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Pt multilayer magneto-optical recording media
Journal of Magnetism and Magnetic Materials 120 (1993) 271-273 North-Holland
Ahm
Controlling the Curie temperature of Co/Pt multilayer magneto-optical recording media H.W. van Kesteren and W.B. Zeper Philips Research, P.O. Box 80000, 5600 JA Eindhoven, The Netherlands
The Curie temperature of Co/Pt multilayers could be lowered by reducing the Co layer thickness, i.e., increasing the amount of 2-dimensional character and disorder or by the addition of transition metals such as Os or Re. The second method had the advantage over the first in that the inherent decrease in the Kerr effect was lower.
C o / P t multilayers (MLs) have proven to be applicable as magneto-optical (MO) recording media. However, lowering the Curie temperature (Tc) and thereby the thermomagnetic switching temperature is an important issue, because it will allow to increase the data rate. Furthermore, a lower switching temperature increases the number of e r a s e / w r i t e cycles that a disk can withstand before it suffers a reduction in recording performance. In this paper we discuss the specific advantages and disadvantages of lowering the Tc either by changing the Co or Pt layer thickness or by the addition of small amounts of transition metals (TMs) to the Co layer. C o / P t MLs were prepared at room temperature by HV electron beam evaporation onto Si and glass substrates and covered with a 50 nm Pt or Pd layer to prevent oxidation. For the TM additions a separate source was used. The multilayer periodicity was checked by X-ray diffraction and the amounts of Co, Pt and addition were determined by inductively coupled plasma spectroscopy. The temperature dependence of the magnetization ( M s) was measured with a Faraday balance, and the anisotropy was determined with a torque magnetometer. The Tc of C o / P t MLs is affected by the Pt as well as by the Co thickness. For a ML with for instance 3.5 ,~ Co, the Tc decreases from 660 to 570 K by increasing the Pt thickness from 10 to 19 A [1]. The effect of the Co layer thickness is shown in fig. 1. The magnetization versus temperature curves show that the T c strongly decreased for Co layer thicknesses of 8.4-2.4 A, becoming as low as 510 K for a (2.4 A Co/13.1 Pt) ML. For comparison, the Tc for a thick, pure Co film is 1400 K. The observed dependence of T c on the
Correspondence to: H.W. van Kesteren, Philips Research, P.O. Box 80000, 5600 JA Eindhoven, The Netherlands. Tel.: + 3140-743559; telefax: + 31-40-744282.
Co layer thickness is in qualitative agreement with the gradual crossover from a 3- to a 2-dimensional system for which the Tc should be strongly reduced according to theory [2]. The actual situation can be more complicated because the polarization of the Pt interlayer induces a ferromagnetic coupling between the Co layers. From the theoretical [2] as well as experimental point of view, however, there are no indications that coupling effects have a strong influence on the Tc. The Tc s for evaporated C o / P t [3] and C o / P d MLs [4] almost coincide, despite the fact that the susceptibilities of Pt and Pd differ by one order of magnitude. Furthermore, for C o / A u multilayers the Tc is even higher than that for C o / P t [3], which is unexpected in view of the nonpolarizability of Au. As discussed by Bloemen et al. [3], these results indicate that interdiffusion effects are important. Because Co and Pt(Pd) form solid solutions, it is assumed that there is a concentration profile over a few atomic layers of the Co in the Pt, and visa versa. For C o / A u MLs the interdiffusion is expected to be smaller because Co and Au do not form solid solutions. We studied the magnetization versus temperature curves for a Co0.z6Pt0.74 alloy film and a (6.8 A C o / 2 5 ,~ Pt) ML with the same overall composition as the alloy, and compared these with a (6 A C o / 4 0 A Au) ML [3]. The Tc was found to be lowest for the alloy and highest for the C o / A u ML. For Pt and Au layer thicknesses larger than about 25 .~, the Tc no longer depends on the interlayer thickness. Therefore, the difference for the C o / P t and C o / A u MLs was most likely due to the presence of Pt in the Co, giving the Co layer some alloy character and thus a lower Tc. The hypothesis that disorder lowers the Tc is also supported by the observation that Ar-sputtered C o / P t MLs, which are more diffuse than evaporated ones, have lower Tc's [5]. Thus, the increasing Tc for C o / P t MLs with thicker Co layers was most likely due to the
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
272
H.W. van Kesteren, W.B. Zeper / Controlling the Curie temperature of C o / P t 1.2
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Temperature (K) Fig. 2. Temperature dependence of the magnetization of 60×( ,Co1_~Rex/13.5 ,~, Pt) MLs. The effective Co thickness is 3.5 A for all samples. The magnetizations are normalized to those at 77 K.
o o
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100
200
300
400
500
600
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Temperature (K)
Fig. 1. Temperature dependence of the magnetization of Co/Pt MLs (60 bilayers) with varying Co thicknesses. The magnetizations are normalized to those at 77 K.
combined effect of the crossover from 2- to 3-dimensional behavior and the diminishing effect of the Pt concentration profile in the Co layer. Lowering the Tc by decreasing the Co layer thickness can be used to a limited extend only for MO recording, because a relatively large perpendicular anisotropy has to be maintained. Suitable perpendicular anisotropies are obtained for 3-4 ,~ Co, which corres,pond to Tcs of about 620-720 K for MLs with 13.5 A P t . Increasing the Pt thickness further to decrease the T c has the disadvantage that the domain shape of the written marks becomes less regular [6]. Apart from adjusting the layer thicknesses, Tc can be lowered by the addition of small amounts of 4d and 5d TMs to the Co layer. For bulk Co, the addition of several percent Ir, Ru or Os lowers the Tc more than adding equivalent amounts of Pt or Pd [7]. For our C o / P t MLs, the addition of 15% Re lowered the T c
by 250 K, as shown in fig. 2. Ru, Rh, Os and Ir lowered the Tc as well, although to a varying extent. A lower Tc related inherently to a lower room-temperature Ms for MLs with fixed Co and Pt thicknesses, but with different additions. Besides their effect on T c and Ms, the additions affected the strength of the anisotropy and the shape of the hysteresis loop. In fig. 3 the t
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os -" 2.0
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I
I
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Demagnetization Energy (MJ/m 3)
Fig. 3. Anisotropy versus room temperature demagnetization energy for 60X(3.8 /k Co0.94TMo.06/13.5 ,~ Pt) Ml_s with various 4d (triangles) and 5d (squares) TMs. As a reference, data for a 60×(3.8 ~, Co/13.5 ,~ Pt) multilayer are given (star).
Table 1 The magnetization per Co (Ms), per ML volume (m s) and the Kerr rotation (OK) for a ML with a Tc of 640 K as well as for MLs with a 130 K lower To The MLs have a thickness of 100 nm and a Pt or Pd top layer of 50 nm Composition
Tc (K)
Ms (MA/m)
m s (MA/m)
8r (deg)
3.8 ,~ Co/13.5 ,~ Pt 2.4 A Co/13.1 ~kPt 3.0 A Co/25 ,~ Pt 3.8 A Coo.9aReo.o6/13.5 APt
640 510 510 520
1.72 1.47 1.43 1.39
0.38 0.23 0.15 0.28
0.24 0.13 0.06 0.17
H.W. van Kesteren, W.B. Zeper / Controlling the Curie temperature of Co/Pt
anisotropy energy K u and demagnetization energy 1/~0M2 are plotted for various additions to the ML. In the figure, a low demagnetization energy corresponds to a low Tc. For an effective perpendicular anisotropy, the ratio (Q) of the intrinsic anisotropy and demagnetization energy should be larger than one. The TM additions studied fulfil this requirement. As a reference, the point for a pure C o / P t ML is included ( a = 1.9). Another criterion is the shape of the hysteresis loop. For data stability in MO recording the coercivity (/arc) should be large and the rectangular ratio r = Hn/Hc, where H , denotes the nucleation field, close to one. Increasing the total thickness of the ML shears the loop and decreases r due to the Kooy-Enz effect [8]. To compensate for this effect in our C o T M / P t MLs, which are much thicker than the MLs used for MO recording, we normalized r to that of a C o / P t ML with the same large thickness. The relative r versus M s for the C o T M / P t MLs are shown in fig. 4. In particular, Re and Os had high rectangular ratios as well as appreciable effects on the Tc. Lowering the Tc of C o / P t MLs by increasing the Pt or decreasing the Co layer thickness had a twofold
273
effect on the magnetization per ML volume (ms). First, the lower Tc related inherently to a lower M s at room temperature of the Co itself. Second, the Co content of the film decreased, leading to a lower m s. Table 1 summarizes the effects on Ms, m s and the Kerr rotation of lowering the Tc of a C o / P t ML by approximately 130 K to 510 K, respectively, by decreasing the Co layer thickness, increasing the Pt thickness, and the addition of Re. The Co magnetizations of the MLs with Tcs of 510-520 K were all comparable and lower than the magnetization for the ML with a Tc of 640 K. There was a clear difference, however, in the magnetizations per ML volume and in the Kerr rotations. The highest Kerr rotation and m s were found for the ML with the Re addition to the Co. Thus, at the cost of a slight reduction in anisotropy and rectangular ratio of the hysteresis loop, the addition of small amounts of Re and Os was found to be an effective method to lower the Tc. The authors would like to thank J. Kerkhof and H.C. Donkersloot for the ML preparation and A.J. Mud, G.J.M. Poodt and E. Janssen for magnetic and magneto-optic characterization of the samples. References
I
COo.94TMo.0s/Pt
O co r~ m
I
D nOs 0.5
zx Rh
Re
O3 c
olr g¢
0.0
&Ru
re-
-0.5 0.5
t 1.0
I 1.5
2.0
Magnetization (MNm)
Fig. 4. Relative rectangular ratio versus the room temperature Co magnetization of 60x(3.8 ,~ Co0.94X0.06/13.5 .~, Pt) MLs with various 4d (triangles) and 5d (squares) TMs.
[1] W.B. Zeper, F.J,A.M. Greidanus and P.F. Carcia, IEEE Trans. Magn. 25 (1989) 3764. [2] P. Bruno, J. Magn. Soc. Jpn. 15 S1 (1991) 15. [3] H.J.G. Draaisma, F.J.A. den Broeder and W.J.M. de Jonge, J. Appl. Phys. 63 (1988) 3479. [4] P.J.H. Bloemen, W.J.M. de Jonge and F.J.A. den Broeder, J. Magn. Magn. Mater. 93 (1991) 105. [5] S. Hashimoto, Y. Ochiai and K. Aso, J. Appl. Phys. 67 (1990) 2136. [6] H.W. van Kesteren, A.J. den Boer, W.B. Zeper, J.H.M. Spruit, B.A.J. Jacobs and P.F. Carcia, J. Appl. Phys. 70 (1991) 2413. [7] J. Crangle and D. Parsons, Proc. R. Soc. 255 (1960) 509. [8] HoJ.G. Draaisma and W.J.M. de Jonge, J. Appl. Phys. 62 (1987) 3318.
Ahm
Controlling the Curie temperature of Co/Pt multilayer magneto-optical recording media H.W. van Kesteren and W.B. Zeper Philips Research, P.O. Box 80000, 5600 JA Eindhoven, The Netherlands
The Curie temperature of Co/Pt multilayers could be lowered by reducing the Co layer thickness, i.e., increasing the amount of 2-dimensional character and disorder or by the addition of transition metals such as Os or Re. The second method had the advantage over the first in that the inherent decrease in the Kerr effect was lower.
C o / P t multilayers (MLs) have proven to be applicable as magneto-optical (MO) recording media. However, lowering the Curie temperature (Tc) and thereby the thermomagnetic switching temperature is an important issue, because it will allow to increase the data rate. Furthermore, a lower switching temperature increases the number of e r a s e / w r i t e cycles that a disk can withstand before it suffers a reduction in recording performance. In this paper we discuss the specific advantages and disadvantages of lowering the Tc either by changing the Co or Pt layer thickness or by the addition of small amounts of transition metals (TMs) to the Co layer. C o / P t MLs were prepared at room temperature by HV electron beam evaporation onto Si and glass substrates and covered with a 50 nm Pt or Pd layer to prevent oxidation. For the TM additions a separate source was used. The multilayer periodicity was checked by X-ray diffraction and the amounts of Co, Pt and addition were determined by inductively coupled plasma spectroscopy. The temperature dependence of the magnetization ( M s) was measured with a Faraday balance, and the anisotropy was determined with a torque magnetometer. The Tc of C o / P t MLs is affected by the Pt as well as by the Co thickness. For a ML with for instance 3.5 ,~ Co, the Tc decreases from 660 to 570 K by increasing the Pt thickness from 10 to 19 A [1]. The effect of the Co layer thickness is shown in fig. 1. The magnetization versus temperature curves show that the T c strongly decreased for Co layer thicknesses of 8.4-2.4 A, becoming as low as 510 K for a (2.4 A Co/13.1 Pt) ML. For comparison, the Tc for a thick, pure Co film is 1400 K. The observed dependence of T c on the
Correspondence to: H.W. van Kesteren, Philips Research, P.O. Box 80000, 5600 JA Eindhoven, The Netherlands. Tel.: + 3140-743559; telefax: + 31-40-744282.
Co layer thickness is in qualitative agreement with the gradual crossover from a 3- to a 2-dimensional system for which the Tc should be strongly reduced according to theory [2]. The actual situation can be more complicated because the polarization of the Pt interlayer induces a ferromagnetic coupling between the Co layers. From the theoretical [2] as well as experimental point of view, however, there are no indications that coupling effects have a strong influence on the Tc. The Tc s for evaporated C o / P t [3] and C o / P d MLs [4] almost coincide, despite the fact that the susceptibilities of Pt and Pd differ by one order of magnitude. Furthermore, for C o / A u multilayers the Tc is even higher than that for C o / P t [3], which is unexpected in view of the nonpolarizability of Au. As discussed by Bloemen et al. [3], these results indicate that interdiffusion effects are important. Because Co and Pt(Pd) form solid solutions, it is assumed that there is a concentration profile over a few atomic layers of the Co in the Pt, and visa versa. For C o / A u MLs the interdiffusion is expected to be smaller because Co and Au do not form solid solutions. We studied the magnetization versus temperature curves for a Co0.z6Pt0.74 alloy film and a (6.8 A C o / 2 5 ,~ Pt) ML with the same overall composition as the alloy, and compared these with a (6 A C o / 4 0 A Au) ML [3]. The Tc was found to be lowest for the alloy and highest for the C o / A u ML. For Pt and Au layer thicknesses larger than about 25 .~, the Tc no longer depends on the interlayer thickness. Therefore, the difference for the C o / P t and C o / A u MLs was most likely due to the presence of Pt in the Co, giving the Co layer some alloy character and thus a lower Tc. The hypothesis that disorder lowers the Tc is also supported by the observation that Ar-sputtered C o / P t MLs, which are more diffuse than evaporated ones, have lower Tc's [5]. Thus, the increasing Tc for C o / P t MLs with thicker Co layers was most likely due to the
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
272
H.W. van Kesteren, W.B. Zeper / Controlling the Curie temperature of C o / P t 1.2
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Temperature (K) Fig. 2. Temperature dependence of the magnetization of 60×( ,Co1_~Rex/13.5 ,~, Pt) MLs. The effective Co thickness is 3.5 A for all samples. The magnetizations are normalized to those at 77 K.
o o
I
I
I
I
I
L
100
200
300
400
500
600
700
Temperature (K)
Fig. 1. Temperature dependence of the magnetization of Co/Pt MLs (60 bilayers) with varying Co thicknesses. The magnetizations are normalized to those at 77 K.
combined effect of the crossover from 2- to 3-dimensional behavior and the diminishing effect of the Pt concentration profile in the Co layer. Lowering the Tc by decreasing the Co layer thickness can be used to a limited extend only for MO recording, because a relatively large perpendicular anisotropy has to be maintained. Suitable perpendicular anisotropies are obtained for 3-4 ,~ Co, which corres,pond to Tcs of about 620-720 K for MLs with 13.5 A P t . Increasing the Pt thickness further to decrease the T c has the disadvantage that the domain shape of the written marks becomes less regular [6]. Apart from adjusting the layer thicknesses, Tc can be lowered by the addition of small amounts of 4d and 5d TMs to the Co layer. For bulk Co, the addition of several percent Ir, Ru or Os lowers the Tc more than adding equivalent amounts of Pt or Pd [7]. For our C o / P t MLs, the addition of 15% Re lowered the T c
by 250 K, as shown in fig. 2. Ru, Rh, Os and Ir lowered the Tc as well, although to a varying extent. A lower Tc related inherently to a lower room-temperature Ms for MLs with fixed Co and Pt thicknesses, but with different additions. Besides their effect on T c and Ms, the additions affected the strength of the anisotropy and the shape of the hysteresis loop. In fig. 3 the t
E
3.0
I
os -" 2.0
.,.,- ""
Id.I c
o
d
C°O.94TMo.06/Pt
ORe
. .~-'""
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Rh Q=I
u Air RU
~
1.0
0.0
~"
I
I
I
0.5
1.0
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2.0
Demagnetization Energy (MJ/m 3)
Fig. 3. Anisotropy versus room temperature demagnetization energy for 60X(3.8 /k Co0.94TMo.06/13.5 ,~ Pt) Ml_s with various 4d (triangles) and 5d (squares) TMs. As a reference, data for a 60×(3.8 ~, Co/13.5 ,~ Pt) multilayer are given (star).
Table 1 The magnetization per Co (Ms), per ML volume (m s) and the Kerr rotation (OK) for a ML with a Tc of 640 K as well as for MLs with a 130 K lower To The MLs have a thickness of 100 nm and a Pt or Pd top layer of 50 nm Composition
Tc (K)
Ms (MA/m)
m s (MA/m)
8r (deg)
3.8 ,~ Co/13.5 ,~ Pt 2.4 A Co/13.1 ~kPt 3.0 A Co/25 ,~ Pt 3.8 A Coo.9aReo.o6/13.5 APt
640 510 510 520
1.72 1.47 1.43 1.39
0.38 0.23 0.15 0.28
0.24 0.13 0.06 0.17
H.W. van Kesteren, W.B. Zeper / Controlling the Curie temperature of Co/Pt
anisotropy energy K u and demagnetization energy 1/~0M2 are plotted for various additions to the ML. In the figure, a low demagnetization energy corresponds to a low Tc. For an effective perpendicular anisotropy, the ratio (Q) of the intrinsic anisotropy and demagnetization energy should be larger than one. The TM additions studied fulfil this requirement. As a reference, the point for a pure C o / P t ML is included ( a = 1.9). Another criterion is the shape of the hysteresis loop. For data stability in MO recording the coercivity (/arc) should be large and the rectangular ratio r = Hn/Hc, where H , denotes the nucleation field, close to one. Increasing the total thickness of the ML shears the loop and decreases r due to the Kooy-Enz effect [8]. To compensate for this effect in our C o T M / P t MLs, which are much thicker than the MLs used for MO recording, we normalized r to that of a C o / P t ML with the same large thickness. The relative r versus M s for the C o T M / P t MLs are shown in fig. 4. In particular, Re and Os had high rectangular ratios as well as appreciable effects on the Tc. Lowering the Tc of C o / P t MLs by increasing the Pt or decreasing the Co layer thickness had a twofold
273
effect on the magnetization per ML volume (ms). First, the lower Tc related inherently to a lower M s at room temperature of the Co itself. Second, the Co content of the film decreased, leading to a lower m s. Table 1 summarizes the effects on Ms, m s and the Kerr rotation of lowering the Tc of a C o / P t ML by approximately 130 K to 510 K, respectively, by decreasing the Co layer thickness, increasing the Pt thickness, and the addition of Re. The Co magnetizations of the MLs with Tcs of 510-520 K were all comparable and lower than the magnetization for the ML with a Tc of 640 K. There was a clear difference, however, in the magnetizations per ML volume and in the Kerr rotations. The highest Kerr rotation and m s were found for the ML with the Re addition to the Co. Thus, at the cost of a slight reduction in anisotropy and rectangular ratio of the hysteresis loop, the addition of small amounts of Re and Os was found to be an effective method to lower the Tc. The authors would like to thank J. Kerkhof and H.C. Donkersloot for the ML preparation and A.J. Mud, G.J.M. Poodt and E. Janssen for magnetic and magneto-optic characterization of the samples. References
I
COo.94TMo.0s/Pt
O co r~ m
I
D nOs 0.5
zx Rh
Re
O3 c
olr g¢
0.0
&Ru
re-
-0.5 0.5
t 1.0
I 1.5
2.0
Magnetization (MNm)
Fig. 4. Relative rectangular ratio versus the room temperature Co magnetization of 60x(3.8 ,~ Co0.94X0.06/13.5 .~, Pt) MLs with various 4d (triangles) and 5d (squares) TMs.
[1] W.B. Zeper, F.J,A.M. Greidanus and P.F. Carcia, IEEE Trans. Magn. 25 (1989) 3764. [2] P. Bruno, J. Magn. Soc. Jpn. 15 S1 (1991) 15. [3] H.J.G. Draaisma, F.J.A. den Broeder and W.J.M. de Jonge, J. Appl. Phys. 63 (1988) 3479. [4] P.J.H. Bloemen, W.J.M. de Jonge and F.J.A. den Broeder, J. Magn. Magn. Mater. 93 (1991) 105. [5] S. Hashimoto, Y. Ochiai and K. Aso, J. Appl. Phys. 67 (1990) 2136. [6] H.W. van Kesteren, A.J. den Boer, W.B. Zeper, J.H.M. Spruit, B.A.J. Jacobs and P.F. Carcia, J. Appl. Phys. 70 (1991) 2413. [7] J. Crangle and D. Parsons, Proc. R. Soc. 255 (1960) 509. [8] HoJ.G. Draaisma and W.J.M. de Jonge, J. Appl. Phys. 62 (1987) 3318.