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Grenoble, France

Lejay P.,CNRS Neel Institute | Canevet E.,Joseph Fourier University | Canevet E.,Institute LaueLangevin | Srivastava S.K.,CNRS Neel Institute | And 3 more authors.
Journal of Crystal Growth | Year: 2011

We report, for the first time, the crystal growth of SrCo2V 2O8 and BaCo2V2O8 compounds using floating zone technique in a mirror furnace under air. We have grown the single crystals of both compounds, with typical dimensions of 3 mm diameter and 5 cm long, which exhibit phase decomposition at about 820 °C. The magnetic susceptibility of both SrCo2V2O8 and BaCo2V2O8 compounds has similar temperature dependences and exhibit AFM ordering with a Neél temperature of 5 and 6 K, respectively. © 2011 Elsevier B.V. All rights reserved.

Mishra D.,Ruhr University Bochum | Petracic O.,Ruhr University Bochum | Petracic O.,Julich Research Center | Devishvili A.,Ruhr University Bochum | And 6 more authors.
Journal of Physics Condensed Matter | Year: 2015

We prepared monolayers of iron oxide nanoparticles via self-assembly on a bare silicon wafer and on a vanadium film sputter deposited onto a plane sapphire substrate. The magnetic configuration of nanoparticles in such a dense assembly was investigated by polarized neutron reflectivity. A theoretical model fit shows that the magnetic moments of nanoparticles form quasi domain-like configurations at remanence. This is attributed to the dipolar coupling amongst the nanoparticles. © 2015 IOP Publishing Ltd.

Alonso J.A.,CSIC - Institute of Materials Science | Martinez-Lope M.J.,CSIC - Institute of Materials Science | De La Calle C.,CSIC - Institute of Materials Science | SAnchez-BenItez J.,CSIC - Institute of Materials Science | And 4 more authors.
Functional Materials Letters | Year: 2011

Many transition-metal oxides in elevated valence states [e.g. Mn(V), Co(IV), Ni(III), Cu(III)] present a metastable character and, given the difficulty of their synthesis, have been relatively little studied. However, they are very interesting materials presenting strong electronic correlations that are bound to exotic properties such as superconductivity, metal behavior, metalinsulator transitions or colossal magnetoresistance. The metastability of these compounds requires special synthesis conditions such as the application of high pressure. In the last years, we have prepared and investigated a good number of materials belonging to several families such as RNiO 3 (R = rare earths), Ba 3Mn 2O 8, (Ba,Sr)CoO 3, La 2(Ni,Co)O 4+δ, etc. In the study and correct characterization of these oxides it has been decisive the use of elastic neutron diffraction, most of the times in powder samples. This technique has allowed us to access the structural details typically related to the octahedral tilting in perovskite structures, the oxygen stoichiometry and orderdisorder of the oxygen sublattice, the distinction between close elements in the Periodic Table, the resolution of magnetic structures and, in general, the establishment of a correlation between the structure and the properties of interest. This letter is organized around the binomial "high-pressure synthesis" and "characterization by neutron diffraction" and illustrated with some selected examples among the metastable materials above mentioned. © 2011 World Scientific Publishing Company.

Michor H.,Vienna University of Technology | Adroja D.T.,Rutherford Appleton Laboratory | Hillier A.D.,Rutherford Appleton Laboratory | Koza M.M.,Institute LaueLangevin | And 4 more authors.
Journal of Physics: Conference Series | Year: 2012

The heavy fermion system CeNi 9Ge 4 exhibits a paramagnetic ground state with remarkable features such as: a record value of the electronic specific heat coefficient in systems with a paramagnetic ground state, γ C/T ≃ 5.5 J/molK 2 at 80 mK, a temperature-dependent Sommerfeld-Wilson ratio, R = χ/γ, below 1 K and an approximate single ion scaling of the 4f-magnetic specific heat and susceptibility. These features are related to a rather small Kondo energy scale of a few Kelvin in combination with a quasi-quartet crystal field ground state. Tuning the system towards long range magnetic order is accomplished by replacing a few at.% of Ni by Cu or Co. Specific heat, susceptibility and resistivity studies reveal T N ∼ 0.2 K for CeNi 8CuGe 4 and T N ∼ 1K for CeNi 8CoGe 4. To gain insight whether the transition from the paramagnetic NFL state to the magnetically ordered ground state is connected with a heavy fermion quantum critical point we performed specific heat and ac susceptibility studies and utilized the μSR technique and quasi-elastic neutron scattering.

Mishra D.,Ruhr University Bochum | Benitez M.J.,Ruhr University Bochum | Benitez M.J.,Max-Planck-Institut fur Kohlenforschung | Petracic O.,Ruhr University Bochum | And 14 more authors.
Nanotechnology | Year: 2012

We have investigated the structure and magnetism of self-assembled, 20nm diameter iron oxide nanoparticles covered by an oleic acid shell for scrutinizing their structural and magnetic correlations. The nanoparticles were spin-coated on an Si substrate as a single monolayer and as a stack of 5ML forming a multilayer. X-ray scattering (reflectivity and grazing incidence small-angle scattering) confirms high in-plane hexagonal correlation and a good layering property of the nanoparticles. Using polarized neutron reflectivity we have also determined the long range magnetic correlations parallel and perpendicular to the layers in addition to the structural ones. In a field of 5kOe we determine a magnetization value of about 80% of the saturation value. At remanence the global magnetization is close to zero. However, polarized neutron reflectivity reveals the existence of regions in which magnetic moments of nanoparticles are well aligned, while losing order over longer distances. These findings confirm that in the nanoparticle assembly the magnetic dipoledipole interaction is rather strong, dominating the collective magnetic properties at room temperature. © 2012 IOP Publishing Ltd.

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