Magnetic properties of Co x Ni1−x /Pt multilayers
R. Krishnan, H. Lassri, M. Seddat, M. Porte, and M. Tessier
Citation: Applied Physics Letters 64, 2312 (1994); doi: 10.1063/1.111628 View online: http://dx.doi.org/10.1063/1.111628
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/64/17?ver=pdfcov Published by the AIP Publishing
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Magnetic properties of Co,Ni, -,/Pt multilayers
R. Krishnan, H. Lassri, M. Seddat, M. Porte, and M. Tessier
Laboratoire de Mug&time et Mat&iaux Magnktiques, C.N.R.S. 92195 Meudon, France (Received 12 November 1993; accepted for publication 3 February 1994)
We have prepared Co,Nir-JPt multilayers by evaporation under ultrahigh vacuum conditions and studied their magnetic and magneto-optical properties. The addition of Co to Ni leads to an increase in both the surface anisotropy and T,. It is found that the surface anisotropy K, of Co,Ni,-,/Pt multilayers can be expressed as the sum of K, of Ni/Pt and of Co/l? times their respective concentration. In Co,,Ni,,/Pt multilayers, for t(Co,Ni)=0.45 nm, one observes a perpendicular M-H loop with a good rectangularity and a coercivity of 1 kOe and T, of 180 “C. These characteristics are very interesting for application in magneto-optic storage.
One of the most promising applications of magnetic multilayers (ML) is for use as magneto-optic (MO) storage media and multilayers such as Co/Pt1,2 and Co/Pd3 have at- tracted much attention recently since they are potential can- didates for such applications. One way to increase the stor- age density is to operate in the blue region of the optical spectrum where the polar Kerr rotation of Co/Pt, for ex- ample, is relatively large as compared to rare-earth-transition metal amorphous films which are currently used. However, the Curie temperature (T,) of Co/Pt, with typical Co layers of the order of 0.4 rnn thick, is close to 400 “C which is rather high for practical applications. This would not only require high power lasers for writing but also would encour- age interfacial diffusion during the write process at such tem-
peratures, eventually destroying the multilayered structure.
Therefore, there is some need to search for other materiaIs with lower Tc . We have recently reported the presence of uniaxial anisotropy in Ni/Pt ML for Ni layers of the order of 1.5 nm thick, which arises from the surface anisotropy.”
Similar results have also been found for Ni/Pd ML by Flevaris.5 We found that the Curie temperature of Ni/Pt ML with t(Ni)<l nm is close to roomtemperature. This result led us to the idea that by alloying Co with Ni one could raise the T, of the ML and indeed tailor it to suit any application.
We describe in this paper our study of the magnetic proper- ties of Co,Ni,-,/Pt multilayers and we focus our attention on the uniaxial anisotropy.
thicknesses. Magnetization was measured using a vibrating sample magnetometer (VSM) and the anisotropy was deter- mined by a homemade sensitive torque meter. The M-H loops were also taken using the VSM. The above properties were measured in the temperature range 6-300 K. In order to determine Tc, magneto-optical Faraday loops were also taken at the laser wavelength of 633 nm in the temperature range 300-600 K.
First let us discuss the anisotropy in these materials. As is well known, the magnetic layer thickness dependence of the measured anisotropy (K,J in the ML can be expressed as
K,,=KV-t2Ks t-l, (1)
where the volume anisotropy K,= Kc,,,-2z-M2 (Kc,,,st arises from the crystalline anisotropy and also the magneto- elastic one and 2rAJ2 is the demagnetization energy) and K, the surface anisotropy. Therefore, by plotting the product KeE t(Co,Ni) vs t(Co,Ni), one obtains a straight line whose slope gives K, and the intercept on the ordinate gives 2K,.
Such an analysis for Ni/Pt ML at 5 K, gave a surface anisotropy of $0.17 erg/cmm2. The perpendicular M-H loop of the Ni/Pt sample {9,20} 32 at 6 K was found to be rectan- gular with a coercivity as high as 4.7 kOe, as reported in Ref.
4.
As the Co concentration increases, the effect on the an- isotropy of the ML is clearly ‘seen. For x=0.1, perpendicular The multilayers were prepared by sequential dual
e-beam evaporation under ultrahigh vacuum conditions. The details of the deposition system can be found in Ref. 4. For depositing Co,Ni,-, alloy layers, we have first prepared an alloy ingot and used it as a source. We have subsequently determined the composition by analyzing a lOO-nm-thick layer. No difference in the composition between the source and the layer could be. detected. Both glass and silicon were used as substrates and were held at 30 “C during deposition.
All the samples were grown on Pt buffer layers 10 nm thick.
The Pt layer thickness t(Pt) was kept constant at 15 nm and that of Co-Ni was varied in the range 0.4 to 3.0 mn. Some samples were also made where t(Pt) was 0.8 nm. The growth parameters will be designated as {t(Co,Ni),t(PT)}N, where N stands for the number of bilayers.
4
-
Ii B sK o z95K
Low and high angle x-ray diffraction studied were made
to verify the periodic structure and to calculate the layer FIG. 1. The variation of the product K,Ert(CoO,lNi,,) as a function of t(Co&Iii.~) in (Co,#ii&Pt ML at 295 and 5 K.
2312 Appl. Pbys. Lett. 64 (17), 25 April 1994 0003-6951/94/64(17)/2312/3/$6.00 Q 1994 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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TABLE I. Volume and surface anisotropy of some (Co-Ni-)/Pt ML samples at 5 K. For all samples reported here t(Pt)=lJ nm.
Kv K,
No. ii (106erg cm 3, {erg cm-‘)
1 1 -ll.oa 0.60”
2 0.3 -4.1 0.33
3 0.1 -2.2 0.25
4 0 -1.9 0.17
%lues at 295 K.
anisotropy appears for t(Co,Ni)<l nm even at room tem- perature and even though T, is still low for thin magnetic layers. Figure 1 shows the variation of KeE t(Co,Ni) as a function of t(Co,Ni) for x=0.1 at 295 and 5 K. It is seen that KS increases considerably at 5 K and particularly for thinner layers. This indicates the proximity of T, to 295 K. Also, it is noteworthy that at 295 K the data points fall below the straight line for layers thinner than 1 nm which again is attributed to the relatively lower Tc. However, at 5 K the data points are better aligned along the straight line. This also indicates that the interface quality is maintained for lay- ers as thin as 0.5 mn. By linear regression we find that at 5 K, KS=+025 erg cm-‘, which is higher than that for Ni/Pt.
The addition of 10 at. % of Co also increases the bulk an- isotropy as shown in Table I. This arises from the contribu- tion of Co where the crystalline anisotropy is considerably higher than that of Ni. The variation of the product Ken t(Co,Nij as a function of t(Co,Ni) for x =0.3 is shown in Fig. 2 both at 295 and 5 K. It is seen that the Curie tempera- ture for this series is already high enough and hence the relative increase in the anisotropy at 6 K is much less than in the case for x=0.1. The surface anisotropy KS has further increased to +0.33 erg cm-‘. The results are shown in Table I, where we have also included those for Ni/Pt at 5 K and Co/Pt ML at 295 K. It is recalled that, since the Curie tem- perature of Co/Pt samples are higher and around 400 “C, the
FIG. 2. The variation of the product K,Ext(Coo~3Ni,,,) as a function of t(Co~.3Ni& in (Coo,3Ni&F’t ML at 295 and 5 K.
FIG. 3. The Cc concentration dependence of the surface anisotropy KS. The straight line is the calculated one.
anisotropy at 5 K is practically the same as that at 300 K, as we have shown earlier.6 It can be seen that KS increases roughly linearly with the Co concentration and one can ex- press the surface anisotropy of (Co-Ni)/Pt ML by
k -;““Y=K~ (x)+Kr (1-x) ,
where Kp, KY represent the surface anisotropy in Co/Pt and Ni/Pt ML. The straight line in Fig. 3 is obtained with e=O.6 and K,N’=O.17 erg cm-=, respectively. The data points for the alloy multilayers agree well the calculated val- ues. Of course, the study of more alloy compositions are needed to obtain a good confirmation of this and such studies are underway.
Figures 4 and 5 show the perpendicular M-H loops at 295 K for t(Co0.iNirJ=0.7 nm, t(Pt)=1.5 nm, and t(Co0,aNia7j=0.45 nm, t(Pt)=0.8 nm, respectively. The co- ercivity of the latter where t(Pt)=0.8 rmt is found to be 1.4 kOe, which begins to be interesting for application. More work is needed to optimize the layer thicknesses for applica- tion.
We measured the temperature dependence of the Farady rotation in order to determine the Curie temperature. This method is more sensitive than that of magnetization. in these measurements, the contribution from the substrate has been subtracted. Figure 6 shows, as a typical example, the tem- perature dependencies of the Faraday rotation (proportional
to the Iw) and the coercivity H, typical for t(Co,,1Ni0,g)=0.9 nm t(Pt)=1.5 nm, and t(Co,,Ni,,)=0.45 nm, t(Pt)=1.5 nm, respectively. It is interesting to note that the coercivity de- creases faster than the magnetization and this is more so for
1 kOe
FIG. 4. Perpendicular M-H loop for the ML for x=0.1 and with (0.7,l.S) 25.
Appl. Phys. Lett., Vol. 64, No. 17, 25 April 1994 Krishnan et a/. 2313
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1 kOe
FIG. 5. Perpendicular M-H loop for the ML for x=0.3 and with {0.4.5,0.83}
x30.
thinner layers. It is noteworthy that the Curie temperature of the sample t(Co,Ni)=0.45 nm and t(Pt)=lS nm is 180 “C, which is in the right range for applications.
In conclusion, we have prepared Co,Ni,-,/Pt ML and studied their magnetic properties. It is shown that at 5 K the surface anisotropy increases linearly with the Co concentra- tion. The ML sample (0.450.8) 30 with x=0.3 presents a perpendicular loop with good rectangularity and a coercivity of 1.4 kOe and has a Curie temperature less than 200 “C.
These characteristics make this material attractive for appli- cations, e.g., as magneto-optic storage media of very high density.
Detailed studies on the temperature dependence of the magnetic properties and polar Kerr magneto-optical spectra are being completed will be published in the near future.
This work was performed under the BRm EURAM Contract No. Breu-0153 which is gratefully acknowledged.
j a 8
"0 4.0
;b 0.8
a?
i3 i: 0.6
2 2.0
do 0.4
2
2 0.2
!-&
0 100 200
Q B
T(‘C)
FIG. 6. Temperature dependence of the Faraday rotation and the coercivity H, in ML for x=0.1 with t(Co,Ni)=lJ.9 MI and for x=0.3 with t(Co,Ni)
=0.45 nm, respectively. For both the samples t(Pt)= 1.5 nm.
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2314 Appl. Phys. Lett., Vol. 64, No. 17, 25 April 1994 Krishnan et al.
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