Bénédicte Vertruyen

Magnetotransport properties of a single grain boundary in a bulk La–Ca–Mn–O material

Besides the “intrinsic” colossal magnetoresistance effect observed in single crystals, the polycrystalline manganate compounds also exhibit an “extrinsic” magnetoresistance related to the presence of grain boundaries. We report electrical transport and magnetic measurements carried out on a bigrain sample extracted from a floating zone method-grown rod of calcium doped lanthanum manganate. Electrical resistance was measured both within a grain and across the grain boundary, between 20 and 300 K and from 0 to 8 T. Magnetoresistance values up to 99% are reached within the grain. The temperature dependence of the resistance across the grain boundary displays a “foot-like” feature towards the bottom of the transition. Low field and high field magnetoresistance effects are examined. We compare our results for a “bulk” grain boundary to those obtained by other authors for bicrystal thin films and bulk polycrystalline materials.

ac magnetic behavior of large-grain magnetoresistive La0.78Ca0.22Mn0.90Ox materials

We report a detailed set of ac magnetic measurements carried out on bulk large grain La-Ca-Mn-O samples extracted from a floating zone method-grown rod. Three samples with

Synthesis of CMR manganate compounds: the consequences of the choice of a precursor method

{\mathrm{La}}_{0.78}{\mathrm{Ca}}_{0.22}{\mathrm{Mn}}_{0.90}{\mathrm{O}}_{x}

La0.7Ca0.3MnO3 / Mn3O4 composites: Does an insulating secondary phase always enhance the low field magnetoresistance of manganites?

stoichiometry but differing in their microstructure were investigated by electrical resistivity and ac susceptibility measurements: (i) a single grain sample, (ii) a sample containing two grains, and (iii) a polycrystalline sample. We show that the superimposition of dc magnetic fields during ac magnetic susceptibility measurements is an efficient way for characterizing the magnetic transition of samples with different microstructures. Whereas both single grain and polycrystalline samples display a single susceptibility peak, an additional kink structure is observed in the case of the double grain sample. The temperature dependence of the ac susceptibility measured with superimposed dc magnetic fields is analyzed in the framework of second-order phase transition ideas. The relations between the critical exponents

Magnetic properties of Ga-doped lanthanum manganite with controlled oxygen stoichometry

(\ensuremath{\beta}+\ensuremath{\gamma}\ensuremath{\sim}1.5,

Unusual resistivity hysteresis in a bulk magnetoresistive ferromagnetic/ferrimagnetic composite (La0.7Ca0.3MnO3∕Mn3O4): Role of demagnetization effects

\ensuremath{\delta}\ensuremath{\sim}2.5)

Magnetic properties of nanosized MgFe2O4 powders prepared by auto-combustion

are found to be close to those of the mean-field model for all samples. This is attributed to the disordering caused by unoccupied Mn sites.

Hydrogel nanocomposites: a potential UV/blue light filtering material for ophthalmic lenses

We have studied the electrical transport properties of manganate samples synthesized by different precursor methods. Differences in the synthesis process of the materials can result in samples with similar grain size but very different physical properties, contrary to what happens in samples prepared by the same method.

Chemical interactions between Bi2Sr3CaO7(Bi-2310) and Bi2Sr2Ca0.8Dy0.2Cu2O8 (Bi-2212(Dy))

Composites of magnetoresistive La0.7Ca0.3MnO3 (LCMO) with insulating Mn3O4 are useful as a model system because no foreign cation is introduced in the LCMO phase by interdiffusion during the heat treatment. Here we report the magnetotransport properties as a function of sintering temperature Tsinter for a fixed LCMO/Mn3O4 ratio. Decreasing Tsinter from 1250 °C to 800 °C causes an increase in low field magnetoresistance (LFMR) that correlates with the decrease in crystallite size (CS) of the LCMO phase. When plotting LFMR at (77 K, 0.5 T) versus 1/CS, we find that the data for the LCMO/Mn3O4 composites sintered between 800 °C and 1250 °C follow the same trend line as data from the literature for pure LCMO samples with crystallite size >∼25 nm. This differs from the LFMR enhancement observed by many authors in the “usual” manganite composites, i.e., composites where the insulating phase contains cations other than La, Ca or Mn. This difference suggests that diffusion of foreign cations into the grain bounda...