Synthesis of nanocrystalline permalloy by electrodeposition

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ScriptaMetallurgicaet Materialia,Vol. 31, No. 6, pp. 735-740, 1994 Copyright© 1994 ElsevierScienceLtd Printed in the USA. All rights reserved 0956-716X/94 $6.00 + 00

SYNTHESIS OF NANOCRYSTALLINE PERMALLOY BY ELECTRODEPOSITION C. Cheung*, G. Palumbo t and U. Erb* tMaterials & Metallurgical Engineering, Queen's University, Kingston, Ontario, Canada. KTL 3N6 *Ontario Hydro Research Division, 800 Kipling Ave., Toronto, Ontario, Canada. MSZ 5S4

(Received May 12, 1994)

Introduction Permalloy, among other Ni-Fe alloys such as HyMu, Supermalloy and Mu-Metal, is a commercial alloy used in soft magnetic applications. Permalloy is nominally a binary alloy (approximately 80wt%Ni-2Owt%Fe) containing alloying elements such as Cr, Cu, Mo and/or Mn in small amounts [1] to further enhance certain magnetic properties such as coercivity and magnetostriction; or to increase the electrical resistivity which is desirable in soft magnetic materials in order to reduce Eddy current losses [2]. Conventional polycrystalline permailoy has been used for many years in the manufacture of magnetic recording head materials. This is because permalloy has a high saturation magnetization and a high initial permeability which are required for this application. However, it is not expected that conventionally processed permalloy will meet the more stringent requirements for future recording head applications due to its poor long term wear and corrosion performance characteristics [3]. These drawbacks, however, may be overcome by nano-processing of permalloy. In recent communications, we have discussed the application of nano-processing in the development of soft magnetic materials with improved performance characteristics [4,5]. For pure nickel, it has been shown that nanoprocessing enhances several of the properties required for high performance recording head materials. These include increased hardness [6], wear resistance [7] and improved corrosion performance in terms of reduced Iocalised attack [8]. Furthermore, it has been shown that in the case of electrodeposited nanocrystalline nickel, the room temperature electrical resistivity is increased by a factor of 3 [9] and the saturation magnetization is essentially the same as for conventionally available nickel [10]. In other words, the saturation magnetization of electrodeposited Ni is independent of grain size. This is in contrast to measurements reported for gas condensed nanocrystalline nickel for which a 50% reduction in the saturation magnetization was observed for grain sizes of about 10 nm [11]. It is believed that this reduction observed for gas-condensed materials is due to the high intrinsic porosity and the associated nickel oxide formation on the internal surfaces [10,11]. Electrodeposited materials, on the other hand, have negligible porosity and therefore, relatively constant saturation magnetization. This finding is in agreement with recent calculations which showed that the structural disorder introduced in nickel by grain boundaries has only a minor effect on the average magnetic moment and, thus, the saturation magnetization [12]. For practical applications as recording head materials, however, the saturation magnetization and permeability of pure nickel may be too low. It is expected that both properties can be enhanced by alloying nickel with iron and other elements to achieve nano-processed alloys with composition close to conventional permalloy. Preliminary results on the synthesis of nanocrystalline permaUoy by electrodeposition have already been reported elsewhere [5]. The purpose of the present paper is to present details on the synthesis, microstructural characterization and hardness measurements for these materials.

Ex~rimental A proprietary plating electrolyte was used containing nickel, iron and chromium salts (as chlorides), boric acid and ammonium chloride. Boric acid is used (1) to buffer the bulk pH, (2) to prolong the plating bath life [13] and 735

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(3) to produce more uniform and satisfactory deposits [14]. Ammonium chloride was used to increase the electrical conductivity of the electrolyte. Chromium salt in various concentrations (0 - 75 g/L) was added to the electrolyte in an attempt to co-deposit Cr with Ni and Fe. Chromium in the deposit is expected to increase the electrical resistivity of permalloy. Electrolysis was carried out in a 1 litre beaker with a dimensionally stable anode made of high density graphite. The cathode material was copper. Agitation was provided by a magnetic stirrer operated at about 200 rpm. The pH of the electrolyte was 1. Electrodeposits (up to 20/an thickness) were produced at 23"C _+ IoC and a current density of 0.05 A/cm2. The morphology and composition of the deposits were analyzed using scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDS). Microstructural analysis was carried out using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Thin foils for TEM examination were prepared by ion-thinning. The hardness of the material was determined by Vickers microhardness indentation using a load of 50 grams.

Results and Discussion The effect of bath chromium concentration on the composition of the alloy deposits is shown in Figure 1. The composition of the electrodeposit in the absence of chromium chloride is about 85wt%Ni-15wt%Fe. With increasing chromium salt addition to the bath, the Ni content drops, going through a minimum of about 75wt% at a chromium salt concentration of 50 g/L and increases to about 80wt% at the maximum chromium salt concentration of 75 g/L. The Fe content shows the reverse trend, starting at about 15wt%, going through a maximum at 50 g/L of chromium salt and dropping back to about 20wt% at 75 g/L chromium chloride concentration. It is interesting to note that the Cr content in the deposits over the entire range of chromium salt concentration studied is about 0.5 wt% even at the highest value of 75 g/L. In other words, an anomalous deposition behaviour is observed; i.e., a linear increase of the ratio of metal ions in the bath does not result in a proportional change in the ratio of the co-deposited metals. However, despite the low Cr content in the electrodeposits, the amount of chromium salt addition to the bath has a considerable effect on the Ni to Fe ratio in the deposits. This could be of importance in the optimization of magnetic properties such as magnetization and permeability, which, for Ni-rich alloys, are strongly dependent on the Ni to Fe ratio. In order to explain the observed changes in the deposit composition, further research is required in particular to study complex formation in the bath and details of the co-deposition process. 9O "-'-

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Figure 2 shows a typical X-ray diffraction pattern of an electrodeposit. It contains reflections from the alloy as well as the (200) and (400) peaks of the underlying rolled Cu substrate. When comparing the intensities of the alloy deposit with the intensities of a standard powder with random crystallographic orientation, it can be concluded that the electrodeposit has a (111) fibre texture. ZOOO84.RAW NANOCRYSTALLINE PERMALLOY

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X-ray diffraction pattern of a typical nanocrystalline permalloy electrodeposit.

The observed peak broadening of the (111) alloy peak was measured to calculate the grain size of the electrodeposits using the Scherrer formula. In Figure 3, the grain size is plotted against the chromium salt concentration in the solution. Following this analysis, all of the deposits are in nanocrystalline form having average grain sizes ranging from 7 nm to about 16 nm. 2O 16 _e. 14 "''-,

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Grain size of the permalloy electrodeposits obtained as a function of chromium chloride concentration in the electrolyte.

From the results of compositional analyses (Figure 1) and grain size measurements (Figure 3), a direct inverse correlation between the Fe content in the electrodeposits and the grain size of the alloy is found. As the Fe content increases from 14wt% to about 20wt%, the grain size of the deposits decreases from 15 nm to about 10 nm. The Fe

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content reaches a maximum of about 24wt% at a chromium chloride concentration of 50 g/L, and the grain size reaches a minimum of about 7 nm at this chromium chloride concentration. At 75 g/L of Cr salt, the Fe content of the deposit decreases back to a value of about 20 wt% resulting in a grain size of about 9 nm. In other words, the deposits obtained at 20 g/L and 75 g/L have the same composition and roughly the same grain size, indicating the grain size in these deposits is mainly controlled by the amount of co-deposited iron. Figure 4(a-c) shows brightfield and darkfield transmission electron micrographs as well as the electron diffraction pattern of an electrodeposit produced from a bath containing 10 g/L chromium salt. The smallest grain size observed in the daridield micrograph is in good agreement with the results obtained from X-ray line broadening measurements. It should be noted that some of the larger bright areas in the darkfield micrograph consist of many small grains in similar orientation indicating the presence of a microtexture over small distances. Similar microtexture features have been previously reported for nanocrystalline nickel which was also produced by electrodeposition. The clustering of small grains in similar orientation may be responsible for the nodular features (50 - 200 nm diameter) observed by scanning electron microscopy on the otherwisesmooth surface of the deposit (see Figure 4(d)).

Figure 4(a)

TEM brightfield micrograph of a nanocrystalline permalloy.

Figure 4(b)

Darkfield transmission micrograph.

Figure 4(¢)

Electron diffraction pattern.

Figure 4(d)

SEM micrograph.

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The Vickers microhardness of the electrodeposits is shown in Figure 5 in the form of a Hall-Perch plot [15,16]. The hardness increases initially with decreasing grain size; however, after reaching the maximum hardness of about 900 VHN, an inverse Hall-Perch behaviour is observed. Over the past few years, the hardness of materials with

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extremely fine grain structures has attracted considerable attention [17-42]. While several studies have shown that the Hall-Petch relationship is exhibited down to the smallest grain size [17-25], others have reported a transition from regular to inverse Hall-Peteh relationship [26-34], similar to the one observed in the present study. A number of contributing factors for the transition from regular to inverse Hall-Petch behaviour have been discussed including texture changes in the material [26], diffusional creep [27], triple junctions [28], dislocation network formation [35] and pile-up [40,42], annealing effect [36], decrease in interfaeial excess volume [37], bow-out of Frank-Read sources [38,39] and the grain boundary source model [41]. At this time, it is not known which of these factors are responsible for the observed hardness behaviour of nanocrystalline permalloy. This remains to be addressed in future work.

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Hall-Petch plot for nanocrystalline electrodeposited permailoy.

Conclusions Nano-processing of permalloy has been proved viable by electrodeposition. From the results presented, it can be concluded that nanocrystailine permalloy can be produced using a metal chloride solution at 23"C and 0.05 A/cmz. The grain size of the electrodeposits ranges from 7 nm to about 16 nm. Nano-processing could potentially provide an improved magnetic alloy for the manufacture of a magnetic recording head material enhancing properties such as coereivity, electrical resistivity, hardness, corrosion behaviour and wear characteristics without compromising saturation magnetization.

Acknowled~,ements The authors would like to acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada and Mr. P. Nolan for his help with transmission electron microscopy.

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