High‐quality monodisperse metallic cobalt nanorods are obtained by the reduction of carboxylate salts of CoII in 1,2‐butanediol using a rapid, simple, and solid‐template‐free procedure. In this polyol process, particle shape can be controlled via the growth rate, which depends on three parameters: i) the nature of the cobalt carboxylate, ii) the temperature ramp, and iii) the basicity of the medium. Cobalt in the hexagonally close‐packed phase favored the growth of anisotropic particles. Magnetic measurements of the cobalt nanorods indicate they are ferromagnetic at room temperature. They have a very high coercivity of 9.0 kOe at 140 K, much higher than that observed for wires prepared with solid templates. This can be attributed to their small mean diameter and high crystallinity.

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We present the fabrication of metallic magnetic nanowires using a low temperature chemical process. We show that pressed powders and magnetically oriented samples exhibit a very high coercivity (6.5 kOe at 140 K and 4.8 kOe at 300 K). We discuss the magnetic properties of these metamaterials and show that they have the suitable properties to realize "high temperature magnets" competitive with AlNiCo or SmCo permanent magnets. They could also be used as recording media for high density magnetic recording.Comment: 5 pages, 5 figure

Cobalt and cobalt−nickel nanoparticles were synthesized by reducing mixtures of cobalt and nickel acetates in sodium hydroxide solution in 1,2-propanediol. The particle shape depends strongly on the sodium hydroxide concentration and the Co/Ni composition of the particles. For cobalt-rich content, agglomerated rods, nanowires with a mean diameter of about 8 nm, and platelets were successively observed when [NaOH] was increased in the range 0−0.2 M. For the Co50Ni50 composition and [NaOH] in the range 0.1−0.18 M, nanodumbbells are formed that consist of a central column richer in cobalt capped with two terminal platelets richer in nickel. The shape of the dumbbells strongly depends on the basicity; long dumbbells are obtained for the lowest NaOH concentration, and short dumbbells and diabolos for the highest. To understand the role of the sodium hydroxide concentration and the different reactivities of cobalt and nickel, we analyzed the equilibrium between the Co2+ and Ni2+ ions in solution and the intermediate unreduced solid phase. For the Co80Ni20 composition, we show that increasing the sodium hydroxide amount lowers the Co2+ and Ni2+ ions in solution through the precipitation of the intermediate solid phase, suggesting that the nanowires are obtained with a higher growth rate than the platelets. The analysis of the solid intermediate phases revealed a Co(II) alkoxide and a Ni(II) hydroxy-acetate showing strong differences in the chemistry of the these two ions in basic solutions of 1,2-propanediol. These differences can explain the two well-separated growth steps originating the Co50Ni50 nanodumbbells.

CoNi magnetic nanowires with a mean diameter of 7 nm and a mean length in the range 100-300 nm have been prepared by reduction of a mixture of cobalt and nickel salts in a sodium hydroxide solution in a liquid polyol. The shape control is related to the particle growth rate which is strongly dependent on the basicity and on the Co-Ni composition. We show that high growth rate favours cobalt urchinlike particles while lower growth rate favours dumbbell-like bimetallic cobalt-nickel particles. Wire formation corresponds to an optimization of the growth conditions and is obtained in a narrow range of basicity and Co-Ni composition. Structural studies showed well crystallized wires with their long axis corresponding to the crystallographic c-axis of the hexagonal close-packed (hcp) phase. Multipods are also observed in which several wires have grown from a single nucleus presenting a cubic structure. Addition of surfactants (trioctyl phosphine and oleic acid) allows the modification of the wire surface and favours their dispersion in toluene. Square hysteresis curves are obtained on wires aligned under magnetic field and frozen in toluene or in PMMA with remanence to saturation ratio close to 1 and coercivity of 6.5 kOe. This value is much higher than was previously obtained on anisotropic particles prepared by the polyol process and results from an improvement of the shape control and the good crystallinity of the wires

Magnetic nanowires could be the building bricks in the fabrication of composite magnetic materials because of their large intrinsic shape anisotropy. We investigate the relation between the detailed shape of a magnetic nanowire and its magnetic coercivity. We have performed three-dimensional micromagnetic simulations on various types of nanowires synthesized during the past few years such as cylinders, dumbbells, or diabolos. The calculations, performed on individual model objects, show that the wire tip plays a key role in the reversal mechanism and on the magnitude of the coercive field and that the aspect ratio plays a much lesser role. Ellipsoidal or cylindrical shapes favor a coherent rotation of the magnetization and thus large coercive fields. Complex tip shapes act as nucleation points and significantly reduce the coercive field. Thus, in order to optimize the shape of magnetic nanowires for permanent magnet applications, the focus should be put on the detailed shape of the wire tips and thus on the growth mechanism rather than on the aspect ratio. The numerical modeling results on individual wires are compared with the experimental data obtained on various types of wires synthesized by soft chemistry methods.

International audienceWe present in this paper the structural and magnetic properties of high aspect ratio Co nanoparticles (~10) at high temperatures (up to 623 K) using in-situ X ray diffraction (XRD) and SQUID characterizations. We show that the anisotropic shapes, the structural and texture properties are preserved up to 500 K. The coercivity can be modelled by µ0HC = 2(KMC + Kshape)/MS with KMC the magnetocrystalline anisotropy constant, Kshape the shape anisotropy constant and MS the saturation magnetization. HC decreases linearly when the temperature is increased due to the loss of the Co magnetocrystalline anisotropy contribution. At 500K, 50% of the room temperature coercivity is preserved corresponding to the shape anisotropy contribution only. We show that the coercivity drop is reversible in the range 300 - 500 K in good agreement with the absence of particle alteration. Above 525 K, the magnetic properties are irreversibly altered either by sintering or by oxidation

Cobalt nanorods and wires were prepared by reduction of a cobalt salt in a liquid polyol. These particles crystallize with the hcp structure and the growth axis is parallel to the crystallographic c‐axis. The kinetic control of the growth allows to vary the mean diameter of the rods and their aspect ratio. Dumbbell like shape particles consisting of a central rod with two conical tips were also obtained. Magnetization curves of oriented wires present very high coercivity (up to 9 kOe) resulting from both a high shape anisotropy and the high magnetocrystalline anisotropy of the hcp cobalt. Micromagnetic simulations showed that the magnetization reversal is shape dependent. The conical tips of the dumbbell particles strongly contribute to the coercivity decrease and must be precluded for permanent magnet applications. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)