We recently reported the electroplating of Fe-Pt thick films using plating baths with varying concentrations of NaCl, suggesting the potential for coercivity enhancement by Na ions. In the present study, our focus shifted to not Na ions but Cl ones, and we investigated the effect of the Cl ions on the crystal structures and magnetic properties of electroplated Fe-Pt films. With the increasing concentration of Cl ions, the coercivity of the films decreased. Furthermore, XRD analysis suggested that the Cl ions in the plating baths prevent the L10 ordering of the Fe-Pt crystalline phase, leading us to conclude that a Cl-free bath is favorable for preparing Fe-Pt thick-film magnets with high coercivity.

L10 Fe-Pt thick films (>1 µm) are attractive materials as film magnets for medical devices due to their favorable hard magnetic properties and high biological safety.1,2 While various fabrication methods, including dry processes (sputtering and vapor deposition) and wet processes (electroplating), are employed to prepare Fe-Pt films, typical dry processes are not acceptable for preparing thick films due to their low deposition rate. Consequently, researchers have turned to electroplating to produce L10 (Fe, Co)-Pt thick films, expecting higher deposition rates, and have reported good hard magnetic properties.3–6 

In our previous investigations, we also electroplated (Fe, Co)-Pt films and successfully achieved Fe-Pt thick films (>10 µm) on a Ta substrate with a high coercivity (>700 kA/m) and a remarkably high deposition rate (>40 µm/h).7,8 In a recent study, we investigated the effect of NaCl concentration in plating baths on the coercivity of the electroplated Fe-Pt films and confirmed the potential for coercivity enhancement by Na ions.9 Varying the NaCl concentration led to changes not only in Na ion concentration but also in Cl ion one, implying that Cl ions may also enhance coercivity. In the present study, we, therefore, electroplated Fe-Pt films from plating baths with varying Cl ion concentrations and evaluated the structural and magnetic properties of the films.

We fabricated hard magnetic Fe-Pt films using electroplating. The plating bath conditions are shown in Table I. Although we employed FeSO4·7H2O as an iron reagent in our previous studies,10,11 FeCl2·4H2O was used to change the Cl ion concentration in Experiment A. In Experiment B, we changed the Cl ion concentration by pre-electroplating. The details for the procedures of Experiment B are shown later. Citric acid and NH4NH2SO3 were added to suppress the formation of iron oxides during the plating and to improve the solubility of Pt(NO2)2(NH3)2, respectively. The electroplating conditions are shown in Table II.

TABLE I.

Bath conditions of Experiment A.

ReagentsContents
FeSO4·7H212-x [g/L] 
FeCl2·4H2x = 0–7 g/L 
Pt(NO2)2(NH3)2 10 g/L 
Citric acid(C6H8O7·H2O) 30 g/L 
NH4NH2SO3 25 g/L 
ReagentsContents
FeSO4·7H212-x [g/L] 
FeCl2·4H2x = 0–7 g/L 
Pt(NO2)2(NH3)2 10 g/L 
Citric acid(C6H8O7·H2O) 30 g/L 
NH4NH2SO3 25 g/L 
TABLE II.

Electroplating conditions.

ConditionsValues
Bath temperature 70 °C 
Anode/Cathode (Substrate) Pt mesh/Cu plate (500 μm in thick) 
Plating area 5 mm × 5 mm 
Plating time 5-15 min 
ConditionsValues
Bath temperature 70 °C 
Anode/Cathode (Substrate) Pt mesh/Cu plate (500 μm in thick) 
Plating area 5 mm × 5 mm 
Plating time 5-15 min 

The as-plated Fe-Pt films had disordered fcc structures, and this magnetic phase shows low coercivity. To transform from the disordered phase to the ordered L10 (fct) one, we employed 700 °C annealing in a vacuum (≈10−3 Pa) using a furnace. The temperature gradually increased from room temperature to 700 °C at a heating rate of 100 °C/min and then kept at 700 °C for 5 minutes.

The thickness and composition of the as-plated film were measured at 25 points and nine points, respectively, using a micrometer (Mitutoyo CPM15-25MJ) and a scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) system (Hitachi High-technologies S-3000). We averaged the measured values and employed the averaged values as the film thickness and composition. The hysteresis loops of the annealed Fe-Pt films were obtained using a vibrating sample magnetometer (VSM). The maximum applied field for the loop measurements was approximately 2 MA/m, and coercivity values were extracted from the loops. Additionally, the X-ray diffraction (XRD) patterns of the annealed Fe-Pt films were analyzed using an X-ray diffractometer with Cu-Kα radiation (Rigaku, Miniflex600-DX).

Figure 1 shows the coercivity of the annealed Fe50Pt50 films as a function of the FeCl2 concentration. The composition of the film was controlled at Fe50Pt50 by the change in the amount of Fe reagents (FeSO4 and FeCl2) in the bath. As shown in Fig. 1, the coercivity decreased with increasing the FeCl2 concentration. This result suggests that the Cl-free bath obtains the Fe-Pt thick-film magnets with high coercivity and implies that the Cl ion affects the coercivity. To confirm the effect of the Cl ion on the ordering process, we decided to carry out an XRD analysis.

FIG. 1.

The coercivity for the annealed Fe50Pt50 films as a function of the FeCl2 concentration. The inset indicates hysteresis loops of the films prepared from the baths with and without Cl ions (7 and 0 g/L of FeCl2).

FIG. 1.

The coercivity for the annealed Fe50Pt50 films as a function of the FeCl2 concentration. The inset indicates hysteresis loops of the films prepared from the baths with and without Cl ions (7 and 0 g/L of FeCl2).

Close modal

Figure 2 shows the Δ2θ, the diffraction peak splitting between fct [200] and [002], as a function of the FeCl2 concentration. Since the diffraction peak of fcc (200) for the as-plated film is split into two peaks of fct [200] and [002] by the development of the L10 ordering, we estimated the degree of the L10 ordering from the peak splitting Δ2θ. Degree parameter S is generally used to estimate the degree of the L10 ordering Fe-Pt crystalline phase. In our results, although we calculated S using the results of XRD analysis, S sometimes showed greater than one. From the TEM observations demonstrated in our previous study,12 we confirmed that the annealed Fe-Pt film consists of more than two magnetic phases (Fe80(PtCu)20 phase and Fe40Pt40Cu20 one). In addition, as we varied Cl ions concentrations in the present study, it is expected that the compositions of these magnetic phases and the degree of impurities had been changed. We, therefore, considered that it is difficult to discuss the magnetic properties of the annealed Fe-Pt films using S. For these reasons, we used Δ2θ to estimate the degree in this paper. As shown in Fig. 2, the Δ2θ slightly decreased with increasing the FeCl2 concentration, implying that the increasing Cl ion tends to suppress the L10 ordering.

FIG. 2.

The Δ2θ which is diffraction peak splitting between fct [200] and [002], for the annealed Fe50Pt50 films as a function of the FeCl2 concentration. The inset indicates XRD patterns of an as-plated Fe-Pt film and the annealed one.

FIG. 2.

The Δ2θ which is diffraction peak splitting between fct [200] and [002], for the annealed Fe50Pt50 films as a function of the FeCl2 concentration. The inset indicates XRD patterns of an as-plated Fe-Pt film and the annealed one.

Close modal

We considered that the results for Figs. 1 and 2 imply that the coercivity was influenced by the Δ2θ. To confirm the relationship between coercivity and Δ2θ, the coercivity values from Fig. 1 were represented in Fig. 3 as a function of Δ2θ. As shown in Fig. 3, we observed a strong correlation between coercivity and Δ2θ for the Cl-added bath. The results for the Cl-free bath showed a different correlation than that for the Cl-added bath. This disagreement suggests that the presence of Cl ions significantly affects the ordering process.

FIG. 3.

The coercivity of the annealed Fe50Pt50 films as a function of the Δ2θ.

FIG. 3.

The coercivity of the annealed Fe50Pt50 films as a function of the Δ2θ.

Close modal

In Experiment A, we varied the Cl ion concentrations in the plating baths by changing the amount of FeCl2 reagent. In Experiment B, we investigated the variations of the pre-plating time.

In our plating baths, the following reactions occur on the anode surface.
2H2OO2+4H++4e
(1)
2ClCl2+2e
(2)

Since the generation of Cl2 gas is much easier than that of O2, Cl2 gas is generated during the plating from the anode for Cl-added baths. In other words, the Cl ion concentration in the plating bath decreases with increasing the plating time. We, therefore, employed pre-electroplating to vary the Cl ion concentration.

Firstly, we prepared the plating bath shown in Table I again. In Experiment B, FeSO4·7H2O, FeCl2·4H2O, and NH4NH2SO3 were fixed at 0, 5 and 20 g/L, respectively. We call this bath a “Fresh bath” in this paper. The Fe-Pt film was electroplated for tpre minutes as a dummy film to reduce the Cl ion concentration in the bath. tpre indicates pre-electroplating time. After the pre-plating, we electroplated Fe-Pt film on the Cu substrate using the same plating bath. This bath is described as a “Used bath.” The effect of the pre-plating time on the coercivity of the annealed Fe-Pt films was investigated in Experiment B.

Figure 4 shows the coercivity of annealed Fe50Pt50 films as a function of pre-electroplating time. In Fig. 4, the results for the plating time of 5 and 15 min are shown. The difference in coercivity values of the films for 5 min and 15 min is attributed to the thickness of the films. We have already confirmed that the Cu diffusion from the substrate during the annealing for the L10 ordering reduces the coercivity.12 Since the thickness for 5 min is thinner than that for 15 min, the effect of the Cu diffusion on the magnetic properties for 5 min is much larger than that for 15 min. Therefore, the coercivity for 5 min is lower than that for 15 min. As shown in Fig. 4, the coercivity increased with increasing the pre-plating time up to 15 min, and this result suggests that the reduction in the Cl ions increases the coercivity.

FIG. 4.

The coercivity of the annealed Fe50Pt50 films as a function of the pre-electroplating time.

FIG. 4.

The coercivity of the annealed Fe50Pt50 films as a function of the pre-electroplating time.

Close modal

From Figs. 1 and 4, we found that the Cl ions are not suitable for obtaining the Fe-Pt films with high coercivity, and we concluded that Cl ions have no enhancement effect of coercivity.

In conclusion, we investigated the effect of the Cl ions on the magnetic properties and the crystal structure of the electroplated Fe-Pt thick-film magnets. The obtained results are summarized as follows:

  1. The Fe-Pt films prepared from the baths with high Cl ion concentration tended to show low coercivity and small Δ2θ, and we confirmed a good correlation between the coercivity and the Δ2θ.

  2. The Cl ion prevented the L10 ordering and consequently the coercivity was reduced.

  3. The Cl-free bath is useful for obtaining the Fe-Pt thick-film magnet with high coercivity.

The authors have no conflicts to disclose.

Takeshi Yanai: Investigation (equal). Daiki Fukushima: Investigation (equal). Ryuta Narabayashi: Investigation (equal). Naoki Ogushi: Investigation (equal). Yuito Yamaguchi: Investigation (equal). Akihiro Yamashita: Investigation (equal). Masaki Nakano: Investigation (equal). Hirotoshi Fukunaga: Investigation (equal).

The data that support the findings of this study are available from the corresponding author or the first author upon reasonable request.

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