J. Chil. Chem. Soc, 52, N° 4 (2007), págs: 1322-1325

EFFECT OF PHOSPHOROUS ACID AND UREA ON ELECTRODEPOSITED CONIP MAGNETIC FILMS

R.N.EMERSOW^A)*

Department of Physics, Karunya University, Coimbatore-641114, Tamilnadu, India
Department of Chemistry, Karunya University, Coimbatore-641114, Tamilnadu, India

ABSTRACT

The properties of hard magnetic CoNiP films electrodeposited in the presence of urea as additive and phosphorous acid as a precursor with varying concentrations were studied with respect to thickness of the films. Films were electrodeposited in various current densities and for different time in order to get different thickness and uniform deposits. Elemental composition of the films was obtained using atomic absorption spectrometry. Vibrating sample magnetometric studies indicate that additives has favourable impact on the magnetic properties of these films. Structural and surface analysis was carried out using X-ray diffractometry and scanning electron microscopy respectively. Reasons for variation in magnetic properties and structural characteristics are discussed. Hardness and adhesion of the films were also studied.

Keywords: Electrodeposition; Additives; CoNiP magnetic film; Magnetic properties

1. INTRODUCTION

The importance of electrodeposition as a fabrication technology in the electronics industry is large and growing. With the current trends towards miniaturization, cost competitiveness and high performance packaging, electrodeposition has become the dominant manufacturing technology in many new applications and remains firmly established in others such as micro electro mechanical system (MEMS) devices, magnetic recording head, reading heads and data storage media [1-4]. The electrodeposition technique is quite advantageous due to its cost effectiveness, easy maintenance and quality deposits.

Various studies have been carried out to develop CoNiP films, which have hard magnetic properties [5]. Electrodeposition provides an easy way to produce CoNiP as well as other magnetic films [6]. Murford et al [7] have studied the morphology and magnetic properties of_CoNiP films electrodeposited on silicon. They have found a convenient method for fabricating thin magnetic films and magnetic sensors with silicon technology. Numerous applications of CoNiP films were reported by many researchers [8-10].

Electroless deposition process has been studied for developing CoNiP magnetic thin films for magnetic recording media applications [11, 12]. However, the effects of electrodeposition conditions and organic additives on the magnetic properties of the deposits have received little attention so far [5].

In the present study we investigated in detail the effects of electrodeposition conditions on the magnetic properties of CoNiP. The effects of concentration of urea and phosphorous source material in the bath were also investigated.

2. EXPERIMENTAL DETAILS

A copper substrate of size 1.5 X 5.0 cm as cathode and pure nickel of same size as anode were used for galvanostatic electrodeposition experiments. Current for electrodeposition was passed from a regulated direct current unit. Analytical reagent grade chemicals were used to prepare baths. An adhesive tape was used to mask off all the substrate except the area on which deposition of film was desired. Each substrate was buffed for removing scratches in a mechanical polishing wheel using a buffing cloth coated with aluminium oxide abrasive. Buffed substrates were degreased using acetone. Before electrodeposition these substrates were electrocleaned in an alkaline electrocleaning bath. The bath contained sodium hydroxide: 7.0 g l^-1; sodium carbonate: 20.0 g l^-1; trisodium phosphate: 9.0 g l^-1 and sodium metasilicate: 24.0 g l^-1. The bath was operated at 70° C and current density applied was 3.0 A dm^-2. After electrocleaning the substrates were rinsed in distilled water. Electrodeposition was carried out on the cleaned substrates using different current density and time of deposition.

CoNiP magnetic thin films were electrodeposited from a bath contained CoCl₂: 0.20 M, NiCl₂: 0.20 M, NH₄C1 : 0.25 M, here in after bath-A. 0.2 and 0.4 M of phosphorous acid (H₃PO₃) and 2.5 and 5.0 g l^-1 of urea were added to this bath and their effect on the properties of CoNiP films was investigated. All electrodeposition studies were carried out at room temperature.

The pH of all electrodeposition baths used in this work was found to be 3.0. The thickness of the deposits was tested using digital micrometer (Mitutoyo, Japan). Magnetic properties of deposited films were studied using vibrating sample magnetometry. In this technique the material under study was contained in a sample holder, which was centered in the region between the pole pieces of a laboratory magnet. A slender vertical sample rod connects the sample holder with a transducer assembly. The transducer converts a sinusoidal alternating current drive signal into a sinusoidal vertical vibration of the sample rod. Coils mounted on the pole pieces of the magnet pick up the signal resulting from the sample motion. X-ray diffractometry (XRD) Rich Seifert, Germany of model 3000 and scanning electron microscopy (SEM) Mosumy Electronics Japan make JEOL were used to study the structure and morphology of these magnetic films respectively. From XRD data crystallite size of the deposited CoNiP and film stress were calculated. Percentage of elements such as cobalt, nickel and phosphorous present in the deposits were obtained as follows. For elemental analysis CoNiP film was electrodeposited on stainless steel substrate to ensure easier peeling-off of the film. After deposition the film was peeled off from the substrate. It was dissolved in 3:1 v/v of H₂S0₄ and HN0₃ and the percentage composition was obtained using atomic absorption spectrometry (AAS). Hardness of the deposit was obtained using Vicker's hardness tester using diamond intender method. Adhesion of the film was tested by bend and by scratch or chisel test. These tests are widely used in the field of electroplating [13].

3. RESULTS AND DISCUSSION

3.1 Thickness study

Table. 1 shows the effect of concentration of H₃PO₃, urea, current density and duration of electrolysis on the thickness obtained under different experimental conditions. The thickness of the film increased with increase in current density as well as increase in duration of electrolysis. The concentration of H₃PO₃ or urea did not have much effect on thickness.

3.2 Structural analysis

Electrodeposited CoNiP films were subjected to XRD studies. The X-ray wavelength used was 1.5405 A° of Cu Ka radiation. Films obtained from experiment number 6, 15, 24, 33 and 42 of Table. 1 were studied for their structural characteristics. XRD data obtained are presented in Table.2.And also XRD pattern is presented in Fig. 1. These data were compared with Joint committee for powder diffraction_studies data (File number: 71/2336). CoNiP films had hexagonal close packing structure and exhibited (201) plane primarily. (201) plane peaks in the data for films obtained from experiment number 24 and 42 were shifted because of the film stress. It was known that film stress will shift XRD peaks [14]. Stress of the films were calculated from XRD data using the formula i.e., Youngs modulus = stress / strain. The results are shown in the Fig.2. Stress was low for film obtained from a bath contained 2.5 g l^-1 of urea. It increased on increasing the concentration of urea to 5.0 g l^-1. This was due to the incorporation of decomposed products of additive in to the film. Crystallite sizes were also low for films obtained from 2.5 g l^-1 urea. These were calculated from XRD data using the formula [15] i.e., crystallite size = 0.9 λ Bcosθ (Scherrer equation), where λis the wavelength of X ray radiation,(>v=l.540598 Á), B is the peak full-width at half-height and 0 is the diffracted angle. Crystallite sizes thus obtained were in the nano scale and it was shown in Table.3.

Electrodeposited CoNiP films obtained from experiment number 6, 15, 24, 33 and 42 of Table. 1 were subjected to SEM. The micrographs are presented in Fig.3. In general microstructure ofthe CoNiP was affected by the percentage of phosphorus content. The film with very low concentration of phosphorus, Fig. 3 (a) (b) and (e) appeared to have a crevice pattern. The film obtained from a bath contained 5.0 g l^-1 was cracked through substrate due to stress ofthe film as shown in Fig.3 (c) and (f). It was also observed in Table.3 that film obtained from bath contained 5.0 g l^-1 of urea had high stress.

3.4 Mechanical properties

CoNiP films, which were selected for XRD and SEM studies, were tested for their Vicker's hardness number. The results are reported in Table.3. Higher concentration of urea in the bath decreased the hardness ofthe film. It was due to the stress present in the film, which caused cracks in the structure. Adhesion ofthe film with the substrate was found to be good.

3.5 Elemental analysis

Table.3 also presents the results of AAS. It was observed that all the films obtained from various baths had less than 2.0 % phosphorous. Even with low phosphorous content the films showed high magnetic properties. It was due to the addition of urea in the bath, which improvedjhe crystalline structure of CoNiP films.

3.6 Magnetic properties

In general H₃PO₃ addition was found to have no effect on the thickness of the film. However the magnetic properties of the film were found to increase with current density and duration of deposition. The morphology of the film however was found to be poor when no urea was added in the electrodeposition bath.

The effect of addition of urea in to the bath-A along with H₃PO₃ was investigated. With the addition of low concentration of urea the deposit characteristics as well as its magnetic properties improved significantly. Under the best conditions involving addition of 0.2 M of H₃PO₃ and 5.0 g l^-1 of urea at a current density of 7.5 mA cm^-2 and time of deposition 60_minutes, the thickness ofthe film was found to be 3.2 µm with coercive and remanent values of 135280 A m^-1 and 0.10 Am² [experiment.number:27] respectively. With further increase in H₃PO₃ concentration the thickness of the films found to be 3.5 µm with coercive and remanent values of 87534 A m^-1 and 0.20 Am² [experiment. number:45].

Increase in magnetic properties of the films is mainly due to urea. The electrodeposited films were uniform and bright. The urea molecules thus are found to have leveling effect, which ensures uniform orientation of crystals during electrodeposition. On increasing the concentration of H₃PO₃ and urea magnetic properties of the films decreased. It was because of the stress present in the films, which was caused by the inclusion of decomposed products of additives.

4. CONCLUSIONS

A CoNiP film with high hard magnetic properties can be obtained by galvanostatic electrodeposition process. The bath required for electrodeposition contained CoCl₂ : 0.2 M, NiCl₂ : 0.2 M, NH₄C1 : 0.25 M, H₃PO₃ : 0.2 M and urea : 2.5 g l^-1. The current density for the deposition was 5.0 - 7.5 mA cm^-2. Addition of urea increased the coercive value of the film. The high coercive value obtained in this work was 135280 Am^-1. This is because the urea moleculesjhus are found to have leveling effect, which ensures uniform orientation of crystals during electrodeposition. 5.0 g l^-1 of urea was found to be the optimum concentration in the bath in order to obtain a CoNiP film with improved magnetic, structural and mechanical properties.

ACKNOWLEDGEMENTS

The authors thank the founders, Dr. D.G.S. Dhinakaran and Dr.Paul Dhinakaran and the management of Karunya University for their support and kind permission to publish this paper.

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