Role of Morphology on the Large Coercive Behavior in Co80Ni20 Nanowires

A. Gaul1, N. Ouar1, S. Mercone*1, F. Zighem1, F. Schoenstein1, B. Leridon2, S. Farhat1 and N. Jouini1 1LSPM-UPR3 3407, CNRS, Université Paris 13, Sorbonne Paris Cité, 99 Avenue J.-B. Clément, 93430 Villetaneuse, France 2LPEM, ESPCI-ParisTech, CNRS, UPMC, 10 rue Vauquelin, F-75231 Paris Cedex 5, France

*Corresponding author: silvana.mercone@univ-paris13.fr

ABSTRACT Ferromagnetic metal CoNi-based nano-objects have been synthesized in a polyol media within different elaboration conditions in order to drive their morphology (i.e. enhancing their length-to-diameter ratio d / L( ) , and changing the diameter d ratio over edge T width d / L( ) ). Transmission Electron Microscopy (TEM) studies revealed unexpected effects on the Co80Ni20 nano-objects arising from the magnetic field assisted synthesis. This gave us the opportunity to compare this latter to coming from the variation of Ruthenium (III) chloride hydrate nucleating agent concentration. A Co80Ni20 anisotropic particles elaboration was successfully achieved under zero magnetic field assisted synthesis, while an important percentage of isotropic nanoparticles appeared immediately under the application of a small magnetic field (i.e. H > 500 Oe). In the first case we were able to sharply drive both the aspect ratio and head morphology of nanowires (T and d /T( ) ). The good crystallinity and structures symmetry of all our samples have been proved by X-Ray Diffraction (XRD) pattern analysis. Magnetic static properties showed a ferromagnetic standard behavior with a coercive field efficiency which was strongly dependent on shape parameters. The magnetic static behavior was studied within a standard Stoner-Wohlfart model as a function of the observed morphologies. Our observations are fully consistent with a shape anisotropy origin behavior of the enhanced coercivity measured as function of the decreasing d / L( ) ratio. However, they revealed the presence of contributions to the global effective anisotropy coming from other complex terms then the shape one (i.e. conic head impressiveness, dipolar interactions and magnetocrystalline anisotropy).

INTRODUCTION In past decades much attention has been focused on the synthesis of magnetic nanowires due to their potential application in the field of permanent magnets1, sensors2, high-density magnetic recording3 and spintronic devices like magnetic memory4 or microwave circuits5. Some of those applications are the natural exploitation of intrinsic properties of the anisotropic (1D) nano-objects, such as their large magnetic shape anisotropy which generates a large coercive field. Two main effects have proven (mainly by micromagnetic simulations) to drastically change this intrinsic property: on the one hand the exchange bias effect6 rising from air-contact surface oxidations of the nanowires and on the other hand inhomogeneity of demagnetizing field at the nanowires edges coming from different head-morphologies6. In this work we focus on the pure morphology effect on the magnetic coercivity properties of Co80Ni20 nanowires. Through a change in synthesis conditions (nucleating agent and magnetic fields strength applied during synthesis), we succeeded in achieving a good and sharp control of the nanowires shapes. Over a large number of nano-objects, we found that the so elaborated nanowires present an average

length inside the range L= [65 nm-145 nm] and an average diameter (in the middle of the nano-object) of d = (8.0±1.5) nm. A conic head diameter (named T) was compared to the diameter d of the nanowires. The d /T( ) ratio vary from 0.5 (so-called diabolo-like nanowires) to 0.9 (quite cylinder-like nanowires). Magnetic measurements revealed an unexpected decreasing of coercivity with increasing d /T( ) ratio (limit value being ∼ 7000 Oe to take into account for applications) and gave insights on the magnetization reversal mechanisms (see inset of Fig.1). EXPERIMENTAL DETAILS

The Co80Ni20 nano-objects were synthesized by polyol process via the reduction of cobalt and nickel acetates induced by heterogeneous nucleation according to the method previously7 illustrated. For nanowires synthesis, a reference volume of 250 ml of 1,2 butanediol was used to successively dissolve sodium hydroxide, cobalt and nickel acetate tetrahydrate and Ruthenium (III) Chloride hydrate and then heated at 180°C for 30 min under mechanical stirring. The obtained precipitates were centrifuged, washed in alcohol and dried several times in order to minimize the presence of organic residues for XRD and TEM studies. Proportions of RuCl3in grams were properly changed in order to allow the variation of the molar concentrations of the nucleating agent [Ru] / [CoNi] = (1.8·10-2 / 4.8·10-2) M. For the 2.4·10-2 and 4.8·10-2

concentration of RuCl3 agent, a magnetic field assisted synthesis was performed using a home-made equipment. This latter consists of a close reactor of 50ml (no mechanical mixing) whose core was inserted right in the middle of the air gap of a standard electromagnet. The magnetic field applicable by this apparatus can be changed continuously from 0 Oe to 11200 Oe. Appropriate tests have shown high magnetic field uniformity in the volume of the chemical reactions. Several techniques have been used in order to study the structural and magnetic properties of our samples. XRD patterns were recorded at room temperature using an INEL (equinox 1000) diffractometer (θ-2θ)) with a cobalt anticathode (λ= 1.7809 Å). The structural analysis (crystalline phases) were realized by using MAUD software applying already reported methods8. Nano-objects morphologies and sizes (i.e. length L, diameter in the center d, width at the edge of the nanowire T -conic head- and fraction of isotropic nanoparticles compared to the anisotropic one) have been studied by Transmission Electron Microscopy (TEM - JEOL 2011). The magnetic properties were measured for different temperatures using a Magnetic Property Measurement System (MPMS 3 Quantum Design) capable of applying a magnetic field from -70 kOe to +70 kOe. In order to get rid of the oxidation process, unavoidable for metallic dried powders, we performed magnetic measurements on one-dimensional system being kept inside their polyol synthesis solution. Also, to keep nano-objects inside the polyol solution prevents them to massively aggregate and allows reducing dipolar interaction strength (estimated dilution is 10-4/cm3). The organic solution being non-magnetic and having a glass transition around 220K, gave us the opportunity of performing magnetization curves from 200 K down to 20 K. We performed two different kinds of measurements: zero field cooled blocked nano-objects (ZFC) and field cooled blocked once (FC). In the first case we cooled down the NWs inside their organic liquid solution from 300K down to 200K without any external magnetic field. After waiting for at least one hour to be sure of the liquid-to-solid transition, we performed standard magnetic cycles. In the second case we used the same procedure, but the system has been cooled down under 70 kOe. We can assume that the ZFC procedure leaves the NWs in a completely misaligned configuration, while the FC procedure tends to align them along the field.

DISCUSSION Sharp control of nanowires morphology and shape

Figure 1. Typical XRD pattern observed for our Co80Ni20 nanowires. Typical standard deviation from the model was found to be Rw (%) = 6.0516; (top-right inset) Schematic view of anisotropic nanoparticles.

The phase and purity of the obtained products were determined from the powder XRD pattern. All the picks could be indexed considering a poly-phasic system containing both hexagonal close-packed (hcp) phases and cubic (Fm-3m) one. The hcp phase is attributed to the hcp variety of cobalt. The X-ray diffraction (XRD) data also indicated a minor phase of cubic CoO (∼ 20%) originating from the oxidation of the nano-objects surfaces. The presence of the thin CoO layer at the surface of the nanowires is a well-known phenomenon9. It is probably due to air contact of the dried nano-powders pushed by a small remainder of some organic matter in the powders9-10. Previous studies have shown that this layer does not exceed 15 wt% of the sample9 which is also confirmed by the present studied nano-powder. TEM images of all our samples lead to a statistical analysis of the three characteristics lengths for one-dimensional nano-objects: the length (L) of the nanowires, the diameter in the center of the nanowire (d) and the width of the head at the edge of the nanowires (T), also called the conic head because of its shape. Our statistics were all realized over more than 200 nanowires. Figure 2 presents TEM images of synthesized samples for 2.4·10-2 and 4.8·10-2 concentrations of RuCl3 agent under different strength of the magnetic field conditions applied during the elaboration process.

[Ru]

/[Co

Ni]

(M)

H=0Oe H=500Oe H=5000Oe H= 11200Oe

2.4*

10-2

scale 50nm

scale 100nm

scale 50nm

scale 50nm

4.8*

10-2

scale 50nm

scale 50nm

scale 100nm

scale 100nm

Figure2. TEM images of Co80Ni20 nanowires synthesized at 2.4·10-2 (first line) and 4.8·10-2

(second line) molar concentration of RuCl3 nucleating agent respectively for different magnetic field applied during the synthesis. Figure 2 clearly shows unexpected result coming from the magnetic field assisted synthesis. On the basis of the magnetization ease of one-dimensional nanowires to align along the magnetic easy axis (length of wires, L), one should expect that using an external magnetic field during the nano-objects growth should enhance their length-to-diameter ratio d / L( ) .  However, in our specific elaboration conditions the surprising results show that the increase of the external magnetic field does not provoke a decrease of the percentage of isotropic nanoparticles (50% over 200 nano-objects statistics). Moreover the scattered values of nanowires length L does not seem to improve as clearly as expected. Different factors can influence the nucleation and growth steps of nano-objects, such as mechanical solution mixing and the nucleation agent rate. A high rate of this latter facilitates the nucleation step, while the mechanical mixing can facilitate the particle growth. The influence of all those factors in the presence of an external magnetic field is currently under study. Thereagainst, in Figure 3 we present the good control of morphology obtained by different concentration of the nucleating agent used in this study. In this latter the shape parameters are reported versus elaboration condition to show the sharp successful control achieved. For obvious reasons, all the systems presenting isotropic nanoparticles mixed to anisotropic ones were not taken into account.

Figure3. Diameter in the center of nanowires over Length d / L( ) and diameter over the conic head width d / L( ) versus the nucleating agent concentration for Co80Ni20 nanowires Previous works6,11 have already shown how the hybrid shape of cobalt based nanoparticles can be drastically changed/ modified by changing the polyol chemical and kinetic conditions of

elaboration. Moreover few studies12 show how the presence of a magnetic field during elaboration of Co, Ni and Fe based metal or oxides can lead to very different structures: polycrystalline CoFe2O4 micrometric nanowires, single crystal Fe3O4 nanowires and Co fiber one-dimensional nanostructures. Our results confirm the complex scenario of previous observations showing that thermodynamic conditions (mixing, temperature and precursors introduction) can play an important role even in presence of a magnetic field assisted nanowires nucleation. This work is although a first attempt to a continuous and sharp control of one-dimensional Co based nano-objects, which shows a linear dependence of the diabolo to cylindrical morphology on the nucleating agent concentration. Morphology effect on the large coercive behavior of nanowires

Figure 4. Measured coercive field at 200K and 20K for zero field cooled and field cooled freeze in polyol NWs of Co80Ni20 are reported as function of shape. Magnetic static measurements were performed on nanowires still diluted in their organic solution under the conditions already reported in the “Experimental details” paragraph. Hysteresis cycles have shown typical ferromagnetic behavior that we studied here by the coercive behavior. In Figure 4 this latter has been studied at high (200K) temperature and low (20K) temperature as function of the nanowires shape and analyzed within a standard Stoner-Wohlfart model. Our observations are fully consistent with a shape anisotropy origin behavior of the measured enhanced coercivity in which HC∼Ha-NM with HC being the coercive, Ha the intrinsic anisotropy field, M the uniform magnetization and N the demagnetizing factor along the long axis (we used for simplicity a finite ellipsoid: Nx=Ny=1/2 and Nz≠0 for a finite dimension dominated by shape anisotropy d << L. In this case, N shows a second-order term d / L( )2 dependence13. In Figure 4 one can see that this is fully consistent with the behavior of the coercive field in totally misaligned nanowires (ZFC procedure). For very long wires, (low d /T( ) values) the coercivity obtained on aligned one-dimensional systems (FC procedure) deviate from the simple model. This nevertheless evidences the presence of contributions to the global effective anisotropy of the whole nanowires ensemble, coming from other terms than the simple shape taken into account. In Figure 3 one can notice that the lowest d / L( ) values correspond to the most pronounced conic head (lowest d /T( ) values) nanowires. This means that, henceforth, the inhomogeneity of magnetization at the edge of nanowires has to be taken into account on the

shape anisotropy contribution as suggested by previous micromagnetic simulations6. Moreover, at this stage of the analysis, it cannot be excluded that other terms like the magnetocrystalline anisotropy and dipolar interactions could be part of the tricky scenario driving the magnetization reversal mechanism in one-dimensional nano-objects.

CONCLUSIONS Ferromagnetic metal CoNi-based nano-objects with a sharp control of their morphology have been studied. Magnetic static properties have shown a ferromagnetic standard behavior with coercive field efficiency strongly dependent on shape parameters. Our observations are fully consistent with a shape anisotropy origin behavior of the measured enhanced coercivity as function of the decreasing d / L( ) ratio. They nevertheless show the presence of contributions to the effective anisotropy coming from other terms (i.e. conic head impressiveness, dipolar interactions and magnetocrystalline anisotropy). ACKNOWLEDGMENTS The magnetic measurements at ESPCI have been supported through grants from the Ile-de-France Regional Council. Authors thank F. Cardarelli for careful reading of the manuscript.

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