Tuning the Crystal Structure and Magnetic Properties of CoNiFeB Thin

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Tuning the Crystal Structure and Magnetic Properties of CoNiFeB Thin Films Amin Azizi,*,†,‡,§ Amin Yourdkhani,# David Cutting,† Gabriel Caruntu,# and Noshir S. Pesika*,† †

Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118, United States Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States § Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States # Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana 70148, United States ‡

ABSTRACT: CoNiFeB thin films with a high saturation magnetic flux density (Bs) and a low coercivity (Hc) are used as core materials in high-density recording heads. Electroless deposition has proven to be an excellent process for the fabrication of metallic thin film structures, because it does not suffer from limitations associated with current density distribution and can therefore be used to create uniform coatings on substrates with fine and/or complex geometries. The crystal structure of CoNiFeB films, which is dependent on their chemical composition, can be controlled by changing the bath composition; however, to our knowledge, there is no report on tuning the chemical composition, crystal structure, and magnetic properties of CoNiFeB thin films only by the optimization of the electroless deposition parameters while maintaining the same concentration of salts in the bath. In this study, through the use of complementary techniques including AFM, SEM, XRD, EDS, and VSM, we show how the crystal structure of the CoNiFeB films can be tuned from a single-body-centered cubic (BCC) phase to a single facecentered cubic (FCC) phase with intermediate mixtures of BCC/FCC phases by only changing the pH and temperature of the electroless bath. KEYWORDS: thin films, magnetic recording heads, electroless deposition, crystal structure, magnetic properties surfaces and usually leads to film structures of uniform thickness and composition in the submicrometer scale. At the other end of the spectrum, electroless deposition methods are much more attractive for use in the fabrication of metallic thin film structures, since they do not present any limitations associated with current density distribution and usually lead to uniform coatings on substrates with fine and complex geometries.9,10 Although earlier attempts8,9 to synthesize CoNiFeB coatings via the electroless deposition method have reported interesting results, to our knowledge, there is no report on tuning the film composition and structure using electroless deposition from the same bath composition, and it is mentioned as a drawback of the electroless process.11 As suggested by previous studies, the crystal structure has a great impact on grain size and magnetic properties of CoNiFe films.6,12 A body-centered cubic (BCC) structure offers a high Bs,13 while a mixture of BCC and facecentered cubic (FCC) structures can improve the coercivity of the films showing a good combination of soft magnetic properties.3,8,12 In this study, we will discuss the role played by

1. INTRODUCTION With the tremendous rapid technological progress that our society has witnessed over the past few decades, magnetic recording media (i.e., hard disk drives (HDDs)) have gained considerable attention, because of their advantages in capacity, speed, and fabrication cost.1 Increase in the areal density of HDDs has been achieved via developments in miniaturizing the read-write head, reducing the head-medium spacing, and decreasing the size of the constituting grains of the magnetic thin film structures.2 Soft magnetic thin films with low coercivity (Hc), high saturation magnetic flux density (Bs), and high electrical resistivity (ρ) are used as the core material of high-density recording heads.1,3 Many soft magnetic films of such transition metals as Fe, Co, and Ni and their alloys have been proposed as suitable candidates for integration into magnetic data storage technology.3−6 NiFe alloys with low Hc and relatively high Bs were used for the first time as the core material in inductive heads by IBM.7 However, the interest in cobalt-containing alloys, such as CoNiFe, has recently grown, because of their promising characteristics, such as higher saturation flux density values.3,4,8,9 The most common technique for the fabrication of magnetic devices such as write-head cores with high aspect ratio is electrodeposition.3 However, this method is limited to conductive © XXXX American Chemical Society

Received: March 15, 2013 Revised: May 17, 2013

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dx.doi.org/10.1021/cm400861h | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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various deposition parameters on the film composition, crystal structure, morphology and the magnetic properties, and show for the first time how the crystal structure of the CoNiFeB thin films can be controlled in order to go from a single-BCC phase to a single-FCC phase or have a mixture of BCC/FCC phases and tune the ratio only via optimization of the deposition parameters while maintaining the same concentration of salts in the bath.

(PANalytical X’pert PRO) was utilized to determine the crystal structure of the samples. The compositions and microstructures of the films were explored using a high-resolution scanning electron microscopy (SEM) system (Hitachi 4800) equipped with an energy-dispersive X-ray spectrometry (EDS), and the magnetic properties of the CoNiFeB coatings were measured with a vibrating sample magnetometer (VSM) (Princeton Measurements Corporation, MicroMag 3900) at room temperature.

3. RESULTS AND DISCUSSION The chemical reactions describing the CoNiFeB film formation during the electroless plating process are the following:10

2. EXPERIMENTAL PROCEDURE CoNiFeB thin films were deposited on n-type silicon wafer via an electroless deposition method. Prior to the deposition, the silicon substrates were degreased with acetone for 5 min and rinsed with deionized (DI) water to remove the dust and the organic substances. Then, 2-nm titanium and 100-nm gold layers were sputtered onto the silicon wafers to act as a catalytic surface for the electroless deposition. The Ti/Au sputtered silicon substrates were dipped into an electroless plating bath (volume = 40 mL) for 60 s. The bath was alkaline in nature and contained sulfates of nickel (0.019 M), cobalt (0.019 M), and iron (0.013 M) as sources of cations and dimethylamine borane (DMAB, 0.07 M) as the reducing agent. The CoNiFeB coatings were obtained from solutions maintained at different values of the altered pH and temperature, respectively. First, samples were deposited from solutions with a constant temperature of 70 °C, whereas the pH was varied from 8 to 9.5. Ammonium hydroxide solution (NH4OH, 28.0%−30.0% NH3 basis) was added to the solution in order to reach the desired pH. The deposition then was performed at the fixed pH of 9 while the temperature of the bath was altered from 60 °C to 90 °C. During the plating process, the bath solution was stirred at 250 rpm with a magnetic agitator. The topography and thickness of the thin film samples were characterized by atomic force microscopy (AFM) (Asylum Research MFP-3D), whereas X-ray diffraction (XRD)

Co2 + + 2e− = Co

E 0 = −280 mV

(1)

Ni 2 + + 2e− = Ni

E 0 = −250 mV

(2)

Fe2 + + 2e− = Fe

E 0 = −440 mV

(3)

3Me2 + + 3R 2NHBH3 + 6H 2O = 3Me 0 + B + 3R 2NH+2 ⎛9⎞ + 2B(OH)3 + ⎜ ⎟H 2 + 3H+ ⎝2⎠ (4)

in which Me can be one of the Co, Ni, and Fe metals, R2NHBH3 is DMAB, and R is the (CH3) group. Metal ions in the solution consume the electrons liberated by the oxidation of DMAB and deposit on the substrate. AFM measurements indicated that the thickness of the CoNiFeB films obtained by the electroless deposition method are 60, 68, 74, and 93 nm for samples deposited

Figure 2. Chemical composition of the films versus (a) the pH and (b) the temperature of the treatment solution.

Figure 1. Deposition rate versus (a) pH and (b) temperature of the bath. B



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available at higher temperatures that can have a greater impact on iron than nickel to facilitate overcoming its larger reduction potential. On the other hand, Fe(OH)2 is more stable than iron at higher pH, according to the Pourbaix diagram of iron,14 which can be the reason for the lower content of iron in the films deposited from solutions with higher pH. It is worth noting that, although the concentration of Ni2+ and Co2+ ions in the bath are the same, the cobalt contents of the coatings are higher than their nickel contents. This indicates an anomalous behavior of cobalt, which preferentially deposits from the coating bath, despite the less-noble electrochemical potential of cobalt, compared to nickel. The low overpotential of cobalt deposition has been suggested, by T. Saito et al.,15 to be the reason for this phenomenon. XRD results presented in Figure 3 show an interesting relationship between the crystal structure of the films and the bath condition and film composition. Body-centered cubic (BCC) and face-centered cubic (FCC) phases are indexed in the XRD patterns. A decrease in BCC(110)/ FCC(111) phase ratio can be observed by increasing the pH of the bath in such a way that the BCC single-phase film deposited at pH 8 goes to the FCC single-phase film by increasing the pH of the bath to 9.5. It can be attributed to the nature of iron and nickel having BCC and FCC crystal structures, respectively. In contrast, the BCC(110)/FCC(111) phase ratio is increased by raising the

for 60 s from bath solutions with pH values of 8, 8.5, 9, and 9.5, respectively. The thickness of the films determines precisely the deposition rate of each particular experiment. The increase in the deposition rate with pH can be attributed to the decrease in the concentration of H+ ions in the treatment solution which can be presumably ascribed to the fact that the increase of the pH facilitates the direct reaction (reaction 4) between the metal ions and the reducing agent. It is also seen that the deposition rate increases with the temperature (Figure 1b) with a quasi-exponential variation consistent with the exponential dependence of the deposition rate with the energy common to electroless nickel-plating systems.10 The variation of the chemical composition of the CoNiFeB films with the pH and temperature of the bath (Figure 2) indicates some very interesting trends. Specifically, the iron content of the film structures decreases with the pH and increases with the temperature of the solution, whereas the nickel content follows an opposite trend and the cobalt content remains almost constant. Interestingly, the absolute values of the increase and decrease of the nickel content with the pH and temperature is comparable to those of the decrease and increase in the iron content, respectively. Comparing the standard reduction potential of iron (−440 mV) and nickel (−250 mV), it can be perceived that iron is more difficult to be reduced than nickel. The higher content of iron in the films deposited at greater temperatures may be due to more thermal energy

Figure 4. Atomic force microscopy (AFM) images of the electroless CoNiFeB films from the baths with different pH values (pH 8, 8.5, 9, and 9.5; T = 70 °C).

Figure 3. XRD results of the electroless CoNiFeB films from the bath with different (a) pH and (b) temperature. C



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temperature of the bath (from 60 °C to 90 °C), which is consistent with the increase in the iron content of the films. M. Saito et al.16 also reported the dependence of the BCC(110)/ FCC(111) phase ratio of the electrodeposited CoNiFe films on the iron content. Figure 4 shows the AFM images of the electroless CoNiFeB films fabricated from the solutions with different pH. As is seen, all the samples are very uniform and the average size of the particulates becomes smaller by increasing the pH, which can be attributed to the increase in the deposition rate. However, the sample deposited at pH 9 with the mixture of BCC/FCC phases has the smallest particulate size, which can be ascribed to the competitive growth between the two phases, which significantly limits the particle size. AFM results also indicated that the surfaces of the films are obviously smooth, showing roughness (RMS) values of 2.54, 1.59, 1.59, and 3.72 nm for the samples deposited at pH 8, 8.5, 9, and 9.5, respectively. Three-dimensional (3D) and two-dimensional (2D) AFM images of the films deposited from the baths with different temperature are presented in Figure 5. Decrease in the average

70, 80, and 90 °C, respectively, which is desirable for device applications. The dependence of surface morphology on the pH and temperature of the bath can be observed in SEM images (Figure 6). They confirm the AFM data indicating the change in the average particle size by altering the bath condition.

Figure 6. SEM images of the films deposited from the solutions with different pH and temperature. Figure 5. AFM images of the electroless CoNiFeB films from the baths with different temperature (T = 60, 70, 80, and 90 °C; pH 9).

However, the samples having the mixture of BCC/FCC phases show small particle size. The surface morphology also varied significantly with the pH of the bath, and the polyhedral morphology can be only seen for the film deposited at pH 8. The larger particle size of the film deposited at 60 °C can be ascribed to its slower deposition rate which allows the initial nuclei to grow. A reasonable relationship between the magnetic properties of the samples and their crystal structure was found. Many

particle size of the films by raising the temperature can be observed. It can be attributed to the increase in the deposition rate, as the nucleation rate increases with the deposition rate and precedes the grain-growth rate. According to the AFM data, all the coatings are visibly smooth having RMS values of 1.97, 1.59, 1.55, and 1.33 nm for the samples deposited at 60, D



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with the pH and temperature of the bath were observed. It was shown that the crystal structure of the CoNiFeB films which is connected to their chemical composition can be controlled in such a way that an increase in the BCC(110)/FCC(111) phase ratio was observed by decreasing the pH and raising the temperature of the bath. The correlations between the film composition, crystal structure, morphology, and magnetic properties were also discussed.

other factors such as grain size, surface roughness, stress, and thickness also influence the magnetic properties of thin films.3,17,18 However, VSM measurements (Figure 7)

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AUTHOR INFORMATION

Corresponding Author

*E-mail addresses: [email protected] (A.A.), npesika@tulane. edu (N.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF), under the NSF EPSCoR Coorperative Agreement No. EPS-1003897, with additional support from the Louisiana Board of Regents and the Gulf of Mexico Research Initiative.

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REFERENCES

(1) Sugiyama, A.; Hachisu, T.; Osaka, T.; Datta, M.; ShachamDiamand, Y. Magnetic Thin Films for Perpendicular Magnetic Recording Systems. In Electrochemical Nanotechnologies; Springer: New York, 2010; pp 87−98. (2) Ross, C. Annu. Rev. Mater. Res. 2001, 31 (1), 203−235. (3) Osaka, T.; Takai, M.; Hayashi, K.; Ohashi, K.; Saito, M.; Yamada, K. Nature 1998, 392 (6678), 796−798. (4) Zhang, Y.; Ivey, D. G. Chem. Mater. 2004, 16 (7), 1189−1194. (5) Andricacos, P. C.; Robertson, N. IBM J. Res. Dev. 1998, 42 (5), 671−680. (6) Osaka, T.; Asahi, T.; Kawaji, J.; Yokoshima, T. Electrochim. Acta 2005, 50 (23), 4576−4585. (7) Chiu, A.; Croll, I.; Heim, D. E.; Jones, R. E., n., Jr.; Klaassen, K. B.; Mee, C. D.; Simmons, R. G. IBM J. Res. Dev. 1996, 40, 283. (8) Rohan, J. F.; Ahern, B. M.; Reynolds, K.; Crowley, S.; Healy, D. A.; Rhen, F. M. F.; Roy, S. Electrochim. Acta 2009, 54 (6), 1851−1856. (9) Yoshino, M.; Kikuchi, Y.; Sugiyama, A.; Osaka, T. Electrochim. Acta 2007, 53 (2), 285−289. (10) Mallory, G. O.; Hajdu, J. B. Electroless Plating: Fundamentals and Applications (sponsored by American Electroplaters and Surface Finishers Society); The Society: Orlando, FL, 1990. (11) Pané, S.; Pellicer, E.; Sivaraman, K. M.; Suriñach, S.; Baró, M. D.; Nelson, B. J.; Sort, J. High-performance electrodeposited Co-rich CoNiReP permanent magnets. Electrochim. Acta 56, (24), 8979−8988. (12) Nakanishi, T.; Ozaki, M.; Nam, H.-S.; Yokoshima, T.; Osaka, T. J. Electrochem. Soc. 2001, 148 (9), C627−C631. (13) Okada, Y.; Kudo, K.; Yoshida, N.; Fuyama, M.; Hoshiya, H. IEEE Trans. Magn. 2002, 38 (5), 2256−2258. (14) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; National Association of Corrosion Engineers (NACE): Houston, TX, 1974. (15) Saito, T.; Sato, E.; Matsuoka, M.; Iwakura, C. J. Appl. Electrochem. 1998, 28 (5), 559−563. (16) Saito, M.; Ishiwata, N.; Ohashi, K. J. Electrochem. Soc. 2002, 149 (12), C642−C647. (17) Jung, H. S.; Doyle, W. D.; Matsunuma, S. J. Appl. Phys. 2003, 93 (10), 6462−6464. (18) Rhen, F. M. F.; Roy, S. J. Appl. Phys. 2008, 103 (10), 103901− 103904.

Figure 7. Magnetization curves of the deposited CoNiFeB films from the baths with different pH values (pH 8, 8.5, 9, and 9.5; T = 70 °C) and temperatures (T = 60, 70, 80, and 90 °C; pH 9).

demonstrate that the saturation magnetic flux density (Bs) of the CoNiFeB films decreases and increases with the pH and temperature of the bath, respectively, which can be attributed to the change in the iron content and, consequently, the BCC/FCC phase ratio of the samples. The increase in the Bs value of CoNiFe films with the BCC/FCC phase ratio have been also reported by other groups12,16 and the larger magnetization of BCC than FCC structure has been proposed as the reason.12 However, the saturation magnetization of the sample deposited at 80 °C is larger than that of the sample deposited at 90 °C, which can be due to contribution of other factors rather than their BCC/FCC phase ratio. On the other hand, the coercivity (Hc) of the coatings roughly decreases by increasing both the pH and temperature of the bath which can be due to the smaller average particle size of the samples deposited at higher pH and temperature.13

4. CONCLUSIONS The tuning of the chemical composition, crystal structure and magnetic properties of CoNiFeB thin films only by the optimization of electroless deposition parameters and not changing the concentration of salts in the bath was studied. Interesting trends in the variation of the chemical composition of the films E


Tuning the Crystal Structure and Magnetic Properties of CoNiFeB Thin

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