M. Hervieu

Bio: M. Hervieu is an academic researcher from University of Caen Lower Normandy. The author has contributed to research in topics: Perovskite (structure) & Electron diffraction. The author has an hindex of 56, co-authored 478 publications receiving 13010 citations. Previous affiliations of M. Hervieu include AeA & University of Huddersfield.

Misfit-layered cobaltite with an anisotropic giant magnetoresistance: Ca 3 Co 4 O 9

Abstract: Combining electron diffraction, x-ray diffraction, and high-resolution electron microscopy techniques, a structural model for the cobaltite $``{\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}''$ has been found. This compound is a misfit-layered oxide consisting in two monoclinic subsystems with identical a, c, and \ensuremath{\beta} parameters, but different b parameters: $a=4.8376(7)\AA{},$ $c=10.833(1)\AA{},$ $\ensuremath{\beta}=98.06(1)\ifmmode^\circ\else\textdegree\fi{},$ ${b}_{1}=4.5565(6)\AA{},$ and ${b}_{2}=2.8189(4)\AA{}.$ The structure is built up from the stacking along c of triple rock salt-type layers ${\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}$ (first subsystem) with single ${\mathrm{CdI}}_{2}$-type ${\mathrm{CoO}}_{2}$ layers (second subsystem). Two different sets of Co-O distances are involved which are interpreted as the existence of cobalt with three different oxidation states 2+, 3+, and 4+, in agreement with x-ray appearance near-edge structure spectra at the Co K edge. At about 420 K, both resistivity and susceptibility show an anomaly which results from a spin-state transition of cobalt at this temperature. Below 300 K, the resistivity measured along the ${\mathrm{CoO}}_{2}$ layers shows a metal-insulator transition as T decreases, whereas the much larger out-of-plane resistivity values show the anisotropic behavior of this phase. The application of a magnetic field induces a negative magnetoresistance which reaches -35% for 7 T. Moreover, thermoelectric power measurements yield a high positive value of \ensuremath{\approx}125 \ensuremath{\mu}V ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ at 300 K with a weak temperature dependence in between 100 and 300 K. This result contrasts with the metallic in-plane resistivity.

Magnetic phase diagrams of L 1 − x A x MnO 3 manganites ( L = P r , S m ; A = C a , S r )

Abstract: The magnetic phase diagrams of four ${L}_{1\ensuremath{-}x}{A}_{x}{\mathrm{MnO}}_{3}$ series $(L=\mathrm{P}\mathrm{r},\mathrm{S}\mathrm{m};$ $A=\mathrm{C}\mathrm{a},\mathrm{S}\mathrm{r})$ have been established combining neutron diffraction, electron microscopy, and magnetotransport measurements in order to understand and optimize the colossal magnetoresistance (CMR) properties of these compounds. A complementary study of the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ phase diagram $(xg~0.8)$ is also performed. The comparison of these diagrams demonstrates that low $〈{r}_{A}〉$ values are required to obtain CMR properties on the ${\mathrm{Mn}}^{4+}$ rich side, i.e., when a competition between charge order and cluster glass phases occurs ${(L}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3};$ $L=\mathrm{P}\mathrm{r},\mathrm{S}\mathrm{m}).$ As a consequence of the low $〈{r}_{A}〉$ values on the rich ${\mathrm{Mn}}^{3+}$ side $(xl0.5$ in ${L}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}),$ the ferromagnetic metallic (FMM) state is never reached, whereas for the same x values, systems with larger $〈{r}_{A}〉$ ${(L}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3};$ $L=\mathrm{P}\mathrm{r},\mathrm{S}\mathrm{m})$ exhibit CMR properties related to the FMM state. A particular attention is paid to the $x\ensuremath{\sim}0.5$ compositions of these ${L}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ series for which the Curie and N\'eel temperatures are ${T}_{N}l{T}_{C}$ and ${T}_{C}l{T}_{N}$ for Pr and Sm, respectively. The importance of the chemical factors on the charge ordering (studied vs temperature by electron diffraction) and on the magnetic structures (from neutron diffraction) is also emphasized.

Track formation in SiO2 quartz and the thermal-spike mechanism.

Abstract: [alpha]-quartz has been irradiated with heavy ions: [sup 19]F, [sup 32]S, and [sup 63]Cu at an energy of about 1 MeV/amu in order to cover a range of electronic stopping powers [ital dE]/[ital dx] between 2.4 and 9 keV/nm and [sup 58]Ni, [sup 86]Kr, [sup 128]Te, [sup 129]Xe, [sup 181]Ta, and [sup 208]Pb between 1 and 5.8 MeV/amu for [ital dE]/[ital dx][gt]7 keV/nm. The extent of the induced damage is determined using Rutherford backscattering ion channeling with a 2-MeV [sup 4]He beam. The damage cross section [ital A] is obtained using a Poisson law [ital F][sub [ital d]]=1[minus]exp([minus][ital A][phi][ital t]), where [phi] is the flux and [ital t] the irradiation time. This damage cross section is linked to the effective radius [ital R][sub [ital e]] through the relation [ital A]=[pi][ital R][sub [ital e]][sup 2], where [ital R][sub [ital e]] is the radius of an equivalent cylinder of damage. Using high-resolution electron microscopy, cylinders of amorphous matter have been observed, whose radius corresponds to [ital R][sub [ital e]] when the track is continuous (i.e., for [ital A][ge]1.3[times]10[sup [minus]13] cm[sup 2]; [ital R][sub [ital e]][ge]2 nm). A thermal-spike model is applied to calculate the radii of the observed tracks assuming that themore » observed amorphous cylinders correspond to a rapid quench of a molten liquid phase along the ion path. The model is applied only when the latent track is continuous and cylindrical. A good agreement is obtained taking into account that the initial spatial energy deposition on the electrons depends on the ion velocity.« less

Ultrasharp magnetization steps in perovskite manganites.

Abstract: We report a detailed study of steplike metamagnetic transitions in polycrystalline Pr0.5Ca0.5Mn0.95Co0.05O3. The steps have a sudden onset below a critical temperature, are extremely sharp (width <2x10(-4) T), and occur at critical fields which are linearly dependent on the absolute value of the cooling field in which the sample is prepared. Similar transitions are also observed at low temperature in non-Co doped manganites, including single crystal samples. These data show that the steps are an intrinsic property, qualitatively different from either previously observed higher temperature metamagnetic transitions in the manganites or metamagnetic transitions observed in other materials.

Perspective on the Development of Lead-free Piezoceramics

Abstract: A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Emergent phenomena at oxide interfaces

Abstract: Recent technical advances in the atomic-scale synthesis of oxide heterostructures have provided a fertile new ground for creating novel states at their interfaces. Different symmetry constraints can be used to design structures exhibiting phenomena not found in the bulk constituents. A characteristic feature is the reconstruction of the charge, spin and orbital states at interfaces on the nanometre scale. Examples such as interface superconductivity, magneto-electric coupling, and the quantum Hall effect in oxide heterostructures are representative of the scientific and technological opportunities in this rapidly emerging field.

Complexity in Strongly Correlated Electronic Systems

Abstract: A wide variety of experimental results and theoretical investigations in recent years have convincingly demonstrated that several transition metal oxides and other materials have dominant states that are not spatially homogeneous. This occurs in cases in which several physical interactions-spin, charge, lattice, and/or orbital-are simultaneously active. This phenomenon causes interesting effects, such as colossal magnetoresistance, and it also appears crucial to understand the high-temperature superconductors. The spontaneous emergence of electronic nanometer-scale structures in transition metal oxides, and the existence of many competing states, are properties often associated with complex matter where nonlinearities dominate, such as soft materials and biological systems. This electronic complexity could have potential consequences for applications of correlated electronic materials, because not only charge (semiconducting electronic), or charge and spin (spintronics) are of relevance, but in addition the lattice and orbital degrees of freedom are active, leading to giant responses to small perturbations. Moreover, several metallic and insulating phases compete, increasing the potential for novel behavior.

REVIEW ARTICLE: Colossal magnetoresistance

Abstract: We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.