135 research outputs found
Lattice deformations at martensite-martensite interfaces in Ni-Al
The atomic configurations at macrotwin interfaces between microtwinned martensite plates in material are investigated using high resolution transmission electron microscopy (HRTEM). The observed structures are interpreted in view of possible formation mechanisms of these interfaces. A distinction is made between cases in which the microtwins, originating from mutually perpendicular \{110\} austenite planes, enclose a final angle larger or smaller than , measured over the boundary. Two different configurations, one with crossing microtwins and the other with ending microtwins producing a step configuration are described. The latter is related with the existence of microtwin sequences with changing variant widths. Although both features appear irrespective of the material’s preparation technique, rapid solidification seems to prefer the step configuration. Depending on the actual case, tapering, bending and tip splitting of the small microtwin variants is observed. Sever lattice deformations and reorientations occur in a region of 5 – 10 nm around the interface while sequences of single plane ledges gradually bending the microtwins are found up to 50 nm away form the interface. These structures and deformations are interpreted in view of the need to accommodate any remaining stresses
Superconductivity in a new hexagonal high entropy alloy
High entropy alloys (HEAs) are the new class of materials with an attractive
combination of tunable mechanical and physicochemical properties. They
crystallize mainly in cubic structures, however, for practical applications,
HEAs with hexagonal close-packed (hcp) structure are highly desirable in
connection to their in general high hardness. Herein, we report the synthesis,
structure and detailed superconducting properties of
ReNbTiZrHf-the first hexagonal
superconducting high entropy alloy (HEA) composed of five randomly distributed
transition-metals. Combination of room temperature precession electron
diffraction, precession electron diffraction tomography and powder X-ray
diffraction is utilized to determine the room temperature crystal structure.
Transport, magnetic and heat capacity measurements show that the material is a
type-II superconductor with the bulk superconducting transition at =
4.4 K, lower critical field (0) = 2.3 mT and upper critical field
(0) = 3.6 T. Low-temperature specific heat measurement indicates that
ReNbTiZrHf is a phonon-mediated
superconductor in the weak electron-phonon coupling limit with a normalized
specific heat jump = 1.32. Further,
hexagonal to cubic structural transition is observed by lowering the valence
electron counts and follows crystalline-like behaviour.Comment: 6 Pages, 9 Figures. arXiv admin note: text overlap with
arXiv:1804.1009
Transport and magnetic properties in YBaCo2O5.45: Focus on the high-temperature transition
The electronic transport properties and the magnetic susceptibility were
measured in detail in . Close to the so-called metal-insulator
transition, strong effects of resistance relaxation, a clear thermal hysteresis
and a sudden increase of the resistance noise are observed. This is likely due
to the first order character of the transition and to the underlying phases
coexistence. Despite these out of equilibrium features, a positive and linear
magneto-resistance is also observed, possibly linked to the heterogeneity of
the state. From a magnetic point of view, the paramagnetic to ordered magnetic
state transition is observed using non linear susceptibilty. This transition
shows the characteristics of a continuous transition, and time dependent
effects can be linked with the dynamics of magnetic domains in presence of
disorder. Thus, when focusing on the order of the transitions, the electronic
one and the magnetic one can not be directly associated.Comment: accepted for publication in PR
Structure determination of a brownmillerite Ca2Co2O5 thin film by Precession Electron Diffraction
Calcium cobaltite thin films with a ratio Ca/Co=1 were grown on (101)-NdGaO3
substrate by the pulsed laser deposition technique. The structure of the
deposited metastable phase is solved using a precession electron diffraction 3D
dataset recorded from a cross-sectional sample. It is shown that an ordered
oxygen-deficient Ca2Co2O5+d perovskite of the brownmillerite-type with lattice
parameters a= 0.546nm, b=1.488nm and c=0.546nm (SG: Ibm2) has been stabilized
using the substrate induced strain. The structure and microstructure of this
metastable cobaltite is further discussed and compared to related bulk
materials based on our transmission electron microscopy investigationsComment: 13 pages, 10 figures, 2 tables, accepted in Phys. Rev.
Tunable magnetic interaction at the atomic scale in oxide heterostructures
We report on a systematic study of a number of structurally identical but
chemically distinct transition metal oxides in order to determine how the
material-specific properties such as the composition and the strain affect the
properties at the interface of heterostructures. Our study considers a series
of structures containing two layers of ferromagnetic SrRuO3, with
antiferromagnetic insulating manganites sandwiched in between. The results
demonstrate how to control the strength and relative orientation of interfacial
ferromagnetism in correlated electron materials by means of valence state
variation and substrate-induced strain, respectively
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 10K 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.Comment: accepted in APL 201
Absence of long-range Ni/Mn ordering in ferromagnetic La2NiMnO6 thin films
Epitaxial La2NiMnO6 thin films have been grown on (001)-oriented SrTiO3 using
the PLD technique. The thin films are semiconducting and FM with a TC close to
270K, a coercive field of 920Oe, and a saturation magnetization of 5muB per
f.u. TEM, conducted at RT, reveals a majority phase having "I-centered"
structure with a=c=1.4asub and b=2asub along with a minority phase-domains
having "P-type" structure (asub being the lattice parameter of the perovskite
structure). A discusion on the presence of Ni/Mn long-range ordering, in light
of recent literature on double perovskites La2NiMnO6 is presented.Comment: To be published in Applied Physics Letter
Caractérisations structurales, phases modulées et transitions de phases: le cas des phases d'Aurivillius
Bien que mon activité de recherche a été principalement axée sur la caractérisation structurale des phases d'Aurivillius en relation avec leurs propriétés ferroélectriques, je me suis aussi intéréssé à divers aspects liés à la synthèse de ces phases que ce soit sous forme de monocristaux, de poudres ou encore de films (voie chimique). Une première section consacrée à la synthèse ouvrira donc cette deuxième partie. Dès mon arrivée au SPCTS, une part importante de mon activité a été consacrée à l'étude des intercroissances à longues périodes qui ont pu être observées dans divers systèmes mais dont les caractéristiques structurales restaient inconnues. Sur la base de nos observations expérimentales, un modèle cristallographique généralisé utilisant le formalisme des groupes de super-espace a été développé pour les stuctures de type Aurivillius. Ce travail est issu d'une collaboration étroite avec l'équipe du Prof. J.M. Perez-Mato de l'Université de Bilbao. Cette nouvelle approche cristallographique et son utilisation constitueront la deuxième section. L'étude des relations structure/propriétés dans ces composés a constitué une autre part importante de mes recherches au SPCTS avec, notamment, le co-encadrement de la thèse de Jenny Tellier. Les divers aspects de la transition ferroélectrique-paraélectrique (FE-PE) rencontrés dans les composés de type Aurivillius ont été abordés que ce soit dans le cas des termes simples, des composés d'intercroissances que dans le cas, moins classique, des composés présentant un comportement ressemblant à celui de "relaxeurs" tel que Pb(Mg1/3Nb2/3)O3. Ce travail visant à décrire les mécanismes structuraux associés à la transition FE-PE fera l'objet de la troisième et dernière section
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