Effect of an annealing on magnetic properties of FeNi films electroplated in citric-acid-based plating baths

We have already reported Fe-Ni films with good soft magnetic properties prepared by using an electroplating method. In the present study, we employed an annealing for further improvement in soft magnetic properties of the electroplated Fe-Ni films. The annealing reduces the coercivity of the films, and the reduction rate of the coercivity depended on the Cl- ion concentration in the bath. The Fe22Ni78 films prepared in the plating bath with high Cl- ion concentration showed large reduction rate of the coercivity, and we found that the annealing is more effective for high Cl- ion concentration bath since much lower coercivity value can be obtained compared with that for low Cl- ion concentration one.


I. INTRODUCTION
Fe-Ni films have been applied to magnetic cores of fluxgate sensors, 1,2 and an electroplating method is one of effective methods to obtain soft magnetic films due to the high economic viability of the process. We reported Fe-Ni films with good soft magnetic properties prepared by an electroplating method, 3 and many researchers also reported electroplated Fe-Ni soft magnetic films. [4][5][6][7][8] In general, residual internal stress in soft magnetic materials degrades their soft magnetic properties, and the materials are typically annealed to release the stress and improve the magnetic properties. It is well-known that as-plated films also exist internal stress, and some additives in plating baths are employed to reduce the stress. Thus, it is expected that an annealing improves magnetic properties of the electroplated Fe-Ni films.
Recently, we have reported the effect of Clion concentration in plating baths, and confirm that moderate low Clconcentration is preferred for obtaining the Fe 22 Ni 78 films with good soft magnetic properties. 9 In electroplating of Ni film, Watts-type plating baths are widely used, and the baths contain a small amount of Clions to enhance Ni anode corrosion and prevent anode passivation. Moreover, it is well-understood that high Clion concentration in the Watts-type baths increases tensile stress in as-plated Ni films. Therefore, we considered that the Clion concentration affects degree of the improvement in soft magnetic properties by the annealing for stress relaxation. In the present study, we investigated an improvement in soft magnetic properties of the electroplated Fe-Ni films using an annealing, considering Clion concentration in plating baths.

A. Electroplating of Fe-Ni films
We electroplated Fe-Ni films on Cu substrates using a direct current. The components in plating baths are shown in Table I   concentrations in the plating baths. The plating conditions were determined based on our previous studies for Fe-Ni films, 3 and summarised in Table II.

B. Annealing of Fe-Ni films
The films were annealed in a vacuum (< 10 -2 Pa). In this experiment, we varied the annealing temperature from 100 to 400 • C. The temperature was ramped from room temperature to the annealing temperature for 5 min, and then kept at constant for 60 min.

C. Measurements
The dc-hysteresis loops of the sheet-shaped Fe-Ni films (15 mm in length, 5 mm in width, and approx. 15 µm in thick) were measured with a B-H tracer (Riken Denshi BHS-40) operated at a field sweep rate of 50 mHz. The maximum excitation field of approximately 4 kA/m was used for the B-H measurements, and the coercivity values were obtained for the measured loops. The compositions and crystal structures of the films were analyzed with an energy dispersive X-ray spectrometry (Hitachi High-technologies S-3000) and X-ray diffraction patterns (Miniflex 600-DX), respectively. The thicknesses of the as-plated Fe-Ni films were measured with a micrometer (Mitutoyo, CPM15-25MJ). As we fixed the current density and the plating time, all films showed almost the same thickness (approx. 15 µm).

III. RESULTS AND DISCUSSION
To determine an annealing temperature, we annealed an electroplated Fe 22 Ni 78 film, and evaluated the coercivity value of the annealed one. Figure 1 shows coercivity of the annealed Fe 22 Ni 78 film as a function of annealing temperature. The coercivity for the as-plated films (before annealing) is also shown as a dashed line. As shown in Fig.1, the coercivity slightly decreases with increasing the annealing temperature from 100 to 300 • C, and then dramatically increased.
From the result for Fig.1, we determined the annealing temperature as 300 • C. Figure 2 shows the coercivity of the as-plated Fe-Ni films and the annealed ones as a function of NaCl concentration in the bath. In this experiment, we fixed FeSO 4 ·7H 2 O concentration at 50 g/L. As shown in Fig.2, the coercivity for the as-plated Fe-Ni film increases with increasing the NaCl concentration whereas that for the annealed one decreases. Figure 3 shows reduction ratio of the coercivity, ∆H c /H c , and Fe content of the Fe-Ni films as a function of NaCl concentration in the bath. We calculated ∆H c /H c using the result for Fig.2   concentration in the bath. Since the ∆H c /H c clearly decreases with increasing the NaCl concentration, we found that the Na + and/or Clconcentrations affect degree of ∆H c /H c . As mentioned in Introduction, we have already confirmed Clion concentration affects coercivity of the as-plated Fe-Ni films. Therefore, we focused on Clion concentration in our plating bath, and varied the Clion concentration by a replacement of FeSO 4 with FeCl 2 . Figure 5 shows the coercivity of the as-plated Fe 22 Ni 78 films and the annealed ones as a function of FeCl 2 concentration. For the as-plated films, the coercivity increases with increasing the FeCl 2 concentration. This result implies that lower Clion concentration is preferable to obtain low coercivity for as-plated Fe 22 Ni 78 films, and agrees with our previously-reported result. 9 The annealing reduces the coercivity of the films, and slight decrease in coercivity was observed with increasing the FeCl 2 concentration. From the result for Fig.5, we found that the ∆H c for the bath with high Clconcentration is much larger than that for low Clone. Consequently, the Fe 22 Ni 78 films prepared in the plating bath with high Clconcentration show large reduction rate of the coercivity.
To further understand the effect of the Clion concentration, we calculated the grain size from the measured XRD patterns using the Scherrer's formula. Figure 6 shows grain size of the as-plated films and the annealed ones as a function FeCl 2 concentration in the bath. As the grain growth by the annealing was inhibited for high Clconcentration bath compared with that for low Clconcentration one, we considered that much smaller grain size of the annealed film is a factor to obtain low coercivity, indicating that the grain refinement reduces the effective magnetic crystalline anisotropy of fcc Fe-Ni nanocrystalline phase. 10,11 From the above-mentioned results, we found that the Clconcentration in the bath affects the grain size and the coercivity of the as-plated Fe-Ni films and the annealed ones. In our experimental conditions, a plating bath with low Clconcentration is preferable for the as-plated films, and that with high Clone is suitable for the annealed ones.