Giant Barkhausen jumps in exchange biased bulk nanocomposites sinterd fom core-shell Fe3O4-CoO nanoparticles

International audienceThe magnetic behavior of spark plasma sintered Fe3O4-CoO nanoparticles is studied. The samples sintered at 500°C exhibit density over 90% and average magnetite grain size about 100 nm. When the nanocomposite is field cooled below the Néel temperature (TN=291 K for CoO), hysteresis loops shows the expected shift with an exchange field of 80 mT at 100 K that drops down to zero approaching TN. The coercivity at 100 K reaches 0.4 T, ten times larger than nanostructured magnetite prepared in the same conditions. When the sample is zero field cooled down to 90 K, the hysteresis loops exhibits giant Barkhausen jumps, an anomalous feature never observed before to our knowledge. The density of jumps gradually decrease on heating and disappear between 150 and 170 K. The stochastic character of the jumps is visible in the plot of the differential permeability. This new phenomenon is thought that it could be related to self-field cooling

Spin-glass state in Ni 5 (OH) 6 (C n H 2n−4 O 4 ) 2 metal-organic frameworks ( n = 6, 8, 10, 12)

We present an extension of a previously published work (J. Solid State Chem. 181 (2008) 3229) concerning Metal-Organic Frameworks (MOFs) of general formula Ni5(OH)6(CnH2n-4O4)2. A modified synthesis procedure comprising a room-temperature step prior to the hydrothermal treatment was employed. This preliminary step made use of peristaltic pumps allowing a slow mixing of the reactants at a constant pH value. Samples of better purity and crystallinity were consequently obtained. In particular, the better crystallinity allowed us to work on two other members of the series, n = 10 and n = 12, which were characterized using synchrotron powder X-ray powder diffraction. These two compounds are isoreticular with the n = 6 and n = 8 compounds. The crystal structure incorporates the long alkane dioic acid molecules as pillars between complex inorganic layers. Samples of better purity for n = 6 and 8, as well as those of the new compounds with n = 10 and 12, gave us the opportunity to revise the magnetic behavior of these MOFs. We found similar magnetic behaviors, independently of the interlayer spacing. We show that, below 19 K, these materials most probably enter a spin-glass or cluster spin-glass state rather than a three-dimensionally long-range ordered state. We link this behavior to the complex topology of the magnetic exchange interactions within the inorganic layers which is very likely to be the source of magnetic frustration

On the limits of Reactive-Spark-Plasma Sintering to prepare magnetically enhanced nanostructured ceramics: the case of the CoFe2O4-NiO system

International audienceMagnetic materials are crucial for the efficiency of the conversion-storage-transport-reconversion energy chain, and the enhancement of their performance has an important impact on technological development. The present work explores the possibility of preparing hetero-nano-structured ceramics based on magnetic oxides, by coupling a ferrimagnetic phase (F) with an antiferromagnetic one (AF) on the nanometric scale. The field-assisted sintering technique or SPS (Spark-Plasma Sintering), adopted at this purpose, ensures the preservation of nano-sized crystals within the final solid structure. The aim is to establish how exchange bias may affect the resulting nano-consolidates and to investigate the potential of this process to increase the total magnetic anisotropy of the CoFe 2 O 4 grains, and thus their coercive field, while keeping the saturation magnetization the same. The structure, microstructure and magnetic properties of the ceramics obtained were studied by several techniques. The results show that the sintering process, along with its typical reductive atmosphere, modifies the composition of the constituents. A new metallic phase appears as a consequence of the reciprocal diffusion of Co and Ni cations, leading to a change in the amount and structure of the AF phase. We propose a schematic representation of the atomic movements that hinder an exchange bias effect between the F and AF phases

Co₄(OH)₂(C₁₀H₁₆O₄)₃ Metal–Organic Framework: Slow Magnetic Relaxation in the Ordered Phase of Magnetic Chains

Reported here are the synthesis and structural and topological analysis as well as a magnetic investigation of the new Co₄(OH)₂(C₁₀H₁₆O₄)₃ metal–organic framework. The structural analysis reveals a one-dimensional inorganic subnetwork based on complex chains of cobalt­(II) ions in two different oxygen environments. Long alkane dioic acid molecules bridge these inorganic chains together to afford large distances and poor magnetic media between dense spin chains. The thermal dependence of the χ<i>T</i> product provides evidence for uncompensated antiferromagnetic interactions within the cobaltous chains. In zero-field, dynamic magnetic susceptibility measurements show slow magnetic relaxation below 5.4 K while both neutron diffraction and heat capacity measurements give evidence of long-range order (LRO) below this temperature. The slow dynamics may originate from the motion of broad domain walls and is characterized by an Arrhenius law with a single energy barrier Δ_τ/k_B = 67(1) K for the [10–5000 Hz] frequency range. Moreover, in nonzero dc fields the ac susceptibility signal splits into a low-temperature frequency-dependent peak and a high-temperature frequency-independent peak which strongly shifts to higher temperature upon increasing the bias dc field. Heat capacity measurements have been carried out for various applied field values, and the recorded <i>C</i>_P(<i>T</i>) data are used for the calculation of the thermal variations of both the adiabatic temperature change Δ<i>T</i>_ad and magnetic entropy change Δ<i>S</i>_m. The deduced data show a modest magnetocaloric effect at low temperature. Its maximum moves up to higher temperature upon increasing the field variation, in relation with the field-sensibility of the intrachain magnetic correlation length

Exchange-Biased Fe 3− x O 4 -CoO Granular Composites of Different Morphologies Prepared by Seed-Mediated Growth in Polyol: From Core-Shell to Multicore Embedded Structures

International audienceMagnetically contrasted granular hetero‐nanostructures are prepared by seed‐mediated growth in polyol, properly combining two oxide phases with different magnetic order, ferrimagnetic (F) partially oxidized magnetite Fe3−xO4 and antiferromagnetic (AF) cobalt oxide. Spinel Fe3−xO4 nanoparticles are first synthesized and then used as seeds for rock salt CoO nanocrystals growth. Three different hetero‐nanostructure designs are realized, acting on the content ratio between the seeds and the deposit's precursors during the synthesis. For all of them, the spinel and the rock salt phases are confirmed by X‐ray diffraction and high‐resolution transmission electron microscopy. Both phases are obtained in high‐crystalline quality with a net epitaxial relationship between the two crystallographic lattices. Mössbauer spectrometry confirms the cobalt cation diffusion into the spinel seeds, giving favorable chemical interfacing with the rock salt deposit, thus prevailing its heterogeneous nucleation and consequently offering the best condition for exchange‐bias (EB) onset. Magnetic measurements confirm EB features. The overall magnetic properties are found to be a complex interplay between dipolar interactions, exchange anisotropy at the F/AF interface, and magnetocrystalline anisotropy enhancement in the F phase, due to Co2+ diffusion into iron oxide's crystalline lattice. These results underline the powerfulness of colloidal chemistry for functional granular hetero‐nanostructured material processing

Experimental and theoretical evidence for oriented aggregate crystal growth of CoO in a polyol

International audienceMonodispersed about 5 nm sized CoO crystals were prepared by forced hydrolysis of cobalt(II) acetate in diethyleneglycol (DEG) solvent. The adsorption of the solvent molecules on these primary nanocrystals caused their in-situ oriented aggregation resulting in the precipitation of textured submicrometer-sized polycrystals. X-ray diffraction, Infrared spectroscopy, Transmission Electron Microscopy and Thermogravimetry analyses coupled to ab-initio modeling were applied to understand the adsorption mechanism of the alcohol moieties and the role of the molecule-to-surface and molecule-to-molecule interactions in the crystal growth mechanism of these polycrystals. We showed that DEG moieties are mainly adsorbed at the top of the cobalt (100) surface atoms and the process does not involve the whole molecule but only one of its extreme oxygen atoms. As a consequence, adsorbed DEG molecules exhibit an extended configuration which is favorable to intermolecule hydrogen bonding from one covered nanocrystal to another. Interestingly, at high surface coverage, the energy required for DEG attachment to the crystal surface is found to be 18.6 kJ/mol per molecule, while that required for hydrogen bonding between a bearing molecule and a neighbor one is found to be 36,4 kJ/mol per molecule, meaning that the collective departure of an assembly of DEG from the surface of CoO nanocrystals is therodynamically easier, leading thus to the observed final morphology

Giant Exchange-Bias in Polyol-Made CoFe 2 O 4 -CoO Core-Shell Like Nanoparticles

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Combining Soft Chemistry and Spark Plasma Sintering to Produce Highly Dense and Finely Grained Soft Ferrimagnetic Y 3 Fe 5 O 12 (YIG) Ceramics

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Combining Soft Chemistry and Spark Plasma Sintering to Produce Highly Dense and Finely Grained Soft Ferrimagnetic Y 3 Fe 5 O 12 (YIG) Ceramics

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Effect of High Pressure Spark Plasma Sintering on the Densification of a Nb-Doped TiO2 Nanopowder

International audienceSintering under pressure by means of the spark plasma sintering (SPS) technique is a common route to reduce the sintering temperature and to achieve ceramics with a fine-grained microstructure. In this work, high-density bulk TiO 2 was sintered by high pressure SPS. It is shown that by applying high pressure during the SPS process (76 to 400 MPa), densification and phase transition start at lower temperature and are accelerated. Thus, it is possible to dissociate the two densification steps (anatase then rutile) and the transition phase during the sintering cycle. Regardless of the applied pressure, grain growth occurs during the final stage of the sintering process. However, twinning of the grains induced by the phase transition is enhanced under high pressure resulting in a reduction in the crystallite size