Nanocrystalline Fe and Fe-riched Fe-Ni through electrodeposition

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Pergamon PI1 SO9659773(99)00065-3

Materials, Vol. 12, pp. 55-60, 1999 Elsevier Science Ltd Q 1999 Acta Metallurrka Inc. Printed in the USA. All tights kserved 0965-9773/99/$-see front matter

NANOCRYSTALLINF, Fe AND Fe-RICEIED Fe-Ni THROUGH ELECTRODEPOSITION

Michel L. Trudeau Emerging Technologies, Hydra-Quebec, 1800 Boul. Lionel-Boulet, Canada, J3X 1S 1, [email protected]

Varennes, Quebec,

ABSTRACT - There is a constant need for new softer magnetic materials, especially with high saturation moments. A number of studies have shown that by decreasing the average crystal size of a material below 20 nm, it is possible to reduce its magnetic losses. This was clearly demonstrated in nanostructured magnetic alloys based on the thermal crystallization of amorphous ribbons. However, this demonstration has been very diflcult for other nanostructured materials, mainly because most of them are based on powder processing. This study presents the synthesis andproperties of nanostructured magnetic materials produced by pulsed electrodeposition. Surprisingly, compared to other synthesis techniques, electrodeposition has received little attention for producing large quantities of fully dense nanostructured materials. In this work we describe some of the electrodeposition parameters leading to the production of nanostructured Fe and Fe-riched Fe-Ni alloys. Some magnetic properties obtained using a vibrating sample magnetometer are also presented. 01999 Acta Metallurgica Inc.

INTRODUCTION The field of nanostructured materials has grown considerably in the last fifteen years. More and more researchers realize that the control of materials structure on the nanometer scale is the key to unique properties. At the same time, high throughput synthesis processes have been developed. It is interesting to look briefly back at one area that was, 10 years ago, clearly at the far front of the present nanostrnctnre revolution. In 1988, Yoshizawa et al. [l] presented a study on nanocomposite magnetic materials produced by the thermal annealing of amorphous ribbons. They showed at that time that, through a judicious choice of the materials composition, it was possible during thermal annealing to favor an increase in the nucleation rate and a reduction of the grain growth. The final end products were nanocomposite ribbons composed of a-Fe(S) crystals, having an average size of the or&r of 15 mu, that are surrounded by an amorphous phase. From the time of their discovery, the large interest for these materials was related to their very low coercivity that is combined with their large saturation magnetization associated with the presence of a-Fe crystals. These nanostructured alloys were seen as the perfect materials for low loss magnetic applications. Following this 55

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work, Herzer published a series of articles [2,3] based on the random anisotropy model (RAM) [4] that explained the relation between the grains size and the coercive fields. This model indicates that below a critical value, L, (the ferromagnetic exchange length), the magnetization will not follow the randomly oriented easy axis of the individual grain but is forced to align by the exchange interaction. The local anisotropy average out over with the increasing number of grains. In these circumstances the value of the coercive field, Hc, should then decrease with the crystallite size as D6. Up to now, most of the experiments that were successful to obtain this D6 dependence have been for thermally annealed amorphous ribbons. Up to now, it has not been possible to reproduce non-ambiguously such a behavior for nanostructured materials synthesized through the densifkation of nanostructured powders. Two main explanations exist for that. The first one is that soft magnets need to be fully densified and relax to have good properties. The other difficulties is that most powders that have been produced up to now always end up with a small contamination layer at the surface of the crystals, most often an oxide, that is detrimental to the magnetic softness. In view of all these reasons, we are presently investigating the synthesis and properties of magnetic materials, with high saturation fields, through pulse electrodeposition. In the present paper we describe some of the parameters as well as some properties for pure iron and iron rich Fe-Ni electrodeposited alloys.

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The magnetic materials were produced by pulse electrodeposition. In pulse electrodeposition a D.C. current is applied for a short period time, ton, that is followed by a period of time when no current is applied, to8 Typical values for to, and &fare between 1 and 40 ms and between 40 and 300 ms respectively. Through the use of a high current density as well as some grain growth inhibitors, such as saccharine, it is possible to increase the nucleation rate and reduce grain growth. Detailed on Fe-Ni pulse electrodeposition have been reviewed recently by Grimmet et al. [5,6]. They found that alloys with composition from pure Ni to about 92 at.% Fe, with grain size around 10 nm, cotild be obtained using a Ni sulfamate and Fe chloride solution. The structure of the materials prepared was investigated using x-ray diffraction using MO or Cu K, radiation and with a high resolution S-4700 SEM from Hitachi. The volumeweighted average column length and internal strain of the crystals were obtained from x-ray spectra, after correction for & and instrumental broadening, based on the equation described in Klug and Alexander [7], considering that the broadening due to strain could be approximated by a Gaussian distribution, while that due to the crystallite size is better described by a Lorentzian. The error in size determination is quite important using this technique, of at least 20-30%. A semi-quantitative assessment of the Fe and Ni concentration was done using energy dispersive x-ray. The surface chemistry was studied using a PHI-5500 XPS from Perkin-Elmer, using Al K, radiation. The peak position were referenced to C-(C,H) contaminants at 284.8 eV. Magnetic measurements was done using a vibrating sample

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FexNI,mxElectroplated FCC

22 JJ E E $$ E E

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-50% Fe

b

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Figure 1 X-ray spectrum for three different Fe-Ni alloys prepared using pulse electrodeposition magnetometer. Finally, obtained using a DSC-7 As mentioned, work of Grimmett et al.

the recrystallization temperature of the nanocrystalline materials was from Perkin-Elmer. the bath composition for the FeNi electrodeposition was based on the [5,6] and was composed mainly of 0.75 M Nickel Sulfamate 0.25 M Ferrous Chloride 0.5 M Boric Acid 0.5 g/l sodium lauryl sulfate 1 g/l ascorbic acid

Saccharine was added between 1 to 10 g/l as a gram refining agent. The pH of the solution was varied between 2 and 3, while the temperature was controlled between 22 and 65 OC. For their parts, t, was set between 1 and 40 ms while & was varied 100 to 360 ms. The peak current density was varied from 0.2 to 1.2 A/cm*. Ni and Fe-Ni anodes were used. Most of the deposits were obtained without solution stirring. For iron electrodeposition, the composition of the solution was based on a sulfatechloride bath [S] and was composed mainly of 250 g/l Ferrous sulfate 42 gfl Ferrous chloride 10 g/l amonium chloride Again, in some cases 1 to 10 8/l of saccharine was added to the solution in or&r to reduce the grain size, In some circumstances other additives were used between 0 and 1 gjl

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Figure 2 Hysteresis loops for two nanostructured Fe-Ni alloys prepared by pulse electrodeposition Na lauryl sulfate, from 0 to 30 g/l Boric acid and from 0 to 15 g/l sodium carbonate. The temperature of the solution was varied from 25 to 65 “C and the pH from 3.5 to 6.0. Peak current density was of the order of 1 A/cm*. In most cases, depositions were made on titanium cathode in or&r to facilitate their removal from the substrate which allows to perform experiments on self supported samples. One should also mention that the more additives are added to the solutions the larger is the contamination problem of the samples. For this reason, most of the experiments were done with the lowest amount of additive as possible. Finally, for both systems some samples were also prepared in normal D.C. mode without pulses. Under these condition the current density used was between 3 and 10 AMm2.

FtESULTS AND DISCUSSION Figure 1 presents the x-ray spectrum for three different electrodeposited Fe-Ni alloys. The crystallite size for the different samples was found to be around 10 run. The iron atomic concentration for the fee alloy was found to be around 55 %, while for the bee is was around 75 %. At concentration of about 65 at.% (sample b) a mixed fcc/bcc phase was obtained. With most of the other parameters equals, plating at room temperature seems to lead more to the formation of bee (high iron) alloys, while electrodeposition at temperature around 50 OC lead to the formation of the fee phase. As expected, high peak current density seems to be necessary to the formation of a nanostructured materials. Figure 2 presents the hysteresis curves for nanostructured (10 nm) fee and a bee samples. As can be expected the value of B, increases with the Fe content when going from the

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Figure 3 X-ray spectrum for two different Fe samples prepared by electrodeposition with (7=0.7) and without pulses (y-1). fee to the bee phase. Also, as seen on this figure the value of the coercive field is quite low, and this especially for the fee alloy. Without any annealing treatment, I-L was found to be about 4 Oe for the bee alloy and 0.5 for the fee. For pure iron, reduction of the crystallite size below 20 mn was found to be more difficult. Figure 3 a and b present the x-ray spectrum for samples obtained with and without current pulses. Interestingly, the average crystallite size for the iron obtained without pulses at pH 3.5, 43 “C and for a current density of 6.4 AAim was found to be about 45 run, quite comparable to the value obtained for a sample prepared with t, and LB of 3 and 40 ms respectively and with a peak current density of 47 A/dm2. Impurities analysis revealed the following contaminants concentration in most materials produced without additives in the solutions: 0, at 0.09 wt%, N2 at 0.03 wt%, C at 0.05 wt%, S at 0.02 wt% and H2 at 0.08 wt%. This indicates that electrodeposition can lead to very pure nanostructured materials, with less contaminants than in most materials produced through the different powder synthesis techniques. With a higher peak current densities or with the addition of saccharine, smaller crystallites were obtained (below 10 mn) but this was normally associated with a higher contaminants concentration or burning of the samples. It was also found that the higher the bath temperature (at least at pH 3.5) the lower was the average size (between 25 and 60 “C). Details on the influence of the different electrodeposition parameters will be published elsewhere. Compare to the Fe-Ni alloys, the Fe deposits were found to be much more brittle. The surface of samples that were produced without pulses was found to be covered by cavities, due most probably to hydrogen evolution. On the other hand for the pulsed-prepared samples no surface cavities were apparent. Similar results were recently reported by El-Sherik et al. [9] for Ni electrodeposition. Hystreresis measurements gave values for the saturation magnetization of

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about 2.2 Tesla, as expected for iron, and coercive fields of about 4 Oe for non-annealed samples. Without saccharine in the solution, no preferential texture was observed for the iron deposits produce by pulsed deposition. On the other hand, as seen in figure 3, without pulses some textures along the 110 direction is apparent (for y = 1). However, with the presence of saccharine in the bath, the samples were found to be highly texture along the 211 direction. The thermal stability of the different materials was investigated. In general the stability was found to decrease with the increased amount of Fe in the alloys. For the fee materials, the recrystallization temperature was found to be around 420-450 “C, while it was more around 390-400 “C for the bee samples. The surface of some of the samples were examined by XPS. In most case, the surface was covered by a thin oxide layer. However, metallic iron was observed at the surface indicative of the small thickness of this oxide layer.

CONCLUSION Nanostructured Fe-rich soft magnetic materials could present some major technological advantages because of their predicted low coercivity and high saturation magnetization. However, in or&r to obtain such materials it is necessary to produce fully dense and relax alloys, with an average crystallite size well below 20 nm and most probably even below 10 nm. Pulse electrodeposition is probably the only technique that can synthesize such materials, at least economically. All, other synthesis techniques, based on powder processing, will not be able to give fully densifled materials without grain growth or without the presence of large amount of impurities (such as oxygen or hydrogen), The results presented in this work show clearly the great potential of this technique. It has been demonstrated here that fully dense iron-based nanostructured materials, with high B, values and low coercivity, can be easily synthesized using pulse-electrodeposition. Moreover, electrodeposition is not limited to the production of plates, since electroforming can be used easily to produce, in a single step process, complex shapes for particular applications.

REFERENCES [l] Y. Yoshisawa, S. Oguma and K. Yamauchi, J. Appl. Phys. 64,6044, (1988) [2]G. Herzer, Mater. Sci. & Eng. m, 1 (1991). [3]G. Herzer, Script. T49,307, (1993) [4]R. Alben, J. J. Becker and M.C. Chi, J. Appl. Phys. 49, 1653, (1978). [5]D.L. Grimmett, M. Schwartz and K. Nobe, J. Electrochem. Sot. 137,3414, (1990). [6]D.L. Grimmett, M. Schwartz and K. Nobe, J. Electrcchem. Sot. 140,973, (1993). [7]H.P. Klug and L.E. Alexander, X-ray D@uction Procedures for Polycrystalline and Anorphous Materials, 2nd ed., Wiley, New York, (1974) [S]Modem Electroplating, ed. By F.A. Lowenheim, Wiley & Sons, 2”d ed. (1967). [9]A.M. El-She&, U. Erb and J. Page, Surf. And Coatings Tech. 88,70, (1996).