noble metal multilayer films

Journal of Magnetism and Magnetic Materials 126 (1993) 320-322 North-Holland

A n n e a l i n g effects o n C o / n o b l e

m e t a l multilayer films

Haruki Yamane, Yoshinori Maeno and Masanobu Kobayashi Research Laboratory, Oki Electric Industry Co., Ltd, 550-5 Higashiasakawa, Hachioji, Tokyo 193, Japan Co/noble metal (Pd, Au, etc.) multilayer films exhibit large perpendicular magnetic anisotropy. After annealing in air, the coercivities of Co/Pd multilayer films markedly increased as a result of the formation of Co oxides. On the other hand. for Co/Au multilayer films, the uniaxial magnetic anisotropy constant increased as the Co-Au interfaces became sharper. 1. Introduction C o / P d , C o / P t and C o / A u multilayer films exhibit large perpendicular magnetic anisotropy. They have attracted attention as new materials for the next generation of high-density magneto-optical recording media, and are also being studied for applications in other areas [1-5]. Recently, we reported the effects of annealing in air on the magnetic properties of sputtered C o / P d and C o / A u multilayer films [6]. In this paper, we report on the relation between the effects of annealing in air on the magnetic properties and the multilayer film structure. 2. Experiments The dual-source rf magnetron sputtering method was employed in fabricating the films, using Ar as the sputtering gas. The magnetic properties were measured with a vibrating-sample magnetometer and a torque magnetometer. The magnetic anisotropy:Kerf was averaged by the total thickness. The film structure was observed by XRD, AES, TEM, and EDS. The samples were annealed in air at temperatures of 100350°C for 2 h.

of H c increase markedly with annealing at 250-350°C to 2-3 kOe. The increase in H{. is observed only during annealing in air, and not in a vacuum, probably because of the influence of film oxidization. On the other hand, for the C o / A u films, Keff increases with annealing temperature, and shows a maximum value of 2.6 x 106 e r g / c m ~ at 200°C. At temperatures higher than 250°C, Keff decreases and becomes negative. The value of Hc increases monotonously with annealing temperaturc, but remains low.

3.2. Structure of multilayer films annealed in air (a) Multilayered C o / P d f i l m . In table 1, the low-angle X-ray diffraction intensities of the as-sputtered and annealed C o / P d films are compared with the calculated intensities by the three-step model as a function of the alloyed layer thickness. The intensities of the second-order reflection appear to decrease with annealing temperature. This result agrees with the calculated resolution increase in the atomic plane thickness of alloyed layer (CoPd solid solution). Therefore, the second-order peak intensity reductions are caused by disturbances of the periodic structure associated with the diffusion of layer interfaces. We assume that the

3. Results and discussion 4

3.1. Magnetic properties of multilayer films annealed in air Fig. 1 shows the relationships between annealing temperature and changes in Keff and H c of the C o / P d and C o / A u films. For C o / P d films, the as-sputtered value of Keff is maximum, and Ken- decreases monotonously with annealing temperature. The values

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Electric Industry Co. Ltd., 550-5 Higashiasakawa, Hachioji, Tokyo 193, Japan. Tel: 0426-63-1111; telex: 2862681; fax: 0426-65-9616.

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Fig. 1. Magnetic properties of Co/Pd and Co/Au multilayer films after annealing in air for 2 h.

0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. (North-Holland)

H. Yamane et al. / Annealing effects on Co/noble metal films Table 1 Values of d = A / 2 sin 0 and relative intensities (second order) of low-angle multilayer reflections for as-sputtered and annealed films, compared with calculated values for a threestep model; dalloy is the atomic plane thickness of the alloyed layer (calc.: three-step model) Order

First

321

Periodicstructure

Second

d (2~)

I (%)

d (A)

1 (%)

As-sputtered Calc. (dalloy = 2.79 A)

13.0 13.1

100 100

6.46 6.54

3.11 3.77

T,m.= 200°C Calc. (dalloy = 3.00 A)

12.8 13.1

100 100

6.55 6.54

1.26 1.33

Tan = 300°C Calc. (dalloy = 3.22 ,~)

13.0 13.1

100 100

6.54

0 0.10

d e c r e a s e in K~ff (in fig. 2) is the r e a s o n for this fact. However, after a n n e a l i n g at 300°C, w h e n the values of H c were n e a r maximum, a low-angle X-ray p e a k is observed, indicating t h a t the periodic structure is maint a i n e d in the C o / P d films. Fig. 2 shows the c h a n g e s in the cross-sectional film s t r u c t u r e m o d e l with a n n e a l i n g in air. Generally, C o / P d films fabricated by s p u t t e r i n g are known to form c o l u m n a r structures (fig. 2a) [6]. T h e ratio of Co was m e a s u r e d by EDS. W i t h as-sputtered, the ratio of Co existence was constant, but, after a n n e a l i n g in air, Co was f o u n d to be p r e s e n t m o r e at the b o u n d a r i e s of the columns t h a n at t h e i r centers. W e assume that the r e a s o n for this Co diffusion was the influence of Co oxidation at the c o l u m n b o u n d a r i e s . A f t e r a n n e a l i n g in air, oxygen e n t e r s the interiors of the films, passing t h r o u g h the b o u n d a r i e s of the c o l u m n a r structures. A t this time, the internal structure of the films assumes a formation, with column b o u n d a r i e s s u r r o u n d e d by Co

Column-boundary~ ~

(a) as sputtered

(b) annealed Fig. 2. Change in C o / P d multilayer film structure after annealing in air.

oxides (fig. 2b). As a consequence, d o m a i n wall binding occurs in the segregated areas of the oxides in the column boundaries, t h e r e b y increasing the coercivities (fig. 1).

(b) Multilayered C o / A u film. In table 2, the X-ray diffraction p e a k intensities of the a s - s p u t t e r e d a n d a n n e a l e d C o / A u films are c o m p a r e d with the intensities calculated by the e x t e n d e d t h r e e - s t e p model [7] as a function of the alloyed layer thickness. In this case, we assumed that the alloyed layers were f o r m e d by the mixture of the C o A u (Co : A u = 1 : 1). In this study, we e x t e n d e d the t h r e e - s t e p model to take into account the

Table 2 Values of d = A / 2 sin 0 and relative intensities (low-angle second order and high-angle + 1 and - 1 order) of multilayer reflections for as-sputtered and annealed at 200°C, compared with calculated values for an extended three-step model; Nalloy is the number of atomic planes in the alloyed layer Observed

Calculated

As-sputtered

Low angle

First Second

High angle

- 1 0 +1

Annealed

Na,oy = 2

Nalloy = 3

d (,~)

I (%)

d (~,)

I (%)

d (,~)

I (%)

1 (%)

26.75 14.15

100 4.67

27.42 16.79

100 6.00

26.93 13.47

100 10.9

100 0.30

2.47 2.28 2.11

52.6 100 3.70

2.45 2.35 2.27

68.2 100 6.20

2.45 2.24 2.07

67.4 100 4.20

52.4 100 3.40

322

H. Yamane et al. / Annealing effects on Co / noble metal films 4. Conclusions

Fig. 3. TEM image of a cross-section of annealed Co/Au film.

fluctuations in the atomic plane thicknesses for the alloyed layers. From table 2, it can be seen that the intensity ratio of the low-angle second-order reflection and the high-angle satellite refractions ( + 1 and - 1 order) increases after annealing at 200°C. These facts can be explained in terms of the calculated resolution decreased of the alloyed layer (CoAu mixture) thickness. In other word, C o - A u interfaces become sharper by an effective negative or uphill diffusion [3], since Au and Co do not form a solid solution. The increase in K~ff (in fig. 1) can be attributed to the same term. Fig. 3 shows a TEM observation of the cross-sectional structure of the annealed C o / A u films at 200°C. It can be seen that the periodic structure becomes disordered with annealing in a part of multilayer film. With annealing at higher than 250°C, the oxidation of the Co layer, the disordering of the multilayer structure, and the segregation of Au and Co30 4 occur. These results show that the multilayer structure of C o / A u films is less stable than that of C o / P d films. We guess that the reason why the marked increase in H c was not observed during annealing in air is the disordering of the film structure before the sufficient formation of Co oxides in the column boundaries.

For C o / P d multilayer films, the uniaxial magnetic anisotropy constant (Keff) and coercivity ( H c) of the as-sputtered films were 3.7 × 106 e r g / c m ~ and 0.5 kOc, respectively. The value of K~tf monotonously decreased with the annealing temperature, caused by the disturbance of the periodic structure associated with the diffusion of the layer interfaces and the oxidation of multilayer films. On the other hand, the value of H, increased with annealing temperature and showed a maximum value of 3.0 kOe at 300°C. After annealing in air, the internal structure of films assumes formation in which column boundaries are surrounded by Co oxides. As a consequence, domain wall binding occurs in the segregated areas of the oxides in the column boundaries, thereby increasing the coercivities. For C o / A u multilayer films, Kcf t- and H~, of the as-sputtered films were 7.8 × 10 5 e r g / c m 3 and 5(10e, respectively; after annealing at 200°C these values increased to 2.6 × 106 c r g / c m 3 and 150 Oe, respectivcly. The increase in Koff can be explained in terms of the C o - A u interface becoming sharper by an effective negative or uphill diffusion, since Au and Co do not form a solid solution. The multilaycr structure of C o / A u films is less stable than that of C o / P d films and is destroyed before the sufficient formation of Co oxides in the column boundaries. Therefore H,, did not become so large with annealing. References [1] P.F. Carcia, J. Appl. Phys. 63 (1988) 5066. [2] Y. Ochiai, S. Hashimoto and K. Aso, Jpn. J. Appl. Phys. 28 (1989) L659. [3] F.J.A. den Broeder, D. Kuiper, A.P. van de Mosselaer, and W. Hoving, Phys. Rev. Lett. 60 (1988) 2769. [4] H. Yamane, Y. Maeno and M. Kobayashi, IEEE TJMJ 6 (1991) 921. [5] Y. Maeno, H. Yamane and M. Kobayashi, Appl. Phys. Lett. 60 (1992) 510. [6] H. Yamane, Y. Maeno and M. Kobayashi, IEICE Tech. Rep. MR91-59 (1991). [7] H. Yamane, Y. Maeno and M. Kobayashi, to be published.