Magnetic properties of cobalt ferrite films with perpendicular magnetic anisotropy

Magnetic properties of cobalt ferrite films with perpendicular magnetic anisotropy

Jownalof magnetism and magnetic ~ H materials •il • Journal of Magnetism and Magnetic Materials 176 (1997) 31 35 ELSEVIER Magnetic properties of co...

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Jownalof magnetism and magnetic ~ H materials

•il • Journal of Magnetism and Magnetic Materials 176 (1997) 31 35

ELSEVIER

Magnetic properties of cobalt ferrite films with perpendicular magnetic anisotropy Nobuyuki Hiratsuka, Masayuki Nozawa, Koichi Kakizaki* Graduate School (~[Science and Engineering, Saitama UniversiO,. 255 Shimo-ohkubo, Urawa 338, Japan

Abstract Cobalt ferrite films with perpendicular magnetic anisotropy were fabricated by facing targets sputtering and their magnetic properties and recording characteristics were studied. When a film was deposited at Po_, = 0 mTorr and annealed at T, = 300cC, a (CoFe204)-(Co CoO) composite film with perpendicular magnetic anisotropy was formed. The composite film, which had He± = 1.5 kOe, Her = 3.5 and Sq± = 0.46, had magnetic properties superior to those in a CoFe204 single-phase film. The recording characteristics of the (CoFe204~(Co-CoO) composite film were measured. The reproduced waveform was a dipulse type, indicating that the film had a perpendicular magnetic anisotropy. The value of Dso was 48.5 kFCI. Keywords: Thin films - cobalt ferrite; Anisotropy

perpendicular; Facing targets sputtering; Recording characteristics;

Post-annealing

1. Introduction

Much research has been done recently on the suitability of Co-Cr-based films [1, 21 and Ba ferrite films [-3,4] as perpendicular magnetic recording media. However, relatively high substrate temperature and annealing temperature are needed in the formation of Ba-ferrite films. It was hoped that the Co-ferrite thin films investigated in this study could be formulated at a lower annealing temperature. The cobalt ferrite thin films studied in other reports showed perpendicular magnetic anisotropic *Corresponding author. Fax: + 81 48 858 3099; e-mail: [email protected]

properties, but their reproduced output was insufficient on account of their low squareness ratio. The perpendicular magnetic anisotropic properties of the film are as a result of their predominantly columnar structure [5, 6]. In this study, a cobalt ferrite thin film which incorporated cobalt's strong magnetic anisotropic properties was prepared. The film was deposited by facing targets sputtering, and its perpendicular magnetic anisotropy, magnetic properties and recording characteristics were investigated.

2. Experiment

The films were deposited using the facing targets sputtering method. Cobalt chips were attached to

0304-8853/97/$17.00 ~i 1997 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 t 0 0 6 2 9 - X

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N. Hiratsuka et aL / Journal of Magnetism and Magnetic Materials 176 (1997) 31-35

a Fe disc target so that the surface area ratio of Fe to Co was 2:1. The chamber was evacuated to pressure less than 2 x 10-s Torr, and then 02 gas with a partial pressure of 0-0.20 mTorr, where the total pressure of the gas mixture of O2 and Ar was 15 mTorr, was fed into the chamber. The target was sputtered for 2 h at an applied voltage of 4.4 kV and current of 10 mA. The substrate was a glass slide or a 3.5 in glass disc, which was rotated at 5 rpm. The substrate temperature was not controlled. The deposited film was then annealed in air at 200-500°C for 5 h and cooled slowly. The crystal structure, magnetic properties and the electronic state of the various elements in the film were investigated using X-ray diffraction (XRD), vibrating sample magnetometer (VSM) with an applied magnetic field of 7 kOe, and X-ray photoelectron spectroscopy (XPS), respectively. The recording characteristics were measured for the (CoFe204)-{Co-CoO) composite film deposited on a glass disc which had been polished by buff and coated with a lubricant of 20 nm. The conditions for measuring the recording characteristics were: 7.83 m/s velocity, 8 mm track width for the disc, and 0.3 mm gap length for the thin-film ring head.

3. Results and discussion

Fig. 1 shows the X-ray diffraction patterns at various annealing temperatures (Ta) of films deposited at a partial O2 gas pressure (Po2) of 0 mTorr. In the as-deposited film the Fe(1 10) is strongly oriented and present in a crystal phase. The cobalt is present in a fine crystalline structure. In the film annealed at T~ = 200°C, the Fe peak disappeared, Cobalt became more crystalline, and fine crystals of spinel phase CoFe204 and CoO phase were also present. For the film annealed at Ta = 300°C, the predominant phase is (2 20) CoFe204 which has a relatively high degree of orientation, and the Co phase is also present. For the film annealed at T, = 400°C or above there is no Co phase, this having been replaced by a non-magnetic Co304 phase. The CoFezO4 phase increases with increase of T,, and a film is formed with the <22 0) oriented perpendicular to the film surface.

Po2=OmTorr

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• •

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• •

~2



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........_..200°

~i~

~7

© c"

A --J

~

20304050607080

As-depo. L__J___J.__

20 (degrees)

Cu-Ka

Fig. 1. X-ray diffraction patterns of Po2 = 0 m T o r r films annealed at various temperatures.

Fig. 2 shows the X-ray diffraction patterns of films prepared at Po, = 0.20 mTorr and annealed at various temperatures. The CoO phase is present in the as-deposited film. This phase is not present in the film deposited at Po2 = 0 reTort. For films prepared at Ta = 300°C or below, the Co and CoO phases crystallize. Furthermore, in contrast to films deposited at Po2 = 0 mTorr, no ( 2 2 0 ) oriented CoFe204 is present. Fig. 3 shows temperature dependence ofcoercivity (He±) measured perpendicular to the film surface, and the coercivity ratio (Her) for a film prepared at Po2 = 0 and 0.20 mTorr. Her is calculated as the ratio of He and the in-plane coercivity (Hell). He± increases when T a increases, and its maximum value is reached when T a - - - - 4 0 0 ° C . However, H¢r decreases as Ta increases, and when Ta = 400°C or above, the film is virtually isotropic. For all annealing temperatures, values of Hc and Her are higher in films deposited at Po2 = 0 mTorr rather than those at Po2 = 0.20 mTorr. This is due to the large influence of the non-magnetic properties of the crystalline Co and CoO phases which are formed early in the films deposited at Po~ = 0.2 mTorr. The film prepared at Po2 = 0 mTorr and annealed at Ta = 300°C produces the maximum value of perpendicular magnetic anisotropy.

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N. Hiratsuka et al. ,/Journal of Magnetism and Magnetic Materials 176 (1997) 3l 35

Po2=0mTorr

Po2=O,2OmTorr oon

~(311)

OCoFe204 OC OCo304 I ~Tc~-Fe]

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....

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~7

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20

30

I

40 20

I

I

I

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50

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Fig. 2. X-raydiffraction patterns of Po~ = 0.20 mTorr films annealed at various temperatures.

2.0

,

,

805 800 795 790 785 780 775 770

Binding energy (eV) Fig. 4. Annealing temperature dependence of the binding energy of C02v electron of the Po~ = 0 mTorr films.

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, 200

, 300

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Fig. 3. Annealing temperature dependence of H~l and He, for Po: = 0 and 0.20 mTorr films.

The magnetization value of the film made in these conditions is 220 emu/cc. Fig. 4 shows the XPS spectra of Co ions for films annealed at various temperatures when Po2 = 0 mTorr. In the as-deposited film Co exists mostly in the metallic phase, but as Ta increases Co z + ions increase. In the film deposited when T a - - - - 5 0 0 ° C there is no metallic Co present. However, in the film deposited at T, = 300°C large quantities of both Co 2+ ions and metallic Co were detected.

Combining this result with those from Fig. 1, we can conclude that the film annealed when Ta = 300°C is a ( C o F e 2 0 ¢ ) ~ C o - C o O ) composite film. Fig. 5 shows the hysteresis loops for (a) a ( C o F e 2 0 4 ~ ( C o - C o O ) composite film and (b) a single-phase CoFe204 film prepared under different conditions from this experiment. For the CoFe20¢ single-phase film in Fig. 5b, He± = 1.6 kOe, Her = 2.5 and the squareness ratio (Sq±) measured perpendicular to the film surface is 0.35. For the (CoFezO¢)-(Co-CoO) composite film in Fig. 5a, He± = 1.5 kOe, Her = 3.5 and Sq± = 0.46. The ( C o F e 2 0 4 ) - ( C o - C o O ) composite film has a superior squareness to the CoFe204 single-phase film, where the low squareness ratio is a vulnerable point. This suggests that the perpendicular magnetic anisotropy of the (CoFezO4)-(Co CoO) composite film is also greater than in the CoFe204 film. Fig. 6 shows a torque curve of (CoFe20¢)( C o - C o O ) composite film. Because the torque curve is assigned to a sin 20 curve, the film has uniaxial magnetic anisotropy perpendicular to the film surface. Large uniaxial magnetic anisotropy

34

N. Hiratsuka et al. / Journal of Magnetism and Magnetic Materials 176 (1997) 31-35 IIIIIII

IIIIIII

~N----~'.4-

IIIIIII -7

IIIIIII 0

+ z

Applied field (kOe)

(a)

IIIIIII

IIIIIII

Fig. 7. Reproduced waveform of(CoFe204)-(Co-CoO)composite film disk.

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CI 0 C

= 0

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IJlllll

-7

Illllll

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(b)

Fig. 5. Hysteresisloopsof(a) (CoFezO4)-(Co-CoO)composite film and (b) CoFe204 film.

xll 3.0 0 U

E 0

c

>"O

2.0

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1.0

0

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180

270

360

e (degrees) Fig. 6. Magnetic torque curve of (CoFe204)-(Co CoO) composite film.

constant, Ku of 2.58 x l0 s erg/cc (2.58 x 10 4. J/m 3) was obtained. The recording characteristics were measured for a (CoFe204)-(Co-CoO) composite film annealed onto a 3.5 in disc prepared at Po2 = 0 mTorr, and annealed at Ta = 300°C. The thickness of the film was approximately 500 nm. Fig. 7 shows the reproduced output waveform measured at frequency of 0.5 MHz for the disc as stated above. The waveform is of the dipulse-type unique to perpendicular magnetic recording media. This shows that the film possesses a magnetic component oriented perpendicular to the film surface. The dipulse ratio of 0.34 is considerably lower than the value of 0.8-0.9 obtained from other perpendicular magnetic recording media. This is thought to be caused by the low degree of perpendicular magnetization of this film. Fig. 8 shows the properties of the recording density of the reproduced output of the film formulated on a disc substrate. When the recording density exceeds 30 kFCI the reproduced output (Ev) decreases rapidly, and Dso = 48.5 FCI. The output remains low and does not reach high recording density because the film surface is not smooth and the flying height of the magnetic head is large. Therefore, if we remove impurities occurring during the annealing process and lessen the flying height to some extent, we can expect the recording characteristics to improve considerably.

N. Hiratsuka et al. /Journal of Magnetism and Magnetic Mawrials 176 (1997) 31 35

where the squareness value was inferior. The (CoFe20,,)-(Co CoO) composite film has Hc = 1.5kOe, H e r = 3.5, Sq =0.46 and uniaxial magnetic anisotropy constant, Ku, of 2.58 x 10s erg/cc (2.58 x 104 j/mS). . When the recording characteristics of the (CoFe204)-(Co CoO) composite film were measured, the reproduced output waveform was of a dipluse type. This shows the film to be of perpendicular magnetic anisotropic type. The dipulse ratio was 0.34 and Ds0 was 48.5 kFCI.

0.20

8 o.lo

..................................................................................

Dso = 48.5 k c~

0

35

10 100 Recording density (kFCl)

Acknowledgements Fig. 8. Recording density characteristics (Co CoO) composite film disk.

of (CoFe.,O4)

4. Conclusions A cobalt ferrite perpendicular magnetic anisotropic film was fabricated using the facing targets sputtering method, and its magnetic properties and recording characteristics were measured. The following results were obtained. 1. Excellent film for perpendicular magnetic anisotropy was obtained when Po: = 0 mTorr and Ta = 300°C. The film structure is a (CoFe204)(Co-CoO) composite film. 2. On account of its high squareness value, the (CoFe20,~)-(Co-CoO) composite film is superior as a perpendicular magnetic anisotropic film to past CoFe204 single-phase films

The authors thank Mr. Makoto Mizukami of Victor Company of Japan, Ltd for his assistance in measuring the recording characteristics for this research.

References [-1] S. lwasaki, Y. Nakamura, K. Ouchi, IEEE Trans. Magn. 15 (1979) t456. [2] Y. Maeda, T. Ohkubo, K. Takei, D.J. Rogers, K.L. Babcock, J. Magn. Soc. Japan 19 (1995) 706. [3] A. Morisako, M. Matsumoto, M. Naoe, J. Magn. Soc. Japan 18 (Suppl. No. SI) (1994) 69. [4] N. Hiratsuka, S. Tojo, E. Koshikawa, M. Fujita, J. Magn. Soc. Japan 18 (Suppl. No. SI) (1994) 315. [5] N. Matsushita, S. Nakagawa, M. Naoe, IEEE Trans. Magn. 28 (5) (1992) 3108. [6] N. Hiratsuka, Y. Kakujima, M. Fujita, M. Sugimoto, Powder Powder Metallurgy 39 (1992) 989.