- Email: [email protected]
Ru multilayers
A A A
Journal of Magnetism and Magnetic Materials 102 (1991) L9-L14 North-Holland
-d
rzz
Letter to the Editor
Antiparallel and perpendicular for Co/Ru multilayers
magnetization
alignment
H.W. van Kesteren, F.J.A. den Broeder Philips Research Laboratories, P.O. Box 80000, 5600 JA Eindhouen, Netherlands
P.J.H. Bloemen, E.A.M. van Alphen and W.J.M. de Jonge Eindhoven University of Technology, Department of Physics, 5600 MB Eindhoven, Netherlands
Received 27 May 1991; in revised form 3 July 1991
Several Co/Ru multilayers were grown in which the magnetization directions of consecutive Co layers were aligned antiparallel and along the film normal. The magnetization curves for these multilayers agree With the curves obtained by minimizing the total enerq including anisotropy and interlayer coupling. Thzse multilayers were part of a series with Co thicknesses of 11 and 22 A and Ru thicknesses varying between 5 and 20 A. Torque and magnetization measurements showed that the multilayers with 11 A Co had a perpendicular easy axis while for the 22 A Co series an in-plane preferential orientation was observed. A strong antiferromagnetic interlayer coupling was obtained for Ru thicknesses of about 8 A.
Magnetic multilayer films based on ultrathin layers of transition metals attracted a great deal of attention during the last decade due to a number of novel properties. Depending on the composition of the multilayer, large perpendicular anisotropy [1,2], giant magnetoresistance [3] and oscillations in the magnitude [41 and sign [5] of the exchange coupling between adjacent ferromagnetic layers can be obtained. The attention comes from researchers interested in both the origin of these novel effects, as well as in using them in applications. For example Co/Pt multilayers, which have a perpendicular anisotropy for thin Co layers, are promising candidates for future magneto-optical recording media [6]. Multilayers consisting of alternating thin layers of Co and Ru form an interesting system. Parkin et al. [4] has shown that for Co/Ru multilayers with 18 A Co layers, the interlayer exchange coupling oscillates as a function of the Ru thickness. Furthermore, Sakurai et al. [7] has obtained a perpendicular anisotropy in this system for Co 0304-8853/91/$03.50
layers thinner than 18 A and a fixed Ru thickness of 20 A. In this letter, we report that by appropriate choice of the Co and Ru thickness, multilayer films could be obtained which have both a perpendicular anisotropy as well as antiferromagnetic coupling, resulting in a perpendicular and antiparallel alignment of the magnetization in consecutive layers. Results of Panissod et al. [8] for Co/Ru multilayers and of Bennett et al. [9] for the Fe/Cu system also indicate the existence of such a state. Co/Ru multilayers were deposited by e-beam evaporation at ambient temperature on glass and on Si substrates. The pressure during vapor deposition was 2 x lo-’ mbar and the Geposition rate for Co and Ru between 1 and 4 A/s. Following Parkin [04]and Sakurai[7], a base layer consisting of 500 A Pd and 100 A Ru was deposited on the substrate before deposition of the Co/Ru multilayer. Two series of six samples were prepared: one series with a Co thickness of 11 A and the other series with 22 A Co. For both series, the
0 1991 - Elsevier Science Publishers B.V. All rights reserved
LlO
H. I+!van Kesteren et al. / Magnetization aligntnent for Co /Ru multilayers
Ru thjckness varied between 5 and 20 A in steps of 3 A with a total number of 10 repetitions for all the samples. The structure of the films was examined using X-ray diffraction at low and high angles with the scattering vector perpendicular to the film plane. The Pd base layer showed besides a strong (111) peak, also a roughly 7 times weaker (002) peak indicating a partial [OOll,, orientation of the Pd. The period determined from the superlattice
diffraction was typically within 5% of the values set during growth. The rocking curve of the main high angle multilayer peak indicated that the films were polycrystalline with a spread in [ill] orientation of 20 o (fwhm). For the magnetic characterization of the samples, we used a Vibrating Sample Magnetometer (VSM) and a torque magnetometer. Magnetization curves versus in-plane and perpendicular magnetic field are shown in figs. la-f. These
lOx(llA
Co+l4A
Ru)
_
I-
(a)
I
-1
0
1
2
Applied field poH (T)
Applied field poH (T)
I
-0.25
0 Applied field p,H
Fig. 1. Perpendicular
(c) 0.5
0.25
-2
(T)
and in-plane
0
-1
1
Applied field poH (T)
hysteresis
loops for Co/Ru
multilayers
with thicknesses
as indicated.
2
Lll
H. W van Kesteren et al. / Magnetization alignment for Co / Ru multilayers
-1 .o
-0.5
0.0
0.5
1.0
Applied field voH (T)
0.5
0.0
-0.5
1.0
Applied field poH (T)
Fig. 1. (continued).
hysteresis loops depicted the main effects observed for the two series of Co/Ru multilayers. 4s can be seen from figs. la-c, the films with 11 A Co had an easy axis perpendicular to the film plane, while figs. Id-f show that the multilayers with 22 A Co had an in-plane easy magnetization direction. The in-plane and perpendicular saturation fields for the multilayers with 5 and 8 A Ru were over 1.7 T, the maximum field available in the VSM. These high saturation fields were clear signatures of an antiferromagnetic interlayer exchange coupling. The (extrapolated) perpendicular and in-plane saturation fields for all the samples are given in table 1. For the 11 A as well as for the 22 A Co series the saturation fields become much lower for the0 multilayers with Ru thickness$s larger than 8 A. For the multilayers with 20 A Ru there is again a rather small increase of the in-plane saturation field. However, the clear oscillation in the saturation field versus Ru thickness a observed by Parkin et al. for a 20 A Co series is less clear for our samples. This is most likely related to the different deposition techniques used i.e. sputtering and evaporation, which lead to different interface structures. For the Co/Cu system, Bloemen et al. noted that the deposition technique can have a strong influence
on the coupling behaviour [lo]. A better signature for the antiferromagnetic interlayfr coupling0 than the saturation field for the 11 A Co/20 A Ru multilayer is the shape of the magnetization curve. The crossing of, the in-pkane and perpeadicular lpops for the 11 A Co/8 A Ru and 11 A Co/20 A Ru multilayers (figs. la and c) is interpreted as due to the combination of a perpendicular anisotropy and an antiferromagnetic coupling. This is illustrated in fig. 2 where the theoretically expected magnetization curves are shown. The linear in-plane curve represents a process of rotation of the magnetizations against the antiferromagnetic coupling and the magnetic anisotropy. The perpendicular curve shows a zero magnetizaTable 1 In-plane (H, ,,) and perpendicular (H, I ) saturation fields for Co/Ru multilayers with various Co and Ru thicknesses
t ..=llA
t,=22A
tR” (AI
Hq (T)
H,, (‘0
Hs,, 0)
H,, (T)
5 8 11 14 17 20
>2 = 3.5 21 0.5 0.6 0.6
>2 2.5 0.6 0.2 0.2 0.2
>2 1.8 0.5 0.2 0.2 0.3
>2 2.1 1.0 0.5 0.5 0.5
H. W. van Kesteren et al. / Magnetization
Applied field poH
Fig. 2. Theoretical magnetization curves for a multilayer with a perpendicular anisotropy and an antiferromagnetic interlayer exchange coupling.
tion at low fields (antiparallel perpendicular alignment), followed by a spirrflop transition at a certain switching field resulting in a jump of the magnetization, and for high fields a linear part which corresponds again to rotation of the magnetizations towards the field direction. The underlying model and a more quantitative treatment of the loops will be discussed later. Clearly the loops of the 11 A Co/8 A Ru and 11 A Co/20 A Ru multilayers (fig. la and c) are qualitatively in agreement with these curves. The switching fields were not sharp probably due to a distribution of coupling strengths resulting from variations in the Ru thicknesses within the samples. The switching field as well as the saturation field of the 11 A Co/20 A Ru multilayer were much lower than the corresponding fields for the 11 A Co/8 A Ru multilayer, indicating a strong decrease of the coupling wit,h increasgg Ru thickness. The loops of the 11 A Co/5 A Ru multilayer were also characteristic of antiferromagnetic coupling but the switching field was too poorly defined to reliably estimate its value. This is related to the fact that the thickness fluctuations for thinner Ru layers cause a larger spread in switching fields.
alignment for Co / Ru multilayers
The hysteresis loops of the multilayers with 11 and 17 A Ru were similar to the ones of the 14 A Ru multilayers and also to e.g. ferromagnetically coupled Co/Pd multilayers [l], indicating a mainly ferromagnetic coupling for these Co/Ru multilayers. The saturation magnetization pOMS per unit Co volume of 1.4 T for the multilayers with 11 8, Co and of 1.6 T for the series with 22 A Co was low compared to the bulk Co magnetization of 1.76 T. This indicated either a lower Curie temperature for the Co/Ru multilayers, interdiffusion of Co and Ru, or moment reduction of Co interface atoms by the interaction with Ru. In the latter case the lower magnetization could be explained by assuming the presence of approximately two interface atomic Co layers per Co layer which had lost their magnetization. The effective magnetic anisotropy energy K was determined by torque measurements. To prevent complications from a strong antiferromagnetic coupling in these measurements, only the Co/Ru multilayers with relatively low saturation fields were measured. For the series with t = 11 and 22 A Co we found anisotropy energies per unit Co volume of 0.12 and -0.29 MJ/m3 respectively. A rough estimate of the volume (K,) and interface (K,) contribution to the anisotropy could be obtained from these two values assuming the linear relation [l] Kt=K,t=2K,.
(1)
Here, K, includes the shape anisotropy. This yielded an interface anisotropy K, of 0.5 mJ/m’ and a volume anisotropy contribution K, of - 0.7 MJ/m3. These values are comparable to those obtained for Co/Pd multilayers with a 11111texture [ll]. With our two data points the in-plane to perpendicular cross;over for Co/Ru multilayers is estimated at 13 A Co. This value is slightly lower than the number of 18 A reported by Sakurai et al. [7], mainly by a more negative volume contribution to the anisotropy for our samples. To interpret the observed shapes of the Co/Ru magnetization curves more quantitatively, we considered a sandwich structure consisting of two
H. W. van Kesteren et al. / Magnetization alignment for Co / Ru multilayers
identical magnetic layers separated by a nonmagnetic layer as an approximation of a multilayer with a large number of periods. The energy E per unit volume of the ferromagnetic material is given by WI E = - ~poMsH[cos -J
cos(41 - 4,)
41 + cos &] +
Eanis
(2)
with 4i and & the angles between the applied field H and the magnetization directions of the two layers and J the coupling energy. J can be converted to an interlayer coupling per unit interface area using Ji =Jt with t the thickness of the ferromagnetic layer. The term describing the anisotropy energy Eanis depends on the direction of the easy axis and the direction of the applied field with respect to the easy axis. The anisotropy for in-plane rotation will be neglected. We will consider three cases. For an in-plane field and a film with an in-plane preferential orientation (K < O), the anisotropy energy is constant and can be set equal to zero Eanis = 0.
(3)
For an in-plane field and a film with a perpendicular anisotropy (K > 01, the anisotropy energy is given by Eanis = - iK[sin24,
+ sin24,] .
Finally, for a perpendicular energy equals Eanis = - OK [ COS2~1
(4)
field, the anisotropy
+ COS2~2]
(5)
both for in-plane as well as for perpendicular anisotropy. By minimizing E and using & = -+2 one derives easily the expressions for the magnetization along the field in case of an antiferromagnetic interlayer coupling (J < 0). From eqs. (2)-(41, we obtain for an in-plane field M/M, = -/.L&~H/~J
(K < 0)
(6)
and M/M,=
-p,,M,H/2(2J-K)
For a perpendicular in M/M, = j~~A4,H/2(2J
(K>O).
(7)
field, eqs. (2) and (5) result + K)
(8)
L13
restricted to - 25 > K. Expressions for the saturation fields can be easily obtained from eqs. (6)-(8) by setting M equal to h4,. For multilayers with a perpendicular anisotropy, expression (8) is only valid for fields higher than the field required to “switch” both magnetizations in the direction of the applied field. This switching becomes energetically favourable for field strengths for which the energy given by (2) and (5) (with & = -42) becomes lower than the energy for the antiparallel perpendicular alignment ( -K + J). This switching field is given by
(9) again restricted to - 2 J > K. The theoretical magnetization curves for a sandwich with a perpendicular easy axis and an antiferromagnetic interlayer coupling as given by eqs. (7)-(91, are shown in fig. 2. For the 11 A Co/8 A Ru multilayer the interlayer coupling energy could be determined from the perpendicular switching field of about 0.65 T and the previously determined anisotropy and saturation magnetization with eq. (9), yielding Ji = -0.7 mJ/m2. This value is slightly smaller than the - 0.9 mJ/m2 obtained from the extrapolated perpendicular and in-plane saturation fields as giveain table t. Using the saturation fields for the 22 A Co/8 A Ru multilayer one obtaines an interlayer coupling of - 1.3 mJ/m2. This is an average value in view of the different low and high field slopes of the magnetization curve for this sample. Using the high field slope, an interlayer coupling of - 1.5 mJ/m2 is found. In computing Ji from the high field slope, we corrected for the volume fraction of 75% involved. This fraction was obtained from the percentage of the magnetization saturating with the high field slope with respect to the saturation magnetization of 1.6 T. Applying the a!alysis to the antiferromagnetically coupled 11 A Co/20 A Ru multilayer yielded an exchange-coupling energy of about -0.08 mJ/m2, i.e. an order of magnitude lower. The exthange coupling per unit interface area for an 8 A Ru interlayer has been found to be about twice as large for the 22 A Co sample as
L14
H. W. uan Kesteren et al. / Magnetization alignment for Co / Ru multilayers
for the 11 A Co sample although a comparable coupling is expected. One should be careful however, in drawing conclusions from these data only, because the exchange coupling varies by a factor of two for a 1 A Ru thickness variation in the 6-9 A Ru range as shown by Parkin et al. In conclusion, we have shown that Co/Ru multilayers have a perpendicuiar magnetization for a Co layer thickneoss of 11 A and an in-plane magnetization for 22 A Co. The estimated crossing over from perpendicular to in-plane anisotropy occurred at a Co thickness of about 13 A. The interlayer exchange coupling was found to oscillate with Ru thickness. Several Co/Ru multilayers were studied which had a perpendicular anisotropy in combination with an antiferromagnetic interlayer exchange coupling. The magnetization curves for these multilayers are in qualitative agreement with the model presented and were analysed to obtain the magnitudes of the interlayer exchange coupling for various Ru and Co thicknesses.
Acknowledgements
We wish to thank H.C. Donkersloot and J.M. Kerkhof for the vapor deposition of the Co/Ru multilayers.
References
111H.J.G. Draaisma, W.J.M. de Jonge, and F.J.A. den Broeder, J. Magn. Magn. Mater. 66 (1987) 351.
121F.J.A. den Broeder, W. Hoving and P.J.H. Bloemen, J. Magn. Magn. Mater. 93 (1991) 562. 131 M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [41 S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [51 P. Griinberg, S. Demokritov, A. Fuss, M. Vohl and J.A. Wolf, Proc. of the MMM Conf., San Diego (19901, J. Appl. Phys. (in press). [61 W.B. Zeper, F.J.A.M. Greidanus and P.F. Garcia, IEEE Trans. Magn. MAG-25 (1989) 3764. [71 M. Sakurai, T. Takahata and I. Moritani, Conf. on Magnetics, Japan (1990). k31 P. Panissod, K. Ounadjela, A. Dinia, D. Muller, G. Suran, F. Petroff, A. Arbaoui and C. Meny, First Kyoto-Duisburg Workshop, Duisburg (1991). [91 W.R. Bennett, W. Schwanacher and W.F. Egelhoff, Jr., Phys. Rev. Lett. 65 (1990) 3169. DO1I P.J.H. Bloemen, E.A.M. van Alphen and W.J.M. de Jonge, First Kyoto-Duisburg Workshop, Duisburg (1991). illI I F.J.A. den Broeder, D. Kuiper, H.C. Donkersloot and W. Hoving, Appl. Phys. A 49 (1989) 507. [12] W. Folkerts, J. Magn. Magn. Mater. 94 (1991) 302.
Journal of Magnetism and Magnetic Materials 102 (1991) L9-L14 North-Holland
-d
rzz
Letter to the Editor
Antiparallel and perpendicular for Co/Ru multilayers
magnetization
alignment
H.W. van Kesteren, F.J.A. den Broeder Philips Research Laboratories, P.O. Box 80000, 5600 JA Eindhouen, Netherlands
P.J.H. Bloemen, E.A.M. van Alphen and W.J.M. de Jonge Eindhoven University of Technology, Department of Physics, 5600 MB Eindhoven, Netherlands
Received 27 May 1991; in revised form 3 July 1991
Several Co/Ru multilayers were grown in which the magnetization directions of consecutive Co layers were aligned antiparallel and along the film normal. The magnetization curves for these multilayers agree With the curves obtained by minimizing the total enerq including anisotropy and interlayer coupling. Thzse multilayers were part of a series with Co thicknesses of 11 and 22 A and Ru thicknesses varying between 5 and 20 A. Torque and magnetization measurements showed that the multilayers with 11 A Co had a perpendicular easy axis while for the 22 A Co series an in-plane preferential orientation was observed. A strong antiferromagnetic interlayer coupling was obtained for Ru thicknesses of about 8 A.
Magnetic multilayer films based on ultrathin layers of transition metals attracted a great deal of attention during the last decade due to a number of novel properties. Depending on the composition of the multilayer, large perpendicular anisotropy [1,2], giant magnetoresistance [3] and oscillations in the magnitude [41 and sign [5] of the exchange coupling between adjacent ferromagnetic layers can be obtained. The attention comes from researchers interested in both the origin of these novel effects, as well as in using them in applications. For example Co/Pt multilayers, which have a perpendicular anisotropy for thin Co layers, are promising candidates for future magneto-optical recording media [6]. Multilayers consisting of alternating thin layers of Co and Ru form an interesting system. Parkin et al. [4] has shown that for Co/Ru multilayers with 18 A Co layers, the interlayer exchange coupling oscillates as a function of the Ru thickness. Furthermore, Sakurai et al. [7] has obtained a perpendicular anisotropy in this system for Co 0304-8853/91/$03.50
layers thinner than 18 A and a fixed Ru thickness of 20 A. In this letter, we report that by appropriate choice of the Co and Ru thickness, multilayer films could be obtained which have both a perpendicular anisotropy as well as antiferromagnetic coupling, resulting in a perpendicular and antiparallel alignment of the magnetization in consecutive layers. Results of Panissod et al. [8] for Co/Ru multilayers and of Bennett et al. [9] for the Fe/Cu system also indicate the existence of such a state. Co/Ru multilayers were deposited by e-beam evaporation at ambient temperature on glass and on Si substrates. The pressure during vapor deposition was 2 x lo-’ mbar and the Geposition rate for Co and Ru between 1 and 4 A/s. Following Parkin [04]and Sakurai[7], a base layer consisting of 500 A Pd and 100 A Ru was deposited on the substrate before deposition of the Co/Ru multilayer. Two series of six samples were prepared: one series with a Co thickness of 11 A and the other series with 22 A Co. For both series, the
0 1991 - Elsevier Science Publishers B.V. All rights reserved
LlO
H. I+!van Kesteren et al. / Magnetization aligntnent for Co /Ru multilayers
Ru thjckness varied between 5 and 20 A in steps of 3 A with a total number of 10 repetitions for all the samples. The structure of the films was examined using X-ray diffraction at low and high angles with the scattering vector perpendicular to the film plane. The Pd base layer showed besides a strong (111) peak, also a roughly 7 times weaker (002) peak indicating a partial [OOll,, orientation of the Pd. The period determined from the superlattice
diffraction was typically within 5% of the values set during growth. The rocking curve of the main high angle multilayer peak indicated that the films were polycrystalline with a spread in [ill] orientation of 20 o (fwhm). For the magnetic characterization of the samples, we used a Vibrating Sample Magnetometer (VSM) and a torque magnetometer. Magnetization curves versus in-plane and perpendicular magnetic field are shown in figs. la-f. These
lOx(llA
Co+l4A
Ru)
_
I-
(a)
I
-1
0
1
2
Applied field poH (T)
Applied field poH (T)
I
-0.25
0 Applied field p,H
Fig. 1. Perpendicular
(c) 0.5
0.25
-2
(T)
and in-plane
0
-1
1
Applied field poH (T)
hysteresis
loops for Co/Ru
multilayers
with thicknesses
as indicated.
2
Lll
H. W van Kesteren et al. / Magnetization alignment for Co / Ru multilayers
-1 .o
-0.5
0.0
0.5
1.0
Applied field voH (T)
0.5
0.0
-0.5
1.0
Applied field poH (T)
Fig. 1. (continued).
hysteresis loops depicted the main effects observed for the two series of Co/Ru multilayers. 4s can be seen from figs. la-c, the films with 11 A Co had an easy axis perpendicular to the film plane, while figs. Id-f show that the multilayers with 22 A Co had an in-plane easy magnetization direction. The in-plane and perpendicular saturation fields for the multilayers with 5 and 8 A Ru were over 1.7 T, the maximum field available in the VSM. These high saturation fields were clear signatures of an antiferromagnetic interlayer exchange coupling. The (extrapolated) perpendicular and in-plane saturation fields for all the samples are given in table 1. For the 11 A as well as for the 22 A Co series the saturation fields become much lower for the0 multilayers with Ru thickness$s larger than 8 A. For the multilayers with 20 A Ru there is again a rather small increase of the in-plane saturation field. However, the clear oscillation in the saturation field versus Ru thickness a observed by Parkin et al. for a 20 A Co series is less clear for our samples. This is most likely related to the different deposition techniques used i.e. sputtering and evaporation, which lead to different interface structures. For the Co/Cu system, Bloemen et al. noted that the deposition technique can have a strong influence
on the coupling behaviour [lo]. A better signature for the antiferromagnetic interlayfr coupling0 than the saturation field for the 11 A Co/20 A Ru multilayer is the shape of the magnetization curve. The crossing of, the in-pkane and perpeadicular lpops for the 11 A Co/8 A Ru and 11 A Co/20 A Ru multilayers (figs. la and c) is interpreted as due to the combination of a perpendicular anisotropy and an antiferromagnetic coupling. This is illustrated in fig. 2 where the theoretically expected magnetization curves are shown. The linear in-plane curve represents a process of rotation of the magnetizations against the antiferromagnetic coupling and the magnetic anisotropy. The perpendicular curve shows a zero magnetizaTable 1 In-plane (H, ,,) and perpendicular (H, I ) saturation fields for Co/Ru multilayers with various Co and Ru thicknesses
t ..=llA
t,=22A
tR” (AI
Hq (T)
H,, (‘0
Hs,, 0)
H,, (T)
5 8 11 14 17 20
>2 = 3.5 21 0.5 0.6 0.6
>2 2.5 0.6 0.2 0.2 0.2
>2 1.8 0.5 0.2 0.2 0.3
>2 2.1 1.0 0.5 0.5 0.5
H. W. van Kesteren et al. / Magnetization
Applied field poH
Fig. 2. Theoretical magnetization curves for a multilayer with a perpendicular anisotropy and an antiferromagnetic interlayer exchange coupling.
tion at low fields (antiparallel perpendicular alignment), followed by a spirrflop transition at a certain switching field resulting in a jump of the magnetization, and for high fields a linear part which corresponds again to rotation of the magnetizations towards the field direction. The underlying model and a more quantitative treatment of the loops will be discussed later. Clearly the loops of the 11 A Co/8 A Ru and 11 A Co/20 A Ru multilayers (fig. la and c) are qualitatively in agreement with these curves. The switching fields were not sharp probably due to a distribution of coupling strengths resulting from variations in the Ru thicknesses within the samples. The switching field as well as the saturation field of the 11 A Co/20 A Ru multilayer were much lower than the corresponding fields for the 11 A Co/8 A Ru multilayer, indicating a strong decrease of the coupling wit,h increasgg Ru thickness. The loops of the 11 A Co/5 A Ru multilayer were also characteristic of antiferromagnetic coupling but the switching field was too poorly defined to reliably estimate its value. This is related to the fact that the thickness fluctuations for thinner Ru layers cause a larger spread in switching fields.
alignment for Co / Ru multilayers
The hysteresis loops of the multilayers with 11 and 17 A Ru were similar to the ones of the 14 A Ru multilayers and also to e.g. ferromagnetically coupled Co/Pd multilayers [l], indicating a mainly ferromagnetic coupling for these Co/Ru multilayers. The saturation magnetization pOMS per unit Co volume of 1.4 T for the multilayers with 11 8, Co and of 1.6 T for the series with 22 A Co was low compared to the bulk Co magnetization of 1.76 T. This indicated either a lower Curie temperature for the Co/Ru multilayers, interdiffusion of Co and Ru, or moment reduction of Co interface atoms by the interaction with Ru. In the latter case the lower magnetization could be explained by assuming the presence of approximately two interface atomic Co layers per Co layer which had lost their magnetization. The effective magnetic anisotropy energy K was determined by torque measurements. To prevent complications from a strong antiferromagnetic coupling in these measurements, only the Co/Ru multilayers with relatively low saturation fields were measured. For the series with t = 11 and 22 A Co we found anisotropy energies per unit Co volume of 0.12 and -0.29 MJ/m3 respectively. A rough estimate of the volume (K,) and interface (K,) contribution to the anisotropy could be obtained from these two values assuming the linear relation [l] Kt=K,t=2K,.
(1)
Here, K, includes the shape anisotropy. This yielded an interface anisotropy K, of 0.5 mJ/m’ and a volume anisotropy contribution K, of - 0.7 MJ/m3. These values are comparable to those obtained for Co/Pd multilayers with a 11111texture [ll]. With our two data points the in-plane to perpendicular cross;over for Co/Ru multilayers is estimated at 13 A Co. This value is slightly lower than the number of 18 A reported by Sakurai et al. [7], mainly by a more negative volume contribution to the anisotropy for our samples. To interpret the observed shapes of the Co/Ru magnetization curves more quantitatively, we considered a sandwich structure consisting of two
H. W. van Kesteren et al. / Magnetization alignment for Co / Ru multilayers
identical magnetic layers separated by a nonmagnetic layer as an approximation of a multilayer with a large number of periods. The energy E per unit volume of the ferromagnetic material is given by WI E = - ~poMsH[cos -J
cos(41 - 4,)
41 + cos &] +
Eanis
(2)
with 4i and & the angles between the applied field H and the magnetization directions of the two layers and J the coupling energy. J can be converted to an interlayer coupling per unit interface area using Ji =Jt with t the thickness of the ferromagnetic layer. The term describing the anisotropy energy Eanis depends on the direction of the easy axis and the direction of the applied field with respect to the easy axis. The anisotropy for in-plane rotation will be neglected. We will consider three cases. For an in-plane field and a film with an in-plane preferential orientation (K < O), the anisotropy energy is constant and can be set equal to zero Eanis = 0.
(3)
For an in-plane field and a film with a perpendicular anisotropy (K > 01, the anisotropy energy is given by Eanis = - iK[sin24,
+ sin24,] .
Finally, for a perpendicular energy equals Eanis = - OK [ COS2~1
(4)
field, the anisotropy
+ COS2~2]
(5)
both for in-plane as well as for perpendicular anisotropy. By minimizing E and using & = -+2 one derives easily the expressions for the magnetization along the field in case of an antiferromagnetic interlayer coupling (J < 0). From eqs. (2)-(41, we obtain for an in-plane field M/M, = -/.L&~H/~J
(K < 0)
(6)
and M/M,=
-p,,M,H/2(2J-K)
For a perpendicular in M/M, = j~~A4,H/2(2J
(K>O).
(7)
field, eqs. (2) and (5) result + K)
(8)
L13
restricted to - 25 > K. Expressions for the saturation fields can be easily obtained from eqs. (6)-(8) by setting M equal to h4,. For multilayers with a perpendicular anisotropy, expression (8) is only valid for fields higher than the field required to “switch” both magnetizations in the direction of the applied field. This switching becomes energetically favourable for field strengths for which the energy given by (2) and (5) (with & = -42) becomes lower than the energy for the antiparallel perpendicular alignment ( -K + J). This switching field is given by
(9) again restricted to - 2 J > K. The theoretical magnetization curves for a sandwich with a perpendicular easy axis and an antiferromagnetic interlayer coupling as given by eqs. (7)-(91, are shown in fig. 2. For the 11 A Co/8 A Ru multilayer the interlayer coupling energy could be determined from the perpendicular switching field of about 0.65 T and the previously determined anisotropy and saturation magnetization with eq. (9), yielding Ji = -0.7 mJ/m2. This value is slightly smaller than the - 0.9 mJ/m2 obtained from the extrapolated perpendicular and in-plane saturation fields as giveain table t. Using the saturation fields for the 22 A Co/8 A Ru multilayer one obtaines an interlayer coupling of - 1.3 mJ/m2. This is an average value in view of the different low and high field slopes of the magnetization curve for this sample. Using the high field slope, an interlayer coupling of - 1.5 mJ/m2 is found. In computing Ji from the high field slope, we corrected for the volume fraction of 75% involved. This fraction was obtained from the percentage of the magnetization saturating with the high field slope with respect to the saturation magnetization of 1.6 T. Applying the a!alysis to the antiferromagnetically coupled 11 A Co/20 A Ru multilayer yielded an exchange-coupling energy of about -0.08 mJ/m2, i.e. an order of magnitude lower. The exthange coupling per unit interface area for an 8 A Ru interlayer has been found to be about twice as large for the 22 A Co sample as
L14
H. W. uan Kesteren et al. / Magnetization alignment for Co / Ru multilayers
for the 11 A Co sample although a comparable coupling is expected. One should be careful however, in drawing conclusions from these data only, because the exchange coupling varies by a factor of two for a 1 A Ru thickness variation in the 6-9 A Ru range as shown by Parkin et al. In conclusion, we have shown that Co/Ru multilayers have a perpendicuiar magnetization for a Co layer thickneoss of 11 A and an in-plane magnetization for 22 A Co. The estimated crossing over from perpendicular to in-plane anisotropy occurred at a Co thickness of about 13 A. The interlayer exchange coupling was found to oscillate with Ru thickness. Several Co/Ru multilayers were studied which had a perpendicular anisotropy in combination with an antiferromagnetic interlayer exchange coupling. The magnetization curves for these multilayers are in qualitative agreement with the model presented and were analysed to obtain the magnitudes of the interlayer exchange coupling for various Ru and Co thicknesses.
Acknowledgements
We wish to thank H.C. Donkersloot and J.M. Kerkhof for the vapor deposition of the Co/Ru multilayers.
References
111H.J.G. Draaisma, W.J.M. de Jonge, and F.J.A. den Broeder, J. Magn. Magn. Mater. 66 (1987) 351.
121F.J.A. den Broeder, W. Hoving and P.J.H. Bloemen, J. Magn. Magn. Mater. 93 (1991) 562. 131 M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [41 S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [51 P. Griinberg, S. Demokritov, A. Fuss, M. Vohl and J.A. Wolf, Proc. of the MMM Conf., San Diego (19901, J. Appl. Phys. (in press). [61 W.B. Zeper, F.J.A.M. Greidanus and P.F. Garcia, IEEE Trans. Magn. MAG-25 (1989) 3764. [71 M. Sakurai, T. Takahata and I. Moritani, Conf. on Magnetics, Japan (1990). k31 P. Panissod, K. Ounadjela, A. Dinia, D. Muller, G. Suran, F. Petroff, A. Arbaoui and C. Meny, First Kyoto-Duisburg Workshop, Duisburg (1991). [91 W.R. Bennett, W. Schwanacher and W.F. Egelhoff, Jr., Phys. Rev. Lett. 65 (1990) 3169. DO1I P.J.H. Bloemen, E.A.M. van Alphen and W.J.M. de Jonge, First Kyoto-Duisburg Workshop, Duisburg (1991). illI I F.J.A. den Broeder, D. Kuiper, H.C. Donkersloot and W. Hoving, Appl. Phys. A 49 (1989) 507. [12] W. Folkerts, J. Magn. Magn. Mater. 94 (1991) 302.