Effect of intercalating Ni layers on the magnetic and magneto-optic properties of Co/Pt multilayers

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ELSEVIER Journal of Magnetism and Magnetic Materials 174 (1997) LI7 L20

Journal of magnetism and maonotlc materials

L e t t e r t o t h e E d i t o r

Effect of intercalating Ni layers on the magnetic and magneto-optic properties of Co/Pt multilayers

R. Krishnan a'*, A. D a s a, N. Keller a, H. Lassri b, M. Porte a, M. T e s s i e r a

~Laboratoire de Magn~tisme et d'Optique de l'Universitk de Versailles, CNRS URA 1531, 45, Avenue des Etats Unis, 78035 Versailles Cedex, France

~ Laboratoire de Physique des Matkriaux et de Micro~lectronique, Facultb des Sciences, UniversitO de Hassan II, B.P. 5366, Ain Chock, Casablanca, Morocco

Received 2 June 1997

Abstract

Co/Pt/Ni/Pt/multilayers have been prepared by RF sputtering to study the effect of Ni layer thickness on the magnetic and magneto-optical properties. The magnetization and the Faraday rotation have been interpreted in terms of the properties of Ni layers in Ni/Pt and a good agreement is obtained. The squareness of the loop of Co 5 ,~/Pt 15 A increases from 0.7 to 1.0 with the addition of very thin Ni layers (~< 5 A). For thicker Ni layers the loop shows in-plane character.

Keywords." Multilayers; Magnetization; Magneto-optical properties

C o / P t multilayers have been studied intensely because they are potential candidates for magneto- optical recording in the short wavelength region [1 3]. This material research was followed by work by several authors including ourselves to develop multilayers with lower Curie temperature because the Tc of C o / P t is relatively high. Writing at such high temperatures demands a high power laser which could potentially destroy the M L structure.

One practical solution is to use a C o - N i alloy layer instead of pure Co [4-6]. The addition of Ni to Co

* Corresponding author. Tel.: + 33 1 3925 4658; fax: + 33 1 3925 4652; e-mail: krishnan@physique.uvsq.fr.

not only reduces Tc, but also contributes to the surface anisotropy and shifts the Kerr rotation peak to lower energies, which is beneficial for ap- plications in the blue wavelength [6]. F o r instance, we have shown that the multilayers sample (Coo.3sNio.65)/Pt shows very interesting properties for magneto-optic information storage with Hc =

1.2 kOe and a K e r r rotation of 0.28 ° near 3 eV which is higher than what one gets with C o / P t multilayers.

It is therefore interesting to prepare structures where a Ni layer can be intercalated in a C o / Pt structure, such as P t / C o / P t / N i / P t / C o and to study its inclusion on the magnetic properties.

P l l S0304-88 5 3(97)00 1 99-6

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thicknesses to stabilize the perpendicular magnetization and vary the Ni thicknesses to see the effect on the various properties. In this paper we describe some results of our studies.

The multilayer samples were RF sputter deposited on water-cooled glass substrates. The system was first pumped down to about 6 x 10 .8 Torr followed by overnight baking. The RF power was about 50 W and the sputter rate was in the range 10-15A/min. The Ar pressure was fixed at 7 mTorr. The layer thickness was measured in situ using a quartz oscillator. First, a buffer Pt layer 100 A was deposited on which the multilayer was grown. The last layer was 50 A thick Pt which served as a protective layer. Most of the samples were prepared keeping the Co layer thickness t(Co) and t(Pt) fixed at 5 and 15 A, respectively. The variable parameter was t(Ni) which ranged from 0 to 40 A. A few samples were also made where t(Pt) was reduced to 5 A in order to increase the strength of the interlayer coupling, the details of which will be described later. The unit structure of the sample corresponds to Co 5 A/Pt 15

A/tNi/Pt

^{15 A and}

7 such units were grown. Some C o / P t and Ni/Pt samples were also made with t(Pt) = 15 ,~ and with different values of t(Co) and t(Ni) for reference purpose, such as X-ray diffraction studies for cali- brating the quartz oscillator.

Magnetization and M H loops were obtained at 295 K using a vibration sample magnetometer.

Faraday rotation loops were taken at the laser wavelength of 6323A using a magneto-optic set-up employing a photoelastic modulator with a sensitivity of about (5 x 10-4) °. The Curie temperature Tc of the samples was determined by studying the Faraday rotation as a function of temperature.

First let us analyze the magnetization results. As a first approach the magnetization (M) was calculated taking into account both t(Co) and t(Ni) and the result is shown in Fig. I. The magnetization initially drops with increasing t(Ni) and then it stabilizes. This trend could be expected from the dilution effect. However, the variation in M cannot be quantitatively accounted for in this simple man- ner. The main reason is that the M of the Ni layer also varies with t(Ni) and it is known from our earlier studies that samples with t(Ni) < 10,~ in

1 6 0 0

1200 E

800 400

i

0

i i

o t(Co+Ni)

• t ( C o ) ' c a l

i 0 i i

O i i O

@ 0

0

I t I I

t (Ni) in

0 10 20 30 40 50 60

Fig. 1. The variation of the magnetization with t(Ni) at 295 K.

Both experimental and calculated data are shown.

Ni/Pt are non-magnetic at room temperature [7].

Even in the present system the above result should be still valid as a first approximation because there can be no exchange coupling between Ni and Co considering their separation. So for samples with t(Ni) < 10 A, it is not justifiable to take into account t(Ni) for calculating M.

We, therefore, calculated M for the multilayers in the following way. M(Co) was obtained from C o / P t sample with t(Co) = 5 A. The value of M(Ni) as a function of t(Ni) at room temperature was taken from our earlier work [7]. The result of the calcu- lation is also shown in Fig. 1. The agreement is fairly good except for the samples with t(Ni) ~< 5 k,.

This discrepancy can be minimized if M for these samples is calculated neglecting t(Ni) as shown in Fig. 1. However, it can be seen in Fig. 1 that for t(Ni) = 8.4 A, neglecting t(Ni) gives a very large M indicating it is contributing to M in contrast to the case of Ni/Pt multilayers [7].

The Curie temperature of a thin Ni layer could be increased by introducing ferromagnetic coupling between the Ni and Co layers. This can be achieved by decreasing t(Pt). So we prepared three multilayer samples with t ( P t ) = 5 A and t ( N i ) = 5, 8.4 and 12 A, respectively, keeping t ( C o ) = 5 A. The separation between Ni and Co being considerably smaller, this should favor exchange coupling. In- deed, M for these samples showed an increase of about 40% with respect to that with t(Pt) = 15 A,

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R. Krishnan et al. / Journal of Magnetism and Magnetic Materials 174 (I997) L17 L20 LI9

indicating that even a 5 A thick Ni layer now be- comes magnetic.

The variation of the F a r a d a y rotation (Or) with t(Ni) resembles that of M as shown in Fig. 1, where the rotation is expressed in terms of t(Co + Ni). As we had discussed in the case of M similar argu- ments are also valid in this case, if we neglect, of course, the effect arising from multiple reflections.

So, using the additive rule and expressing the rotation as arising from both Co and Ni layers, we can write that Ov (measured) = 0v(Co) + 0F(Ni). The first term is experimentally known from the sample Co5 A / P t l 5 ,~ and the second term was obtained from measurements on N i / P t samples with 8.4 ~<

t(Ni) ~< 30 A. (Table 1). The calculated points are shown in Fig. 2 The agreement is fair.

Let us consider now the magneto-optical loops.

The loops are modified with increasing t(Ni). The remanence ratio (MR~Ms) is 0.7 for t ( N i ) = 0, but increases to 1.0 for t(Ni) < 8.4 A and for thicker Ni layers MR~Ms starts decreasing and the loops get more and more in-plane type, as shown in Fig. 3.

This is due to the decrease in the contribution to the surface anisotropy (Ks) as the Ni layer thickness increases because it is known that it varies as 1/t.

The increase in MR~Ms also indirectly shows that thin Ni layers are p r o b a b l y magnetic. The coerciv- ity (Ho) also increases for thin Ni layers then decreases, as shown in Fig. 3. We have not optimized the deposition parameters to increase Ho.

In order to determine Tc, we studied the temperature dependence of the F a r a d a y rotation for samples with t(Pt) = 15 ~.. The upper limit of the

"T

._=

16

12

8 o

°o

4

@ ©

i

o Exp

, c a l

' ? d o ® ,

0 I ~ i I

0 10 20 30 40 50

t (Ni) in

Fig. 2. The variation of the Faraday rotation OF with t(Ni) at 2 = 6323 ~,. Both experimental and calculated data are shown.

~t3

1.2 1 0.8 0.6 0.4 0.2 0 0

i

w - ~ - - - n .

,, • ',,,

I

4 8

• MR / MS

• H c

"'e

I I

12 16 t(Ni) in A

250

200 150 ~=

50 0 20

Fig. 3. The variation of the remanence ratio MR/Ms and Hc with t(Ni) at 295 K. The dotted line is a guide to the eye only.

Table 1

F a r a d a y rotation in N i / P t multilayer samples with t(Pt) = 15 ,~

measured at )~ = 6323 ,~. The values given are from interpola- tion from the curve of t(Ni) dependence of Or, obtained on a series of samples with varying t(Ni)

t(Ni) in ,~ 0v in 105 d e g c m -1 _+ 5%

5.0 0

8.4 0.2

12.0 1.05

18.0 1.5

24.0 1.75

30.0 2.0

40.0 2.0

temperature was kept at 500 K, because when heated beyond this point some irreversible changes take place, due to interdiffusion. Therefore, we ex- trapolated the data by assuming the well-known power law Ov oc T °~ which is valid for T / T c >~

0.75, and hence, determined the T c. F o r t(Ni) = 0, Tc was 593 K and with t(Ni) ~< 5 A, Tc decreases to about 540 K. However, for thicker Ni layers Tc increases again in order to stabilize at a value of (280 _+ 10)°C. In this respect, this system is less practical to tune Tc than Co N i / P t [6].

Anisotropy measurements are necessary to understand more quantitatively the changes in the

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L20 R. Krishnan et al. / Journal o f Magnetism and Magnetic Materials 174 (1997) L17 L20

loop. Studies at low temperatures could be interesting because then the thin layers of Ni would be- come ferromagnetic and modify effectively the properties. Such studies are in progress and will be reported in the near future.

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