Search results for "Curie"
showing 10 items of 110 documents
Crystal chemical and dielectric properties of Pb0.5(5-x)LaxNb5-xFexO15system with “tetragonal tungsten bronze” structural type
2005
The lanthanum-lead and iron-niobium coupled substitution in PbNb 2 O 6 have allowed to isolate new materials of a general formula Pb 0.5(5-x) La x Nb 5-x Fe x O 15 crystallising with the tetragonal tungsten bronze structural type. The resulting influence on the lattice parameters, the ferroelectric Curie temperature and Cole - Cole plots are presented. A typical ferroelectric - paraelectric behaviour occurs in the composition range 0 ≤ x ≤ 0.33 while, a relaxor behaviour is observed in the composition range 0.33 < x < 1.
Dielectric Permittivity and Conductivity of Li0.07Na0.93Ta0.1Nb0.9O3and Li0.07Na0.93Ta0.111Nb0.889O3Solid Solutions
2011
Results of studies of dielectric properties and conductivity in 290–700 K range of temperature and frequencies from 25 to 106 Hz in ferroelectric perovskite solid solutions Li0.07Na0.93Ta0.1Nb0.9 O3 and Li0.07Na0.93Ta0.111Nb0.889 O3 are reported. A first-order ferroelectric phase transition proceeds in both studied compositions within the observed range of temperature. The co-precipitated Ta2yNb2(1-y)O5 pentoxides being used for synthesis of Li0.07Na0.93Ta0.111Nb0.889O3 the Curie point shifts by ∼75 K to a lower temperature while compared with Li0.07Na0.93Ta0.1Nb0.9O3 synthesised from mechanical mixture of the Ta2O5 and Nb2O5 oxides the high-frequency dielectric permittivity and ion conduct…
Crystal structure transformations induced by surface stresses in BaTiO3and BaTiO3@SiO2nanoparticles and ceramics
2015
International audience; Lattice structure transformations in nanopowders of ferroelectric BaTiO3 and BaTiO3@SiO2 core-shell nanostructured ceramics were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) at the temperatures 120–450 K and particle size of 300 and 500 nm. NMR spectra of all studied samples in the paraelectric phase are identical to the spectra in bulk material indicating their perfect perovskite structure without visible influence of particle surface. However, we have found that surface of particles essentially influence the ferroelectric phase transitions detected by both NMR and EPR techniques. The strongest changes as compared to bulk mat…
Cyano-bridged perovskite [(CH3)3NOH]2[KM(CN)6],[M: Fe(III), Co(III)] for high-temperature multi-axial ferroelectric applications with enhanced therma…
2020
Highly stable ferroelectrics with reversible high-temperature phase transitions and switchable nonlinear optical behaviour are much coveted targets for emerging optoelectronic applications. Here, we demonstrate a cyano-bridged perovskite [(CH3)3NOH]2[KCo(CN)6] (TMAO-Co), a new analogue of the multi-axial ferroelectric [(CH3)3NOH]2[KFe(CN)6] (TMAO-Fe) with improved thermal stability and enhanced second-order nonlinear optical response. Indeed, for TMAO-Co the Curie temperature (Tc) is shifted to a higher value of ca. 416 K (improvement by ca. 10 K versusTMAO-Fe); the separation between Tc and the decomposition threshold is 46 K. TMAO-Co is a biaxial ferroelectric as revealed by P(E) hysteres…
Ten millennia of hepatitis B virus evolution
2021
Hepatitis B virus (HBV) has been infecting humans for millennia and remains a global health problem, but its past diversity and dispersal routes are largely unknown. We generated HBV genomic data from 137 Eurasians and Native Americans dated between ~10,500 and ~400 years ago. We date the most recent common ancestor of all HBV lineages to between ~20,000 and 12,000 years ago, with the virus present in European and South American hunter-gatherers during the early Holocene. After the European Neolithic transition, Mesolithic HBV strains were replaced by a lineage likely disseminated by early farmers that prevailed throughout western Eurasia for ~4000 years, declining around the end of the 2nd…
Comparison of first-principles methods to extract magnetic parameters in ultra-thin films: Co/Pt(111)
2019
We compare three distinct computational approaches based on first-principles calculations within density functional theory to explore the magnetic exchange and the Dzyaloshinskii-Moriya interactions (DMI) of a Co monolayer on Pt(111), namely, (i) the method of infinitesimal rotations of magnetic moments based on the Korringa-Kohn-Rostoker (KKR) Green function method, (ii) the generalized Bloch theorem applied to spiraling magnetic structures and (iii) supercell calculations with noncollinear magnetic moments, the latter two being based on the full-potential linearized augmented plane wave (FLAPW) method. In particular, we show that the magnetic interaction parameters entering micromagnetic …
Superparamagnetism in Ising Clusters
1992
Recent experiments on small ferromagnetic clusters have inspired introduction of a number of seemingly quite different theoretical models. We shall argue that all these models show superparamagnetic behaviour above the blocking temperature but below the Curie temperature. In particular, we shall show that Ising clusters display superparamagnetism and introduce a simple correction to the usual tank behaviour of magnetisation which has to be included for very small clusters. We also discuss the dependence of magnetisation on coordination number.
Itinerant half-metallic ferromagnetsCo2TiZ(Z=Si, Ge, Sn):Ab initiocalculations and measurement of the electronic structure and transport properties
2010
This work reports on ab initio calculations and experiments on the half-metallic ferromagnetic Heusler compounds ${\text{Co}}_{2}\text{Ti}Z$ $(Z=\text{Si},\text{ }\text{Ge},\text{ }\text{Sn})$. Aim is a comprehensive study of the electronic-structure and thermoelectric properties. The impact of the variation in the main group element $Z$ on those properties is discussed. X-ray diffraction was performed on the compounds and the lattice parameters are compared to other ${\text{Co}}_{2}$-based compounds. Hard x-ray photoemission measurements were carried out and the results are compared to the calculated electronic structure. The experimentally determined electronic structure, magnetic propert…
Understanding the Giant Enhancement of Exchange Interaction in Bi2Se3−EuS Heterostructures
2017
A recent experiment indicated that a ferromagnetic EuS film in contact with a topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ might show a largely enhanced Curie temperature and perpendicular magnetic anisotropy [F. Katmis et al., Nature (London) 533, 513 (2016).]. Through systematic density functional calculations, we demonstrate that in addition to the factor that ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ has a strong spin orbit coupling, the topological surface states are crucial to make these unusual behaviors robust as they hybridize with EuS states and extend rather far into the magnetic layers. The magnetic moments of Eu atoms are nevertheless not much enhanced, unlike what was…
Ferromagnetism in small clusters.
1991
Magnetization of small ferromagnetic clusters at finite temperatures has been studied using the Ising model and Monte Carlo techniques. The magnetization of finite clusters is reduced from the bulk value, and increases with the external magnetic field and with the cluster size. The results explain qualitatively the recent observations by de Heer, Milani, and Chatelain of the reduction with decreasing cluster size of the average magnetic moment in small iron clusters.