Effect of Electrolytic Bath Temperature on Magnetic and Structural Properties of Electrodeposited NiFeW Nano Crystalline Thin Films

Introduction

The chemical, mechanical and thermal electrochemical interaction of materials is initiated from materials surface [1]. Therefore, surface is the most important engineering part of a material. The role of technologies which modifying surface in the production and manufacturing process is predictable [2-4].Also the surface technology has vital role in production process. Electrodeposition is an electrochemical process and an old method which applied for surface structure modification [5,6].  Nickel and its alloys have important advantages such as good wear and corrosion  resistance .So deposition of nickel are needed to improve wear and corrosion resistance in the industry, repairing eroded metals, to improve magnetic properties, to change the dimensions of small metal pieces  and preparing organic coating. NiFeW, NiCoW, and NiW are most commonly used magnetic thin film materials in MEMS and NEMS [7-10].  The electroplating of NiFeW thin film is used to obtain enhanced electrical conductivity, good soft magnetic characters, special optical properties and improved corrosion resistance [11-14]. Electrodeposition method is widely used in MEMS,NEMS, communication, optical and sensors industries [15]. In this study, the effects of different temperatures on characters of NiFeW alloy thin films are analysed. This paper explains the preparation and characterization of electroplated Ni-Fe-W thin films.

Experimental Part

The electroplated NiFeW alloy films were prepared for different temperature 30, 50, 70 and 90 ^o C. The time duration of deposition process was 15 minutes. In this investigation, copper and stainless steel substrates acted as cathode and anode respectively with 1.5 cm x 7.5 cm. dimension [16-19]. The NiFeW thin films were prepared from electrolytic solution which contain Ferrous Sulphate (10 g/l), Nickel sulphate (30 g/l), Sodium tungstate (15 g/l), Ammonium Sulphate (40 g/l) , Boric acid(10 g/l) and Saccharin(10 g/l) [20-21].The pH value of electrolytic solution was fixed as 6.0 by mixing ammonia solution and  electroplating process was carried out with current density 3 mA/cm² . The copper or cathode was carefully removed from the bath after 15 minutes and dried for few minutes [22-23]. The surface nature of Ni- Fe- W films was characterized by Scanning Electron Microscope .The elemental content of deposits of films was examined by Energy-dispersive X-ray Spectroscopy and crystal structure of deposits was analyzed by X-ray diffraction study. The micro hardness of films was measured by Vickers Hardness Test. The important magnetic properties of deposits are saturation magnetization and coercivity .Analysis of thin films with Vibrating Sample Magnetometer gives magnetic properties.

Results and Discussion

Composition of Ni Fe W films

Energy-dispersive X-ray analyser (EDAX) gives elemental composition of deposits. Table 1 shows weight percentage of Fe, Ni and W at different electrolytic bath temperature by EDAX analysis. From results, films prepared at 90˚C have high content of tungsten. The highest ferrous content of 22.67 wt% was found at electrolytic temperature of 30˚C. While electrolytic temperature increases, the content of nickel and tungsten also increase. The higher nickel content of 61.41 wt% was found at 90˚C temperature.

Table 1: EDAX analysis of thin films

S. No	Temperature	Fe Wt%	Ni Wt%	W Wt%
1.	30˚C	22.67	48.52	28.81
2	50˚C	18.12	52.73	29.15
3	70˚C	11.38	57.29	31.33
4	90˚C	05.33	61.41	33.26

 

Morphological Study of Nifew films

The surface structure of Ni- Fe- W thin films with temperature 30, 50, 70 and 90˚C were analysed by Scanning Electron Microscope (SEM) images and they are shown in Fig 1. The thin films are bright and uniformly coated on the surface. They are crack free by appearance.

Figure 1: Ni FeW films – SEM images at (a) 30˚C (b) 50˚C (c) 70˚C (d) 90˚C Click here to View figure

 

Structural Analysis of Ni Fe W films

Crystal structure of Ni- Fe- W alloy films was analyzed by powder X-ray diffraction (XRD). Fig 2 shows diffraction patterns of Ni- Fe- W films prepared with different temperature. The occurrence of sharp peaks in X-ray diffraction pattern exhibit crystalline nature of deposits. The XRD patterns of all samples deposited at 30, 50, 70, and 90˚C are being indicative of the (111), (200) and (220).Also XRD result reveal the existence of FCC type crystal. The particle sizes of Ni- Fe- W deposits are 27.15 nm, 22.18nm, 20.72nm and 17.28 for the bath temperature 30, 50, 70, and 90 ˚C respectively.  So it is concluded that crystal size of thin film deposits decreases by increasing electrolytic temperature.  Also deposits of thin films reveals nano scale and average crystalline size is around 22 nm. When bath temperature increases, strain value increases from 1.333 x 10^-3 to 2.095 x 10^-3 dislocation density increases from   13.57 x 10¹⁴ / m² to 33.49 x 10¹⁴ / m².

Figure 2: NiFeW films – XRD patterns at (a) 300C (b) 500C (c) 700C (d) 900C Click here to View figure

 

The crystal size, strain value and dislocation density of Ni-Fe-W alloy films are shown in table 2. The crystalline size of deposits decreases due to onset orientation of crystals while bath temperature increases. Figure 3 shows that crystal size reduces with increasing bath temperature.

Table 2: Structural characteristics of Ni-Fe-W alloy thin films

S.No	Bath Temperature (˚C)	2θ (deg)	d (A0)	Particle Size(D) (nm)	Strain (10-3)	Dislocation Density (1014 / m2)
1	30	50.83	1.783	27.15	1.333	13.57
2	50	50.76	1.792	22.18	1.632	20.33
3	70	51.13	1.818	20.72	1.747	23.29
4	90	51.02	1.825	17.28	2.095	33.49

Figure 3: Variation of crystal size with different electrolytic bath temperature Click here to View figure

 

Mechanical Properties of Ni Fe W films

Micro hardness measurement of deposits was done by Vickers hardness tester. The hardness values of thin films prepared for temperature 30, 50, 70 and 90 ^o C are 114,139,187,221 VHN respectively. So hardness test shows that the micro hardness increases by increasing electrolytic bath temperature due to lower stress associated with thin films. Fig 4 shows the variation of hardness with increasing bath temperature.

Figure 4: Variation of micro hardness with different electrolytic bath temperature  Click here to View figure

 

Magnetic Properties of Ni Fe W films

The important magnetic characters coercivity, saturation magnetization, retentivity and Squareness of NiFeW alloy films were obtained by VSM and they are given in table 3.The hysteresis curves for thin film NiFeW with various temperature are shown in figure 5.The elemental composition of deposits, applied potential, additive materials, temperature and electrolytic bath are the deciding factors of thin film magnetic properties.

When electrolytic temperature is changed as 30˚C, 50˚C, 70˚C and 90˚C, the magnetization increases from 38.713 x 10^-3 emu/cm² to 152.824 x 10^-3 emu/cm²   .Also when temperature is increased from 30˚C to 90˚C, the coercivity decreases from 220.634 G to   121.82 G.  From result of vibrating sample magnetometer, it is concluded that films obtained at higher temperature reveals high value of magnetization and low value of coercivity. The enhanced value of saturation magnetization and reduced value coercivity promote magnetic properties of alloy coating.

Table 3: Magnetic Properties of NiFeW deposits

 

S.No	Bath Temperature (˚C)	Coercivity Hs (G)	Magnetization Ms (emu/cm2) 10-3	Retentivity Mr (emu/cm2) 10-3	Squareness S(Mr/ Ms)
1	30	220.63	38.713	3.4703	0.089642
2	50	163.16	87.216	2.1315	0.024439
3	70	144.14	108.715	1.1760	0.010817
4	90	121.82	152.824	5.1027	0.033389

Figure 5:  Hysteresis curves for NiFeW film at electrolytic bath temperatures (a) 30˚C (b) 50˚C (c) 70˚C (d) 90˚C Click here to View figure

 

The magnetization and other important magnetic property coercivity can be changed by decreasing the grain size of deposits .The film stress is reduced by increasing bath temperature. The Ni-Fe-W thin films prepared at 90˚C have lower coercivity and higher magnetization due to low stress formation during deposition. So it is concluded that soft magnetic nature of NiFeW thin films are enhanced by increasing bath temperature.

Conclusion

The NiFeW alloy thin films with different temperature were prepared by electrodeposition by maintaining current density 3 mA/cm² and pH of solutions 6.0. The thin films are bright and uniformly coated on the surface. The XRD result reveal the existence of FCC type crystal .The crystalline size of deposits decreases due to online orientation of crystal while increasing bath temperature. The hardness increases with increasing bath temperature due to lower stress associated with thin films. When bath temperature is increased from 30˚C to 90˚C, the magnetization increases from 38.713 x 10^-3 emu/cm² to 152.824 x 10^-3 emu/cm²   .Also when temperature is increased from 30˚C to 90˚C, the coercivity decreases from 220.634 G to 121.82 G. This is due to nano crystalline structure of deposits. The addition of tungsten (W) with NiFe alloy in electroplating can enhance its magnetic, mechanical and structural properties and this alloy films can be used in NEMS,MEMS and memory devices.

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