ELSEVIER Journal of Magnetism and Magnetic Materials 133 (1994) 453-456
journal of magnetism
~pqand magnetic materials
Magnetic and magneto-optical properties of Co-Ni/Pt multilayers
R. Krishnan *, H. Lassri
Laboratoire de Magn~tisme et Mat~riaux Magngtiques, CNRS 92195 Meudon, France
Abstract
We have prepared CoxNi a x / P t multilayers by evaporation under ultrahigh vacuum conditions and studied their magnetic and magneto-optical properties. Addition of Co to Ni increases the surface anisotropy and T c. For instance, in Co0.3Ni0.y/Pt for t(Co, N i ) = 4.5 ,~, one observes a perpendicular rectangular M - H loop with a coercivity of 800 Oe and T c of 180°C. The polar Kerr rotation of this sample shows a peak value of 0.3 ° at the photon energy of 4 eV. These characteristics are very interesting for realising magneto-optic storage media capable of working in blue light.
1. Introduction
The properties of magnetic metals are greatly modi- fied when they are present as ultrathin layers or as multilayers. Multilayers (MLs), which are artificially prepared materials, are receiving increasing attention from many researchers, due to the novel properties that they could manifest [1]. One of the interesting aspects of MLs arises from surface effects. N6el [2]
pointed out as early as 1953, the possibility of the existence of surface anisotropy. Indeed, one observes in some ML systems a contribution from the surface atoms to the uniaxial anisotropy. In fact it is interesting that classical soft magnetic metals such as, Fe and Ni become magnetically hard when prepared as multilay- ers. In this paper we discuss the case of multilayers with Ni and N i - C o alloys, both of which are known to be soft magnetic materials in bulk form. One of the most promising applications of multilayers is for MO recording as storage media. In order to increase the storage density it is necessary to operate in the blue region of the optical spectrum; the present rare earth-transition metal amorphous alloy films are no longer suitable for this purpose because of their poor
Kerr rotation. Multilayers such as C o / P t [3,4] and C o / P d [5] have attracted much attention recently and are promising candidates for application in blue light.
For instance, C o / P t ML, with Co and Pt layer thick- nesses of about 10 A present adequate properties for MO applications. However, the Curie temperature (T c) of the above ML is of the order of 400°C, which is rather high for practical applications and there is a need to search for other materials with lower T c. We recently reported, for the first time, the presence of the uniaxial anisotropy in N i / P t ML for Ni layers of
o
the order of 9 A, although the Curie temperature of these materials is below room temperature [6]. This is of fundamental importance because so far Ni has not been thought to show such uniaxial anisotropy. This result suggests that by alloying Co and Ni one could tailor the T¢ for any application. In this paper we describe the results of our study on CoxNi I x / P t multilayers. For the sake of discussion we will also recall briefly some of the results on pure C o / P t and N i / P t , that is for x = 1 and 0.
2. Experimental details
* Corresponding author. Fax: + 33 (1) 4507 5822.
The multilayers were prepared by sequential evapo- ration by dual e-beam under ultrahigh vacuum condi- tions. The pressure during film deposition was in the 0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSDI 0304-8853(94)00179-U
454 R. Krishnan, H. Lassri /Journal o f Magnetism and Magnetic Materials 133 (1994) 453 456 10 9 T o r r range. T h e evaporation rate and the thick-
ness were individually controlled by precalibrated quartz crystals. Both glass and silicon were used as substrates and were held at t e m p e r a t u r e s in the range 30-200°C. Some samples were grown on Pt buffer layers 100 ,~ thick. Generally the Pt layer thickness t(Pt) was kept constant in the range 15-20 ~ , but t(magnetic layer) was varied in the range 4 - 4 0 A. T h e growth parameters will be designated as {t(Co, Ni), t(Pt)} × N, where N is the n u m b e r of bilayers. The details of the preparation can be found in Ref. [6]. T h e composition of alloy layers was d e t e r m i n e d by electron probe microanalysis.
Low- and high-angle X-ray diffraction studies were m a d e to verify the periodic structure and to calculate the layer thickness which agreed with that obtained from the precalibrated quartz crystals.
Magnetization and M - H loops were m e a s u r e d us- ing a vibrating sample m a g n e t o m e t e r and the anisot- ropy was d e t e r m i n e d by a h o m e - m a d e sensitive t o r q u e meter. T h e above properties were m e a s u r e d in the t e m p e r a t u r e range 6 - 3 0 0 K. Magneto-optical K e r r and Faraday loops were taken at the laser wavelength of 6328 A in the t e m p e r a t u r e range 6-600 K. F o r some selected samples the magneto-optical Kerr effect spec- tra w e r e studied at r o o m t e m p e r a t u r e in the photon energy range 1.5-5.2 eV.
3. Results and discussions
W e will only briefly recall some of the relevant results here on C o / P t and N i / P t as they have been published earlier [4,6]. F o r C o / P t series, one obtains p e r p e n d i c u l a r magnetization for t(Co) < 10 A. F o r the sample {4, 14} × 15 grown at 423 K on a Pt buffer layer the loop is perfectly rectangular with the coercivity as high as 1.9 kOe. T h e Kerr loop of the above sample is shown in Fig. 1. T h e surface anisotropy in the M L can be calculated based on the p h e n o m e n o l o g i c a l model according to which the effective anisotropy Kef f that one measures can be written as:
Kef f = K v + 2Ks, (1)
where the volume anisotropy K v = Kc~y~t + 2"rrM 2 (Kcryst is the crystalline anisotropy and 2TrM 2 is the
H c = 1.9 kO
Fig, 1. Polar Kerr loop of C o / P t with {4, 14}× 15.
~ H
H c 7.80c Fig. 2. In-plane M H loop of N i / P t with {42, 20} × 11.
demagnetization energy, which is taken with a negative sign) and K s is the surface anisotropy. By plotting the product Keff X t(Co, Ni) versus t(Co, Ni), one obtains a straight line whose slope gives K v and the intercept on the ordinate gives 2K~. Such an analysis was m a d e for C o / P t M L and the surface anisotropy was found to be 0.6 erg cm 2 at 295 K [4]. T h e Curie t e m p e r a t u r e of this sample was well above 650 K and could not be d e t e r m i n e d because interdiffusion starts at these tem- p e r a t u r e s and the periodic structure is destroyed.
T h e interface plays an important role in determin- ing the properties of the multilayers and in the case of C o / P t , because the affinity for C o - P t to form an alloy is well known. Many attempts have b e e n made to characterize the interface. By studying the N M R spin- echo of Co in C o / P t , we could identify the interfacial alloy layer as Co3Pt [7].
As for N i / P t , the samples with t ( N i ) > 30 A are magnetically soft. T h e in-plane loop of the sample {42, 20} × 11 with an easy direction coercivity (H~) of 7 . 8 0 e at 295 K is shown in Fig. 2. However, as the t(Ni) is decreased the material becomes more and m o r e magnetically hard and the easy axis (normal to the film plane) H c increases remarkably to reach a few k O e for very thin layers. T h e M - H loop of the sample {9, 20} × 32 at 6 K is found to be rectangular with a coercivity as high as 4.7 kOe. A strong uniaxial anisotropy is found for samples with t ( N i ) < 20 ,~.
arising from the contribution from the surface anisotropy. D u e to the decrease in the exchange inter- action along the z-axis, the Curie t e m p e r a t u r e s of such samples decrease and are either close to or even below r o o m temperature. O n e finds K S = 0.17 erg cm -2 at 5 K [61,
In o r d e r that such multilayers could be of practical interest the Curie t e m p e r a t u r e has to be increased and so we p r o c e e d e d to investigate C o x N i I x / P t ML. W e discuss here the results for two different series with x = 0.1 and 0.3, which illustrate well the advantage of
such alloying.
F o r these two series of samples, the Pt layer thick- ness was kept fixed at 15 A and the samples were grown at 300 K on 100 A thick Pt buffer layer. The ( l i d texture could be seen from X-ray diffraction.
As the Co concentration increases, the effect on the
anisotropy of the M L is clearly seen. For x = 0.1,
perpendicular anisotropy appears for t(Co, Ni) < 10 A
R. Krishnan, H. Lassri / Journal of Magnetism and Magnetic Materials 133 (1994) 453-456 455
1.(
0.5
©
._=
0
-0.5
-1.0
\
* 1 Ni09Co0] /Pt
• 2 Nio7COo3 / Pt o 3 C o / P t
\\
o\
' ~ 20 40
I
~ , ~ ~ ( N i , C o ) in A
" I
Fig. 3. Variation of the product Kef f X t(Co, Ni) as a function of t(Co, Ni) in (CoxNi t _x)/Pt ML for x = 0.1 and 0.3.
even at r o o m t e m p e r a t u r e . First we discuss the anisotropy results in these two series.
W e ascertained that the Curie t e m p e r a t u r e of the thinnest samples studied is above 300 K which is im- p o r t a n t since otherwise the data have to be considered at 4 K. Fig. 3 shows the variation of Kef f × t(Co, Ni) as a function of t(Co, Ni) for x = 0.1 and 0.3. Also we show in the same figure for comparison our result for C o / P t taken from Ref. [4]. W e recall that the results for N i / P t M L can not be d e p i c t e d here because the Curie t e m p e r a t u r e for these samples is relatively lower.
Several points can be noted. First of all, it is seen that K s increases with an increase in Co concentration. By linear regression we find K s = 0.13 and 0.19 erg cm 2 for x = 0.1 and 0.3, respectively, and it reaches a value of 0.6 erg cm -2 for p u r e C o / P t . A t this point it is interesting to recall that Co0.3Ni0. 7 alloy is known to be
0 kOe
Fig. 4. Perpendicular M - H loop for the ML for x(Co)= 0.1 and with {7, 15}x25.
magnetically very soft with initial permeabilities as high as 105 [8]. However, the same material in the form of a very thin layer becomes magnetically hard as shown by our present studies. T h e slope of the straight line also increases with Co concentration indicating the increase in K v which arises from the contribtution of Co. Table 1 summarizes the anisotropies of some samples.
Figs. 4 and 5 show the p e r p e n d i c u l a r M - H loops for the samples with t(Co0.1Ni0. 9) = 7 A and t(Co0.3Ni0. 7) = 4.5 A respectively. T h e coercivity for the latter is found to be 800 Oe, which begins to be interesting for applications. O f course, more work is n e e d e d to optimize this result.
W e shall discuss the magneto-optical properties which are not only interesting from the f u n d a m e n t a l point of view but also for application. In o r d e r that these multilayers can be used for magneto-optical stor- age devices it is also necessary that the K e r r rotation be reasonably high. T h e polar K e r r loops taken at the laser wavelength of 6328 A are similar to the M - H loops and hence are not shown here. Earlier we re- p o r t e d [9] M O K e r r spectra in C o / P t M L and showed that there is an e n h a n c e m e n t of Kerr rotation at 4.25 eV that arises from the interaction of Pt and Co, in a g r e e m e n t with the literature [10]. This was further confirmed by us by showing that in annealed sample the peak height increases due to increased P t - C o interaction caused by the interdiffusion [9]. F o r this work we consider the M O K e r r effect spectra only for the sample with x = 0.3 and with growth parameters {4.5, 15} X 30, which is the most interesting. T h e M O K e r r effect spectra are shown in Fig. 6 and one ob- serves an increase in the M O effect towards higher energy. T h e Kerr rotation shows a peak value of 0.3 ° near 4 eV. In pure C o / P t such a peak occurred at 4.25
Table 1
Volume and surface anisotropy of some ML samples
No. Co K v K s
(x) ( × 1 0 6 e r g c m -3) (ergcm -3)
1 1 - 11.0 0.60
2 0.3 -4.1 0.19
3 0.1 - 1.2 0.13
M1
- - ' - " ~ H H c = 0 . 8 k O e
Fig. 5. Perpendicular M - H loop for the ML for x ( C o ) = 0.3
and w i t h {4.5, 15} × 30.
456 R. Krishnan, H. Lassri /Journal of Magnetism and Magnetic Materials 133 (1994) 453-456
T i . i . n ' i
0.0 " ~ ° o elliptieity
0 ° o
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0
¢~ 0
o o
-0.2 ~ _ rotcttion °o ~
" ~ O o , °o • I
£ °Oo o" ~1
- O. 2, %°OooooO°°
/
2 3 4 5
E n e r g y leVI - ~
Fig. 6. Magneto-optical Kerr effect spectra of the sample for x =0.3 and with {4.5, 15}×30.
eV, as we m e n t i o n e d earlier. So the present results are also to be attributed to similar interaction at the inter- face. T h e K e r r rotation value of 0.3 ° is only slightly smaller than for p u r e C o / P t sample of similar thick- ness, which makes the present sample quite interesting for applications for M O recording in blue light.
It is known that the magnetization is proportional to M O rotation and t h e r e f o r e we m e a s u r e d the tem- p e r a t u r e d e p e n d e n c e of the Faraday rotation to deter- mine the Curie t e m p e r a t u r e . In these m e a s u r e m e n t s the contribution from the substrate was taken into account which is negligible for T < 60°C but could a m o u n t to as m u c h as 20% n e a r the Curie t e m p e r a - ture. Fig. 7 shows the results for the series with x = 0.1 and 0.3 and for different magnetic layer thickness. T h e
• - - X = 0 . 3 •
4.0 "~
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