M.-J. Casanove

Chemisorption on nickel nanoparticles of various shapes: Influence on magnetism

Nanoparticles of nickel have been obtained in solution through reaction of an organometallic precursor, Ni(COD)2 (COD=cycloocta-1,5-diene), with dihydrogen, in the presence of various stabilizing agents. X-ray diffraction studies evidence a face-centered-cubic structure for all samples. Spherical isolated superparamagnetic nanoparticles (d∼4.5 nm) are produced in the presence of poly(N-vinylpyrrolid-2-one). They display a magnetization value comparable to that of bulk nickel, as determined from superconducting quantum iinterference device (SQUID) measurements. Exposure of the surface of the nanoparticles to CO, leading to CO coordination as monitored by infrared spectroscopy, or to methanol, strongly reduces their magnetization. This reduction corresponds respectively to one or two magnetically inactive layers of nickel atoms at the nanoparticles surface. The production of elongated nanoparticles was favored when either trioctylphosphineoxide or hexadecylamine (HDA) were used as stabilizing agents. In this case, SQUID measurements show that only HDA preserves the magnetization of the surface. This stabilizing agent was selected for the production of nickel nanorods. The influence of ligands on the effective anisotropy is discussed.Nanoparticles of nickel have been obtained in solution through reaction of an organometallic precursor, Ni(COD)2 (COD=cycloocta-1,5-diene), with dihydrogen, in the presence of various stabilizing agents. X-ray diffraction studies evidence a face-centered-cubic structure for all samples. Spherical isolated superparamagnetic nanoparticles (d∼4.5 nm) are produced in the presence of poly(N-vinylpyrrolid-2-one). They display a magnetization value comparable to that of bulk nickel, as determined from superconducting quantum iinterference device (SQUID) measurements. Exposure of the surface of the nanoparticles to CO, leading to CO coordination as monitored by infrared spectroscopy, or to methanol, strongly reduces their magnetization. This reduction corresponds respectively to one or two magnetically inactive layers of nickel atoms at the nanoparticles surface. The production of elongated nanoparticles was favored when either trioctylphosphineoxide or hexadecylamine (HDA) were used as stabilizing agents. In thi...

Structural, magnetic, transport, and magneto-optical properties of single crystal La2/3Sr1/3MnO3 thin films

We have grown La0.7Sr0.3MnO3 thin films by reactive facing target radio frequency sputtering on SrTiO3 (001) substrates with the nominal stoichiometry and the perovskite structure down to nanometer-scale film thickness. The films are found to be perfectly expitaxied on SrTiO3, without misfit dislocations. The surfaces of the layers were found to be highly flat with terraces of ≈4 A height corresponding to the manganite unit cell. Magnetic measurements evidence a decrease of the Curie temperature for decreasing thickness. Low temperature magnetoresistance is very small, confirming the absence of grain boundaries in the films but it reaches ≈−50% at room temperature. Finally, magneto-optical Kerr effect measurements between 20 and 400 K reveal the onset of ferromagnetic transition via the coercive field increase and the Kerr rotation and ellipticity measurements.

Magnetoresistive oxides: new developments and applications

Abstract We review progress on development of applications of manganese perovskites and double perovskites based on polycrystalline films. We will show that grain boundaries and interfaces, either in polycrystalline materials or epitaxial films are far from being chemically or electronically homogeneous and this may represent a serious handicap for spin polarized tunneling transport across interfaces. We will report on successful strategies of further raising T C .

Growth and stress relaxation of Co/NiO bilayers on MgO(001)

Co/NiO bilayers have been grown on a MgO(001) surface in an ultrahigh vacuum sputtering chamber under various deposition conditions. Investigations by x-ray diffraction, atomic force microscopy, and high resolution transmission electron microscopy give evidence for a close relationship between structure and deposition conditions for the two different layers. The role of the deposition rate on the oxide layer surface roughness is particularly emphasized. At high deposition rates, surface roughness increases with temperature, whereas the surface remains flat for low deposition rates. Whatever the growth conditions, the NiO layer grows epitaxially on the MgO substrate with the orientation relationship NiO(001)[100]//MgO(001)[100]. The misfit strain (about 1%), elastically accommodated in the thinner layers (3 nm), is fully relaxed in 50-nm-thick layers. A temperature dependence of the cobalt layer structure is observed: at room temperature it grows in its high temperature face-centered cubic structure, where...

Structural and magnetic properties of FeFe2N sputtered multilayers

Abstract We have prepared iron-iron nitride multilayers by sequential reactive-nonreactive sputtering. The structure of the FeFe 2 N interfaces has been studied and found to be correlated with the magnetic properties of such samples. They are found to remain sharp enough for direct observation of multilayers either by transmission electron microscopy or by small angle X-ray scattering despite a significant shift of the Fe(Fe,N) interfaces towards the iron layers as a result of nitrogen diffusion.

Magnetic properties of CoNRhM nanoparticles: experiment and theory

The magnetism of Co–Rh nanoparticles is investigated experimentally and theoretically. The particles (≈2 nm) have been synthesized by decomposition of organometallic precursors in mild conditions of pressure and temperature, under hydrogen atmosphere and in the presence of a polymer matrix. The magnetic properties are determined by SQUID, Mossbauer spectroscopy, and X-ray magnetic circular dichroism (XMCD). The structural and chemical properties are characterized by wide angle X-ray scattering, transmission electronic microscopy and X-ray absorption near edge spectroscopy. All the studied Co–Rh clusters are magnetic with an average spin moment per atom µ that is larger than the one of macroscopic crystals or alloys with similar concentrations. The experimental results and comparison with theory suggest that the most likely chemical arrangement is a Rh core, with a Co-rich outer shell showing significant Co–Rh mixing at the interface. Measured and calculated magnetic anisotropy energies (MAEs) are found to be higher than in pure Co clusters. Moreover, one observes that the MAEs can be tuned to some extent by varying the Rh concentration. These trends are well accounted for by theory, which in addition reveals important spin and orbital moments induced at the Rh atoms as well as significant orbital moments at the Co atoms. These play a central role in the interpretation of experimental data as a function of Co–Rh content. A more detailed analysis from a local perspective shows that the orbital and spin moments at the Co–Rh interface are largely responsible for the enhancement of the magnetic moments and magnetic anisotropy.

Structural and magnetic study of bimetallic Co1−xRhx particles

Abstract We studied the structural and magnetic properties of cobalt-rhodium nanoparticles dispersed in a polymer matrix. It is shown that the structure depends on the composition, high cobalt content leading to a non-periodic structure. Magnetic measurements confirm the dispersion, particle size, and bimetallic character of the particles. Evidence is given of enhanced magnetisation for the composition x =0.5.

X-ray magnetic circular dichroism and element-selective magnetic hysteresis in Fe/Cu/Co/Cu multilayers

Abstract A Fe/Cu/Co/Cu multilayer exhibiting an hysteresis loop with two characteristic coercive fields which can be assigned separately to the two magnetic 3d metals has been studied. XMCD measurements are able to follow the field dependent element-specific magnetisation. The photon helicity was reversed using a diamond quarter-wave plate in Laue geometry. XMCD and macroscopic magnetisation measurements are in agreement and show that in this multilayer system the Co layer reverses magnetisation direction at a lower field than the Fe layer.

Sputter growth and magnetic properties of exchange-biased La1/4Ca3/4MnO3–La2/3Sr1/3MnO3 epitaxial bilayers

We have grown La1/4Ca3/4MnO3–La2/3Sr1/3MnO3 (LCMO–LSMO) thin films by reactive facing target radio frequency sputtering on SrTiO3(001) substrates. After 1 bar oxygen annealing at 800 °C, the films have the desired oxygen stoichiometry and perovskite structure. Transport properties of single LCMO films reveal the charge order (CO) transition between 100 and 180 K depending on the LCMO layer thickness while LSMO films have a Curie temperature greater than 330 K. Kerr effect measurements show evidence of an increase of the LSMO coercive field HC from ∼50 Oe at room temperature up to 110 Oe at 40 K and a significant decrease in the squareness of the hysteresis loop. This effect is accompanied by a slight shift of the hysteresis loop HE due to exchange bias between the antiferromagnetic charge ordered LCMO layer and the LSMO ferromagnet which reaches 20 Oe at 40 K. Both the onset of HE and the drop in squareness are observed to be close to the charge order transition of a similar LCMO single layer and are assu...

Synthesis of Cobalt Nanoparticles, Nanorods and Nanowires assisted by Oleic Acid and Oleylamine based Mixtures.

In this paper we report on the synthesis in solution of cobalt nanoparticles, nanorods and nanowires achieved through the decomposition of an organometallic precursor Co(η 3 -C 8 H 13 )(η 4 -C 8 H 12 ) and stabilized by carboxylic acid / amine based mixtures. Through decomposition of the precursor in the presence of a 1/1 mixture of oleylamine / oleic acid during 3 hours, the formation of 3 nm spherical nanoparticles is observed while hcp-cobalt nanorods are obtained if the reaction is pursued for 48 hours. Increasing the amount of oleic acid to 2 equivalents with respect to the amine results in the formation of hcp-cobalt nanowires, micrometers in length and 6 nm mean diameter‥ By using amines with various chain lengths we have observed a modification of the aspect ratio of the cobalt nanorods 1 . For example, the decomposition of Co(η 3 -C 8 H 13 )(η 4 -C 8 H 12 ) in the presence of octadecyl-, hexadecyl-, dodecyl- and octylamine in association with oleic acid allows to selectively produce nanorods of varying aspect ratio. Physical characterization using TEM and HRTEM, shows that the nanoparticles, nanorods and nanowires are all hcp single crystals. The magnetic properties reveal a saturation magnetization similar to that of bulk and a ferromagnetic behavior at room temperature for the nanorods and nanowires while the 3 nm nanoparticles remain super-paramagnetic.