Improved soft magnetism of amorphous CoZr films

Journal of Magnetism and Magnetic Materials 93 (1991) 247 251 North-Holland

247

Improved soft magnetism of amorphous CoZr films A. R a k i i a, H. L e G a l l a, J. G i e r a l t o w s k i b, L. L o a e c b, A. F e s s a n t b a n d I. P u c h a l s k a ~' ~'CNRS, Laboratoire de Magn~tisme et d'Optique des Solides, 92195 Meudon Cedex, France bUnit'ersit~ de Bretagne Occidentale, Laboratoire des Mat~riaux Magm?iques, 28287 Brest, France

A m o r p h o u s binary Cot00 xZrx alloy films have been investigated for high efficiency recording heads. The static and dynamical properties have been improved from extended analysis of the influence of the sputtering parameters and from the use of both a magnetic field applied on the substrate during deposition and a rotating field annealing after deposition which decrease, respectively, the angular dispersion of the easy-axis direction and the anisotropy field. Quasi-ideal CoZr films have been obtained with 4"rrM~ = 15.5kG, H c - 0.02Oe, H K < 1 0 e , A~ < 10 7, ~'~,u - 50000, #',vi,1 = 1800 and roll-off frequency Fc = 680MHz.

1. Introduction Amorphous CoZr films have been produced from different techniques and their structural and magnetic properties have a strong dependence on the deposition parameters. From the rf diode sputtering deposition, for instance, it has been observed an increase of the coercive field H+ and the anisotropy field H K with the incident rf power density and the sputtering gas pressure [1-3]. From these investigations it was concluded that the argon gas pressure was the most important deposition factor for the CoZrNb films [4]. We have shown recently that the range of optimum conditions for deposition of high quality CoZr films has to combined both the input rf power and the Ar gas pressure. In the present work we report extended analysis of the influence of the sputtering deposition parameters on static (coercivity, in-plane anisotropy field, magnetostriction) and dynamical properties (low and high frequency permeabilities associated with domain wall motion and spin rotation, respectively) in high s a t u r a t i o n m a g n e t i z a t i o n (up to 15.5 k G ) Corn0 +Zr x amorphous films (3 < x < 14) grown in wide ranges of rf power and argon pressure. It is shown that the static and dynamical soft magnetism can be strongly improved first after an-

nealing in a rotating magnetic field (RFA) and second when a static magnetic field is applied during deposition in the film which decreases the ripples in the magnetic domains as observed from Lorentz microscopy.

2. Experiment Thin films of about 1 ~ m thickness were deposited on 2 5 m m - d i a m e t e r microscope cover glass substrates by using a conventional diode sputtering equipment. The substrates were placed on a water cooled table without or with an applied dc field of about 300Oe produced by two opposite magnets. Composite sputtering targets were used from 5 x 5 x 1 mm 3 Zr chips epoxied on 4in. diameter pure Co with three different areal percentages of 14.6, 17.3 and 2 0 % Z r . The base pressure before sputtering was below 5 x 10-Tmbar. The magnetic properties of the films were investigated as a function of the rf power between 50 and 700W and the argon pressure between 10 -3 and 1 0 - 2 m b a r . The amorphous films were analyzed before (as-sputtered) and after annealing at 300°C under a magnetic field of 5 k O e rotating at 190rpm. The amorphous or crystalline state was checked from X-rays, the

0304-8853/91/$03.50 ~') 1991 - Elsevier Science Publishers B.V. (North-Holland)

248

A. Rakii et aL /hnprot'ed sot't magnetism ~( amorphous CoZr films

composition was determined from E P M A technique and the thickness was obtained from a profilometer. The coercivity and the uniaxial inplane anisotropy field were deduced from a BH loopmeter at 50 Hz or from a VSM. FMR technique was used for measurement of H~, the magnetostriction constant A~ and the resonance linewidth at 10GHz. Low frequency permeability between 0.1 and 10kHz was measured from the inductance change of a ferritc coil [5]. The complex permeability spectra at high frequencies were obtained from I MHz to 3 G H z from a new broadband, non-destructive technique using a stripline cell and a network analyzer [6]. Fine structures of magnetic domains associated with ripples were investigated by Lorentz microscopy.

3. Results and discussion

3.1. Soft magnetism of CoZr deposited under zero magnetic .field The magnetic properties of the films are sensitive to the sputtering parameters such as the rf power and the argon pressure. We have shown [7] that the Ar pressure range for optimal deposition decreases with the rf power and the best soft magnetism was obtained at the lowest rf power (50W) and low Ar pressure (2 × 10 ~ mbar). As checked from EPMA, the Zr content was decreasing with the Ar pressure from x = 6.8 to 3.1, for P,f= 100W for instance, due to the decrease of the Ar sputtering efficiency on Zr compared to Co at low pressure and rf power. That explains an increase of H c observed on the low Ar pressure side of the H,.-PAr c u r v e s due to the growth of a microcrystalline structure induced by a too low Zr content. The strong increase of the coercivity on the high Ar pressure side of the curves was also attributed to the growth of a columnar structure with a microcrystaIlization. This structure could be due to an increasing temperature of the substrate induced by high energy particle

bombardment mainly at Zr content lower than the glass-forming concentration. It was concluded that good as-sputtered soft alloys ( H c 0.25-0.5Oe) can be obtained mainly at low rf power and Ar pressure inside a well defined narrow range for these deposition parameters. In what follows it is shown that strong improvement of soft magnetism can be obtained from annealing under a rotating magnetic field (RFA) and from deposition under static field which reduces the ripple structure in the magnetic domain.

3.2. RFA of CoZr deposited under zero magnetic

field Soft magnetism improvement corresponding to a reduction of the coercivity, FMR linewidth and in-plane anisotropy is observed after RFA as shown in fig. 1 for H c. The coercivity can be reduced from 0.09 to 0.06 Oe for films deposited under 50 W and 2 × 10 ~ rob. The anisotropy can be decreased by a factor of about 10 after annealing at 300°C for about 20 rain. That increases the spin rotation permeability /z'~ deduced from the

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dependence along the hard axis obtained from VSM experiment as shown in fig. 2. It is to be noted that the spin-rotation permeability, which is the only effective process in the high frequency range ( > 10MHz), is strongly improved by a factor of about 7 after 30 min annealing time at temperature higher than 250°C. Saturation effect of R F A is observed beyond 30 min (dashed area in fig. 2). The R F A induces similar effect for the domain wall permeability p," measured at low frequency (100Hz) as shown in fig. 3 where an increase of # ~ by about 40% is obtained after RFA. The wall permeability is also strongly frequency dependent for CoZr films. We have shown that beyond 10kHz # " presents a strong decrease associated with the wall relaxation. Static and dynamical soft magnetisms of domain walls are well correlated since the wall permeability increases when the coercivity decreases (fig. 3). The highest wall permeability value (4.8 × 104) is obtained in a very narrow Ar pressure range near 2.2 x 10 3 mbar and under low rf power (50W), which indicates that good soft magnetic films can be obtained only under well defined critical sputtering deposition parameters. M-H

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A. Rakii et al. / Improt'ed

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Fig. 4. Strong and small magnetic ripples for CoZr films deposited, respectively, without (a) and with H~,

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A. Rakii et al. / Improved soft magnetism of amorphous CoZr films

251

3.3. CoZr films deposition under magnetic field

4. Conclusion

Improvement of soft magnetic properties after R F A is enhanced when the amorphous films are deposited under static magnetic field as shown in fig. 1 where a decrease of the coercivity down to very low values ( H c = 0.02Oe) is obtained with films submitted to a dc field of about 300Oe. Such improvement is associated with the strong reduction of the magnetic ripples corresponding to local fluctuations of the stress-induced anisotropy as observed from Lorentz microscopy

Static and dynamical properties of amorphous CoZr films have been improved first from extended analysis of the influence of the sputtering parameters and second from the use of a mag-

in fig. 4a and b corresponding, respectively, to films deposited without and with a magnetic field H a. Both magnetic field deposition and R F A increase the high frequency permeability associated with the spin rotation as shown in fig. 5 for 1 txm thick CoZr film deposited under 5 0 W rf power and 1.9 × 10 -3 mbar and under a magnetic field H a. The roll-off frequencies F c near 650MHz is associated with the eddy current losses in 1 Ixm thick films. We have noted first the strong influence of H a which increases the high frequency permeability by a factor 2 and second the high permeability value (/z'~= 1800) at high frequencies above 100MHz.

netic field during deposition and R F A which decrease, respectively, the angular dispersion of the in-plane easy axis direction and the anisotropy field. CoZr films with 4"rrMs= 15.5kG, H e = 0.02Oe, H K < 1 0 e , As < 10 - 7 , /~'all = 5 0 0 0 0 , /z'spin = 1800 and Fc = 680MHz have been obtained. References [1] Y. Shimada, Phys. Stat. Sol. 83 (1984) 255. [2] J.J. Guzman and M.H. Kryder, J. Appl. Phys. 61 (1987) 3240. [3] A. Materne, J. Geynet and H. Moriceau, J. Chem Res. (S) 1 (1987) 139. [4] K.K. Choh, J.H. Judy and J.M. Sivertsen, IEEE Trans. Magn. MAG-23 (1987) 2539. [5] J. Gieraltowski, J. Loa6c and H. Le Gall, IEEE Trans. Magn. MAG-25 (1989) 4219. [6] A. Fessant, J. Gieraltowski, J. Loafic and H. Le Gall, J. Magn. Magn. Mat. 93 (1991) 242. [7] H. Le Gall, F. Chevrier, A. Rakii, J. Gieraltowski and J. Loa~c, Acta Phys. Polonica A76 (1989) 315.