- Email: [email protected]
(Cr0.2Co0.8-SiO2)n: A multilayered medium for vertical recording
Journal of Magnetism and Magnetic Materials 54-57 (1986) 1683-1684
1683
(Cr0.2Co0.s-SiO2).: A M U L T I L A Y E R E D M E D I U M F O R V E R T I C A L R E C O R D I N G J. I J E S S E R R E
*, J.C. B O U C H A N D
a n d D. J E A N N I O T
Bull, DRTG, rue Jean Jaures, B.P. 53, 78340 Les Claves-sous-bois, France
Perpendicular magnetic anisotropy of CoCr alloy thin films for perpendicular magnetic recording can be reduced by the existence of a longitudinally oriented transition layer or by a negative magnetostrictive effect. Here, we describe some experiments that show these phenomena. A multilayered structure is proposed to reduce those effects.
1. Introduction CoCr alloys thin films are used for perpendicular magnetic recording. The anisotropy of this alloy results mainly from magnetocrystalline and shape effects [l]. The longitudinally oriented transition layer at the substrate interface [2] and the magnetostriction [3] can both decrease the magnetic anisotropy of the CoCr. The importance of the transition layer can be reduced by introducing a convenient underlayer, such as Ge [4]. The effect of the magnetostriction can be decreased by heating the substrate during deposition [5]. The experimental procedure was described elsewhere [6].
graphic orientation ratio 11oo/loo 2 as functions of the thickness of the substrate t~ are shown in fig. 1. Although an optimal value of l~oo/I(~j2 is achieved for a 0.7 mm thick substrate, H e j_ monotonically decreases when t, increases, Two phenomena may be involved in this effect: i) the temperature at the surface of the substrate is strongly influenced by the thickness, since the substrate holder is water-cooled; ii) inner stresses relax for the thinner substrates. The magnetostriction thus influences the properties of these layers. 2.2. Magnetostrictwe effect
With time, the edges of some layers became more and more unstuck from the substrate until all surface was finally removed. The inner stresses are thus im-
2. Results and discussion 2.1. Thickness o f the substrate
A 1 ~tm thick optimized CoCr layer was deposited onto glass substrates with different thicknesses. The perpendicular coercive force He± and the crystallo-
M
(a.u.)
* ENERTEC/Schlumberger; 1, rue Nieuport, BP54, 78141 Velizy-Villacoublay Cedex, France.
10
a 1500
10
1400
~
HC.L (ce)
o
o
1
1300
1100 I o02
(%)
(koe)
0.1
1200
lO I 0.5
I 1
I 1.5
2
t S (mm)
Fig. 1. Dependence of the perpendicular coercive force ( H~ • ) and of the crystallographic orientation ratio (11oo/loo2) on the thickness of the substrate (t~) for a 1 ~tm thick CoCr monolayer. 0304-8853/86/$03.50
Fig. 2, Perpendicular hysteresis loop obtained with a V.S.M. (a) the CoCr monolayer on the substrate; (b) the CoCr monolayer, after removing from the substrate.
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V,
1684
J. Dexserre et al. / A multilavered medium JT)r t,ertical recording
Table 1 last of the studied multilayered structures H 1 2 3
4 5
6 lO 20
1400t:
Structure 1 ~t (7oCr (0.50 btm CoCr +0.02 (0.33 p.m CoCr + 0.02 (0.25 lam CoCr +0.02 (0.20 l~m CoCr+O.02 (0.16 ~m CoCr +0.02 (0.10 ~tm CoCr +0.02 (0.05 ~tm CoCr+O.02
p.m St02) 2 ~ m SiO 2 ) 3 gm SiO:) 4 ~m St02) 5 tam St02 )<~ ixrn St02) m ~m Si02)2o
HC-L
1200I
2.3. Multila3'ered s t r u c t u r e
We divided the CoCr film into several layers whose total thickness was always equal to 1 btm. All monolayers in this multilayer had equal thickness. The CoCr monolayers were separated with 200 A, thick SiO2 layers. The studied structures are listed in table 1. The dependence of t&. • and l i o o / l o o 2 on the n u m b e r of layers n which compose the multilayered structure is presented in fig. 3. The crystallographic orientation improves as n decreases. This is quite consistent with previous results [6]. In effect, for a monolayer, this orientation improves with increasing thickness until a b o u t 1 ~ m ("self-epitaxy"). The improved crystallographic orientation for the monolayer does not generate a better perpendicular magnetic anisotropy. For instance, H,, • presents a maximum between n = 6 and 10. This shows that other p h e n o m e n a occur in these films, such as magnetostriclive effects. The action of intermediate layers, such as SiO, in the present case, is a well-known problem for integrated head deposition [7] and for V.L.S.I. technology.
6
(oe) 1000
I
4
800I 600F;~
portant. The perpendicular hysteresis loops of an asdeposited layer is presented in fig. 2a. The corres p o n d i n g loop, after becoming unstuck from the substrate, is presented in fig. 2b. The " f r e e " layer is clearly more anisotropic than the layer which remained on its substrate. The removal of the substrate did not change the coercive force. X-rays measurements were carried out on all samples by means of diffractometry. Those measurements show that the interreticular distance of the (002) planes varied. It was always larger than d002 for pure Co. The relative ratio varied up to 8 × 10 3 The removal of the edges of the layers and the excess of the d002 of the CoCr vs. Co are both consistent with compressive inner stresses (i.e. from the edges to the center of the surface of the layer).
8 I lOO 1002
(%)
2 1
I
5
10 n
15
20
Fig. 3. Dependence of the perpendicular coercive force (//~, ) and of the crystallographic orientation ratio (Imo/loo2) on the number of monolayers (n) of the 1 txm thick multilayered CoCr. 3. C o n c l u s i o n s
Magnetocrystalline and shape anisotropies both have positive effects on perpendicular magnetic anisotropy of C o C r thin films. The initial transition layer and the magnetostriction both have negative effects on this anisotropy. In this paper, we have presented some results which show the negative magnetostrictive effect. To improve the perpendicular magnetic anisotropy of CoCr thin films, two methods have been proposed. Firstly, to increase the temperature of the substrate during the deposition (to reduce the inner stresses); secondly, to introduce a convenient underlayer, such as Ge (to improve the initial transition layer). Here, we propose a n o t h e r method, which is the multilayered structure where the inner stresses could be " a b s o r b e d " by the insulating layers. We will finally note that, in perpendicular magnetic recording, the output signal is proportional to //~, [g]. The authors t h a n k N. Pichereau and Y. Gaillard for VSM and X-rays measurements, respectively. [1] S. lwasaki and K. Ouchi, IEEE Trans. Magn. MAG-14 (1978) 849. [2] C. Byun, J.M. Sivertsen and J.H. Judy, J. Appl. Phys. 57 (1985) 3997. [3] T. Wielinga, thesis, Twente (1983). [41 M. Futamoto, Y. Honda. H. Kakibayashi and K. Yoshida. Intermag 85, paper DBI. [51 (L Baldin, R. Tsui and H. Hamilton, lntermag 85, paper DB4. [6] D. Jeanniot and J. Desserre, MMA85, paper DY('. [7] J.P. LaT~zari. IEEE Trans. Magn. MAG-14 (1978) 503. [8J P. Bernstein, J. Desserre. D. Jeanniot, C. Gueugnon and M. Porte, IEEE Trans. Magn. MAG-20 (1984) 809.
1683
(Cr0.2Co0.s-SiO2).: A M U L T I L A Y E R E D M E D I U M F O R V E R T I C A L R E C O R D I N G J. I J E S S E R R E
*, J.C. B O U C H A N D
a n d D. J E A N N I O T
Bull, DRTG, rue Jean Jaures, B.P. 53, 78340 Les Claves-sous-bois, France
Perpendicular magnetic anisotropy of CoCr alloy thin films for perpendicular magnetic recording can be reduced by the existence of a longitudinally oriented transition layer or by a negative magnetostrictive effect. Here, we describe some experiments that show these phenomena. A multilayered structure is proposed to reduce those effects.
1. Introduction CoCr alloys thin films are used for perpendicular magnetic recording. The anisotropy of this alloy results mainly from magnetocrystalline and shape effects [l]. The longitudinally oriented transition layer at the substrate interface [2] and the magnetostriction [3] can both decrease the magnetic anisotropy of the CoCr. The importance of the transition layer can be reduced by introducing a convenient underlayer, such as Ge [4]. The effect of the magnetostriction can be decreased by heating the substrate during deposition [5]. The experimental procedure was described elsewhere [6].
graphic orientation ratio 11oo/loo 2 as functions of the thickness of the substrate t~ are shown in fig. 1. Although an optimal value of l~oo/I(~j2 is achieved for a 0.7 mm thick substrate, H e j_ monotonically decreases when t, increases, Two phenomena may be involved in this effect: i) the temperature at the surface of the substrate is strongly influenced by the thickness, since the substrate holder is water-cooled; ii) inner stresses relax for the thinner substrates. The magnetostriction thus influences the properties of these layers. 2.2. Magnetostrictwe effect
With time, the edges of some layers became more and more unstuck from the substrate until all surface was finally removed. The inner stresses are thus im-
2. Results and discussion 2.1. Thickness o f the substrate
A 1 ~tm thick optimized CoCr layer was deposited onto glass substrates with different thicknesses. The perpendicular coercive force He± and the crystallo-
M
(a.u.)
* ENERTEC/Schlumberger; 1, rue Nieuport, BP54, 78141 Velizy-Villacoublay Cedex, France.
10
a 1500
10
1400
~
HC.L (ce)
o
o
1
1300
1100 I o02
(%)
(koe)
0.1
1200
lO I 0.5
I 1
I 1.5
2
t S (mm)
Fig. 1. Dependence of the perpendicular coercive force ( H~ • ) and of the crystallographic orientation ratio (11oo/loo2) on the thickness of the substrate (t~) for a 1 ~tm thick CoCr monolayer. 0304-8853/86/$03.50
Fig. 2, Perpendicular hysteresis loop obtained with a V.S.M. (a) the CoCr monolayer on the substrate; (b) the CoCr monolayer, after removing from the substrate.
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V,
1684
J. Dexserre et al. / A multilavered medium JT)r t,ertical recording
Table 1 last of the studied multilayered structures H 1 2 3
4 5
6 lO 20
1400t:
Structure 1 ~t (7oCr (0.50 btm CoCr +0.02 (0.33 p.m CoCr + 0.02 (0.25 lam CoCr +0.02 (0.20 l~m CoCr+O.02 (0.16 ~m CoCr +0.02 (0.10 ~tm CoCr +0.02 (0.05 ~tm CoCr+O.02
p.m St02) 2 ~ m SiO 2 ) 3 gm SiO:) 4 ~m St02) 5 tam St02 )<~ ixrn St02) m ~m Si02)2o
HC-L
1200I
2.3. Multila3'ered s t r u c t u r e
We divided the CoCr film into several layers whose total thickness was always equal to 1 btm. All monolayers in this multilayer had equal thickness. The CoCr monolayers were separated with 200 A, thick SiO2 layers. The studied structures are listed in table 1. The dependence of t&. • and l i o o / l o o 2 on the n u m b e r of layers n which compose the multilayered structure is presented in fig. 3. The crystallographic orientation improves as n decreases. This is quite consistent with previous results [6]. In effect, for a monolayer, this orientation improves with increasing thickness until a b o u t 1 ~ m ("self-epitaxy"). The improved crystallographic orientation for the monolayer does not generate a better perpendicular magnetic anisotropy. For instance, H,, • presents a maximum between n = 6 and 10. This shows that other p h e n o m e n a occur in these films, such as magnetostriclive effects. The action of intermediate layers, such as SiO, in the present case, is a well-known problem for integrated head deposition [7] and for V.L.S.I. technology.
6
(oe) 1000
I
4
800I 600F;~
portant. The perpendicular hysteresis loops of an asdeposited layer is presented in fig. 2a. The corres p o n d i n g loop, after becoming unstuck from the substrate, is presented in fig. 2b. The " f r e e " layer is clearly more anisotropic than the layer which remained on its substrate. The removal of the substrate did not change the coercive force. X-rays measurements were carried out on all samples by means of diffractometry. Those measurements show that the interreticular distance of the (002) planes varied. It was always larger than d002 for pure Co. The relative ratio varied up to 8 × 10 3 The removal of the edges of the layers and the excess of the d002 of the CoCr vs. Co are both consistent with compressive inner stresses (i.e. from the edges to the center of the surface of the layer).
8 I lOO 1002
(%)
2 1
I
5
10 n
15
20
Fig. 3. Dependence of the perpendicular coercive force (//~, ) and of the crystallographic orientation ratio (Imo/loo2) on the number of monolayers (n) of the 1 txm thick multilayered CoCr. 3. C o n c l u s i o n s
Magnetocrystalline and shape anisotropies both have positive effects on perpendicular magnetic anisotropy of C o C r thin films. The initial transition layer and the magnetostriction both have negative effects on this anisotropy. In this paper, we have presented some results which show the negative magnetostrictive effect. To improve the perpendicular magnetic anisotropy of CoCr thin films, two methods have been proposed. Firstly, to increase the temperature of the substrate during the deposition (to reduce the inner stresses); secondly, to introduce a convenient underlayer, such as Ge (to improve the initial transition layer). Here, we propose a n o t h e r method, which is the multilayered structure where the inner stresses could be " a b s o r b e d " by the insulating layers. We will finally note that, in perpendicular magnetic recording, the output signal is proportional to //~, [g]. The authors t h a n k N. Pichereau and Y. Gaillard for VSM and X-rays measurements, respectively. [1] S. lwasaki and K. Ouchi, IEEE Trans. Magn. MAG-14 (1978) 849. [2] C. Byun, J.M. Sivertsen and J.H. Judy, J. Appl. Phys. 57 (1985) 3997. [3] T. Wielinga, thesis, Twente (1983). [41 M. Futamoto, Y. Honda. H. Kakibayashi and K. Yoshida. Intermag 85, paper DBI. [51 (L Baldin, R. Tsui and H. Hamilton, lntermag 85, paper DB4. [6] D. Jeanniot and J. Desserre, MMA85, paper DY('. [7] J.P. LaT~zari. IEEE Trans. Magn. MAG-14 (1978) 503. [8J P. Bernstein, J. Desserre. D. Jeanniot, C. Gueugnon and M. Porte, IEEE Trans. Magn. MAG-20 (1984) 809.