthorhombic b axis contribute to an electric polarization oriented along the orthorhombic a (respectively c) axis in agreement with the predictions from density functional calculations.

Superspace description of the hexagonal perovskites in the system Ba5Nb4O15-BaTiO3 as modulated layered structures

Abstract A general structural model of B -site deficient hexagonal layer perovskites AB 1− x X 3 having no tilting of anion octahedra in consecutive layers is presented. They are described as modulated layered structures using a (3+1)-dimensional superspace approach. The presence or not of a given atom along the stacking direction is described in superspace through step-like (crenel) occupational modulation functions. Being essentially composition independent, the model is applicable to any compounds related to this structural type. The component γ of the primary modulation vector q =γ c * is connected to the chemical composition by the relation γ =(1+ x )/3. The model is confirmed and the efficiency of the superspace approach is demonstrated by the structural analysis of some of the hexagonal perovskite found in the system Ba 5 Nb 4 O 15 –BaTiO 3 , where long-period intergrowth compounds have been recently identified.

The Role of Trilinear Couplings in the Phase Transitions of Aurivillius Compounds

Some Aurivillius compounds exhibit features beyond the single soft-mode picture valid for other ferroelectrics. In SrBi2Ta2O9 (SBT) two primary distortions are needed to reach the ferroelectric phase, causing the presence of an intermediate phase. However, no intermediate phase has been detected in SrBi2Nb2O9 (SBN), and an avalanche transition seems to happen, with the two order parameters condensing simultaneously. According to ab-initio calculations, the usually assumed triggering mechanism through biquadratic coupling of the two order parameters can be discarded, and we have looked for other possible causes of these avalanche transitions. Using a simple microscopic Hamiltonian (an extended model), mean-field techniques (independent-site approximation) and Monte Carlo simulations, we show that a strong trilinear coupling between the two order parameters and an additional secondary mode could explain the existence of a single first-order transition with the simultaneous condensation of two order parameters.

HRTEM study of cation-deficient perovskite-related AnBn-δO3n (n ≥ 4δ) microphases in the Ba5Nb4O15-BaTiO3 system

Abstract The occurrence of coherent intergrowths of cation-deficient perovskites in the Ba 5 Nb 4 O 15 –BaTiO 3 system has been examined by high-resolution transmission electron microscopy and selected area electron diffraction. Because of their structural similarity, the simple members Ba 5 Nb 4 O 15 ( n =5) and Ba 6 TiNb 4 O 18 ( n =6) form coherent intergrowths—noted 5 P 6 1 —by the juxtaposition along the c -axis of P perovskite-like blocks n =5 and one perovskite-like block n =6, with P =1, 2 and 3. More generally, the ability to form intergrowths in the hexagonal perovskite systems is discussed considering the structural characteristics of the simple members. Examples taken from various systems show that the formation of such intergrowths is highly dependent on the size of the A cation present in simple members.

Constrained ferroelectric domain orientation in (BiFeO3)m(SrTiO3)n superlattice

Ferroelectric domains were investigated using piezoresponse force microscopy in superlattices composed of multiferroic BiFeO3 and SrTiO3 layers. Compared to single BiFeO3 thin films, a reduction in the domains size and a suppression of the in-plane orientation of domains are observed in a superlattice of (BiFeO3)4(SrTiO3)8, suggesting a constrained ferroelectric domain orientation along the out-of-plane ⟨001⟩ direction. Such modification of domain size and orientation in BiFeO3-based heterostructures could play a vital role on engineering the domains and domain wall mediated functional properties necessary for device applications.

Microstructure of epitaxial strained BiCrO3 thin films

The structure and microstructure of fully strained BiCrO3 thin films have been investigated by x-rays diffraction and transmission electron diffraction, at room temperature. Interestingly, three structural variants are simultaneously stabilized within the film. While two of them are consistent with the existing phases in the bulk-below and above the 420 K structural transition, a different phase is identified. The existence of various structures has been attributed to the inhomogeneous distribution of local strains and oxygens resulting from a minimization of the strain-energy at the interface. These findings will open the route to a better understanding of Bi-based perovskites and metastable phases.

Structural transition in LaVO3/SrVO3 superlattices and its influence on transport properties

Measurements of the resistive properties and the lattice parameters of a (LaVO3)[6 unit cells]/(SrVO3)[1 unit cell] superlattice between 10 K and room temperature are presented. A low temperature metallic phase compatible with a Fermi liquid behavior is evidenced. It disappears in the vicinity of a structural transition from a monoclinic to tetragonal phase, in which disorder seems to strongly influence the transport. Our results will enrich the understanding of the electronic properties of complex heterostructures.

The high-temperature and intermediate phases of ferroelectric Aurivillius compounds in superspace. A unified approach

The recently proposed superspace description of the Aurivillius compounds in their ferroelectric phase is here extended to their high-temperature phases. The probable existence of an intermediate phase between the tetragonal high-symmetry and the ferroelectric phases is stressed. The relevant superspace groups for both the tetragonal and the intermediate phases are identified. Consequently, the 3D space groups for the intermediate phases as a function of composition are predicted.